The piston ring has an inherent amount of radial pressure, which we feel as drag when we insert a piston into a bore, or via the force when we check the fit of the rings in the bore before fitting them to the piston. This is often done when confirming or adjusting ring end gaps. Indeed, the radial pressure or ‘ring tension’ is often expressed as a force needed to bring the ring into a form that will fit in the nominal bore size.

This radial pressure is augmented by gas pressure behind the ring, and it is this component of the ring force that can be increased through detailed design. The technique to improve pressure behind the ring is known as ‘gas porting’ or ‘gas jetting’, and the idea is to use the pressure in the combustion chamber to enhance the pressure behind the ring. This can be done by using a series of small drillings directly from close to the periphery of the piston crown through into the top ring groove. These are usually known as vertical gas jets, although they are not necessarily absolutely vertical.

The second type of gas porting, which is more common, is radial gas porting, also known as horizontal gas porting, side gas porting or lateral gas porting. These features are produced using a milling cutter rather than a drill, especially if they are added to existing pistons. The reason for this is that the radial type of gas jet is not a continuous hole, but is a semicircular cut with its axis intersecting with the top side of the ring groove.

Among the reasons that piston rings may not seal is insufficient side clearance between the ring and the groove – that is, across the width of the piston ring. To avoid ring flutter, small clearances are used, and this can prevent sufficient pressure reaching the back of the piston ring groove.

However, the use of low-tension rings and gas ports is often intentional, being a design feature from the start. Through the use of gas porting, the only time there is sufficient radial force on the ring to seal combustion pressure is when pressure is present – when we need it. At all other times, the ring tension alone forces the ring against the cylinder wall, and this means the average radial force is lower by using a low-tension ring and gas porting than would be achieved using a piston ring of higher tension and no gas porting.

Written by Wayne Ward

In many instances, piston pins are kept in their intended position with round-wire clips of varying design. Some are free to rotate within their grooves, while others are designed with features that locate in an appropriate hole or slot in the piston. For racing use, round-wire circlips are usually manufactured from a high-strength wire, and these have been covered in another previous article.

However, as usual, there are alternatives, and the Spirolox clip (often referred to as Spiralock clips or Spiral locks) was mentioned in the article on alternatives to round-wire clips, but not discussed. There are two types – single and dual coil. The single coil is essentially a circlip formed from a small, flat bar, rather than round wire. However, the double-coil type is often used for more demanding applications. Compared to a round-wire clip, the beam section of the Spirolox type gives the single-coil clip much greater stiffness, owing to a greater moment of inertia. It is the double-coil type that is normally fitted in order to retain piston pins.

To ease fitting and help take up any axial clearance in the piston, the double coil is strained axially, so that the flat clip is opened up into a helix. This is then carefully fed into the groove, sometimes with a special tool but often only with fingers. Because the clip has been strained plastically, it retains some elasticity and therefore tries to resume its ‘expanded’ form, thus taking up any clearance in the groove. As this design of clip is flat on its outside diameter, it does not work in the same way as a round-wire clip, which centralises itself within its groove, which is also machined with a round cross-section.

It is common to find two of these clips on each side of the piston pin, although this choice is by no means universal. However, if the piston is provided with a groove designed for two clips to be used then two clips should be used in order to prevent excessive axial movement of the piston pin.

Compared to a round-wire circlip, the Spirolox clip has much more abutment area and bears against the end face of the piston pin. It is the combination of size and position of the circlip groove the chamfers that controls the maximum piston pin end-float where a round-wire clip is used, but this doesn’t apply with Spirolox clips, so the chamfers on the pin are solely for ease of fitting. During manufacture, it is much easier to control the length of a pin than to control the distance between gauge diameter on opposite ends of the pin.

Written by Wayne Ward

With few exceptions, race engines use piston rings of the conventional gapped type – that is, there is a carefully controlled gap between the ends of the piston rings. A report by Jung and Jin (1) shows that, at one engine speed, the speed and direction of piston ring rotation depends on load, and it is not necessarily the case that both rings rotate.

Their study, although weighted toward the technique used in the measurements, monitored oil consumption and was able to correlate this with the relative positions of the piston ring gaps. It is perhaps unsurprising to find that oil consumption rose to a maximum when the two ring gaps were aligned, and fell to a minimum when the ring gaps were spaced 180° apart. This makes sense, as the flow losses would be greatest when the ring gaps are as far apart as possible, so for a given pressure difference there would be less flow through the gaps in the rings.

The speed of rotation of piston rings has been measured experimentally. Shaw and Nussdorfer (2) examined the phenomenon on a large engine and found that, at 1000 rpm engine speed, the piston rings were “observed to rotate as rapidly as 1 rpm”. Jung and Jin reported in more detail: on the engine they used, at 4000 rpm and 2 bar bmep, the rings rotated at 0.6 rpm in opposite directions to each other, with the second ring initially oscillating between two positions before finally beginning to rotate continuously.

At higher load, the top ring didn’t rotate, and the second ring rotated at speeds from 0.5 to 3 rpm. On the same engine and at lower engine speed, the top ring simply moved to a given angular position and then remained stationary at 2 bar load. With an increase to 4 bar, there was a change in top ring position but still no continuous rotation. The second ring was also observed to be stationary at this speed in some tests, some of which found the ring gaps aligned – the condition where oil consumption is highest.

It is clear that the piston rings in any engine lead a mysterious life where, depending on the load and speed conditions that apply, they might rotate continually, oscillate between certain positions or remain stationary. Oil consumption is found to vary with ring position and, where the rings rotate periodically, the rate of oil consumption is a function of the rotation period of the rings. Other than pinning the piston rings to prevent rotation (which is commonly done in two-stroke engines), there is little we can do to influence ring position or speed of rotation. It is clear that whatever position the rings are in during the engine build will not be maintained during service. 

References

1. Jung, S., and Jin, J., “Monitoring of Rotational Movements of Two Piston Rings in a Cylinder Using Radioisotopes”, Journal of the Korean Nuclear Society; vol 31(4); ISSN 0372-7327, August 1999

2. Shaw, M., and Nussdorfer, T., “A Visual and Photographic Study of Cylinder Lubrication”, NACA Technical Report no 850, 1946

Written by Wayne Ward

The role of surface treatments and coatings is important here: if selected correctly, they can significantly increase the durability of the component. If we look at the common example of a steel piston pin, there are a lot of possible surface treatments and coatings. However, we find that racing pins are very often made from a nitriding steel, or other types of steel that respond well to nitriding even if they weren’t ‘designed’ for this purpose.

The nitriding treatment both hardens the surface and puts it into a state of residual compressive stress. The hardening of the surface is an important factor in making the surface resistant to damage, and the compressive residual stress means the pin is much more durable – its fatigue resistance is increased markedly for any given level of service load. The resistance to surface damage is an important part of this fatigue resistance too: as the surface is difficult to damage, there is little risk of a significant stress concentration forming during the piston’s working life.

Coatings are an important consideration as well. As with nitriding, hard low-friction coatings also increase wear resistance and also reduce any tendency for the component to seize in either the con rod or the piston. The most common type of coatings we find on racing piston pins are the diamond-like carbon (DLC) group. If DLC is not properly ‘supported’ by the substrate though, there is a risk that small particles of what is a very hard, sharp and abrasive coating could be introduced into the contact between pin and piston or between pin and rod bore, where they would be likely to cause significant damage.

The effect of strengthening the surface of the component by surface hardening is an effective way to improve the durability of coatings. The hardening process, if properly specified and carried out, prevents the pin surface from yielding. While DLC coatings are very good, however, they are not known for their ability to conform if the substrate deforms plastically.

Nitrided piston pins were very popular before DLC-coated pins started to become commonly used, perhaps 15 years ago, although their superiority over non-hardened pins had been obvious for a long time. With the advent of piston pins with hard coatings that could potentially cause damage if they were to fail, the role of the nitriding is therefore more important than ever.

Written by Wayne Ward

However, a couple of new structural concepts that depart quite strikingly from the usual designs have been developed by prominent piston companies, and both are very different from each other. The first, developed by a US piston company, is a skeletal design that ‘ties’ the skirt to the pin bosses more firmly. The middle part of the skirt, which is generally not well controlled relative to the pin boss, is now connected to the pin bosses by bringing the usual webs closer to the piston centreline. The pin bosses are tied to the lateral areas of the crown by very deep ribs, and these in turn are connected to both the skirt and each other by a circumferential ‘stringer’.

It certainly looks a very stiff design, and it is perhaps easier to describe the piston as a very modern take on a slipper piston, but using modern machining techniques to take weight away where it is not required. The company cites improved stiffness and fatigue among the findings of initial testing.

The second concept comes from a UK piston supplier which has gone in the opposite direction, seeking instead to decouple the movement of the crown and the skirt as much as possible. In a conventional piston, the crown and skirt are intimately linked, and the behaviour of one affects the other. By linking both independently to the pin bosses and removing any real stiffness between the crown and skirt, both are free to move without affecting the deformation of the other.

So, under combustion and inertia loads, the skirt is no longer pushed and pulled by the crown. Consequently the skirt profile can be optimised for this, and it has been found that the skirt area can safely be reduced by a significant amount, and this has led to reduced friction. Skirt lubrication is also likely to be improved compared to a standard piston design as there is much more access for oil to lubricate the skirt-to-liner contact as the piston rises from bottom dead centre.

It is interesting to note that both of these suppliers have noted significant, although different, improvements from their new concepts. By straying from the well trodden development path, both have found new areas of performance to explore. We are likely to see both concepts become commercially available in the near future. 

Written by Wayne Ward

The structure of the piston has also been the subject of a lot of work by various suppliers in recent years, and this comes on top of a great deal of evolution in the previous three decades although, with few exceptions, aluminium is the material of choice for race engine pistons. Originally pistons were basically cylindrical, with a full skirt extending 360° around them. So-called ‘pot’ pistons are still produced for some classic applications, but ‘slipper’ pistons, with much smaller skirts and limited to bore contact on the major thrust faces of the piston, are very much lighter.

Lighter pistons are very good for engine performance, as they allow an engine to run to higher speed because inertia forces are kept as low as possible; for a given crank stroke, inertia forces increase with the square of crankshaft speed. The bearings will have a limiting load/pressure, and this is reached at lower engine speed with a heavier piston. Reduced bearing loads also mean lower frictional losses, and the smaller skirt area leads to reduced friction at the piston-bore contact.

Slipper pistons remain the norm in race engines, and their evolution has headed towards ever-smaller skirts. However, a few new developments have come to light in the past couple of years.

One US piston manufacturer has a range of pistons that aim to restore some of the rigidity of the pot piston while maintaining the low mass of the slipper type. The pistons it has developed could be said to be similar in principle to the pot piston, but with skirt relief over much of the diameter and then much of the relieve area machined for lightness, leaving a number of struts.

A British piston producer has developed a different new concept, which aims to divorce the crown and skirt in terms of structure, and this was studied in the February 2014 issue of Race Engine Technology. In a slipper piston, the skirt is joined to the crown, but the new concept allows the skirt to flex without affecting the crown, and vice versa. Usually, as the piston crown flexes under combustion loads, the skirt contact is significantly affected, and this is partly why the piston skirt is machined in the way that it is. By the deliberate uncoupling of the piston crown from the skirt, the crown can flex without deforming the skirt, which can then be optimised independently.

I saw a similar concept tested in a high-speed engine some time ago, but it lacked the structural support for the piston crown which exists in this new concept. There is a forthcoming focus article on pistons in issue 78 of Race Engine Technology, which will discuss new structural concepts among other subjects.

Written by Wayne Ward

Nowadays, in a four-stroke engine, we would typically find no more than three piston rings used on any piston, whether it be for racing or for passenger car use. The reason which has driven the use of fewer piston rings is friction, but what has allowed it is better sealing between rings and cylinder bores. The friction resulting from using an extra piston ring to improve cylinder sealing is significant, and harms engine performance and efficiency. With improved sealing behaviour owing to a better understanding of piston rings, piston design and cylinder bore surfaces, we can often  manage happily with three rings for most applications. Where a ring is deleted, it is one of the compression rings – in almost all circumstances, an oil control ring is required in a four-stroke engine. There are some heavy-duty diesel applications though that still use three compression rings and an oil control ring.

Knowing that there is a gain to be had from deleting a piston ring in terms of friction is reason enough to dispense with one of them where possible, but the benefits go further. One obvious benefit is that the piston can be developed to be lighter, as we require less depth of material in which the ring grooves are cut. Taking this a step further allows us to decrease the distance between the piston pin bore and the crown. In order to maintain the same compression ratio, we might choose to use a longer con rod, but this also improves efficiency owing to lower rod articulation angles and therefore lower piston thrust forces.

The lighter piston and lower rod articulation means that reciprocating forces are reduced, and this can mean an efficiency improvement as bearing loads will be lower. The con rod, being less highly stressed, can be re-optimised based around a lighter piston to give a mass reduction or a possible gain in durability.

The gains don’t stop here, as it might be possible to reduce the counterweight mass on the crankshaft, leading to lower mass and inertia. In lowering the distance between the pin bore and the crown, we could alternatively have opted to keep the existing rod and choose to reduce the distance between the crankshaft bore and the top of the cylinder block.

By the simple expedient of reducing the number of piston rings, we can make quite significant differences to engine efficiency and mass. Along with the reduction of reciprocating valvetrain mass, piston assembly mass reduction is one of the more potent tools for making the whole engine lighter, as it opens the way for mass reduction in other components. So, where sealing is adequate and durability is sufficient, there is a real incentive to use fewer piston rings. The same logic also applies to two-stroke engines – racing two-strokes generally use only a single compression ring, as oil control rings are not necessary in a conventional two-stroke engine. 

Written by Wayne Ward

The pin bore is a case in point. If we look at it simply, it is little more than a hole through two bosses. Look more closely though and we perhaps notice some other features such as circlip bores and oil grooves; there is often more engineering involved than meets the eye here.

The first obvious aspect to get right with the pin bore is its size. It needs to provide a suitable clearance to the piston pin in all circumstances, at all operating temperatures and with just the right material conditions on the diameter of the pin.

The surface finish of the pin bore is also critical, and honing is the method most often used for finishing pin bores. Honing produces a low-roughness surface finish with a ‘structure’ that retains some oil in the bore. It also makes it easy to remove tiny amounts of material from the bore, so represents a good way to finish the bore to a tight tolerance.

Any axial oil grooves in the bore need to be placed in the correct positions. If they coincide with areas of significant loading then high contact stresses can result on the edges of the grooves, deforming the material and allowing the pin more movement off-centre. The circlip grooves machined into the pin bores are also critical, as they control the axial float of the piston pin.

We must be sure that the pin is not able to float in any circumstances. That means the circlip pin grooves need to have the correct tolerances applied on both size and position, and need to be designed in conjunction with the circlip and the piston pin. It is often the chamfers on the piston pin ends that interact with the circlip, so these seemingly innocuous features need to be tightly controlled.

If you look at the pin bore on a highly stressed engine, you will often see at its bore edges on the inside of the pin bosses a radius that blends as closely as possible with the pin bore – it probably won’t be perfectly tangent, but close. This is again in the interests of the piston pin but also of the piston durability.

The aim here is to provide a ‘soft’ contact. If we leave a sharp edge, when the pin and piston flex, there is a condition of high contact stress at this point, and this can cause fatigue cracks to be initiated. This is particularly important if you are using a piston material with low elongation. At a microscopic level, this design helps to prevent the material exceeding its yield strength and elongation limit, and cracking. People developing pistons from such materials can find that simply copying an existing design results in failure because cracks have started from locations of high local contact stresses.

Written by Wayne Ward

Given the contribution of the piston assembly to overall engine friction, it should come as no surprise that engineers soon tried to apply DLC to pistons, often with poor results. There are peer-reviewed technical papers, such as that by Demas et al*, which find that DLC shows little advantage. However, in the pages of Race Engine Technology we have seen that Formula One engines definitely use DLC. The magazine has run very detailed articles on Toyota and Cosworth Formula One engines, and both show DLC-coated pistons in use. Be under no illusion, if the Formula One engine companies use it, there is an advantage to be had, either in terms of performance, reliability or both. So why do the likes of Cosworth find a gain from DLC coating where researchers like Demas et al find none?

The answer lies in the fine detail. The Demas paper notes that, for their experiment, sections of an existing piston were used. Speak to someone who knows how to get a DLC-coated piston to work and they will tell you that simply applying the coating to an existing piston is most definitely not the way to go. One only has to look at a DLC-coated Formula One piston and compare it to a conventional piston, and there are some obvious differences.

The visual difference is the machining. The skirt of a conventional piston has some very coarse machining, almost like a soft single-cut file. An optimised DLC-coated piston though has a very smooth skirt. If we coat a conventionally machined piston, we create something akin to a file with hard teeth. It can easily wear its counterpart – the cylinder bore – working as a very efficient broaching tool.

Uncoated aluminium pistons wear in a very benign way, with the piston skirt ‘bedding-in’ over time, and the very small amount of material displaced does not cause any detriment. In fact, it was common not so long ago for Formula One pistons to have a resin-bonded polymer skirt coating, precisely to accommodate some skirt wear as the piston bedded into the cylinder bore. DLC coatings are not apt to wear, so the piston doesn’t wear in; rather, it can tend to wear out its counterpart, or the coating fails. It is therefore very important that the DLC-coated piston skirt profile is more highly optimised than its uncoated equivalent. 

* Demas, N.G., Erck, R.A., Ajayi, O.O., and Fenske, G.R., “Tribological studies of coated pistons sliding against cylinder liners under laboratory test conditions”, Lubrication  Science

Written by Wayne Ward

There are two methods of securing the piston pin which are variations on the same theme; mechanically speaking, it is the simplest method of piston pin control as it requires no extra components at all – that is, no circlips or any other method of retention. The use of mechanical interference has been widely used in production engines, and the two variations are to interfere the pin in either the con rod or the piston.

The easiest to achieve is interference between con rod and piston pin. There are three main reasons for this – there is only a small difference in thermal expansion coefficient between the pin and rod, there is a smaller difference in operating temperature between pin and rod compared to the temperature difference between pin and piston, and there is less interference length in the pin and rod contact. All of these mean that, if interference fitting is your chosen method, it requires lower levels of interference and lower levels of insertion force at room temperature. It may also be safer to have interference in the rod.

Assuming that a bronze bush or plain steel rod is used, there is less chance of damage or seizure than there would be with heavy interference between pin and an aluminium piston pin bore in the piston. Many years ago, the interference was achieved by having the small end of the con rod designed as a pinch clamp; the load on the clamp was applied by tightening a threaded fastener. Having a threaded fastener cyclically loaded in such a critical application was risky though, so this method was abandoned decades ago.

The method that is widely used for racing, especially in drag racing applications, is to replace circlips with polymer ‘buttons’. These are fitted into the piston pin and limit the pin’s travel in each direction when the buttons come into contact with the cylinder bore. There are usually only very small forces causing the piston pin to thrust one way or another, so the frictional losses due to the contact between the button and the cylinder bore should be low.

The method has proved successful over a number of years, but it can’t be considered to be the optimal solution in many cases, assuming that the optimal solution is the one that produces the lowest-mass solution. The pin length can be limited to that required to keep contact stresses low enough – this should be the same length as a pin retained by circlips.

In ‘slipper’ type pistons – those with only partial skirts on the thrust and anti-thrust sides of the piston – the buttons would have to be very long and would therefore be much heavier than a wire circlip. Drag race engines generally use ‘pot’ type pistons with a full skirt, and these need a lot of piston pin contact area owing to the huge combustion forces. The buttons are therefore comparatively light and short.

Another popular method of pin retention is the use of spiral locks. These are basically a helical design, being a spring of just less than two coils and made from flat material. They act in the same way as a circlip but are renowned for being very difficult to remove. Many engine builders would say though that this is not a disadvantage, especially those who have had a wire circlip ‘escape’ in the past.

Written by Wayne Ward

However, there is a particular case where piston rings can cause a problem owing to a vibration condition which is due to a combination of factors centred around the fundamental geometry of the engine, the mass of the piston rings, engine speed and the pressure differential across the piston ring.

As the piston approaches bottom dead centre on the exhaust stroke, the combination of inertia forces acting on the piston ring and forces acting on the ring owing to the pressure differential across it act to push the piston ring up against the top side of the ring groove. This effectively causes a seal between the ring and the top of the groove. For the piston ring to work effectively it is energised against the cylinder wall by a combination of its own inherent radial forces when compressed into the bore (often referred to as piston ring ‘tension’) and a force owing to the radial force exerted as a result of the gas pressure acting on the inside of the piston ring.

Let us take the example of a piston ring with an 80 mm inside diameter, and which is 1 mm thick. The cross-sectional area is (80 x p x 1) = 251mm², and if the cylinder pressure is 5 bar then the force on the inside of the ring is 5 x 0.101325 x 251 = 127 N. Robbed of this force, the radial force of the piston ring can become insufficient to allow the piston to seal effectively. This leads to a vibration condition known as ring flutter.

As the speed of the engine increases, so the piston acceleration increases as the square of engine speed, and even a small increase in maximum engine speed cause the onset of ring flutter. If we say that the fundamental geometry of the cranktrain and the new maximum engine speed is fixed (this defines the piston acceleration), and that the pressure differential across the ring is also fixed (this can be a dangerous assumption), then if we want to eliminate ring flutter we can do so using a few different methods, but the aim is to increase the speed at which flutter will occur so that it is above the operating speed of the engine.

We could consider an increase in ring tension, so that the ring still seals even when it is forced against the top of the ring groove. This can lead to significantly higher engine friction though, and adds to the forces which push the ring toward the top of the ring groove.

The level of piston acceleration at which ring flutter begins can be raised by decreasing the mass of the ring. For a given cranktrain geometry, piston acceleration increases in proportion to the square of engine speed, so by increasing the piston acceleration at high flutter starts we can take it out of the engine running range. Engineers tend to like reducing the mass of reciprocating engine components, so this is a solution that will find favour among designers and developers. The problem though is that it means having to change the design of the piston ring and piston. The elimination of flutter is one reason why we have seen a constant decrease in top ring widths in recent years. As engine speeds increase, a thin low-mass ring is important for eliminating flutter as well as reducing the inertia forces acting on the cranktrain.

Another option is to modify the piston so that the cylinder pressure is not ‘denied access’ to the inside diameter of the piston ring. There are machined design features that can be added to pistons to allow the cylinder pressure to act on the inside diameter of the top ring, even when the ring is forced against the top of the ring groove.

Written by Wayne Ward

It is usual though for engines in production cars and motorcycles, and almost all race engines, to be equipped with hollow pins, as these offer sufficient stiffness to resist excessive ‘ovalisation’, which is the term many use to describe the crushing deformation that can cause a pin to bind in its bore. The amount of ovalisation that can be tolerated depends on the clearance between the pin and the bores into which it fits. Larger clearances can tolerate larger deflections without binding, and these larger allowable deflections can lead to a lighter pin. However, this has to be weighed against the maximum stresses in the pin which may increase with more flexible designs.

The bore of a racing piston is very often equipped with a taper at each end. The amount of taper again dictates the amount of ovalisation and the stresses at the ends of the pin. Excessive tapering will lead to a pin with insufficient stiffness and strength, so we need to temper our ambition in the pursuit of low component mass.

At the ends of the outside diameter there are often chamfers and radii to which you may have paid little heed; they are often very tightly controlled and play a critical part in the function of the piston pin and the ease with which it can be installed. The external chamfers may actually be double-angled.

A shallow-angle chamfer is often added to aid easy fitment of the piston pin, allowing the part to slide smoothly through the piston and rod bores. This chamfer is normally short so as not to effectively shorten the pin to bore contact. A more substantial but carefully controlled chamfer, often machined at a 45º angle, will in the case of plain round wire circlips often form the surface that is in contact with the circlips. In such designs the chamfer machining is critical, as the relationship between the chamfers on each end of the circlip dictates the amount of piston pin ‘float’ (potential axial movement) that is possible.

Where any such chamfers meet the outside diameter, it is often the case that the resulting sharp edge is ‘broken’, with either a machined radius or using an alternative abrasive mechanical process. This prevents the likelihood of any material being removed from the pin bores in the piston, or any other damage (burring or scoring) being caused to the bore during assembly.

Surface finish is another important design feature, if you going to design your own pins. It is important though that the surface finish is very fine – piston pins are often very hard, and make excellent cutting tools if the surface finish is too rough.

Written by Wayne Ward

The top class of motorcycle racing had for some time been dominated by two-stroke motorcycles. The capacity limit for the top Grand Prix was 500 cc, and Honda decided – possibly because it would be a difficult engineering challenge – to build a four-stroke 500 cc racer to challenge the dominant two-strokes. What it came up with was the NR500, a machine with a number of innovations, not least of which was a remarkable engine equipped with ‘oval’ pistons. The engine was a V4, which was unremarkable, but it had eight con rods, eight spark plugs and 32 valves, as we might expect in a four-stroke engine with twice the number of cylinders. The engine was essentially a V8, but with four pistons. The crankshaft had four pins, each carrying two rods; continuing with the strange architecture, all four crankpins were coaxial.

The oval piston was not, if fact, oval in the usual sense of the word. If we imagine a circle split in half, with the halves moved apart and a rectangular section inserted tangentially to the separated halves of the circle, this is the actual shape. The aspect ratio of the piston – that is, the ratio of piston length to width – is defined by the number of valves and their size/disposition. In the case of the various Honda oval-piston engines, including the NR500 and both road and race versions of the NR750, each cylinder had four inlet and four exhaust valves.

The greatest technical challenge was perhaps not in the manufacture of the piston, but in making the piston rings work. We know instinctively from handling piston rings, and perhaps from design studies, that squeezing a ring creates a ring tension that is pretty even around the cylinder bore. One can imagine that trying to create the same effect for a piston ring with a distinctly non-circular profile would present a huge challenge.

According to Cameron*, the original skirt piston profile with its flat sides was eventually replaced with one whose curvature was more continuous, as in two ‘ends’ with small radii joined by ‘sides’ of large radii. Allegedly, one strategy for piston rings was not to have a single ring per groove but to have two overlapping ‘half rings’. Piston development was also rumoured to have incorporated a period of magnesium piston development. Honda has not been alone in trying to use magnesium pistons in race engines.

The oval-piston project was not a great success. Against the might of the other manufacturers and their well-developed two-stroke technology, the Honda NR500 struggled. Its only race win was a non-championship 500 km race in Japan. On the Grand Prix circuits, the highlight of the bike's career was to run in fifth position in the 1981 British Grand Prix before breaking down; its highest Grand Prix finish was 13th. It did however spawn a wonderful 750 cc oval-piston endurance racer, and an incredibly expensive 750 cc road bike.

 * Cameron, K., “Classic Motorcycle Race Engines”, Haynes, 2012, ISBN 9-7818-4425-9946

Fig. 1 - Oval-piston engine internals (Courtesy of Honda)

Written by Wayne Ward

The piston crown shape affects the flow of the fuel-air mixture into and around the chamber during the intake stroke. Its shape is particularly important where fuel is injected directly into the chamber, as with diesels and gasoline direct-injection engines. Beyond this, the piston crown shape affects the overall efficiency of the engine. Some of us may recall that to minimise heat rejection, the ratio of combustion chamber surface area to volume should be minimised. In terms of simple geometric shapes, the ideal combustion chamber would be a sphere.

However, in the practical world of engineering, we are limited to something approaching a cylinder, where the ends are complex surfaces defined by the piston crown and combustion chamber. A ‘clean’ flat-topped piston crown with few features such as valve cut-outs or intruders will have the minimum practical surface area. In many cases, our requirement to run high compression ratios means we will add an intruder to the top of the piston, and the valve timing and lift necessary for optimum performance will often give us deep valve pockets. This is far from ideal if we want to minimise combustion chamber surface area.

There are two other disadvantages to piston crowns that have lots of such features. First, sharp external corners will tend to run hot and can be a source of detonation, which can limit performance, owing to the optimum valve timing being within a range that causes detonation. Second, sharp internal corners, as we might find in the base of valve pockets, can tend to act to as flame quench areas, with the flame not burning right into the corner.

If we look further than the crown and examine the top land of the piston skirt profile, there are opportunities here to improve efficiency. There is very thin ring of combustion chamber volume which is bounded by the piston, liner and top ring, and which is open to the rest of the chamber. This acts as a quench volume and plays no real part in the combustion event. However, while its form is not conductive to combustion, there is nothing to prevent the fuel-air mixture entering this ‘crevice volume’. Anyone chasing maximum efficiency will keep this crevice volume to a minimum by maintaining the minimum working clearance between the piston top land and cylinder bore. There is also an incentive to have the top ring as close to the piston crown as is practical.

There are other effects of the crown design on combustion. The squish effect is caused by the piston expelling the working fluids from the periphery of the chamber as the piston approaches top dead centre, and the reverse happens as the piston descends. The squish effect is potent in terms of improving combustion, and the amount of squish clearance is also thought to be important for reducing detonation damage.

Written by Wayne Ward

The material choices for piston rings have remained broadly static for many years, although the budgets available in racing allow us the latitude to use more expensive materials and coatings than can be justified for passenger cars. For many years, plain cast-iron rings were the order of the day, having been well-proven in mass production, while ductile iron also plays an important part in the commercial ring market. Steel rings were the next logical step since, for a similar mass, they are much stiffer and so allow us a greater factor of safety against running into a resonant running condition such as ring flutter.

In terms of commercial steel rings, the elastic modulus (a measure its stiffness) is in the region of 200-230 GPa, while for cast iron the range is about 90-150 GPa. Steel materials are also stronger than cast iron in fatigue. In comparing a cast iron and steel ring material of the same static strength (1300 MPa), published results* show that an 18% chromium steel has a 60% higher fatigue strength (>500 MPa) at 10 million cycles than a cast-iron ring material. Tool steels are offered commercially by some racing ring makers. These offer improved static and fatigue properties compared to commercial ring steels, but come at a premium owing to their composition and manufacturing processes.

Another important advantage of using steels, especially those able to resist softening at the moderate temperatures seen by piston rings, is that piston designers can run steel rings higher on the piston than would be possible for the same reliability with a cast-iron ring. This eliminates some crevice volume and therefore improves combustion efficiency, since less unburned fuel can remain in the crevice volume where the flame is quenched.

The higher the tempering temperature of the ring, the higher the temperature it can operate at, and the higher the ring can be run – although the limiting factor may be the piston material or the design of the piston itself, especially valve pockets. The advantages of having the piston rings positioned closer to the piston crown are more than simply reduced crevice volume, as the compression height of the piston can be reduced to give the opportunity to run a lighter piston and a longer con rod.

Both of these design options have their own benefits. The longer rod can benefit the engine in terms of reduced friction owing to lower maximum rod angularity and lower piston thrust forces. The lighter piston means lower inertia forces at top dead centre and bottom dead centre, and these can allow both a lighter-section con rod to be used and a lower-mass crankshaft. All these benefits can flow from using an improved piston ring material.

* http://www.federalmogul.com/korihandbook/en/img_23_400_279.htm

Written by Wayne Ward

Even in a spark-ignition engine, the piston crown has a big impact on combustion, affecting it right through the engine cycle. If we look at the role of the piston during inlet, the shape of the crown and any features protruding from or formed in it will affect the motion of the mixture and the distribution of fuel within the chamber. As the piston rises on the compression stroke, the valves shut, we ignite the mixture, and the flame spreads from the spark plug outwards and downwards.

In order to force the mixture at the margins of the chamber towards the advancing flame front, engines tend to take advantage of the squish effect. The term ’squish’ refers to the thin part of the combustion chamber at the outside of the piston where the piston most closely approaches the head. The mixture is forced inwards at a velocity affected by various factors including the squish area and shape, engine stroke and engine speed. The reverse happens as the piston descends, with the enflamed volume being drawn out towards the margins of the chamber.

It is inevitable that the piston will absorb some heat from the hot combustion process, and this is also affected by piston crown geometry as well as piston temperature, thermal conductivity and other factors. The rate at which the surface heats up is affected by the ratio of surface area to volume of different areas of the piston - sharp-pointed sections will absorb heat at a greater rate than concave corners. Such hot-spots can provoke uncontrolled combustion, which can damage the piston and cylinder head.

We have mentioned in passing that the piston can affect the distribution of fuel and motion of the mixture. This is true in general for all types of engine, but especially for one class of race gasoline engine, the direct injection spark-ignition engine. Here, the fuel is injected directly into the cylinder, where it impinges on the piston crown. The form of the piston needs to be carefully considered in order to ensure that the fuel is where we want it to be (and in the right proportion compared to the available oxygen) at the start of ignition. If the mixture in the vicinity of the spark plug is either too rich or too weak in terms of air-to-fuel ratio, combustion will not be possible or may be inefficient. The piston crowns used in gasoline direct-injection engines often have a smooth area where the fuel spray interacts with the piston crown.

Written by Wayne Ward

Piston crown coatings are used for various reasons but, as we shall find out, there are generally other positive side-effects that can help us improve efficiency and increase engine output. There are two main types of crown coating - ceramics and metallic.

Ceramic piston crown coatings generally have very low thermal conductivity; this acts as a thermal barrier, reducing the transfer of heat through conduction from the crown to the rest of the piston and engine structure. The heat rejected to any oil used for cooling the piston is also reduced, and if the flows are re-optimised for a ceramic-coated crown then the squirt jet flows can be reduced or perhaps even done away with.

This re-optimisation of squirt jet flows has benefits. The lower the heat rejection then the smaller the cooler needs to be to reject the heat to atmosphere. Not only is this a lighter component, but aerodynamic drag is also reduced. Lower piston cooling flows generally mean lower frictional losses from the bottom end of the engine; there is less oil entrained in the general airflow and less oil hanging around which can generate heat through constant shearing due to solid components passing close by.

Reduction of squirt jet flows can mean that smaller pumps are required, which may also lead to a very slight decrease in frictional losses. With a lower thermal conductivity piston crown, overall heat rejection is reduced and the engine, for a given trapped mass of air and fuel, should be more efficient. Increasing the thickness of the thermal barrier coating minimises the amount of heat rejected through the piston crown.

So why do we not coat all pistons with a generous layer of thermal barrier coating? The result of coating the crown is that the combustion chamber temperatures are increased, and the wall temperatures are also higher. This means that any fresh charge suffers from higher heat transfer from the wall of the combustion chamber, and this leads to a premature increase in pressure, reducing the pressure difference across the inlet valve and thus reducing inlet charge mass flow rates. The result can be a drop in volumetric efficiency. However, the overall fuel conversion efficiency is generally increased. The net effect may be to reduce engine performance, although fuel economy may be improved
Metallic coatings are used widely in turbocharged and supercharged engines. Such forced-induction engines, with high combustion chamber pressures and temperatures, are prone to engine knock, a kind of uncontrolled combustion event. Although many engines continue to increase output as knock ensues, the downside is that rapid engine damage follows, especially where pistons are concerned. In general, it is necessary to change ignition timing to bring the engine out of knock, but performance suffers. By using a hard metallic coating, it is possible to run an engine into knock without suffering the mechanical damage and engine failure often seen with uncoated pistons.

Written by Wayne Ward

I would hope that most of you have not had to survey the wreckage of an engine whose moment of glory has been blighted by a circlip problem, either through component breakage or by a failure to fit the circlip correctly during build. If you do lose or break a circlip, you might be lucky and find that it does little damage itself, but once the piston pin starts to move, time is generally short before serious damage ensues - especially in a modern, highly optimised engine where the piston pin is relatively short, sitting in two narrow pin bosses and passing through a narrow rod. If the pin becomes disengaged from one piston pin boss, then the piston tends to flap around, and can easily break. If not, it is still likely to damage the valves as clearance is reduced and the pin itself can badly damage a liner or the cylinder block.

Piston circlips are generally round wire items, and are made from high-strength spring wire which is carefully formed into the correct shape. The amount of deformation from the 'free state' to the fitted state dictates the circlip load, and for high loads a high-strength wire is required so that the circlip, which is acting as a radial spring, behaves elastically and doesn't lose its load. Drawn wire springs, when subjected to load and relatively modest temperatures, tend to lose load over time.

The shape of the circlip is most commonly a section of a circle, with a small gap in the installed state; variations on this have a sharp oblique cut at the ends of the circlip. Some people will often dress the sharp edges of this type of circlip, while others maintain the sharp edge so that it 'digs' into the groove, preventing circlip rotation. Circlip rotation does happen and, on an engine with extended periods between rebuilds, this can wear the circlip groove so that some of the installed load is lost.

There are circlip designs with anti-rotation features that don't rely on anything so uncontrolled as the circlip end digging into the piston. Many suppliers can make circlips with a tag or ear on one end of the circlip that can locate in a slot or hole, thus preventing rotation.

Besides loss of load owing to rotation and wear, circlips can deform and fail due to vibration. A particular forcing frequency can excite a resonant condition in a circlip which will cause a fatigue failure.

Written by Wayne Ward

Equally, we don't want unburned compressed charge or burned combustion products to flow past the rings into the crankcases. Not only do we lose cylinder pressure, and therefore performance by doing so, we contaminate the oil. Oil dilution by unburned fuel thins the oil, and we end up with a substance that we hadn't intended to lubricate our engine with.

The correct choice of ring materials, heat treatment and coatings is critical to getting the best from an engine. Where you have adapted a production engine for racing, there is probably a mass of information available to you about what works well. For a bespoke race engine, the design engineer has a free choice. The most basic choice available to many people is whether to use cast iron or steel. For high-performance race engines, steel is generally the material of choice. However, cast iron can be more durable in certain situations owing to the fact that it often has graphite at the surface. One well-known engine builder says he finds that steel rings need more oil and a deeper valley in the cylinder honing compared to a cast-iron ring.

The choice of steels used for piston rings in engines ranges from very low-strength, low-alloy steels though stainless steels to high-strength, highly alloyed tool steels. Generally speaking, piston rings made from the steel grades with higher strength are used for more arduous applications.

While 'plain' steel materials of varying strength levels are widely used, it is also common for piston rings to be surface engineered to give improved sealing performance and wear behaviour. Nitrided steel rings are commonly used for racing applications, and both gas nitriding and plasma nitriding methods are used to provide the hard surface layer. Nitrocarburising is also used, but the 'case depths' are shallow compared to a nitrided part. It should be noted that nitrocarburising and nitriding are both used in the production of certain cast-iron piston rings as well.

Tool steel piston rings are generally used where the durability of other materials is insufficient. As a piston ring, they can be an expensive option, but where ring wear or breakage is the limiting factor for engine durability, they can reduce rebuild frequency. For a small extra investment in a piston ring made from a higher-specification material, the cost saving can prove to be substantial.

Tool steels combine toughness and wear resistance with high strength. These qualities also allow engineers to specify ever-smaller piston rings without sacrificing engine longevity. This is important as, in a well-optimised engine, the reciprocating mass savings are not limited only to the piston ring but also to the material around the ring grooves. The overall mass savings can be significant, and with the mass savings come performance and economy.

Piston ring development in racing is one item that benefits roadcars too; there are production engines in widespread use with piston rings that would have been considered pretty 'racy' in a production engine only 10-15 years ago.

Written by Wayne Ward

In chasing minimum mass, engineers look to decrease length and diameter, within the important constraint of having to maintain adequate stiffness. There are other constraints we need to respect, the most important being to ensure sufficient life in the component by keeping stresses within acceptable bounds for the material in question.

The most common material for piston pins in general is steel; the surface is often hardened to improve wear resistance, and the choice of hardening method will dictate the choice of material. There are two main choices for hardening - carburising (also known as case hardening) and nitride hardening. Both have a beneficial side-effect of imparting significant compressive residual stresses to the surfaces of the part, which in turn improve fatigue resistance compared to a part without these stresses.

Carburising steels are characterised by a low carbon content and additions of manganese, nickel and chromium; it is the low carbon content that allows diffusion of carbon into the surface. Nitriding steels have additions of elements such as chromium, aluminium and titanium, which are strong nitride formers. These nitriding steels are similar to - and in many cases the same as - steels that we would normally use to make racing crankshafts. There are a number of higher strength steels not necessarily designed for nitride hardening, but which it is certainly possible to nitride and which would make excellent candidates for piston pins.

The wear resistance of all kinds of steel pins has been improved by using hard, thin coatings. There are various kinds of DLC that have been shown to be beneficial, and pins with these kinds of coating have been in reasonably widespread use in many categories of motorsport for well over a decade.

Steel is not the only choice for piston pins though; a number of companies offer titanium pins. Titanium has a much lower density than that of steel, although its elastic modulus (a measure of stiffness) is also low compared with steel. Titanium Ti-6Al-4V is often used, although pins made from Ti-17 (Ti-5Al-2Sn-4Mo-2Zr-4Cr) are also commercially available.

We should not, however, expect to replace an optimised steel piston pin with one of the same dimensions made from titanium, and expect to find success. Titanium has particularly poor wear behaviour in sliding contacts, so titanium pins would need to be coated to achieve an acceptable level of durability. Again DLC is often used here, but pins are also marketed which have a titanium nitride coating.

A special note should be made of the very exotic pin developed many years ago for Formula One and Pro Stock applications, which used an aluminium beryllium inner and a steel outer sleeve. There is little doubt that this would have been extremely light and stiff.

Written by Wayne Ward

There are reasons why designers of new engines might wish to have the cylinder axis not intersect the crankshaft axis, and these have been used by production vehicle manufacturers in justifying and producing new engines with non-conventional layouts for many years. The two reasons for producing an engine with this layout have opposing results. The first, which does not apply to race engine design, is to improve the noise characteristics of the engine, and specifically to reduce the noise due to piston slap at top dead centre (TDC).

The second reason, which is definitely of more interest to motorsport engineers, is to improve engine output by reducing friction. By having the cylinder axis offset from the crankshaft axis, the aim is to keep rod angularity to a minimum while the cylinder pressure is at a maximum. The reasoning behind the concept is that reduced angularity leads to lower piston thrust forces, and therefore lower frictional losses during the period of maximum cylinder pressure. In practice, the concept is much more easily applied to inline engines than to vee engines. For those whose business it is to develop production-based engines, the concept is difficult, if not impossible, to apply retrospectively.

However, there is an equivalent mechanism that does allow the concept to be applied, and that is to have the pin bore offset in the piston. This allows the same crank/rod pin axis geometry as an engine designed with offset cylinder axes, but within a 'conventional' engines. For those who have pistons custom-made, the option is open to design such a piston, but for some popular engines, pistons with offset pin bores can be bought from motorsport piston manufacturers.

The amount by which the cylinder axes need to be offset depends on many variables, but the main ones are the ratio of crank throw to con rod length and the angle after TDC at which maximum cylinder pressure occurs. The design engineer may have the simple aim to have the con rod parallel with the cylinder axis at maximum pressure, or he may resort to more complex simulation in order to reduce friction over a given range of engine speed.

Depending on the angle after TDC at which maximum cylinder pressure occurs, this may preclude being able to practically apply the desired pin bore offset within the piston. In this case, the engineer will have to settle for the pragmatic approach and be satisfied with whatever gain is observed. The practical limit to moving the pin bore off centre is often the excessive moment due to the large distance between the pin bore axis and the centre of gravity of the piston.

Another point to be aware of is the effect on the engine stroke due to offsetting the pin bore. For a given crankshaft stroke, offsetting the pin bore or cylinder axes increases the stroke, and this may be enough to put the engine beyond the capacity limits of the class in which it is being raced.

Fig. 1 - This production engine piston uses pin bore offsetting

Written by Wayne Ward

Chasing the perfect combination is always the mantra for crew chiefs and tuners in the Top Fuel game - and anyone who doesn't admit that they're chasing the top teams from Al-Anabi Racing and Don Schumacher Racing is, frankly, lying.

Born into the business, Richard Hartman is the tuner and crew chief for journeyman owner/driver Terry McMillen; he is a former driver who finished in the NHRA top ten twice while driving a Funny Car. Hartman previously worked with IHRA T/F champion Bruce Litton before joining McMillen at the start of the 2010 season.

Since the 2012 season began at Pomona in February, Hartman's been chasing the right combination that will help McMillen make it down the 1000 ft dragstrip consistently and competitively. "With the new combination we've been working with this year, we've had an issue with sporadic cylinder scuffing. Basically the sleeve and piston are trying to melt together due to higher cylinder temperatures and increased parts expansion.

"It's not [happening in] any one cylinder," he said. "One run it may be the first three or four cylinders and the next it might be the last two to four - or any combination of all eight. To cure this, we are changing the profile of the piston. The profile we have used for five or more years just wasn't compatible with our new combination."

With a piston manufacturer as one of McMillen's partners, Hartman had several boxes of new pistons in his trailer when we spoke at the third race on the tour in Gainesville, Florida. "We're working on skirts, trying to change the profile of the cam effect on it and we're trying to get a little better clearance," he said. "We're opening it up about 0.003 in, which isn't much but with expansion it could be quite a bit with the heat these engines generate."

Hartman isn't sure just how much the engines cool off from the time they reach the top end of the dragstrip until they return to the pit area. "We really can't measure it, but they have probably cooled off about 300º [F] by the time we get them out of the car. There's no way to know, really."

The Top Fuel cylinder heads normally heat to about 1400-1500º on a run; as they cool off (without an assist) after a run, they'll lose about 300, 400º in temperature. "The fuel does the cooling; there are no oiling jets on the bottom to try and cool the piston," Hartman said.

His piston scuffing problem doesn't evince itself in any one particular cylinder. "The biggest thing is it's not every cylinder doing it every time, so it kind of rotates and picks a new cylinder every run," Hartman explained. The tuning combination Hartman and his crew are working with this year "is putting a lot more heat into the piston and the sleeve, so the sleeve is changing its shape and the piston is changing its shape differently than in years past."

His supplier offered a possible solution while visiting the team at the season opening Winternationals in Pomona, California. "Since Pomona to now [we spoke at Gainesville], they've developed a whole new piston profile to combat the expansion, and it's worked perfectly. We went from three to four scuffed pistons per run at the first two events to one scuffed cylinder for the entire event. Now we can concentrate on the tune-up," Hartman said.

In Top Fuel racing, squish area is not an overall concern, "Because we fill the compression with liquid [nitromethane fuel]. We fill it with liquid where with a naturally aspirated engine they have to create their own compression; we create compression by just putting more fuel in it or more blower on it," he explained.

McMillen's dragster runs a cross-braced triple-ring piston with a thick, flat dome. While he guards many of the dimensions on his piston package, Hartman said the dome height is about 3/8 in, and added that the team was trying some different piston coatings the manufacturer wanted to test in Gainesville. "You'll see some in a grey colour and others that are black. They're basically the same process - as far as how they anodise them and the type of anodising - but the black is supposed to be a little more resilient and not wear off as easily."

Hartman's experience has shown that pistons can run for as few as a single pass - again depending on the tune-up - or as many as 20 or even more. "We run them until they no longer pass our specs. After every run we check for scuffing, for dish where the dome will warp down a little bit and we check for ring tightness. Really, that's it," he said.

Weight on a Top Fuel piston is never an issue. "We want the strongest piston possible and we really don't look at weight; we need strength and reliability in this game," Hartman said. His pistons come in at 774 g each.

While his engine runs really evenly, "Not any one cylinder right now gets beat up more than any other." It looks like this problem has been solved.

Fig. 1 - Scuffed Top Fuel piston (Photo: Anne Proffit)

Written by Anne Proffit

As wonderful as all this may be, when it comes to piston rings we don't do it like that. Piston rings these days are generally either cast or made from some form of strip steel according to the intended application and economies of scale, with the process varying between manufacture and the material used. One ring manufacturer casts its grey iron products as individual rings in a non-circular shape. Others, however, cast theirs as individual pots on a sprue, which makes them look more akin to the design of a futuristic space station rather than the beginnings of a piston ring. Such rings will be cast, using shell moulding techniques in the highest quality resin sand using CNC pattern-making machines to generate the void.

Designed for tight dimensional tolerances and good surface finish with tight draft angles, the result is a near-net shape needing minimal machining to finish. When machining cast rings, one option is to form-turn both the inside and outside diameters at the same time and to the shapes necessary. Referred to as double-cam turning, once a segment is machined away, the remaining free gap will enable the ring to be fitted in the cylinder bore and given an even radial pressure all round.

A better alternative to grey cast iron is SG (spheroidal graphite) cast iron, more commonly these days referred to as 'ductile' iron. Produced from centrifugally spun cast iron, the resulting thick-walled tubes or cylinder can be machined on a normal lathe and sliced into separate rings, after which the free ends are cut using a very thin blade. Thereafter the 'C' shaped devices are placed on a carefully designed mandrel and heat-treated to generate the radial pressure needed once fitted. To finish, the ring profile (barrel, Napier and so on) will be ground to shape and the ring coated if required.

Steel rings, however, tend to be made in an entirely different way. Made from large coils of wire, steel rings are more likely to be rolled to the circular shape required using a system of rollers. Machining the outside oval shape and then slicing out a segment to create the gap and separate the rings, the internal bore is machined fully round once the rings are compressed to the installed gap dimension. If the profile has not been introduced at an earlier stage then this may be ground on before final coating.

The manufacture of piston rings may not be as colourful as the story behind the One Ring, or perhaps as exciting, but at least we don't have to go near the black lands of Mordor.

Fig. 1 - Cast piston ring moulding box

Written by John Coxon

Shaver specifies a box-style piston made of 2618 alloy for these engines, and works with two piston manufacturers - one for standard orders and a second when he needs a quick turnround. While there was a time when he, as the engine builder, would designate the specifications of his pistons, that has changed over the years, and Shaver now relies on his suppliers. "They know exactly what we need and we don't have piston failures any longer," he says. "They improve every year."

His three-ring pistons are all phosphate-coated (a dry-film lubricant that helps with ring and pin break-in) and there are Teflon wear patches on the skirts. He uses two ring packages for these pistons and a horizontal gas porting: one set is 1.5 mm, 1.5 mm, 3 mm; the second pack Shaver has just started using is 0.043 in, 1.5 mm, 3 mm.

The pistons come out with each engine rebuild, which is scheduled after 550 race miles. "We usually rebuild at that time and always replace the pistons. Could they go further? Probably. They always pass crack tests but we just don't put them back in because it is a $1500 part, and if it ever failed our name would be mud," Shaver says.

Whenever he has a piston issue, he returns the part to the manufacturer and usually receives a clear response within eight hours - "Which is doggone good!"

Shaver has been using a 13º chamber, and there is squish on both sides as well as in the middle.

Being a slipper piston with a relatively short skirt, there is quite a lot of rock but it seems much more alarming when the piston is cold. In the hot, running engine it is a much closer fit, and getting the running fit just right is part of the development process.

The struts that come off the pin tower are common now in race pistons, and they provide the necessary stiffness and strength to the crown and ring lands in such a narrow slipper piston. They are an efficient use of material on a component where weight is so important.

Shaver acknowledges that skirt profile development "is kind of like voodoo. The cam profiles we use and the taper of the barrel - skirts are barrelled and not flared - is all done in CAD/CAM as to what's going to wear the best," he says. "In the old days we'd have all kinds of cam cuts but these days, with computers, they can put in anything they want, so we really don't press the manufacturers. We let them do what they think is best for the skirt."

For these sprint car engines Shaver doesn't specify any type of piston cooling. "They don't run long enough to get that kind of heat in them, and methanol keeps the piston temperatures much cooler. Because of that we don't need external cooling systems," he says. Were he to add extra cooling, the piston shapes and profiles would then have to be re-optimised.

His wrist pin is made of 9310 material with 0.120-0.200 in wall thickness. "They DLC-coat them," he says. "The wrist pin is a very important part of the piston. It's rigid and strong to help the piston with their strength. We use a round wire clip for pin retention without a tang on it."

When pistons arrive at Shaver's shop his staff check deck clearances and ring land clearances. They use a micrometer to make sure all dimensions are within spec, but until they come out of the engine, nothing additional is done.

"We always look at the wear and we look to see how they're doing, especially in the ring land area because of the different diameters. Our side ring groove clearances usually start out at a thousandth-and-a-half and the wear is crazy - sometimes they'll come back in at three or four thousandths wear because of the dirt ingested!"

Shaver doesn't really work on weight reduction for his pistons; he has two different compression heights, the standard piston weighs 430 g and the option weighs 455 g. "To a point we try to reduce weight but we're kind of stuck there because the loads we put on them - with the stroke and engine speed - the bottom of the piston has to hold on. We're getting past 10,000 lb of inertial load so we try to keep everything balanced with the piston speed - which is everything! When the piston speed goes past 5000 ft/min, you're in trouble," Shaver says.

He changes spec on the piston every year or so, but when he changes cylinder head specs - as he's about to do - he expects to change piston spec two or three times during development. "Usually we play in the ring area more than anything, trying to get a little more out of it. The top ring's ring location is what we really work on, but we're always chasing something."

Fig. 1 - Ron Shaver's piston valve pockets don't have a cut-out plunged pocket, as that would shroud the valves and hinder performance

Written by Anne Proffit

Precise estimates of this friction vary. Some have reported up to 38% of total losses in a V10 Formula One engine whereas others, in more mundane applications, have suggested that 20% may be a more conservative estimate. Whatever the precise number - and this will depend on many factors including engine speed, load, oil temperature and so on - the true situation is very much different from that when the rings are initially installed during the engine build.

But before we look at some typical installed figures, let us look at the task required. Above all else, the primary task of the piston ring is to seal the gap between piston and bore. Sounds simple enough but of course this has to be achieved with the minimum of drag and cope with slight misalignment between piston and cylinder under all conditions of loading.

Furthermore, the oil necessary to lubricate the system has to be removed from the lower bore and carefully metered so that just enough of it is retained towards to the top (of the bore) such that very little (preferably none) is burned and escapes into the exhaust. Too little oil here and the top compression ring will scuff and fail, but getting it just right has tasked engineers for countless years.

On the induction and exhaust strokes of the piston, and to a large extent that of the compression stroke, the compression rings are tensioned more or less as installed. During the mid-stroke positions the lubrication is hydrodynamic, so the friction is very small. On the firing stroke, however, combustion gas bleeds out past the top ring land and, forcing its way behind the compression ring, pushes it out against the bore.

Far from exerting just a nominal force, the compression rings will exert a force far larger, the exact numerical amount depending on the combustion pressure at that instant and the size and geometry of the gaps. But although the pressure forcing the ring onto the bore may be higher, the actual frictional forces generated mid-bore may still not be significant.

The main contribution to friction, according to modern piston ring and lubrication models, is in the top compression ring around top dead centre between the compression and expansion strokes. Here the hydrodynamic lubrication gives way to boundary conditions, and with much higher ring loadings friction is also greatly increased.

Since oil control rings are not subject to combustion forces in the same way, these rings generally have to be tensioned to much higher levels, sometimes as much as eight to ten times that of a compression ring. With some 'as installed' compression ring tensions as little as 2 to 3 N, and oil rings at 30-35 N, it is therefore little wonder that in the search for low-friction rings the best place to start is the oil control ring.

Fig. 1 - Typical compression and oil ring tension figures.

Written by John Coxon

All steels of course contain some level of nitrogen within them, particularly when in the molten state. With a maximum solubility of around 450 ppm, which drops to nearer 10 ppm once the material has cooled to ambient temperature, nitrogen can have a beneficial effect on steel if it remains in solid solution or precipitates out as a fine coherent nitride at the gain boundary. The presence of molybdenum, chromium or vanadium in the mix can also help the material form metal nitrides. However, as the amount of nitrogen increases, its degree of formability decreases, so it is often better to make the component first, introducing nitrogen into the surface layer at a later time.

Such a process is called nitriding, and is the accepted practise of forcing nitrogen into the surface of a steel component to produce a much harder layer which is more wear resistant. Typically somewhere around 0.001 in ( 0.025 mm) thick, this layer is comparable with other forms of case hardening, and gives a hard outer layer around a softer but tougher central core. Since lower temperatures are generally used, less distortion of the final product is to be expected. Processed after heat treatment and subsequent tempering, in some cases final grinding may be necessary to remove a hard 'white' layer before finishing to size.

More commonly associated with the case hardening of crankshafts and camshafts, there are principally three methods in use. The first is a salt bath that uses cyanide-based salts, and because of that it is no longer in general use. Of the other two methods - gas nitriding and ion or plasma nitriding - both are of potential interest to piston ring manufacturers. Gas nitriding, whereby components are placed in an atmosphere of nitrogen-enriched ammonia gas in an air-tight furnace, will produce all-over surface benefits. Plasma nitriding, however, as a result of its better targeting, can be confined to particular surfaces.

Used in diesel and other automotive OEM engines to replace the toxicity of chromium plating methods, nitrided rings can be used on their own against just about any Nikasil or iron bore surface and in the aggressive environments offered by exhaust gas recirculation. At around 110 Vickers harness (50% greater than the base steel) it has also been shown to offer improved durability over chrome. For competition use, nitrided rings have been shown to produce a higher level of performance over the traditional moly-coated alternative, since the nitriding layer is integral with the base steel and not an 'add-on' coating. In some circumstances molybdenum coatings have been known to flake off as a result of thermal shock experienced in heavily turbocharged engines.

Mindful of the strength benefits that nitriding gives to the base ring material, in some cases manufacturers offer traditional PVD coatings on top of the nitrided layer - a sort of 'belt and braces' approach that produces even higher hardness numbers, with 1400-2200 Vickers being quoted.

Nitrogen may be all around and even within us, but when introduced to the surface layer of a steel piston ring its effects may be more noticeable.

Fig. 1 - A PVD-coated ring after nitriding

Written by John Coxon

Erik Milholland manages Stanton's racing division, and travels with the USAC circuit, handling any powertrain issues its customers might have.

The Toyota and Mopar engines have differing cylinder heads that necessitate a choice of dome shapes on its forged pistons. "With our current cylinder head-camshaft combination," Milholland says, "the lower piston dome of .020 on both our Toyota and Mopar engines performs better because of less disruption of airflow."

The 2618 forged aluminium piston has three rings - the gapless top and second rings are 0.032-and-a-half while the oil ring is 2 mm. The Toyota piston weighs 465 g; the Mopar is 450 g.

"We use a shorter dome because we wanted to flow more air through the combustion chamber. That's our basic priority," Milholland says. "We can do that with smaller combustion chambers on our cylinder heads and still keep our compression up where we want it to be, even as we minimise the dome shape."

The compression on these national USAC Midget engines is 15:1, and the Toyota and Mopar inline four-cylinder engines make upwards of 390 hp on the dyno and about 365-370 hp at the wheel, which is standard for the breed.

With their emphasis on low-end torque for racers competing on differing track sizes and specifications - both asphalt and dirt - engine builders in the USAC National Midget Series also have to deal with a limit of 8700 rpm for pushrod engines like those produced by Stanton.

The shorter piston dome shape came about because Stanton had cracking and breakage problems with its previous manufacturer's design. "Our new partner helped us work on our own piston and ring package designs so that we've been able to eliminate those problems," Milholland says.

In the Toyota and Mopar engines that Stanton produces, he says, "The shapes are different because the cylinder head shapes are a bit different. Still, overall, the pistons - as far as the forgings, coatings and ring packages go - are all primarily the same."

Skirt profile is determined by "trial and error on our end," Milholland confesses. "We've run piston clearances as low as three or four thousandths and as big as eight or eight-and-a-half thousandths. Over the last year-and-a-half or two years, analysing skirt profile, we came up with the height clearance and skirt profile we were looking to achieve."

To accomplish piston cooling, he says, "We run a twin oiler system squirting two jet oilers on the piston and pin at all times with the Toyota, whereas we only run a single oiler on each cylinder with the Mopar. The Toyota is a purpose-built engine specific to the Midget racers, and they did a really good job of building up the oiling system. We have a little machine process we go to with the Mopar engines, so we can go to the main oil gallery and feed that single oiler to the piston."

Wrist pins, which come with pistons from the manufacturer, are Casidiam coated and according to the engine being prepped. "The Toyota was developed with 0.866 pins, and the rods are fitted for that dimension," Milholland says. "The Mopar engines run the standard 0.927-diameter pins everyone else uses." Pin retention is by round wireclip.

Stanton Racing likes to check the pistons after four to six nights of racing and may, depending on the client's budget, re-use them. "We make sure the clearances, pin bores and rings are all within tolerances. We recommend ten to 14 nights on an engine, but that depends on the size and length of the track ,and the type of circuit. We have more wear with dirt, but dirt always gets into these engines," Milholland says.

He reckons 90% of the engines get new pistons each time they're rebuilt. "If they're coming back for a rebuild, you might as well spend another $1000 and get new pistons. A standard rebuild runs to $5000-7000; fresh engines cost around $33,000," he says.

Because the USAC National Midget Series isn't in the best of health - and because this is, after all, a business - Stanton Racing has gone to a heavier piston for its inline fours. "We've been helped out by the rpm rule, and lighter is not always better. We're still able to pull the rpm with our heavier piston, and we've got our longevity. By adding a bit of weight we're able to get our engines to live longer, and we've doubled or even tripled the life expectancy of our engines," Milholland says.

"With the added piston weight, your biggest thing is taking the bob weight, the rotating assembly balance and crankshaft balance into consideration. We're probably giving up 15-20 g - a total of maybe 80 g overall - but we're getting much more life out of it. You've got to keep the cars going and keep the customer happy."

At the Turkey Night Midget Grand Prix on the half-mile Toyota Speedway at Irwindale on Thanksgiving night, Stanton Racing's engines set fast times, a new lap record and won the race. That's keeping the customer happy.

Fig. 1 - Toyota (left) and Mopar pistons have coatings on all but the short dome and in the ring grooves (Photo: Erik Milholland)

Written by Anne Proffit

The Jaguar XKR engine is a 5 litre of about 500 hp, using a pair of air restrictors (29.92 mm) that keep his useable revs to 7000 rpm.

Gentilozzi uses two different manufacturers' pistons as he zeroes in on the engine set-up that will work best in his two racecars. "We like the features of both manufacturers, and we're working on getting it all together with one piston maker," he says.

"We're pretty equal on wear for all eight pistons. We see the most wear in the top groove and work very hard on that. We run a hard anodised coating on, and special ring in, the top groove. There is no coating elsewhere on the engine."

Because the XKR Jaguar coupe and convertible use direct injection on the street, the race engine is using the same induction system. "With direct injection we run very lean, and that's why we're heating up the top ring on the groove; it's relatively substantial."

All pistons are of Gentilozzi's design. "We designed the piston and did some CFD work with Jaguar in the process, and dome development was done in CFD and using high-speed photography on a single-cylinder development engine.

"The most important part of the direct injection system is the dome shape of the piston, allowing the combustion event to work in conjunction with the injection event and allowing them to get the most efficient and most effective burn," he says.

"The spray pattern of the injectors and the dome shape of the piston - or lack thereof - is critical to direct injection. We run both concave and convex dome shapes. For the endurance races we go with a dish piston because it's necessary when running stratified injection, as it allows a very lean burn below 3500 rpm. For sprint races we run a dome piston."

He says rod length is unaffected by the different types of piston, and that the sprint piston dome does not change from port fuel injection to direct injection. ALMS contests last for as little as 2 hours 45 minutes to as long as 24 hours, for those invited to the 24 Heures du Mans.

For the Petit Le Mans 10-hour or 1000 km contest, Gentilozzi fits a dish piston and runs about 12.5:1 compression. "The point of the dish is to focus the charge as near as you can to the spark event; you want an efficient and effective burn of the charge."

The Jaguar XKR pistons go through two rebuilds or roughly 80 hours, as Gentilozzi stipulates rebuilds at 40 hours. "We do a fair amount of testing because we're still in development of both the engine and the car," he says.

The team switched to direct injection at the Mobil 1 12 Hours of Sebring held last March. Gentilozzi says, "We changed our piston spec to accept direct injection. We did our research and development in the off-season, and it's constantly evolving as we learn more. Skirt shape and design hasn't changed all that much; we run a relatively short skirt without coatings and only hard anodise the top ring groove."

Because the Jaguar engine is continuing to evolve, Gentilozzi has different piston combinations on his mind in this off-season. "We're thinking we may try a two-ring piston for endurance and friction reduction, as well as some different bore coatings. We've been using regular nickel silicon carbide coatings, and have stayed away from moly-based coatings because we're not certain about their longevity. We're not about to sacrifice on that end. With our air restrictors you're only going to use a certain amount of air no matter what you do - you're going to hit a wall there, and all you can do from there on is work on friction advantages."

Fig. 1 - Gentilozzi piston (Photo: Tony Gentilozzi)

Written by Anne Proffit

Eventually, however, as engineers we will demand just that little bit extra - greater combustion pressures, smaller piston top lands, higher heat transfer rates, thinner profiles or simply greater durability - and the technology carefully honed over the years needs to be updated. So for many years, while piston rings have come in various thickness, profiles and coatings, the basic material has been cast iron, ductile iron or steel.

Cast iron in its grey form has the best lubricating properties, but is very brittle. Ductile or 'nodular' iron can be roughly twice as strong but while being highly compliant and difficult to break, as well as expensive, its lubricating properties fall some way short of that required. For many years now, however, as rings have become narrower and less weighty, the real growth in ring material has been that of steel. 1.4% silicon, high-carbon (0.6%) spring steels, chromium (13%) steels and 18% chromium martensitic stainless steels have been - and continue to be - routinely used, but the latest version to be added to the list is that of an M2-grade tool steel.

Tool steels are a separate category of engineering steels, characterised by the uses to which they are normally put. As the name implies, tool steels are used in the machine shop to cut and shape other metals. Sometimes referred to as high-speed steels, their main properties are those of high hardness - particularly at temperature - high strength and resistance to wear, with the ability to withstand shock loading. In fact, it is just those characteristics that are needed in the top piston ring in a heavily pressure-charged engine using a highly oxygenated fuel such as alcohol or nitrous oxide.

The problem though is that there are several different classifications of tool steel, each of which is based roughly around the intended application. But since 'piston rings' don't feature highly in any of these applications, how do you go about selecting the appropriate grade to suit? The answer is: very carefully, which I guess is why tool steels - although not altogether new in piston rings - haven't featured highly until now. Selecting the appropriate steel is therefore much like baking a cake, and all about balance; and instead of taste, firmness and lightness of the sponge, the factors are strength, toughness and resistance to wear.

Increasing the amount of carbon in the steel mix along with various alloys (such as molybdenum, chromium or tungsten) contributes to the ultimate strength of the metal, particularly at high temperatures, but this is at the expense of toughness, which tends to decrease as the amount of alloy increases. The carbide particles are also very hard, evenly distributed throughout and give excellent wear resistance. This is enhanced even more by the addition of the alloys.

However, a metal that is too tough might not be able to withstand the inevitable shocks, so the toughness has to be brought back by heat treatment, which may reign in some of its ultimate strength. It sounds simple enough to do, but the challenge in getting just the correct balance of properties for the application, and then making them at prices customers can afford, is not one to be underestimated.

Perhaps the ultimate in high-performance ring materials - and used in knives, dies and twist drills - tool steels are back at the heart of the engine!

Fig. 1 - Comparison of tool steel properties

Written by John Coxon

Garrett Jacobson Motorsports in Northridge, California, is bucking that trend by using former Roush Yates Engines Ford C-3 Nationwide V8s as the basis for its K&N Series NASCAR West entry. Building this engine goes against the grain and is, for chief operating officer Gregg Jacobson, the only way he feels confident of the engines he provides. The objective is to update this older technology as it trickles down to him while keeping up with current technology.

"We can do as much development work as we want to within this class, and we do," Jacobson says. "We are always looking for that competitive edge yet are constrained to the rules and limits set by the sanctioning body, and we can do what we want - as long as we stay within the limits they set."

In his various engine build-ups, Jacobson has been using the same pistons almost exclusively for nearly a decade. He stipulates that the dome of the piston be machined with a radial, concave dish shape, developed "to allow for better load distribution across the face of the piston, in addition to improved flame propagation".

In designating the specifications for this piston, Jacobson asks for a proprietary ceramic dome coating that provides a thermal barrier, helping to reflect heat back into the combustion chamber as well as protecting the piston from the scourge of detonation.

For the piston skirt, Jacobson specifies a graphite film because it can serve as an anti-friction barrier and helps reduce persistent scuffing on the skirt. The piston is a triple-ring set-up, common for this type of V8 engine that sees continually high rev ranges.

Jacobson expects pistons to last a complete season, which could be anywhere from 20-35 races of various lengths, from 50 to as many as 200 laps. "That depends on the environment in which the engine is run," he says. "In a perfect world we don't expect to see any appreciable wear." Jacobson tries to get his engines returned for examination after 800 track miles, but with teams on the road that schedule has to be somewhat flexible.

The most wear Jacobson might see on his pistons is in the pin boss. "We check for out-of-round and signs of any metal transfer," he explains. "We also watch the ring lands, which are prone to wear, particularly with the top ring that takes most of the abuse from combustion forces."

Jacobson is always looking for ways to get newer technology into his older engine package - the C-3 has been out of use in Nationwide competition for nearly a decade - and notes that "piston technology is constantly evolving and we are always looking at current trends in the industry."

Fig. 1 - Garrett Jacobson Motorsports specifies the coatings for its radial dish Diamond piston (Photo: Anne Proffit)

Written by Anne Proffit

In engineering however, we are rather more pragmatic, so things tend to be far less certain. Thus the position of a hole may be 100mm ± 0.5mm from a datum, or the diameter of a bearing journal could be somewhere between 44.980 and 45.000 mm. This lack of precision is called tolerance, and brings with it the simple admission that we can't make things as precise as we might wish.

A lack of precision in size is one thing but engineers as a rule tend to deal in more than one dimension, so the idea of shape - or as metrologists prefer, 'form' - comes to assist. Thus concepts like straightness, cylindricity, roundness and flatness have been defined to describe shapes more completely. And what better shape to describe than that of a piston ring and the groove in which it operates?

In trying to describe the attributes of a piston ring, therefore, we may have a tolerance of ±0.000050 in on the thickness or one of ±0.001 in on the as-installed bore diameter, while at the same time the surface finish may be described as being less than 4 microinches (< 0.000004 in). All very impressive and perhaps the very minimum I might expect in a piston ring, but to understand how a good ring relates to the piston and the groove in which it locates, I talked to a piston manufacturer who has an exemplary record and continues to supply pistons to all classes of motorsport. I must admit, I was unprepared for the response.

"There is a lot of rubbish written and talked about piston ring grooves," says the owner and managing director, who asked to remain anonymous. "Surface finish and flatness - absolutely ridiculous," were his exact words. He went on to explain that under running conditions the ring is a clearance fit inside the groove, and with a couple of 'thou' (0.002 in) or so axial and radial clearance the ring will be moving dynamically and unpredictably inside the piston groove. So while flatness of the grooves in the piston may be important - and by implication that of the ring also - it is not the overriding importance that some care to say.

Having described some of the research he had recently undertaken into logging piston temperatures in a running engine, it was clear that these results had produced an insight into piston design never previously fully appreciated. Using high-speed data logging of the piston temperature at up to eight critical points around the piston, and encoding the data against the crankshaft position, much was learned apparently. Furthermore, by feeding the data into a finite element model of the piston at the different times throughout the engine cycle, the analysis went on to give a dynamic picture of the piston groove and an insight into the resulting geometry, not only of the piston lands but the grooves in which the ring has to operate.

"Rather than the flatness of the groove," stated my contact, "under the very high temperature gradients induced, its shape - particularly that under the precise conditions of running - is the more important factor."
Taking care not to expand on this statement, and unable to divulge any engineering numbers, clearly the results were a revelation.

Fig. 1 - Shape, not flatness, is the key

Written by John Coxon

Neff handles the dual role easily, as he performed tuning duties for 2005 champion Gary Scelzi in the category, and in 2008-9 - his first year of competition - he was the fourth driver for JFR, earning the Auto Club Road to the Future (rookie of the year) honours in.

With direct control over all aspects of his 8000 hp racecar, Neff is able to specify the precise ingredients he needs to get his car down the 1000 ft dragstrip without injury or delay. He specifies Venolia pistons for his Funny Car - as he does for the entire JFR team - using a three-ring (compression, oil and oil expander) aluminium piston, telling Venolia's Tom Prock what he wants. "We get the most wear around the top ring, but usually they last pretty good and don't wear too much," Neff says.

Service life can be anywhere from one to six runs, depending on "how hard we step on it" during a run, says Neff. Compression for the Funny Car engine is only about 6.8:1, "but we might go higher, depending on the atmospheric conditions" he adds, and certainly higher at the Mile High Nationals outside Denver. "Here in Denver you've got to raise it up pretty good, but when you're at sea level you lower the compression, so that number will always vary, but the same amount of pressure is always on the engine," says Neff.

In Denver, Neff says he will also fit a gasket that is anywhere from 25-35 thousandths of an inch thinner to cope with the lower air density that comes from being a mile above sea level; there is not necessarily more pressure on the pistons at altitude. "You still have the same amount of pressure on the engine - that's what you're dealing with by raising and lowering the compression," he says. "The pressure pretty much remains the same.

"What makes it tough at Denver is the engine idles a lot different; you get a hot piston or something and the engine will start working more on one piston than the others," Neff explains. "Generally, if everything gels and you set it up just right, it shouldn't be any worse than at sea level. Idle circuitry is different up here. You tune it up and change the air-fuel mixture; it's just a little different and you have to adjust to it."

At Denver, piston service life is higher because the altitude is harder on parts. "You have to make big changes and it's hard to get it all ironed out in the short amount of time we're here," Neff says. "You're going to hurt more parts up here than say in Norwalk, Ohio," which is much closer to sea level.

While Neff says he might see higher piston wear on the top compression ring, skirt wear is not bad. "We kill the skirt sometimes but not on a regular basis. It's hard to say if it's worse in Denver than anywhere else. You take your best educated guess on a starting point and look at it from there."

JFR hasn't changed its piston spec since going from 1320 to 1000 ft a couple of years ago. "It's really no different. It's just another half-a-second and you're trying to run as quick as you could everywhere else. The fact is, you're just shutting off a little earlier and that doesn't require you to change the engine.

"A thousand feet lengthens our piston life a little bit because you're not on the throttle as long, so you're less likely to hurt anything. It's at the end of the track that you're most likely to hurt a piston. The longer you're on the throttle, the more damage you're going to do, so it definitely helps the engine, parts-wise, going to 1000 ft."

Fig. 1 - Mike Neff specifies the piston he wishes to use at every NHRA event (Photo: Anne Proffit)

Written by Anne Proffit

It is perhaps little wonder therefore that under the high-speed dynamic loads experienced, rings tend to move in ways not originally foreseen. That the ring, whatever its position down the piston skirt - be it top, second or oil control - should be a clearance fit inside its retaining piston ring groove is surely an obvious necessity. Enabling the ring to move in and out, this gap also allows that on the power stroke combustion gases can pass down the top land travel into the gap and push the ring radially outwards against the cylinder bore. This assists sealing on the power stroke, and at the same time reduces the contact force throughout the rest of the cycle when it is not really needed.

Capable of moving in and out as well as up and down within its groove, the position of the ring is governed by its inertia, the gas pressures above it, the frictional forces and the distribution of the other pressures along its flanks as well as those on the running surface. Moving up and down, ordinarily the ring will seal on its lower flank and against the cylinder wall, at which time all these forces as well as the moments induced will be balanced.

However, under some dynamic conditions these can become unbalanced, and the ring can move in ways not originally intended. Unbalanced forces can introduce additional bending moments at various times during the piston cycle, for instance, causing the ring to bend or twist in its groove at various points around its circumference. During this process the flow of combustion gas around and behind the ring may be interrupted, but so long as a net radial force between ring and liner wall continues to exist then the ring will continue to seal. When this isn't the case, and the radial force disappears, the ring will collapse away from the wall, introducing a new and potentially devastating leakage path for both gas going down and oil coming up.

To prevent this, rings can be made to give either a positive or a negative twist depending on the particular problem encountered.

As a particular example, I remember some years ago a durability test identifying excessive crankcase blow-by above a particular high engine speed. Specifying a positive twist compression ring, a chamfer was introduced to its top inside edge, causing it to flex down in response to the introduction of combustion gas pressure. At high engine speeds this gave better sealing and a reduction in blow-by.

To understand a little of what was happening the engineer I was with compressed the ring so that the two free ends came together, at which point the ring took on a slight positive twist. When combustion gas was introduced, or so the theory stated, the ring twisted downwards and perpendicular again to the liner, producing a better seal.

Surely a case of when a twist in the story produces a better ending.

Fig. 1 - A positive twisted ring

Written by John Coxon

The boat runs in two classes of this series, the seven-second class and the unblown fuel jet class. It's leading the latter in points after three events and is in second place in the former category.

The engine powering the boat is a 565 cu in V8 using a pair of four-barrel carburettors and running nitrous oxide. On the dynamometer it makes 1140 hp (without nitrous) and on the water with nitrous it's close to 1800 hp. "That's not a lot on the water," Gaskamp says. "There are fuel boats with a lot more, but for a carburetted motor it runs pretty good."

There are six to eight NJBA races every year and anywhere from 12-15 passes down a quarter-mile (1320 regulation) surface on any given weekend. The team for whom Gaskamp builds engines spends a lot of time testing the boat/engine combination to work on different tune-up scenarios.

"We have the [jet] pump that pushes the boat, and you have to get the impeller right with the rpms of the engine. You have to get enough water in the pump to make the boat move, and there are a lot of variables," Gaskamp says. "There are places with different water that changes the set-up a lot. We can overcome the water and air with nitrous because we're pumping our own air into the engine."

For all his race engines, Gaskamp chooses Venolia pistons (and con rods), doing business with Venolia as long as he's been building engines. Gaskamp uses a 628 g piston with a triple ring land, and finds the most wear on the skirts. "When the piston goes up it scuffs one side, and when it comes down it scuffs the other side. That's the most wear I see, other than the rings that go away real quick because of the nitrous," he says.

The service life on his pistons is customarily as few as 70 passes and as many as 140. "After 70 runs I take the rods off and see how they look. Sometimes we'll go two or three services before changing them out, but usually it's after 70 runs." That would amount to half a season of racing, he says. He installs aluminium rods for their longevity.

Gaskamp has been using the same piston spec for the past two years, and has specified his own dome shape. "I clay a dome for my application to fit the chamber of the head the way I like it. I like a lot of compression - that's what makes horsepower. I'm experimenting with different coatings at this point," he says.

Gaskamp is looking at a Teflon piston coating similar to that currently used in Top Fuel and Funny Car engines (which make close to 8000 hp) for his next batch. That Hardtuf black, hard-anodised coating is similar to that on his back-up piston (shown here). Gaskamp also produced a piston with a ceramic thermal barrier coating on the dome that, Venolia says, increases dome life.

"The top ring always gives up first (on his race-spec pistons). The bottom doesn't wear and the second doesn't really, but boy, the top one really takes a beating," he says. "I know that's normal in most applications but with the nitrous, it really is hard wear. It's gas-ported on the side so it's pushing the ring out hard on the cylinder, and with all the heat the nitrous makes it just wears out that ring."

Gaskamp changed from an accumulator groove because he wanted more material for his first ring land. "I wanted to help the top ring. I wanted more meat here for the first ring so I don't burn it up so quickly - to get it away from the heat a little more - so we put in more material up top so it doesn't burn through."

Fig's. 1 & 2 - Two sample Venolia pistons, one with added material to the first ring land as well as ceramic dome coating; the other with the hard-anodised Hardtuf black coating, both as used by Wayne Gaskamp

Written by Anne Proffit

While the technological push may be for the more sexy sounding diamond-like-carbon - DLC to you and me - there are some friction reduction applications for which DLC in some of its forms are not suitable. So when manufacturers are looking increasingly to coat components such as tappets, valve stems and piston pins, the area where most friction resides - in the piston ring pack, especially when running against a nickel-silicon carbide plated cylinder bore - is often reserved for something much less exotic. Commonly found frequenting the machine shop in the form of machine tool coatings, one of those is titanium nitride or TiN.

Sometimes incorrectly referred to as 'titanium coated', which is the bare metal and something totally different, titanium nitride is a hard crystalline substance very similar to common salt (sodium chloride) in its chemical structure. Golden in colour, the body-centred cubic (bcc) arrangement of the titanium metal atoms is converted into the face-centred cubic (fcc) arrangement of the nitride. Incorporating the much smaller nitrogen atoms increases the density of the product by only 25%, but its overall increase in strength is five times that of the titanium. The resulting compound is extremely hard, with a Vickers hardness claimed by some to be in excess of 2500. And as we know, hard materials (such as titanium nitride) running against much softer media (as in a nickel matrix) are likely to wear least. The presence of oil on the surface retained in the gaps between the silicon carbide particles also helps.

The tribology associated with the passage of a piston ring across the surface of a cylinder bore is a complex affair. The geometry, relative velocity, tangential force and surface finish - not to mention the lubricant and additive technology used - all have a significant influence on the friction generated and the subsequent durability of the system. In offering any coefficients of friction values, therefore, this has to be set against the exact conditions prevailing in the ring pack, which are likely to be very different from those in the test rig.

Since TiN is deposited on the surface to a thickness no greater than around 5 microns, the surface finish of any thin coating (an important factor in tribological study) could reflect that of the surface upon which it is deposited rather than the actual surface of the coating under test. Any friction thus generated could therefore easily be a measure of the underlying surface finish of the substrate rather than the upper surface coating.

Nevertheless, some reliable data I have seen on a standard pin-on-disc test rig running in the absence of oil gives a coefficient of friction of around 0.13 for TiN while a similar compound - chromium nitride - came in at 0.12. Although the same test conditions show the coefficient of friction of a hydrogen-free DLC coating at 0.07, it must be remembered that the piston ring only enters the boundary lubrication regime at each end of its travel, at BDC and TDC. For the rest of the time, the lubrication is essentially hydrodynamic, and is therefore more dependent on other factors such as the viscosity of the oil used and the temperature of the engine.

All of this serves to point out that when selecting a suitable piston ring coating to match your shiny bores, it may be more advisable to look for the most durable solution to suit your application rather than hunting for the lowest friction coefficient.

Fig. 1 - The face-centred cubic TiN crystal

Written by John Coxon

While cars in any particular class will be on the track at the same time, the organisers try to pace each car so that the lead racecar never catches traffic and the second-placed car never hooks up with the first. "There's no traffic involved, and that way everyone gets a clear and clean shot at one lap," Cardona told me.

Engine builder Gary Kubo preps the engines for driver Chris Rado, who races a Scion tC that sports a 2AZFE 2.4 litre all-aluminium engine. "Right now we're running about 1000 horsepower and maybe a little bit more when we compete; we can run up to 50 pounds of boost, depending on the atmosphere and the track, of course," he said.

The team has been working closely with JE Pistons over the past 10 years to get the right package together for their drag and Time Attack engines. When they began racing in Time Attack, Kubo said, "Their off-the-shelf parts are so well developed we've actually, for the first two seasons, run their off-the-shelf pistons.

"We've made some changes because we try to upgrade the car every year, little by little. We try to get more power and more response, so we go back to the data to see what we can do to change that engine response," Kubo related.

He said the changes made annually to piston spec come in skirt design. "You don't want to run the skirt so loose that you obviously have a reliability or oiling problem, yet you want to run them tight enough so you have the ring seal. When you run as much boost as we do - and, mind you, this is a factory block - we're making 300 horsepower per cylinder and the pistons need to stay round. so the skirt design is critical."

WORLD Racing runs a quenched chamber, so it specifies a dome/dish configuration that matches the chamber. "We try to digitise the chamber and reverse-duplicate that same configuration on the piston side. I would say the biggest wear problems we're having is in the skirt because we're running methanol," Kubo said.

"We don't try to shoot for max horsepower on the dyno because it never reflects the real world. We're using 2500 cc injectors multiplied by eight so, for our engine to consume as much fuel as it does, it has to be well over 1200-1300 horsepower because the fuel consumption is so high."

Still, the service life of the JE pistons is about 30 hours, but it would depend on how many races the team runs. According to Kubo, "We've had engines that last all season, a minimum of 12 races plus testing and dynamometer work. We try to change the spec only once a year, coming back each year with a bigger bang.

"WORLD Racing does get a bit of wear on the top-ring group because we run our lateral gas port set-up hard, so there's obviously combustion pressure pushing the ring against the wall and there's some axial movement - not much, we still hold the tolerances. Even if the ring groove opened up maybe half a thou, with a boosted engine that's a minimal difference," Kubo said.

"It's not like there's a book on what we're doing - we're making our own data on this engine."

Fig. 1 - WORLD Racing pays great attention to skirt details on its JE pistons (Photos: Anne Proffit)

Fig. 2 - WORLD Racing uses a quenched dome

Fig. 3 - The team reports some wear on the upper ringland

Written by Anne Proffit

To ensure efficient compression and expansion of the working fluid, the rotor has to be sealed across the apices between the rotor faces and around the sides; and while the system of side seals presents few if any difficulties, the seals at the tip of the tri-pointed rotor (known as the 'apex seals') present some of the biggest issues.

To start with, while the traditional piston ring pack consists of one or two compression rings backed up by an oil control ring scraping the excess oil away from the flooded cylinder bore, lubricating this single apex seal cannot be quite so generous, since this seal is exposed on both sides to combustion events. Effectively therefore it's a total-loss system, and the metering of small amounts of oil into the intake port to lubricate this seal has to be miserly, to keep oil consumption to acceptable levels. Given the marginal lubrication but surprisingly high residence time of this oil in the engine, it is little wonder that the materials selected for the seal require a certain amount of self-lubrication properties if they are to survive.

If the lubrication in this zone could be described only as marginal, the geometry of the situation at least helps. Dynamically, the forces on this seal are comparatively light, set alongside those of the stop-start nature of the piston ring. The lack of a reciprocating motion also ensures that the average sliding velocity of the seal against its housing - much higher than that in a conventional piston engine - should be sufficient, given adequate oil, to maintain a healthy amount of hydrodynamic lubrication at all times.

But as far as seal materials go, the choice these days is very much down to the housing material and specific application. On Norton race motorcycles fitted with rotor housings made from high-strength, high-silicon aluminium alloy (LM13), a two-piece plain cast-iron apex seal is used. This runs directly against a traditional nickel-silicon carbide surface with few if any reported issues. In applications where cast-iron housings are used - perhaps for reasons of cost or durability and where the extra weight is not much of a handicap - the material of choice will most likely be a form of heat-treated acicular cast-iron for normal road-type motoring. It's a higher strength, more shock-resistant material than other cast irons but one that shouldn't be used at temperatures higher than about 300 C.

For more powerful engines, seals of this material will not have enough strength, and in the past carbon-aluminium seals may have been the best approach. However, for the serious racer the only, an albeit somewhat expensive option is to use a fibre-reinforced ceramic apex seal of a type used in the 1991 Le Mans-winning Mazda. With high levels of strength and wear resistance, a single-piece silicon-nitride ceramic strengthened by short silicon-carbide whiskers was developed that was subsequently split into two pieces similar to the earlier Mazda production seal. With the same low weight of the carbon seal, the ceramic seal has much better durability and can be used against the standard housing surface finish, unlike the previous carbon seals.

Often derided for its historic sealing issues the Wankel engine has long since moved on. However, because of the method of lubrication (injecting oil into the intake port), oil consumption - often associated with seal wear - is always relatively high.

Fig. 1 - One-piece apex seal complete with spring

Fig. 2 - Wankel engine cross-section

Written by John Coxon

According to crew member Rob Hauser, "We're trying to adapt to a newer, more late-model tune-up for 2011 than we used in years past; we're going with what a lot of the other teams are running."

When Vandergriff Motorsports was running a limited number of races during the past few years, an aggressive tune-up wasn't necessary. He wasn't trying to secure championship points - as he is now - and didn't have the full-season backing of C&J Energy Services.

To make the adjustments, Vandergriff opted for a Venolia three-ring piston, one he's been using since the start of the 2011 season. "We wanted to be a bit more competitive and give ourselves the best chance we could, so that's why we're using Venolia pistons," Hauser told me.

As Hauser and the balance of the crew strive to gain consistency with their new tune-up "to where we want it - above 75% - we're working on getting faster down the track, and this new piston is helping.

"We're getting a greater number of runs out of the piston and we're not having any kind of lean conditions where we're back-siding the piston and melting the back side, or scuffing. We're getting three to five runs out of our new pistons," he said.

The piston spec changed as Vandergriff's team went to a higher compression than before - about 6.875:1 up to 6.9:1, which is pretty much standard for the class, Hauser noted.

Most of the wear occurs on the left-hand side of the skirt on each piston. It happens on the intake side on the right-hand side of the engine and on the exhaust side on the left.

"We just wish we knew why," he mused. "We're getting some rocking and we haven't figured out exactly why - it's all part of that tune-up thing. So we're working with our pistons, rockers, camshaft and the rods to make sure everything's happy together to run down the track.

"We're getting a little bit of wear above the top compression ring and this, too, is a mystery. The configuration we're using normally shows wear just on the exhaust side on both sides of the motor," Hauser said. "There is a little bit of wear that comes from the rocking we're experiencing [which manifests itself on the top ring land area]."

With the higher compression the team is running, "The piston is out of the cylinder a little bit farther, but then again, our cylinder heads are also a little bit higher," Hauser said. "It's not as much as you might think, but it's still a bit higher compression than what we're used to." The piston weight is 1 lb 11 oz (765.43 g).

The specified piston has a flat dome but does get "a little sink in the middle, so we keep track of that to know how much the chamber and piston combination has grown. We 'cc' every chamber of every head and we need to know if the dome of the piston is still flat, so it takes up the same amount of area from cylinder to cylinder, or if the dome has actually dished," Hauser said.

There are different compressions run on the engine - five cylinders have one compression and the other three have a lower one than the five. Hauser reports that most of the abuse he's seen is going to the number 3 piston in the V8 array.

Because they're still experimenting with the tune-up and getting all parts to the point where they complement one another, the Vandergriff team isn't exactly running the engine on the ragged edge as much as others might.

"The fuel curve has been fantastic with this new piston and they're living for quite a while. It's not too rich and not too lean - the curve is just right for the power level we're at right now," Hauser said.

Fig. 1 - This image shows wear on the flat dome (Photo: Anne Proffit)

Fig. 2 - Scuffing on the skirts occurs when the piston rocks (Photo: Anne Proffit)

Written by Anne Proffit

If all this wasn't enough, while every effort is made to ensure the cylinder bore - the barrel, in two-stroke parlance - is perfectly round, the introduction of intake ports, scavenge ports and exhaust ports in the side of it will combine such that even with the best technology available, the bore will be far from cylindrical in operation. And in crossing such chasms, the movement of the ring and the pressures around it will exert instantaneous changes to the radial and tangential forces never experienced or even considered in the four-stroke.

Pegging it in the groove at least prevents ring rotation and stops the possibility of the outer ends springing out and hitting the edge of a port as it passes. But in doing so the ring still needs to be free to move both in and out and up and down in the groove. Drilling the hole for this steel peg in an aluminium piston is therefore a critical operation - slightly too large and although fitting securely when cold it risks falling out at operating temperature; too small and the problem is one of assembly. No wonder aftermarket piston manufacturers are rarely keen to offer such pistons.

The main issue with two-stroke rings, however, is one of lubrication. Since such engines have no crankcase lube, oil to the piston ring(s) has to be transferred within the fuel, so for it to interfere as little as possible with combustion it has to be as sparing as possible. Oil in the fuel/oil mixture lubricates the bore from the bottom, while once in the combustion chamber and in the moments immediately before combustion it lubricates it from above.

Incomplete combustion of this oil will introduce carbon and varnish into the ring groove, causing a phenomenon also seen in some diesel engines - ring sticking. This is when the build-up of carbon and oil varnish condenses on the walls of the ring groove, preventing the free movement of the ring in its groove.

In competition units this is not normally a problem, since regular removal of the cylinder head and barrel is commonplace and the ring or piston regularly cleaned or replaced. In road-going or motocross applications, where two rings are used (as opposed to the single ring piston of the competition unit), rings using a keystone cross-section are often recommended. These have the upper surface of the ring at a small angle to that of the lower face. Seldom more than a 6-7º taper, this cross-section enables gas to get move easily behind the ring to push it towards the cylinder wall, while at the same time the action of the ring moving in and out cleans the ring groove and substantially prevents this build-up in the first place.

Despite now only regularly seen in karting, the two-stroke engine represents a totally different challenge to the piston ring manufacturer.

Fig. 1 - Keystone ring cross-section

Written by John Coxon

Using Hyundai's Lambda 3.8 litre naturally aspirated V6, as found in the production Genesis coupe and sedan, Millen's team took the stroke out to 93 mm from 87 mm, raising the displacement to nearly 4.1 litres and using a single turbocharger.

Pistons for this engine come from a supplier local to Millen's shop, JE Pistons, in Huntington Beach, California. "They ask us what we're doing and what kind of displacement we're looking for, turbo or non-turbo applications and our combustion," says team manager Eric Cantore. "They look at all that and are able to come up with a great piston.

"We haven't seen any scoring on the skirts, the piston tops look great coming out," Cantore says. "Everything you might need for pushing nearly 700 horsepower and 700 lb-ft of torque, you need a piston that will keep everything cool."

Cantore and JE agreed on a three-ring piston of 331 grammes, a flat top with cuts in for the valves, to get a bit more lift out of them. "With our stroke being a bit higher now, we try to keep everything in the valvetrain as low as possible," says Cantore. "Instead of bringing the stroke down further, we brought it up a bit so we can keep the bottom end of the motor happy running the dry sump system."

The bespoke pistons are machined from 2618-T6 aluminium forging specific to the Hyundai Lambda V6's bore range and valve configuration, according to Sean Crawford at JE Pistons. The flat ring grouping and dish has a volume of -12.8 cc. The valve pockets have been enlarged and made deeper to provide necessary clearances, and the piston crown features a thermal barrier crown coating supplement. This coating, Crawford says, is up to 0.005 in thickness per surface and is designed to wear very slowly, even in the high-stress applications experienced at Pikes Peak and in Formula Drift.

After Pikes Peak, the team decided to continue with the 4.1 litre mill for its Formula Drift programme. "We kept the pistons in for five events after Pikes Peak and ran in a Brazilian race this December, as well," says Cantore. The latter event, on the Serra Do Rio Do Rasto in Santa Catarina mountain, encompassed a 156-turn course in 8 km, rather than Pikes Peak's same number of turns in 12.42 miles!

"Even though we don't do a lot of mileage at our races, we do a lot of testing before and after each event, and do events like the one in Brazil and fun runs for Hyundai," says Cantore. "The engine does get quite a bit of mileage - on an average weekend you're looking at maybe 100 miles.

"We look at our pistons after each race but we don't do a complete teardown until we see something. We'll look inside to see if there are any issues, any unexpected wear. We did a leakdown and compression check before leaving for Brazil and the engine was still showing great numbers. The pistons were so good we just left them in."

While RMR doesn't have a mileage stipulation for its pistons, Cantore says, "We are keeping note on it. The pistons haven't had to be changed because of wear yet, but we'll likely swap them out before starting the new year of seven Formula Drift events."

While the Hyundai engine doesn't go long distances on a race weekend, he says, "We do get temperature real fast, run real hard and then stop. As little as it does run, the engine does have very high rpm - and with Pikes Peak you've got the altitude to deal with and the heat changes."
Practice for Pikes Peak starts at 5am and finishes around 9am. The race, on the other hand, can start as late as noon. "So the race temps have nothing to do with the practice times," says Cantore. "In practice, the ambient temps may be 50-60 degrees and then, at start line you're at 80 degrees, so you have to take that into consideration." As it was, the engine posed no problems for Rhys' (Millen) climb, but other factors relegated him to a third-place finish in the Unlimited class.

Fig. 1 - This triple-ring, flat-top JE Piston is made from 2618-T6 with enlarged valve pockets (Photo: Anne Proffit)

Written by Anne Proffit

One way, I suppose, is to look behind you. If you see a slight blue haze in the rear view mirror then either you have a fancy tinted rear window or the beginnings of a major catastrophe. And should it be the latter, there is no mistaking the smell of burning oil.

To the rest of us, regular checking of the dipstick when the engine is cold and parked in the same level place will give a much better guide. However, this is measuring only one function of the piston ring pack - that of oil control when the oil from the crankcase passes up the bore. What about that other function of the rings, that of sealing the combustion gases and preventing them from going down the bore and entering the crankcase? Engineers call this piston ring blow-by. Oil goes up the bore, piston ring blow-by goes down it.

One way, and perhaps the most reliable of all, is to measure it using a common domestic gas meter. A positive displacement device consisting of two or more sets of chambers separated by a diaphragm, piston ring blow-by coming from the crankcase is directed through a series of valves, which alternately fill and empty the chambers from an uninterrupted flow. As the diaphragms move, mechanical systems transfer the linear motion into the rotary motion of a counter, which registers the amount of gas metered.

When used as a blow-by meter, all the engine breathers around the engine need to be blanked off and the total crankcase flow ducted through the meter and vented to atmosphere. Accurate, reliable and virtually foolproof as well as imposing little back-pressure on the flow of gas, the domestic gas meter has in my opinion few peers for measuring blow-by, with little to go wrong.

With modern test cell data acquisition systems the downside is the difficulty of connecting this meter to a computer. Fortunately, vortex-shedding flow meters came to the rescue, being not only easy to install and integrate with modern computer systems but also insensitive to the high levels of exhaust gas contaminants contained in the flow. These vortex-shedding devices measure the volumetric flow rate using the Karman Vortex principle.

Consisting of a plain tube with a small circular strut within it, when air or gas is drawn past the strut small vortices are produced on the downstream side. Alternating from each side of the strut, these flow down the tube with the gas, and the frequency with which they pass an ultrasonic detector is proportional to the flow of gas. With no moving parts, minimal pressure drop, an electronic output and ability to handle contaminated air, these devices can be inserted almost anywhere into an engine.

However, most race engines these days are fitted with dry-sump systems, whose scavenge pumps extract not only the lubricating oil but also the ring blow-by contained within it. Such blow-by meters, whether domestic gas or vortex-shedding, are therefore impossible to use. In such cases the only method of determining the quality of the seal between ring and bore is the leak-down test.

Applied to a static engine, the procedure consists of removing the spark plugs once fully warm and then, after positioning the piston at TDC firing on each cylinder in turn, measuring the decay in pressure inside the cylinder of a supply of compressed nitrogen or dry air. If the gas is supplied at, say, 100 psi and the pressure gauge from the cylinder reads 95 psi then the leak-down is said to be 5%. While 5% is considered adequate for a roadcar engines, for racing nothing less than 3% is acceptable. However, more importantly, cylinders throughout the engine should not vary more than a couple of psi.

Not totally foolproof, the leak-down test also checks for leaking head gaskets and valves.

Fig. 1 - The domestic gas meter

Written by John Coxon

Dunn has been using Bill Miller Engineering (BME) pistons and con rods for at least 15 years, he says. "Well, I tried them all and I believe his are the best. Ninety percent of the people run his rods and I think some of them run other pistons because they sell them cheaper. I don't do that."
Dunn adds that he needs the strongest pistons he can get because they take all the beating on a 1000-ft catapult down the drag strip. Bill Miller says Dunn specifies a flat top made of 2618 aluminium with a special, proprietary hard coating applied.

Dunn specifies the compression he wants and whether he wants flat or domed tops for his pistons. "I run flat tops because they're less expensive and I really can't see any difference. Most guys run between 6:1 to 7:1 compression; I run 6.9:1 and the flat tops work fine for me," he told me.

According to Dunn, the magic of getting a piston to live is simple. "It all comes down to tuning," he says. "If the engine's happy (with his tune-up) and I ran a 4:08 at 308 [mph], I hurt one piston. Where it hurts pistons is when we drop a cylinder, because when the engine drops a cylinder it gives you an instant two or three more pounds of blower boost, but you still have the fuel going in. All of a sudden, the other seven pistons feel they need to catch it up and they just kill themselves."

Dunn believes it's impossible to stop piston breakage. "That's the game. We have to see how close we can get to blowing them up [the engines]," he says. He finds the most wear on the ring grooves. "If it's unhappy it'll be 'black death', which is like a two-cycle when it seizes - they're seizing but the other pistons have so much power it won't stop the engine enough. We'll beat them so hard we pull the wrist pin right out of the piston," Dunn states.

Dunn used about 200 pistons in 2010's 23-race season. He makes some races; at others he doesn't qualify for Sunday's eliminations. His car seems to run better as the season progresses and that is due to his budgetary concerns. He figures if he makes it through the first round, he can spend that much more money on the second, then the one after that and so on. "Instead of hurting one or two pistons, I'll hurt three or four (in a round). We'll spend that extra $1000 per run to go on," he says.

When NHRA first mandated 1000-foot racing, "Everyone said it was good not to have any oil-downs. I said, 'Give them three weeks and they'll all have 1000-ft fuses (referring to the engines).' Three weeks later, [John] Force had the white smoke coming out, just like he did at 1320."

After a plethora of late-season blow-ups, NHRA is returning to monetary penalties for oil-downs in 2011, forcing teams to be more diligent with their piston choices and their tune-ups. "All that does is hurt the little guys like me," Dunn says of the minimum $5000 fines.

"Everything is about the tune-up because you have to run it so close, and that's where the money comes in. I can't run it that close to breaking on the budget I have," Dunn explains. Sticking to his preferred tune-up, he runs the same design, same compression he's used since 1991. The dome has changed a little bit since then, but the compression height remains the same.

"Anything will work if you make it happy, so you have to have the right blower boost and the right fuel flow going into it. Low compression works and high compression works. It's just about getting the tune-up right - and luck!"

Fig. 1 - Jim Dunn's Bill Miller Engineering piston shows pitting but little other wear (Photo: Anne Proffit)

Written by Anne Proffit

Simple and yet highly effective, there are however two issues created by the inevitable gap in the rings that result. The first is the uneven wear that tends to take place at the edges of the ring next to the gap, while the second is the gap itself.

Examine any well-used compression ring and, close to the edges to the gap, you may see that the witness marks left by the ring moving up and down the bore tend to splay out just before you reach the gap. It's not much of an issue in itself, especially if you're talking about competition engines when rings are regularly replaced, but it is an indication perhaps that the rings twist somewhat differently near the gaps, and may or may not indicate uneven loading. The gap, however, is another issue.

At first thought it might be desirable to have little or preferably no gap at all, but as the piston, the rings and the cylinder bore expand and contract due to changes in temperature and/or mechanical load, these components will expand and contract at different rates. And while a zero gap might minimise combustion gas blow-by, should the two ends 'butt' together then the resulting geometric effect would be to transfer any further relative thermal expansion into additional radial load on the bore. Factor in its associated friction as well and, at worst, the heat generated would be rapidly followed by breakdown of the cylinder lubrication and catastrophic seizure.

Clearly, therefore, this 'gap' is more critical than at first thought, and when set to the supplier's recommended figures it should minimise the chance of seizure.

In service, however, this gap can allow combustion gas to bridge the ring, which not only costs engine performance but creates an instability in the zone between first and second compression rings where combustion gas is essentially 'trapped'. One enterprising company, however, has come up with a novel and unique design.

Protected by a design patent, the gapless ring system consists of an upper ring of such a cross-section that another, narrower lower unit can be assembled coaxially alongside it. Each ring has a traditional ring end gap associated with it but when assembled into the ring groove, so long as the two gaps don't align, no combustion gas can escape through this route. During engine build the gaps in each are set individually and are positioned about 15º apart around the piston ring groove.

Knowing that piston rings tend to rotate, I would have thought differently, but apparently during use the two rings rotate more or less as though they are clamped together to form an assembly. These gaps therefore hardly ever line up. However, even in the worst case, when excessive detonation in say a turbocharged engine causes them to rotate independently, according to the manufacturer the situation can be no worse than a normal single-gap ring. Used in many applications in all the major racing formula with a high degree of success, this design can be used equally successfully in first or second compression ring positions.

Although the manufacturer doesn't say so, these rings may also give a more even loading around the cylinder bore.

Fig. 1 - The gapless ring

Written by John Coxon

So to help their customers in the field, Shaver Specialties Racing Engines (SSRE) of Torrance, California, chooses a robust Mahle Motorsports piston of 2618 alloy that's coated with phosphate; skirts are coated with Mahle Grafal.

Shaver's operation, supplying engines for four-time champion Donnie Schatz and his True Speed team owned by NASCAR Sprint Cup star Tony Stewart, has been using this particular spec - an off-the-shelf part - for about two years, with only a few minor changes. Company owner Ron Shaver had some design input on the piston, according to Dennis Hardesty, his third in command at the workshops.

The engines used in World of Outlaws run between 408 and 410 cu in and, from Shaver's shop, are Chevrolet-based. Horsepower is 850 and Shaver's engines produce more than 700 lb-ft of torque at 6300 rpm, with redline at about 8500 rpm.

"We dyno them to 8000 here," Hardesty says. "We see some wear around the skirt but not around the ring land area unless they've detonated them a lot. Every once in a while we will break a ring, but that is because of detonation."

Pistons used in Shaver's World of Outlaws engines have to be heavier than, say, a NASCAR Sprint Cup pot. "They have to handle a lot more torque, which means they also have to be heavier because of the alcohol (methanol) and because of detonation," Hardesty says. The piston has a small dome on it that helps with strength at the top.

The best drivers ride the rim of the racetrack - the cushion - but when they come across slower drivers, an accident or suffer a mishap of their own, they have to lift the throttle from this single-geared engine. "The engine detonates until it gets back on the cam curve," Hardesty explains. "So our piston has to take a lot more abuse than a NASCAR piston.

"They tend to be a bit heavier so that they can take it. Our Mahle piston weighs 475 grams."

Detonation is the demon for World of Outlaws sprint car pistons. To stop detonation and the resulting wear to the piston skirts, Shaver specifies a piston life of then to 12 races, or about 400 miles for the hard runners.

"We see some wear but it's at a minimum; what we see sometimes is wear to the piston skirt, so that is why we don't run these pistons longer than ten to 12 races," says Hardesty. "We use a box-style piston and skirt length, cam shape and ring land area specified by Mahle.

"These pistons rock at the top and bottom of the bore more than the pistons we used five to ten years ago. This helps the mass load, as the piston is changing direction at the top and bottom of the bore. This has given us excellent reliability with the 16:1 compression ratio the hard runners like to use."

To help customers keep track of piston wear while on the road, Shaver specifies Total Seal rings in the triple-ring land piston it supplies. "That way they can tell if there is anything wrong with the motor," says Hardesty. "We have customers out in the field and you've got to have a way to let them know if they've messed up with their tune-up, so they leak them down and if they're not leaking past the rings or past the valves they can run them again."

Never wanting to stand still, Shaver is experimenting with other manufacturers' pistons, and has used their test pistons in practice sessions as well as races, always trying to find ways to avoid the detonation problems that can ruin an engine. Still, it stays with Mahle for its primary choice because, as Shaver says, it's consistent.

"A motor that sees a lot of rpm needs to get its piston weight down; a motor that doesn't see as much RPM doesn't need to go to that edge," says Shaver. "We need to get as close to that edge as we can."

Fig. 1 - The only type of wear Shaver Specialties Racing Engines sees on its World of Outlaws is the occasional wear in the pin boss area (Photo: Anne Proffit)

Written by Anne Proffit

To piston designers, however, the name means only one thing - a particular design of scraper ring common in many ring packs and used to control the amount of oil on the cylinder bore.

In a typical three-ring piston design the Napier ring is invariably used in the second ring position. The top compression ring takes all the gas loads and heat generated as a result of combustion while the oil control ring at the other end controls the lubricating oil attached to the cylinder bore, returning it through the side of the piston to the sump.

The task of the centre ring, often referred to - quite erroneously in some people's eyes - as the lower or second compression ring, is a combination of both. It further controls oil that has escaped the oil control ring, and at the same time reduces still further the amount of combustion gas travelling towards the sump. In fact recent studies have suggested that up to 85% of its activity is oil control while only 15% is gas control. The ring is therefore essentially scraping off the excess oil and allowing only the minimum through to ensure adequate lubrication of the top ring.

While simple scraper rings can suffice, the Napier ring is perhaps the most common of those used and comes in two basic forms. The first is essentially a rectangular cross-section recessed at the bottom outer surface, while the second is similar but incorporating a slight taper on the outside face. The result is a sort of hook or claw in cross-section as the inner face is undercut.

When installed, this latter version creates a continuous line of contact around the surface of the cylinder bore, maximising the contact pressure but minimising the actual friction load even when the ring twists, as it will invariably do. This narrow line of contact also ensures rapid bedding-in and, shielded from the excessive temperatures of the top ring, it is therefore usually made from cast or ductile iron.

The great advantage of the Napier scraper cross-section in comparison to a simple scraper ring is its ability to store oil just below its lip. This ensures that there is always a space for any excess oil to flow, and prevents any potential for the inter-ring pressure to build up and force its way past. Thus a three-ring piston with a Napier ring as the bottom compression ring will always have better oil consumption than other designs.

This, however, tends to be at the expense of increased exhaust gas blow-by. Furthermore, such designs can be adjusted to suit the application by altering the size of the oil storage volume to ensure oil is removed quickly away from the bore.

John Napier's invention of logarithms simplified the world of mathematics and astronomy. In the piston ring the name 'Napier' is equally significant.

Fig. 1 - Napier ring
Fig. 2 - Cross-section

Written by John Coxon

Johnson himself, who with his son Kurt builds engines for their two-car operation, is known as 'The Professor' for his meticulous and methodical work patterns, as well as for his innovations in the category. He notes that, with the GM engines, "You've got block materials and significant inconsistency in the GM block. You look at the Ford and Mopar blocks, and you see a maximum of 12-thousandths cylinder wall change, where we have to live with stuff that is 200-thousandths cylinder wall thickness changes.

"Because of that cylinder wall change it becomes a real problem, and consequently piston skirt configuration becomes an area you need to address," Johnson advises. "You are constantly addressing skirt wear because of heat, rpm and loading from the different con rod and stroke combinations, so you're constantly changing skirt configuration and end up with, essentially, zero wear."

Over the years, he has changed his piston spec perhaps 14 times from the original box piston he designed in 1990. On a 500 cu in engine that runs about 11,000 rpm, Johnson has specified pistons from Wiseco Performance Products, based in Mentor, Ohio, for the past 15 years.

"They are the only manufacturer out there that does their own forgings and heat treating," he says. "Everything is done under one roof. The other manufacturers farm out our forging and heat treating so we prefer to deal with Wiseco..

"If you are going to have a change, it's nice to have it all performed under one roof. We can specify a forging and have our pistons in sometimes as little as a week-and-a-half, in the right circumstances."

Using a piston for as many as 35-50 passes down the quarter-mile racetrack, Johnson finds that he changes his piston specification as he adapts his cylinder head configuration. "The forging changes because we want a net piston that we don't have to go and do a lot of under-head lightening on," he says.

NHRA requires pistons to weigh a minimum of 460 g, so that under-head weight reduction of the piston weakens the structure of the piston. "We like a net forging that we can set up there in the engine, coming up with a piston that is relatively close to the weight minimum but still having a good service life," Johnson says. While he does not lighten the piston, Johnson's shop performs some additional machining to the gas ports and contours the piston dome to his exacting specifications.

"Thirty-five runs is easy, and even 50 runs too," Johnson says. "Some of the pistons still look good but we change them out for historical purposes. We don't throw anything away because we may have to use those pistons in an emergency."

Johnson uses the same engine configuration for 22 out of the NHRA's 23 races in the 2010 campaign. "We make small adjustments for Denver because of the lack of air there, but that's about it," he says.

Fig. 1 - Warren Johnson specifies his net piston from Wiseco Performance Products, and performs additional machining to the dome contour (Courtesy Warren Johnson Enterprises)

Written by Anne Proffit

But unless you have tried to manoeuvre one of these 50 litre (11 gallon) devices either full or empty, you can't fully appreciate how easily they move. It's all in the shape, you see.

Similarly, another component conforming to a similar shape and one that may perhaps be of more interest to readers here, is the piston top ring. And rather like our ale cask, the barrel-shaped compression ring is the product of continuous development over many years.

The top ring in any piston assembly has the most arduous of tasks. Taking the full brunt of combustion events, it has to seal against an imperfect cylinder bore, coping with huge amounts of pressure variation and conducting large amounts of waste heat away from the piston crown while at the same time minimising any frictional losses.

Working in association with the rings beneath it, all this has to be done many times a minute and with losing the least amount of oil up into the combustion chamber. But as engine speeds get higher and rings thinner, and therefore required to be stronger, the need for the barrel shape has become even greater.

Ground or lapped into shape and then coated with any manner of favoured materials, once installed the ring offers virtually line contact all the way around the bore. And as the ring groove and ring invariably move or twist in response to the dynamic or thermal loads, this line contact will prevail when other profiles might lift in certain places and allow precious gas to escape.

In race engines, where engine life is always limited, bedding in of the ring against the liner needs to take place quickly, preferably in minutes, so as to not use up too much of that valuable running time. Even assuming the best of plateau hones, the narrow line of contact will produce high contact loads during initial running.

These loads will drop very quickly when the high spots on both the liner surface and ring are removed and the line of contact broadens. The big advantage of the barrel shape, however, is in the lubrication between ring and bore on both the upward and downward strokes.

On the downward stroke, as the ring tries to strip away the oil left by the secondary, scraper ring, the shape of the barrel is such as to generate a 'wedge' of oil between the ring and the cylinder. The ring will ride over this very thin layer of oil, and hydrodynamic lubrication with minimal friction (and hence minimal wear) will result.

Likewise, on the upward stroke when the force between the ring and liner surface is not so great, since the high combustion pressures of the downward stroke are no longer forcing the ring outwards, the shape of the upper ring profile generates yet another wedge of oil and friction is minimised yet again. Even as the ring twists and turns through its dynamic movement, the geometry is such that on the upward or downward movement this wedge of oil is still present.

So, as in our cask of ale, a barrel shape has many advantages.

Fig. 1 - Top-ring lubrication

Written by John Coxon

"Typically we use JE three-ring pistons because they are local (less than an hour away from Esslinger's shop) and they do an outstanding job. They work with us and turn stuff around in a hurry. We've found they make fewer mistakes than we do! Sometimes we ask for things we shouldn't and they (the pistons) get stuck in engineering that way. JE generally gives us what we ask," Esslinger says.

Because there have been so many different piston designs going through the Esslinger Engineering shops in South El Monte, California, the company has ended up with product on the shelf. This is also due to the different ring packages, "As the ring manufacturers figure out different things that work better for what we're doing," Esslinger says.

"And some things you try and go straight away from them; some things you stay with. There are different ways of doing gas porting and everything you change changes another three things, it seems. It's hard to put stuff on the shelf and hang on to it for a very long time when there is so much research and development going on," he says.

Esslinger Engineering changed its piston package at the end of the 2009 midget season, and has been using the same combination throughout the 2010 season. "The only change we made was to valve pocket depth, but to do that we had to change the ring pack and shorten things up for the valve relief that runs into the top ring. Everything changes, so you're constantly chasing the set-up," Esslinger says.

He also says piston wear is not much of an issue for his clients. "We typically change them out after 20 runs. The amount of time we spent 'magging' them, it's a certainty thing. After all, if you break a piston, you do pretty big damage. Of all the motors we've done, I can honestly think of only two actual piston failures (in the past seven years) and both of those were due to pulling too many times, so we swap them out willy nilly."

Esslinger used to have an issue with ring land wear, mostly caused by dirt and the type of filter he was using. "We made some changes there and we still see an air filter getting torn off at some point. In 20 races, we're not seeing much in the way of ring wear. It's probably due to the way we prep the cylinder walls; we've got some things kind of figured out. Even in extreme heat situations, we're not seeing scuffing and those kinds of things," he says.

Esslinger uses flat top pistons - "it's simple that way" - and says the combustion chamber works pretty good. "Everything affects everything, and a piston failure isn't always a piston failure a lot of the time. It's a tuning problem or those kinds of things. If we saw a piston that appeared to fail, we'd look harder in places other than the piston. We try not to ignore any of the things that potentially might be causing any problems," he says.

Esslinger's Ford cammer engines run a little over 15:1 compression, and he says they're getting stressed hard. "But compared to our competitors, we turn more rpms, which actually tends to take some load off them. We turn right around 10,000 rpm, depending on the customer. High rpms are hard on some things and easy on others; it's pretty good on pistons," he reveals.

Fig. 1 - New JE piston for use by Esslinger Engineering

Fig. 2 - Used JE piston nearing the end of its service life, as used by Esslinger Engineering
(Photos: Anne Proffit)

Written by Anne Proffit

Ductile iron comes from a class of materials referred to as SG (spheroidal graphite) irons. Similar in composition to the grey cast-irons used in older type piston ring technology, the carbon flakes in grey cast-iron have been converted into a round, nodular form, inhibiting the formation of micro-cracks and making the resultant iron considerably more ductile and very much less brittle.

As a result, however, it loses much of its bearing properties, so to regain these certain modifying elements are included along with suitable heat treatment processing. The result is a centrifugally cast thick-walled tube reminiscent of a cylinder liner, that can be machined and split into individual rings on a normal lathe.

The more usual method of production these days is to cast individual units on a sprue, very much like a Christmas tree. Cast from round with the free-end gap after machining, when the end gaps are brought together the ring is perfectly round.

This is fine when you are making thousands of rings of, say, a nominal 4 in or 83 mm size diameter. But when making a ring or set of rings for a non-standard bore, as in the case of Omega Pistons of Halesowen, England, the options for manufacture are suddenly reduced.

In such cases, the above cast thick-walled tube is machined on a normal lathe, and the free-end gap cut using a very thin blade. The thin, C- shaped products are subsequently placed on a mandrel and heat treated to produce the permanent set we expect from an open ring. The individual rings are then finish-machined and an external profile - whether it be a barrel, taper or Napier scraper - is introduced. All told, from ingot to finished product, some 14-15 operations are needed.

Using such methods, small batches of rings can be made for non-standard pistons. The resulting rings - be they top, secondary or oil control - will be as good as, if not better than, a standard cast-iron one, and considerably stronger.

Extremely hard-wearing ductile iron rings will work directly on any of the usual bore materials without the need for any coating. They will work in a plain iron bore, a chromium-plated bore or a Nikasil bore. And above all, they don't break!

They may not be quite as good as the best steel rings but for most applications they are more than adequate. For example, they were fitted to a team of rally raid vehicles recently, and after 6,500 very tough kilometres the cars finished first, second and third with no hint of any engine issues.

According to the maker, the only limitation is the width of the ring, as anything over 1.2 mm wide will produce too high a radial tension. Because of this limitation the radial depth has to be made smaller in order to open it up sufficiently for it to go over the crown of the piston and into the ring groove.

And if you think the ring is not flexible enough, Fig. 2 shows it before I curled it back to roughly ring shape again.

Fig. 1 - The ductile iron piston ring

Fig. 2 - Before being curled back to ring shape

Written by John Coxon

John Stewart, tuner and crew chief for sophomore Top Fuel racer Shawn Langdon, has a great deal of experience in the sport, which makes him a perfect choice to take care of a newcomer like Langdon. "We use Venolia pistons with a triple ring," he says, made of billet aluminium, "the same everybody else runs.

"We find the most wear occurs on the skirt, because the piston rocks around and puts a hole out. It'll just scuff the backside of the piston a little bit, putting a little mark there and a little light gristle on the gut. It gets to the skirt and starts dancing around, and once that is done - when [the engine] hits the rev limiter or when he's shutting off at the other end (of the track) - it takes all the fuel away, but we're still trying to run, so it scores the skirt," Stewart explains.

Stewart checks his pistons for dish after every run. "They'll run for a while but I don't like to put more than 12 runs on them," he says. "As long as they check out, I'll go forever, but you really have to watch them. Sometimes they start breaking the ring lands, and I'll start chipping that away."

After each run this tuner/crew chief will pull the pistons and check them. "As long as they're free, they're okay and I'll leave them in the rack. The 'kid' (who cleans up the engine parts) will go through and check them all. And as long as they're good we'll keep running them."

Scoring is normal wear and tear on a Top Fuel piston, and Stewart's team takes that condition in stock for its wear checks.

Stewart uses three piston heights. "We run the setback blower with a little opening on the front of the manifold, so the piston in the front has to be a lower piston," he says. "You can't run as much compression because that air is coming straight down on numbers 1 and 2. [Numbers] 7 and 8 are in the back and they are a little bit taller. Seven is taller.

"We use three different heights. [Pistons] One and 2 are the same, while 7 is a little bit taller because it doesn't get the air, so I run more compression there. Number 5 is a little bit lower because it is right in the middle of the block," he says.

Stewart admitted that piston numbers 1 and 2 see the most damage, as well as number 7, because that is where the air is lacking in the engine. "It's all about the way the motor is fed the air," he says. "Those are the ones that tend to get picked on - 1, 2 and 7."

Normal service life for Top Fuel pistons can't really be defined because of the nature of each race. "We'll bring them in and put them up on the rack, go around with the gauge and check the tops for dish. If they are dished more than five thousandths I won't run them again, they'll sag," Stewart says. .

"I finally got my motor to work the way I want, so I haven't been hurting the pistons that much," he's pleased to say. "And that Shawn Langdon is a real driving machine!"

Fig. 1 - Morgan Lucas racing piston

Written by Anne Proffit

The basic U-flex design goes back many years. A one-piece OCR consisting of a complicated U-shaped pressing, the design was widely used even before World War II. The story I was told was that during the Great Depression of the 1930s, being unable to afford to replace their increasingly smoky and worn-out engines, an OCR was developed that conformed better to the these worn-out bores and kept the engines running when other OCRs did not.

While this might be an urban myth, the technology soon became unnecessary as bore technology improved, but the fact remains that designs like this produced exceptionally good oil control. But this was to the detriment of other characteristics.

All OCRs are now generally very good. Available as two- or even three-piece designs, they are configured to apply an even pressure around the bore as they travel up and down. When bores remain round and cylindrical they work very well, even though the radial pressures of the rings have to be higher than those of the compression rings above to compensate for the lack of exhaust gas pushing them out.

But as engines get lighter, cylinder walls thinner, or bores get closer together, these bores begin to distort and the traditional two- or three-piece ring becomes inadequate. Under such conditions the traditional approach does not work, so much more flexible oil rings are needed. As bores distort as a result of reduced cooling efficiency or greater loading due to lighter structures, U-Flex OCRs can therefore show great benefits. But even when distortion is under control, the design can still have advantages.

In any engine the piston ring pack can account for the lion's share of internal friction - 30%, sometimes as much as 50% of the total friction in an engine - and the vast majority can be down to the OCR. Traditional two- and three-piece designs are heavily tensioned to conform to the shape of the bore and therefore create high levels of friction.

The latest U-Flex, however, with its multi-segmented design, will conform to the bore more readily and so the potential exists to reduce this radial ring load. But, functioning much like a gapless one-piece design punched from spring steel and folded to suit, the accuracy has to be very high and matched to the bore if even higher, unintended, radial loads are to be avoided.

In high-revving engines, piston suppliers report poor support around the ring groove, and while the ring gives excellent conformability and minimal oil consumption, wear can be high. But like the great number of different piston ring designs before it, this, the latest in U-Flex designs, will no doubt have its place among the piston ring options.

Fig. 1 - CAD rendering of a U-Flex OCR

Written by John Coxon

Much has changed since then, as HPD became sole engine supplier in the 2006 IndyCar Series season. Now the engines have a service life of 1400 race miles or 1600 absolute miles, as Roger Griffiths, manager of the development division at HPD, explains. So in piston development, he says, "We've gone through quite a lot, as we've looked at two different areas - reliability and performance.

"Obviously, we had to switch between a 3.5 litre and a 3 litre engine, and the piston specification changed on that. The bore size never changed but the stroke did, which led to different pin positioning on the piston pins," he says.

Most of HPD's development went towards keeping the power output steady and extending reliability. "In the past four years it has been about maintaining the same performance level over a longer and longer period," Griffiths says. "These efforts have been in ensuring that when we take the piston out of the engine after 1600 miles, it is in one piece.

"The other challenge we had was when we switched from methanol to ethanol, so we had to learn a little bit about the combustion on the engine using ethanol. Ethanol is more sensitive to detonation, so we just backed off the compression with our pistons.

"We didn't need to change the cylinder head at all, so we changed the shape of the piston dome to use less material. We went a bit flatter so that there was less material in use," he says.

While HPD never reveals its supplier resources, it does acknowledge it has a single supplier, and that the relationship has been in play since the beginning of HPD's engine building experience in the Indy Racing League.

Griffiths also says the team decided on a double ring piston. "We get what we need with two rings," he says. "We've been able to design the piston in such a way that we really can identify trouble spots early in the going, and we don't have any particular issues that we are concerned about right now.

"We may run into some stability issues but over the past three years - based on the work we did in the first four years - it's the same piston we've been running since the start of the 2008 season. We're into our third season with it and pistons have been a non-issue for us."

Most of the aluminium pistons removed from the engine after the duty cycle is complete are crushed after removal. "But when we're examining other parts of the engine we might use a used piston for testing purposes," Griffiths says.

HPD has used various coatings or treatments on its pistons over the course of the programme, Griffiths says. "Sometimes those coatings or processes have changed during the development - we've found something new or better, say, or it was no longer necessary. It's all about the package, developing the right package for the right application. There is no miracle coating you can put on anything."

HPD replaces pistons at each rebuild - between 1400 and 1600 miles - and might carry out upwards of 200 engine builds a year depending on the number of entries for the 17-race season and the all-important Indianapolis 500, where as many as 40 individual race cars could show up to qualify. At this time, all contracts are for the full month, which gives each entered Dallara one engine for practice and qualifying, and another for the final practice and the 500-mile race.

Fig. 1 - Generic double-ring piston

Written by Anne Proffit

At the risk of stating the obvious, gaps in piston rings are a physical necessity to aid assembly and assist with the inevitable expansion of bore and piston. When in place, therefore, a gap of 0.003-0.005 in per inch of bore diameter is generally recommended.

In two-strokes it is common practice to peg the ring to prevent it from rotating, since not to do so would risk the gap edge digging in and scoring the bore as it crosses the exhaust port. In preventing it from rotating though, the risk is that both ring and bore will wear non-uniformly and compromise durability. In four strokes, however, the reverse is true and the rings are not restrained, other than by the groove and liner surface.

These days though, and now somewhat enlightened, I prefer to subscribe to the school of thought that if something can move, it will. So if a nut can fall down inside the tappet chamber when assembling an engine, it will. And of course, if God had intended piston rings not to rotate he would have made sure they physically couldn't in the first place.

The point was brought home to me by a piece of research carried out a surprisingly long time ago now, which involved making a very small section of a ring radioactive and then looking at the gamma radiation given off using a sodium iodide scintillation tube placed next to the cylinder block. During testing, and despite the noisiness of the signal generated, a sinusoidal output could be clearly seen as the ring (presumably) rotated and moved away from and then back towards the detector.

Using more advanced, statistical analysis merely enhanced this characteristic. The frequency of the signal was highly variable but depended on the engine speed and load, with a maximum correlating roughly to engine speed - for every 1000 rpm by the engine, the ring rotated by a maximum of roughly one.

In later work, it was also suggested that not only could the rate of rotation be measured but its direction as well. The theory went that by dosing another portion of the ring with another radionuclide - different from the first and having a gamma-ray signal differentiated from it - and using a second scintillation tube, the direction of rotation could be deduced.

That rings rotate is no longer disputed. In which direction and why, however, is the subject of much debate. Maybe it's all to do with the cross-hatching of the hone, or it's a much more complex issue than we can ever hope to resolve. Either way, there seems little point now in staggering the rings' gaps - but despite our new-found knowledge, we'll still do it.

Fig. 1 - Measuring ring velocity and direction

Written by John Coxon

For co-crew chiefs Rob Flynn, Mike Guger and Todd Smith (who came over from Don Prudhomme's Snake Racing after that outfit closed operations at the end of the 2009 season), the objective is to fit the strongest piston available.

"I don't think we want to make the piston lighter; we need to make it stronger more than anything so that it supports our crankshaft," Flynn confirmed. "We want it to maintain its shape," and running close to 8000 horsepower doesn't help a piston's service life.

The Bernstein trust works with both Venolia and JE pistons, depending on the application. "Everybody's combination is a little bit different but with most combinations, you usually have to watch the No. 3 piston. In the nitro motor, the piston is just not strong enough. You don't get much life out of it, so we have them in for 1-2 runs per piston, but No. 3 is susceptible [to failure] because of the distribution of air and fuel," added Smith. "Still, you can't necessarily build a specific piston to deal with strength for that hole, because you don't want to mess up your balance. That's the hardest running one, for us anyway."

Flynn acknowledged the team has "scuffing in the skirts sometimes, and you do have problems with cracking ring lands. That's just a by-product of the pressures that we put into the cylinder." Guger added, "It sinks the dome. We use a flat-top piston and we sink the domes with all of our pressure."

Even if the manufacturers - who furnish aluminium forged material for these pistons - make the part thicker, it doesn't help. "You can add material and still sink it," Flynn told me. "I think if you make it too thick, you'd lose some of the heat properties that you want to keep in the chamber."

"It's a fine line really," Smith noted. "Many piston manufacturers will play with the forging that is specific to them. We just try to relay to them what we see or what we need and they'll go about it accordingly."

Flynn said he gives his primary manufacturer a specific piston to copy "and they made it the way their material works with our ring configuration and our compression height. We have a different compression height - everybody's a little bit different," he said. "What we are doing is different than what others have and we don't worry too much about what the other guys are doing. We just do our own thing," which is the way the entire paddock operations in NHRA Top Fuel racing.

Thus far in the season, the Bernstein gang has had trouble going rounds - until Gainesville, when they made it as far as the quarterfinals. Flynn, Guger and Smith can see the changes they've made are starting to pay some dividends.

Fig. 1 - Kenny Bernstein Racing is looking for strength in its pistons, not light weight

Words and photo by Anne Proffit

You may always hear that the root of the problem is all to do with the acceleration and deceleration of the piston and is therefore a phenomenon much more likely to happen at high engine speeds. That isn't always the case. If the conditions are right, ring flutter can occur at any speed and is therefore considered now to be more of a dynamic condition as a result of the piston ring moving up and down in the ring groove, twisting as it goes, as well as moving in and out and losing contact with the cylinder wall.

To understand the phenomenon even in part, we need to look at the forces acting on the top ring of the piston. These are the combustion pressure forcing the ring down and the crankcase pressure underneath, counterbalancing it to a limited extent. This ring will also have a slight clearance in the retaining groove in the piston and to assist sealing, a tangential force acting on the cylinder wall. When the engine is operating at full load, the combustion gas will flow between the top of the piston ring and the ring itself and from behind the ring, forcing it outwards. A gas tight seal will therefore result as the ring is seated against the lower face of the piston groove and forced against the cylinder wall.

However under the conditions of flutter when lightly loaded, as the piston slows down, the ring may wish to twist or move and part company with the lower groove face, cutting off the flow of gas to its rear. The ring will then effectively collapse inward, perhaps even breaking contact with the wall and the gas pressure eventually overcoming its now impaired ability to seal, finding its way down towards the crankcase. Passing either side of the piston ring, pressure may build up between the top and second ring grooves, which will then further upset the top ring at certain stages in the cycle and if it hasn't already done so, lift it momentarily off the bore thus making matters even worse. Sometimes accompanied by high oil consumption, the motion is irregular and in a way quite light, like the fluttering of a butterfly. But unlike that (unless you are an advocate of chaos theory) the consequences can be more devastating.

Lest you forget, the piston ring is a major route for heat transfer between the piston crown and cylinder bore, and eventually into the coolant system and anything that interrupts this flow can have serious consequences to the local temperatures in the piston. During the periods when the ring is not in contact with the surface of the bore, no heat can flow and the piston therefore risks overheating leading to pre-ignition in the combustion chamber.

But rather than just the inertia of the ring in the groove or its design, modern thinking suggests that the phenomenon of ring flutter is more down to the incompatibility between the piston ring and the piston itself, than one factor alone. The design of the ring, barrel or otherwise, if it has a bevel on the inside or not and also the quality of the ring groove in the piston are all issues. Incompatibilities in thermal expansion as well as that in thermal conductivity can both influence the condition and if left uncorrected will steadily destroy the geometry of the area and give the wear characteristics so evidently identifiable as ring flutter.

Although heard of comparatively rarely these days, the issue most certainly hasn't gone away.

Fig. 1 - Flutter: (verb) to move or fall with light irregular motion

Written by John Coxon

Forming a seal between the piston and cylinder bore the goal is to prevent the passage of combustion gases between combustion chamber and crankcase on the one hand, and at the same time minimise the passage of oil from the crankcase up into the combustion chamber on the other. During the compression and power strokes the compression rings seal the combustion gases and control the exhaust blow-by while on the downward strokes, the excess oil thrown up onto the cylinder walls is scraped off and returned eventually back to the sump, leaving only a small amount of oil on the cylinder wall. Since the majority of parasitic losses occur in the ring pack in the form of friction, all this has to be done using the minimum of side force between ring and bore throughout the cycle.

The Dykes ring is therefore a lightweight 'L'-shaped ring with only a very low spring tension on the cylinder wall in the static condition. During the intake and exhaust strokes the arrangement will exert little in the way of side force on the cylinder wall but on the power stroke combustion gas can flow down past the increased gap around the top land and through the convoluted space between ring and piston and force the ring out against the bore. In this way, or so the theory goes, friction is only apparent during the combustion stroke. Furthermore, the reduced mass of the ring because of the ultra thin section minimises the chances of ring 'flutter' caused by acceleration and deceleration at high engine speeds. Additionally, machining the grooves into the piston required much more care than other arrangements.

Needing a special piston with groves to match, their asymmetric profile can make them hard to seat leading to higher bore wear. Replaced by more effective designs, other than in certain specialist applications, Dykes rings have now been consigned to history.

Fig. 1 - Cross-section of the Dykes ring.

Written by John Coxon

Still, according to David Currier, TRD's vice president of engine engineering, the specs change on these pistons about three times each season for open configurations and about twice annually on the plate engines. "We change our specifications fairly frequently because if we change our head geometry, that changes the crown shape; we change chamber shapes from time to time, as well as valve lift, which changes pocket depth. Of course we're always working on friction, slip skirt shape and rings so we're continually evolving those things."

Currier admits there are more challenges with the open engine - used far more often than plate configurations - but the pistons, he noted, aren't that difficult to deal with because of the mandated 400-gram weight for all configurations. No matter the configuration, TRD uses its pistons for a single use of one race weekend. "Structurally speaking, there aren't any big problems with the pistons; certainly, wear relative to skirts and optimizing the complex three-cam shape is a continual battle in adjustment. But as the engine changes we change the crown shape and that changes some of the structure so it will flex differently," he told me.

Working together with their supplier, TRD has verbal discussions of "take a little off this area" and occasionally makes a proposal based on their post-race teardowns. "It just depends on who's got the experience and what it is we're seeing," in regard to wear. "If it's something they are better at than we are, then we let them use their expertise. If it's something we feel we have more knowledge about - or a stronger opinion about - then we'll dictate it more."

Currier emphasized that changes to the piston skirt design has to be a balance between cost and logistics, because there are races nearly every weekend between February and November on the NASCAR trail. "We try to do fewer steps but bigger steps" when a piston redesign is called for. "Just because of the costs," Currier said. "Trying to control our costs and get our budgets further refined and further down as they tend to be going. If we change every weekend, then we have spare engines that need to be changed and there is a lot of cost with that."

When TRD does change piston design, "It might be due to other things that cause the piston to be changed, like a combustion chamber change, or the head's a bit different and the valves are different. Those things usually go as part of a package."

Fig. 1 - TRD's piston specifications change a few times each season (Photo courtesy of Toyota Racing Development)

Written by Anne Proffit

According to Stewart Van Dyne II and Stewart Van Dyne III (Tres) of Van Dyne Engineering in Huntington Beach, California, "We tend to like pistons with longer pins and thicker walled pins because they help keep the pistons alive."

Primarily utilising products from local purveyor CP Pistons in Irvine, California, the Van Dyne operation specifies 2618 material weighing 670 grams and goes with three ring lands, with radial gas ports, double pin oilers and wire locks.

"The off-shore pistons get a bigger beating than any of the pistons in our other [big block] engines because, even though they are on and off the throttle a lot, they are on it a lot! The pistons have a tendency to get soft, losing their strength and they end up ripping the wrist pins out of the bottom of the piston."

Because they are limited to 12:1 compression in the Super Cat class, the company's client Craig Ferguson of nearby Garden Grove, who has campaigned The Renegade to two World championships and 11 APBA Western Division titles, ends up with fairly traditional piston specifications.

"We may run a different shape [dome] but it's not that critical because the dome is not too tall. We normally don't use coatings because we haven't seen much value in it, or seen them live longer, or better, because of the coatings. We have done the skirts with black moly coating, but it doesn't seem appropriate for the application as the CP skirts always look perfect," both father and son agreed.

The engineers specify a high deck block, one that is about 0.400 thousandths taller than the standard deck. The engines come out a bit under the maximum allowed size at 3.750 stroke and 4.600 bore. Using a 7.000-inch rod, this also helps piston life, giving a 1.866 rod/stroke ratio. These engines make about 830 horsepower.

The American Power Boat Association races last about an hour with three practice sessions of 20 minutes each the day before the event. This run time allows the pistons to go for four races - sometimes five - before being removed and replaced with new stock, according to the Van Dyne operation.

It's rare that they see much wear, particularly with the current imposed rev limit of 7600 rpm, but they'd rather replace the units than have their client's engine develop problems.

The approximately 10 hours run time on the pistons has become nearly a full season's racing for Ferguson, who has backed off his competition in 2009 with the economic downturn. "The fields are a bit shorter than they had been but Craig seems to win nearly every race he enters."

Fig. 1 & 2 - Pistons life is extended by the cold water that runs through the block and head

Words and photos by Anne Proffit

But what precisely is steel and how does it differ from that other popular piston ring material, cast or even ductile iron? Rather like the focus of the UN Summit meeting, the answer is all in that most politically sensitive of all materials - carbon. And whereas with greenhouse gas emission, the culprit is carbon dioxide - at levels currently around 380 ppm in the atmosphere, with steel that carbon manifests itself in slightly larger amounts but typically less than 1%.

Pure iron, in other words an iron that contains no carbon or impurities, has poor strength and is soft, softer even than aluminium. Silver-grey in appearance, in this form the material is highly reactive in air or moisture but the addition of only small amounts of carbon increases its strength substantially but to the detriment of ductility. When this amount of carbon present exceeds 2%, graphite flakes begin to be formed which when at 3% by weight gives us our typical grey cast iron. Producing a level of inherent lubrication at the surface, these flakes are also stress raisers which reduce the overall strength of the material and make it much more susceptible to brittle fracture. But for its poor strength however, cast iron makes an ideal piston ring material.

However as dynamic loading in the top ring increases - as a result of increases in engine speed, for instance, there comes a point when cast iron, even in its stronger and more ductile nodular carbon form, can no longer cope. The desire to reduce piston weight, especially around and above the top ring also calls for a stronger, thinner and therefore more flexible ring material. Steel in one of its many forms would therefore seem to be ideal having much better tensile and fatigue strengths (sometimes as much as five times of that of cast iron), together with improved hardness. As a result, rings can be made thinner and more flexible reducing both friction and wear. Whereas a typical cast iron top ring would be no thinner than 1.5 mm, its equivalent in steel is typically 1 mm or even less. But although the steel ring may be stronger, in order to minimise friction and wear, the working surfaces will need to be coated with one of any number of coating technologies.

Manufacturers are rarely keen to talk about exact materials but one such used - AINSI 9254, is described as a high-alloy steel with between 0.51 - 0.59% carbon. Classed as a medium to high carbon steel this material was chosen to give excellent wear characteristics with the hardness obtained at the expense of ductility and toughness.

But while UN Summit meeting itself dissolved into confusion and acrimony, somewhat ironically the carbon in the 'Ring of steel' kept all those inside safe from harm.

Written by John Coxon

Wendland said the team uses a variety of pistons, depending on the nature of the circuit. Their primary choice is a hard anodized JE piston, “Because they provide excellent customer service. It’s also part of the combination that was on this car when we came over here. You hate to change a system that already works,” he said.

Trying different combinations during a race weekend – there are only four in-season test sessions allowed over the 24-event campaign – Wendland may want to “raise a piston or lower a piston’s compression,” so with one-week turnaround that is easier to do when they’re on extended road trips. Raising or lowering a piston is the tuner's way of increasing or decreasing compression in a particular cylinder. Such changes are slight, but a cylinder that burns consistently ‘hot’ might become normal with a little less compression by lowering the piston in that hole.

One of the biggest challenges is dealing with dish on the top of the piston. “After a run, depending on the dome thickness that you run, you may see conditions change. Given the tremendous amount of dome heat on the top of the piston, and compression [combustion pressure is concentrated on the dome of the piston, which is hollow underneath] and cylinder pressure, it’s almost trying to knock that piston through its dome [that is, the surface area of the piston exposed to the heat and pressure of the spark and combustion of the fuel mixture]. Then add the heat and it can really knock it through.”

“The problem is,” Wendland continued, “when you develop this piston so thick on top, it doesn’t dish. You then have a heavy top-ended piston, so you have all that weight above the pin height and that sometimes causes the piston to rock. If you get the piston rocking too much, you see a lot of wear on the sides of the piston – and that turns into scuff, which is a nitro tuner’s worst nightmare!

“Once you have scuff, the piston loses ring seal and then you lose compression, blower, fuel – all those things that can cause failure. It’s a fine line. We don’t run a thick belt, like some of our competitors, because of the weight issue,” Wendland told me.

The Matco Tools team’s pistons can last as few as half a 1000-foot run or as many as four passes down the dragstrip. They weigh about 2000 grams and are made of forged aluminium. “Antron can step on the throttle and have the wrong kind of oil and ruin the piston,” Wendland said. How do you ruin the piston with the wrong kind of oil? “If the rings don’t seal well, then it allows that oil to go by and it loses ring seal. Then the piston tightens up and does nothing, and you can do that at the step of the throttle.”

By using Valvoline’s nitro oil with the proper amount of ‘slickum’* put into the product, that determines the ring seal. “Too much ring seal is never a bad thing; too little ring seal is a very bad thing,” he laughed. “We’re experimenting with different types of oil to combat those problems and keep our pistons (and corollary items) happy.

The Matco Tools team is also experimenting with different forms of hard anodizing – using only one thousandths thickness – but Wendland acknowledges, “What it boils down to at the end of the day with our piston wear is tune-up and oil.”

(*‘Slickum’ is slang for ‘ingredients’ or ‘additives’ as it pertains to the new technology in racing oil for nitro cars. Racing oil has to lubricate revolving parts like crank bearings, rod bearings and cam bearings but new technology ingredients are helping increase ring seal in nitro engines. The heavy liquid volume of nitro, compared to the mostly air over fuel ratio of a gasoline or methanol engine, can wash past piston rings and scuff cylinder walls causing reduced ring seal.)

Words and photo by Anne Proffit.

Some time ago I had occasion to view the results of a study into engine friction. The engine, a quad-cam V8 of modern design and a ladder frame bottom-end, was motored in the fully hot condition up to 6000 rpm - the limit of the rig. When converted into Mean Effective Pressure the results revealed a total friction of 1.4 bar at 6000 rpm which roughly equates to a figure just over 11% of the total engine output at this speed. Of this, 11% was in the valve train, 21% in the crankshaft and bearings and a whopping 51% in the piston and ring pack. Now bear in mind that this was a fully developed piston and three-ring assembly using suitably coated steel rings and you can begin to see an emerging story here. On a ring-on-block test rig replicating the surfaces and loadings inside a firing cylinder, the coefficient of friction of the standard arrangement was roughly somewhere near 0.3 –0.4. Assuming only half of the friction stated was in the ring pack itself you can begin to see that in using diamond-like-carbon coatings with friction coefficients of less than 0.1, the potential to release valuable horsepower, especially at speeds well in excess of the 6000 rpm tested, is a challenge worth taking.

But what precisely is DLC and how does it differ from that other carbon-based coating, graphite?

To start off with, DLC is the generic name for any number of diamond-like-coatings. Made in two ways, plasma assisted chemical vapour deposition (PACVD) or physical vapour deposition (PVD), diamond-like-coatings are a mixture of sp2 and sp3 bonded carbon atoms of different forms having no dominant crystalline lattice structure. The sp2 and sp3 refers to the position of the electron in the carbon atom, sp2 being that for graphite and sp3 diamond. And while graphite is a soft slippery substance, diamond is the hardest substance known to man. Clearly the more sp3 or diamond in the structure, the harder it will be and the more resistant to wear. However, this will bring with it the disadvantage of higher compressive stresses in the body of the coating (which is around 2-5 microns thick), and in extreme cases, can lead to problems with adhesion and/or delamination of the film.

By introducing so-called ‘fillers’ like the sp2 graphite, hydrogen or other substances (both metals and/or non-metals) other desirable properties can be introduced. Properties like toughness, wear resistance, adhesive properties and coefficient of friction can all be manipulated by altering the deposition regime to give the exact balance of properties deemed necessary. In the case of one particular variety, an a-C:H coating consisting of only carbon and hydrogen and deemed suitable for a piston ring, when run in a dry steel-on-ball test at a load of 10N and a speed of 170 cm/second, ran for over 100,000 cycles before failure. If we were to add in a lubricant, as you might expect as in the case of a piston ring in a cylinder bore, and the dry coefficient of friction which was originally quoted as 0.10, would drop further and failure, if it happened at all, would take much, much longer.

With these characteristics, no wonder everybody is raving about the stuff.

Written by John Coxon.

“As rings get closer and closer to the top of the piston, they get so hot that the aluminium melts and adheres to the piston. Not long after that, the piston begins to fail.” What Randolph and ECR have done to circumvent this problem is hard anodising on the top ring land of the piston. “It works pretty well, but it still wears away the groove. When that happens, the groove starts sagging and then the ring starts twisting in the groove.”

In order to circumvent this type of problem, Randolph works with his suppliers on utilizing different types of materials. “There are different types of hard anodising and other alternative approaches such as Keronite, an electrolysis coating. We’re always looking at different materials that can cope with the pressures, the temperatures and the power levels, all of which are going up. You work that ring harder and harder and harder,” he told me.

While the pistons, which are procured from BME and Capricorn, come without this type of ring land anodising, ECR usually “runs them through a machine to quantify exactly what the form is – how flat it is – and what the clearances are of the groove. Then we send the piston out for treatment.”

Through the 2009 season, ECR have tested four different types of hard anodised coatings from three different international vendors. “Everybody has this problem and there are different ways of looking at it as different people have different solutions. Is hard anodising a better solution? I like it; it’s been working pretty well,” Randolph admitted.

“The thing about hard anodising is that it is not a coating; it actually has depth to it. If you want to get half a tenth build-up of hard anodising, normally it will also sink into the metal about the same amount,” he said. “So it is tough stuff. It’s not a coating where pieces come off; it s a transformation of material properties.”

The hard anodising of a piston has an effect on wear. “Mileage is one of the factors we look at, and it varies from track to track. At Loudon, for instance, the duty cycle is pretty low, at about 50 percent; when you get out to Fontana the duty cycle is more like 75 percent. That is a lot higher average temperature and pressure that the ring is going to face, along with higher engine speeds,” Randolph said.

Keeping the top piston ring land intact and trying to avoid micro-welding is an ongoing dilemma. “This is one of our big constant obstacles – how to keep the top ring land and piston intact. It is not an easy thing to do.”

Written by Anne Proffit.

Photo caption: Hard anodising on the top ring land of the piston can lengthen the duty cycle of the piston and stop it from micro-welding.

As we are all aware, the top compression ring is the main barrier to the combustion gases escaping into the crankcase and the simple practicalities of the design require it to be not only circular to match the cylinder bore, but also capable of being fitted into the piston groove with the minimum of complexity. Thus the familiar circular ring with a gap is almost ubiquitous and ring manufacturers will advise that this gap when fitted should be nominally something like 0.005 inches per inch diameter of the bore. Too large then the combustion gas will escape, exhaust gas blow-by will increase, and engine horsepower as a direct consequence will suffer. The most obvious solution is therefore to keep this gap to a minimum.
But we, as development engineers know it isn’t always that easy. In performance engines, reducing this end face clearance can bring with it other issues.

By their very nature, competition engines tend to run harder for longer periods. The heat flow around the piston top land is therefore much greater and the temperature around the piston ring much higher. In the cylinder liner, particularly near the top, temperatures are to a certain extent constrained by the presence of the cooling water and as such the temperature gradients in this zone can be considerably increased. Add to this the effect of the occasional detonation cycle if, as a competition unit it is ‘tuned’ to the limit, and we have all the ingredients for the rings to expand much more than normal, filling the void and eventually butting together. At this point any further increase in temperature as a result of say, a slight weakening of the mixture or increased detonation, can have a devastating effect producing more friction followed by the breakdown of the lubricant film, eventually leading to scuffing on both the cylinder bore and ring outer face. In extreme cases, rings will seize, resulting in catastrophic damage when the piston top land breaks away.

But if there is one thing that I have learned over the years, it is that engines don’t always react as they should. Reducing ring gaps, particularly those of the top ring should, in theory, minimise blow-by and generate just that extra little bit of power. However if combustion is marginal and detonation, however slight a possibility, then it might be better to stay on the safe side and maintain or even slightly increase them. But whatever you do, follow the manufacturers guidance.

Oh and when getting off the ‘tube’, do be careful!

Written by John Coxon.

If the pair are getting driver Ashley Force-Hood down the track without hurting any parts, they can easily stay with the same piston stagger for well more than one race. “It could be for as many as four races or maybe all year, if it’s friendly,” Antonelli said.

This team “has at our disposal a lot of different compression heights that we can play with,” Douglas noted, “but we try not to change it if it’s working for us, which it is currently.” They are in second place in the Funny Car category’s Countdown to One, with five races to go at this writing.

Inspection of their pistons is all-important for Antonelli and Douglas. “There is a lot that goes into inspection after a run. We are always looking at the piston to see what happened to it on that particular run and also, in our inspection process, we want to make sure that piston is able to make another run so we don’t have a failure,” Douglas said.

“We get a good product from our piston supplier but we don’t leave everything to their end. We inspect the product, measure and go through the pistons when they’re new – just like we do after a run,” he continued. “Nothing gets put in the engine unless it’s stamped ‘A OK’ by our team.”

On average, this team replaces its pistons after three runs, although they can be replaced after a single pass. “Sometimes they’re good for five; it kind of varies, depending on conditions. The pistons we put in for the Friday night runs maybe don’t come back in as good condition as the pistons we use for a hot track run where you don’t have as much fuel and you’re not pulling on it as hard,” Douglas added. “Each run is its own animal.”

“We allow ourselves to run the pistons a bit harder on the night runs,” Antonelli concurred. “We’re not as concerned when we use up a couple more on those runs, but if we are hurting them and we’re going slow, then we’re more concerned. We try not to over-react for testosterone or anything else. If the conditions are there, we’ll stand on it; if they’re not, we’ll make the safe call to go down the track.”

The decision on which piston to use occurs between rounds and that call comes well before engine warm-up. “We’ll go out there and evaluate [conditions] and do what we think we can run. We’ll see what the track looks like and we’ll talk about it,” Douglas said. “I don’t think we need to change that approach.”

“This year we have a master plan to try and get our cars more like we all were a couple of years back so we’re working that way the last five races or so and it seems to be working,” Antonelli said of their operation. “The cars have gotten more similar with the Countdown coming and we’ve fed off one another.”

Written by Anne Proffit.

Unlike the compression ring/rings above, there is little or no combustion gas pressure to find its way into the void behind the ring and force it against the cylinder wall and so these rings have to use a totally different method of maintaining some kind of seal. The single piece oil ring, for example, with its two outer lands parallel to each other relies totally on the inherent tension in the ring itself. These rings can have chamfered edges on either the outer edges of the lands – to increase the contact pressure – or chamfered edges on that part of the land facing the combustion chamber, which will also assist oil consumption through improved oil scraping from the bore. With slots to allow the oil to flow back through the piston, whatever the detail design, these are somewhat boxy affairs and find it difficult to contort and conform to the shape of the bore, which is often far from round during cylinder firing. Rings of this design are very rarely used today having been superseded by the most part by two and three piece designs.

Similar in some ways to the single piece ring, the two-piece version applies its pressure onto the cylinder bore using a coil spring in compression. Cylindrical and often made from heat-treated spring steel, this coil locates into a semi-circular groove or V- shape at the back of the ring and acts uniformly around the whole of the circumference. While not only helping to positively locate the spring this also reduces the cross-section enabling it to twist and conform more to the shape of the bore. At one time slotted holes were popular but now drilled holes have taken over because of their more uniform strength and even contour. Made from high chromium nitrided steels, with a variety of chamfers or more pronounced bevelled edges, these rings are more frequently found in diesel applications.

In high performance engines and especially those with thinner wall cylinder liners, three-piece oil control rings are a popular solution. Consisting of two thin steel rings (the rails), held apart by an expanding spacer, which also presses them against the cylinder wall, these three ring systems are designed to be fully flexible and ‘hug’ the cylinder wall as closely as possible. Having low inertia around their cross-section, these separate elements can also twist and squirm in response to the dynamic shape of the bore allowing minimum oil leakage to the compression rings above and at the same time helping to stabilise the piston in the bore. This conformability of the rails can be improved by increasing the tangential load or by decreasing its moment of inertia. However, this is not always desirable since the higher tangential loads increase engine friction and reducing the section can in turn introduce other durability issues.

In racing engines where oil control is not quite so important, friction is the enemy. The trade-off between oil control and friction will therefore inevitably fall in favour of low-tension rings and high oil consumption.

Some things just never change.

Written by John Coxon.

To keep things consistent, there’s not much out of the ordinary that he’s ready to do with his Bill Miller and Mahle pistons at this point in the game. “We do a lot of testing with different moly coatings on the rings and ring lands,” he told me at Chicagoland Speedway the final weekend of August. “It’s all about how the ring seals – some of them are not as compatible with the aluminium as others, so sometimes you have to coat them to make them survive.”

Since NASCAR does not allow exotic materials such as beryllium or titanium for the CWTS pistons, the sanction looks for a parity weight rule, requiring pistons to weigh a minimum of 400 grams. “Depending on your rpm limits and what your power level is, that’s how you decide how aggressive you want to go with the piston.”

There’s a wide variety of moly coatings available to Smith and to him, “It seems like every week somebody comes up with a new coating for us to try.” Of course, when you change the piston coating, it’s easy to cause imbalance elsewhere in the driveline. “It’s never a ‘freebie.’ If you do something to the piston it usually affects something else. There is different growth in different areas so you have to be careful,” particularly with ceramic coatings.

“You change different designs on the pistons because of where the heat is. We try to test as much as we can with significant time on the dyno and then we’ll take it to a short track, somewhere with throttle changes where you’re on and off the throttle quite a bit to test the parameters.” NASCAR has scotched testing at series circuits.

In the CWTS, the trucks run pretty much ‘flat’ at a place like Chicagoland Speedway, a 1.5-mile oval. “They are wide open all the time and it’s pretty tough on the pistons, so short track testing doesn’t bear on Chicagoland,” Smith said. “Sooner or later you have to put it in the vehicle,” so he finds other teams to test new pistons for him. “I hate to put something brand new into Ron’s or Matt’s trucks because they are in the points chase. You can’t take a chance on that stuff.”

He’s not the only one testing his pistons in this manner. “Everybody else is testing like this, too. You just have to be smart where you change things. When you test at a short track, you don’t get the same things you get on a mile and a half like this. Tracks like this are pretty significant for a truck, because they do run so wide open.”

NASCAR mandates fixed gear ratios for the trucks that keep them around 8000-8200 rpm. “That’s about all we’ve got, but we used to run about 9500 so we’ve got less friction now because of the revolutions. All those things come into play with our pistons but the big thing is making them live at a track like this because they are running wide open all the time.”

Written by Anne Proffit.

But there is one technique that can generate much more accurate wear rates, and in real time as well, and that is the process of thin layer activation. TLA, SLA (surface layer activation) or as it is known in Germany, RTM is a radioactive method and relies on a very small part of the surface under investigation made radioactive to depths typically between 10 – 300 microns. This enables activation to be confined precisely to those components under study and where critical wear can occur. In carefully selecting the activation depth, the amount of radioactivity can be minimised while still maintaining high wear sensitivity but at the same time minimising any hazards.
Applicable to many components in an engine, these are made radioactive using a charged particle beam to generate low levels of the appropriate radionuclide in the near surface. This is a specialised business using a cyclotron particle accelerator or tandem Van de Graaff accelerator usually found in specialised universities.

The radionuclide used depends on many factors, the main one of which is the material in the surface under investigation. In the case of a piston ring, a plain cast iron ring might use Co56 (Cobalt) or if chromium plated, Cr51(Chromium) could possibly be the best choice. For other, more exotic surfaces, other radionuclides can be chosen but these can often be dependant on the equipment available and it having a practical half-life (the time to reduce its radioactivity by half). Co56 for instance has a half-life of 79 days while that of Cr51 is only 28 days. With a sensible working life of three half-lives this gives more than enough time for most tests.

The ? radiation given off is typically monitored using a minimum of three inorganic scintillation detectors incorporating sodium iodide (NAI) crystals normally around 50 mm in diameter. Together with the photomultiplier and associated electronics, this makes for a rather large (200-300 mm) unit which when temperature stabilised using a water cooling system can be placed as close as possible to the top ring reversal point adjacent to but not touching the block. This detector will measure the loss of radioactivity from the ring while a second and third instrument in suitable housings can be situated in the oil circuit measuring the build up of radioactivity in the oil and at the filter. When recorded and analysed using PC based equipment, wear level rates can be calculated instantaneously.

While the indirect oil and filter measurements might not compare exactly with the direct ring loss, the sensitivity of the readings can generate wear data resolved down to nanometre (10-9 metres , 0.001 microns) levels and because such data is generated in real time so much more information can be derived. And the technique is not just applicable to one component. Provided radionuclides can be discriminated using a more advanced spectroscopic detection system, two or more surfaces can be investigated. This gives the potential to investigate the relative wear from each of two surfaces, say a piston ring and associated liner and the wear rate output I am sure would be very interesting.

Scintillating information or what?

Written by John Coxon.

According to Green, all their coating manufacture is performed in-house. This, he told us, can provide shortened lead times and reduced costs when compared to shipping pistons to outside coating sources. It also “gives us much greater control over maintaining tight tolerances and ensuring our customers get the best system for their application.”

The wide variety of custom pistons produced for the NASCAR market typically have the company’s Grafal coated skirts and hard anodised ring grooves, Green explained. This particular pattern on the skirt provides dynamic oil control in the piston skirt and cylinder wall interface.

“We maintain ultra flat tolerances on the ring grooves – whether or not they are anodised. The anodising [of pistons] presents special challenges in maintaining the groove quality, in order to provide the optimum sealing of the piston, ring and cylinder bore power cell,” he said.

Green notes that their Ferroprint coating, for use with aluminium cylinder bores, “is embedded with iron particles, thus allowing an aluminium piston to run against an aluminium liner.

“Typically,” Green continues, “if similar materials are run against one another in this type of fashion, the end result would be the piston ultimately friction-welding itself to the liner. With the advent of this coating, that isn’t the case. The engine builder can, in turn, reduce the mass of the overall engine package as a result of [using] this simple coating.”

The company recently received patent approval for creating a patterned skirt coating for its racing pistons. The pattern enhances dynamic oil control along the piston skirt, resulting in increased efficiencies of the piston-to-liner interface, according to Green.

“In using our new compressible Grafal coating, the patterns would also allow coating of specific highly loaded areas of the piston skirts to help eliminate issues with secondary motion, as the piston changes direction.

“This compressibility helps to reduce overall stress in the piston, potentially resulting in greater durability.” While this technology is still in its initial implementation for NASCAR stock car applications, “The benefits are readily apparent.” Using these new patents, “This is yet another feature to set us further apart as we continue to demonstrate our efforts remain on the leading edge of technology.”

Written by Anne Proffit.

It has long been acknowledged that the piston ring, particularly the top ring, has one of the most arduous tasks in the internal combustion engine. Having to seal the gap between the piston and bore from the passage of unwanted blow-bye gases, transfer huge amounts of heat from the piston crown into the cylinder liner and do all this reliably at mean piston speeds approaching 5000 ft per minute through the tiniest of contact zones, is no mean task. It is hardly surprising therefore, if abused or if the lubrication of this thin line representing the contact point breaks down, then scuffing, scoring or galling, call it what you will, will inevitably occur. But even if this catastrophic situation isn’t reached there is still the less major issue of durability to be addressed.

In any metal-to-metal surface such as this, there are basically four types of potential wear mechanism: adhesive wear (scuffing, galling, cold welding, seizure), abrasive wear (grinding, scratching, polishing or ploughing), corrosive wear (thermo-chemical wear, electro-chemical wear and oxidation), or surface fatigue (pitting, spalling, flaking…..). In the case of the top ring to cylinder bore contact zone, of these four only two are of interest to us - adhesive and abrasive wear. And for most of the time, that factor affecting durability at the top and bottom ring reversal points, it is most probably abrasive wear. Under these conditions, hydrodynamic lubrication ceases and boundary lubrication, with its much higher friction levels, takes over.

Now there are certain rules that engineers can use to mitigate the situation. Clearly if the hardness of each mating surface is the same then wear will eventually take place on both. The harder the surface, the less wear will take place but since in the early stages, soon after start up,
the rings and liner will need to bed into each other, removing those minor imperfections of manufacture, there is a practical limit to the hardness.

But if we know that wear will take place then why not introduce elements of a soft surface as a sacrificial component against the harder surface. And this is exactly what we are doing when a chromium-plated ring is used in a cast iron liner. The soft graphite element in the cast iron liner will wear away much quicker than the ring, adding a degree of lubricity as well. Thus for comparatively soft cast iron liners almost any ring coating material will suffice.

Another way of coping with this inevitable wear is by embedding hard particles in a much softer matrix. Such is the case in the many aluminium liners where nickel ceramic coatings are electrolytically deposited on the bore surface. The ring coatings running over the small embedded hard silicon carbide inclusions in the coating produce very little wear debris and the presence of the oil in the oleophilic material greatly assists. Nitrided steel, titanium nitride or chrome nitride coatings can often be used and even cast iron if high wear rates are acceptable. However for such surfaces chrome is not recommended, nor is the use of DLC. Thus, although friction levels with many of these surfaces can be low, as with the compatibility issue of our high performance lady, great care needs to be taken.

In the end, much like the eventual choice of bride, proceed with caution, and seek advice along the way. When it comes to piston ring coatings you can’t be too careful!

Written by John Coxon.

This allows the machining characteristics of the new materials to be considerably closer to that of non-reinforced aluminium alloys using conventional carbide tools, when compared to the machining difficulties encountered with silicon carbide reinforced aluminium alloys. The aluminium matrix alloy can either be taken from the current 2000, 6000 or 7000 series chemistries. (The exact aluminium alloy chemistry used from any one of these three series is proprietary.)

According to Del West program director, Len Gibbs, “The 2000 series chemistry materials will be suitable for use in piston manufacturing because it has a lower thermal expansion than convention aluminium piston alloys such as 2618 and 4032, by as much as 25 percent in the case of 2618.

“The lower thermal expansivity of the new material will permit better sealing at the piston: cylinder wall, thereby enhancing performance. The new alloy composition also has better stiffness and elevated temperature strength than 2618 and heat transfer similar to 4032,” he said.

“The lower thermal expansion of the new metal should lead to a benefit in piston ring life,” Gibbs mused. “More than anything else, I think the piston ring will gain longer life and a better seal, in particular because this material expands more slowly than the conventional aluminium alloys. The four important properties of this material are its better machinability, low rate of thermal expansion, its high tensile strength and its greater stiffness.”

There has been extensive testing on a 7000 series matrix alloy using Spintron to ensure validity of the material under the highly stressed condition associated with a valve spring retainer. “I’d say we’ve had about six million cycles on a retainer using a Michigan (International Speedway) NASCAR Sprint Cup track loading profile. What is interesting, Gibbs said, “is that retainers made of this material weigh about 20 percent less than titanium retainers.”

With the capability to blend its own alloys, the company can vary the amount of AI203 reinforcement volume in the process, according to Gibbs. “We can vary the percentage of reinforcement and work with the properties to increase strength and stiffness for use in piston manufacture,” he said.

Work continues with several partners to determine proper piston design using the new material, which is made on-site in Valencia, California. The material is applicable for racing series that permit metal matrix materials, and Gibbs can envision diesel applications for the products.

It has taken a long time to develop the alloy and modify the chemistry for use in piston manufacture, as well as to standardise the manufacturing process. “There are many requirements we have to meet for the manufacturing of this alloy.” When will it be available to customers? Gibbs is not ready to make any promises, but more information is expected in the coming months.

Written by Anne Proffit.

For sprint engines to minimise friction this job is done by a single steel or cast iron ring. But when durability is an issue, this role will be shared with a second ring of a totally different design, reflecting its dual role supporting the oil control ring below it.

Since the primary task of the top ring is to seal, many believe that this is because of radial tension holding it against the cylinder wall. While it is true that this tension is an absolute necessity, it is the combustion gas pressure flowing past the piston top land and getting behind the ring that is the key action. As well as ensuring that no gas gets past the ring at its outer contact with the cylinder wall, the clearance between the top of the ring and the piston, usually referred to as the ‘side clearance’ has to be sufficient to allow the gas to flow quickly through into the space at the back. If this side clearance is too large then under the dynamic loads the ring maybe able to move and distort uncontrollably to the detriment of the seal at the cylinder wall. Such a phenomenon is known as ‘ring flutter’ and usually occurs at high engine rpm when it will inevitably lead to excessive piston blow-by and reduced engine performance. Eventually this will lead to burnishing of the ring faces when heat cannot be dissipated and in the extreme can lead to melted pistons and that sickening sound of an engine when things go drastically wrong. Piston ring manufacturers generally recommend that this side clearance should be something around 0.001 to 0.002 inches (0.025 – 0.05 mm) depending upon the piston and ring profile.

Along with this ‘side clearance’ comes the ‘back clearance’. In order to ensure that the gas can get behind the ring it is imperative that this ‘back clearance’ should be at least 0.005 inches. Much more than this and it will take longer for the gas to accumulate and push the ring outwards on the power stroke. Any less and it may not function as designed at all. However, for all the above to work as planned it is imperative that the underside of the ring and corresponding ring groove surface are both flat and smooth to the highest standards. This should produce a gas tight seal and ensure that the gas doesn’t escape past the ring land and into the crankcase via a different route.

In certain applications where the ultimate in ring control is required (such as some forms of drag racing when very high engine running speeds are normal), this side clearance is reduced to a minimum and a number of very small ‘gas ports’ drilled vertically down from the piston crown into the back of the ring groove. These 1 mm diameter holes when drilled equidistant around the crown will enable the combustion gas to get behind the top ring without the need to travel down the piston top land and by way of the side clearance. Consequently this side clearance can be reduced but since these gas ports are likely to fill up with soot very quickly, this approach should only be entertained when engines are rebuilt on a highly frequent basis.

As racers we’re all too familiar with the idea of checking the engine top-end on a regular basis. However in view of the critical influence of the compression ring, shouldn’t we be checking this as well?

Written by John Coxon.

Most people will appreciate that the piston ring exists principally to reduce the amount of combustible gases escaping into the crankcase, but I’m afraid there is a lot more to it than that simple statement. To start off with, the seal has to move at a speed of something up to 5000 ft per minute (26 m/sec) and cope with piston

temperatures of 300 deg C and more. In combination the ring pack also stabilises the piston as it moves up and down the bore minimising the chance of point contact on the piston top land and lower skirt. As well as controlling the blow-bye going down the gap between piston and cylinder and conducting around three quarters of the heat in the piston into the cylinder block, it also has to control the engine lubricant travelling in the opposite direction and do all this with minimum contact force and do it reliably and repeatedly for hours on end.

Fortunately we do not expect a single ring to do all of this. This would be expecting too much and although some predominately sprint racing applications use only two rings, conventional pistons tend to use three rings. To achieve this over the years, many different designs of rings have been developed often in response to particular characteristics or problems in the engine and or piston design.

The top ring is designed purely as a compression ring. Rectangular or ‘L’ shaped ‘Dykes’ rings all have their applications but it is the barrel faced top ring that is favoured in most racing power units. Giving purely line contact around the circumference of the bore, this design would appear to give the best sealing pressure for any given initial radial tension. At light load, this radial tension should be sufficient to seal the gap but at higher loads it is the gas pressure itself seeping into the void behind the ring that does the work and is the reason why the side clearance between ring and piston can be highly critical. The barrel shape is quicker to bed in and can also control the oil flow more effectively. Barrel profiles can also be symmetrical if desired and can enhance oil control under certain circumstances. While a few years ago flash coatings around 2-3 microns of titanium nitride may have been optimum, today, and depending on your bore surface, these are becoming increasingly replaced by DLC.

The second compression ring has more of a dual role. Generally in the form of some kind of scraper ring, it completes the seal, trapping any gas bypassing the top ring on the power stroke, and in addition, supports the oil control ring in removing or scraping off any excess oil missed by the oil control ring below it. Usually a rectangular ring with the outer face tapered such that the bottom edge contacts the cylinder, the downward stroke removes the excess oil with the upward stroke in theory riding over any oil on the bore.

Oil control rings can be one, two or even three piece affairs, with upper and lower side rails bordering some kind of corrugated spacer and remove any excess oil from the bore and return it through the piston back to the crankcase.

But rather like a duet, only when the pistons and rings are working in harmony, can the music begin.

Written by John Coxon.

There are many issues to work out with the steel insert and Coil’s efforts stalled by the fact that NHRA limits testing during the 2009 season to only four days per car. “We have rules against testing in-season, so this project is on the back-burner until next winter when we can test again,” he said. “The reason we are looking into this is because pistons get pretty hot. Aluminum gets weak really quickly and when it gets hot, the ring lands bend and/or they crack. That causes us to replace an awful lot of pistons,” Coil said. “And frequently, that is a limiting factor on the tune-up. So, if the piston were just a little bit more rugged, you wouldn’t bend the ring lands and you might be able to run the same distance – let’s say 10 or 20 runs instead of just one or two.”

To Austin Coil’s mind, the benefits would far outweigh the initial extra development cost and the added weight of a steel insert into ring lands. “This could be a safety benefit and, eventually, it would be a cost savings. It might even be a little bit of a performance advantage too.”

There is but one such piston in existence. “We made the piston and supplied the process for making the steel ring insert fit,” advised Tom Prock of Venolia. “The beauty of it is that this is a billet piece of aluminum, forged to an 800-degree temperature and cylinder head temps exceed that. It might just work,” he said.

“The first product is a little too heavy and it is only coated in some areas. I intended to have the aluminum parts coated so they won’t hurt the cylinder walls,” Coil said. “We don’t know what the future will bring, but it certainly works in diesels and it certainly worked for Honda back in the Senna days, in the 1980s when Honda first came into Formula One with McLaren.”

Coil’s Venolia prototype is about 200 grams heavier than the race-ready products JFR uses from Tom Prock at Venolia. “We think we can get it down another 100 grams. It is probably doable,” Coil said. Trying to run the piston “to see what it might tell us, the problem is that this piston is so much heavier than the other ones that I wouldn’t want to run one of them; you’d have to make a whole set.”

Coil spoke with the engineers who made the Honda pistons for their Formula One project. “When I started talking about doing it, they said that might be really expensive. In Formula One there is no budget limit, if a piston costs a million dollars, it costs a million dollars. And we don’t have a million-dollar budget for pistons,” Coil laughed.

Written by Anne Proffit.

The burden of piston research and development is predominantly the responsibility of the Tier 1 component suppliers. Their scope of products ranges from lawnmower and small stationary engines through to Formula One gasoline, high speed and racing diesel engines. High speed diesel applications present a particular challenge to the engineers as the current and future generation of engines start to exceed the 200 bar cylinder pressure threshold. It’s at these extreme pressure domains that aluminium based pistons find it difficult to survive.In an effort to overcome the material limitations of aluminium based pistons, manufacturers have been actively investigating the use of steel as the base material for use in small capacity diesels. In the world of large diesel engines however, the use of steel is not uncommon. Typically these heavy duty pistons comprise a two part articulated assembly, the steel section embodying the combustion chamber, ring pack and pin bosses and the aluminium part providing the main skirt section and piston to liner thrust face.However, these complex two part piston assemblies are very expensive to manufacture and typically extremely heavy and robust. And so they need to be, as they are expected to have a life expectancy in large commercial vehicles exceeding 10 years and covering up to million kilometres.Although the articulated piston provides a solution for heavy duty diesels, weight and cost considerations make it unviable for small capacity diesel engines, so for a good many years now companies like Mahle, Federal-Mogul and Kolbenschmidt have been working to produce viable steel piston solutions for passenger car sized engines.Mahle seems to have the technological edge in this area and when Audi and Peugeot decided each to use a diesel engine in their Le Mans Prototype, it was to Mahle that they both turned for an extreme duty steel race piston.The Mahle ‘Monotherm’ race piston is made of a single steel forging. It is extremely compact for a diesel piston and can achieve compression heights near to 50% of the cylinder diameter. As steel is more thermally stable than aluminium, reduced fitting clearances can be used leading to a better guidance of the piston and reduced levels of friction. In the case of a Le Mans Prototype application it is likely that some form of DLC coating was also used. The steel construction also provides better stability in the ring area, resulting in reduced oil consumption and blow-by.At present it’s unlikely we will see steel pistons used in gasoline race engines as the inertial loads, especially when one considers the 18,000 rpm operating limit of the current generation of Formula One engines, would be unacceptable. However, if the FIA has its way and we see the introduction of a four-cylinder, 2.0 litre ‘world engine’ in the near future, who knows? We may yet see the day when steel pistons are used in Formula One!

In four-stroke gasoline racing engines, the accepted wisdom is to fit one or two compression rings and an oil control ring, sometimes known as an oil-scraper. It is widely understood that the piston assembly is the source of a large percentage of the frictional losses in a racing engine and that the piston rings account for much of this loss. If it is possible to dispense with one of the compression rings whilst maintaining low blow-by, then this is generally done, thereby gaining an increase in performance and, in these straitened times, a welcome reduction in piston assembly cost, albeit slight.In general, the compression rings in a Formula One engine are steel and may be surface hardened before the application of a surface coating.

Some years ago the trend was to have a coating of another metal such as molybdenum on the outside diameter of the ring, but these have been replaced by the new generation of engineering coatings with which we have become familiar. Titanium nitride has been a common choice of coating for many years, but this has been largely supplanted by the use of DLC coatings in recent times. I have seen other coatings tested on motored rigs which promise even lower friction, but not with sufficient success to be used for serious testing or racing. The general trend in compression rings is to use as thin a ring as possible, with the attendant benefit of lower piston mass.The design of the compression ring is, generally speaking, a thin ring having a width which is several times the height of the ring, and the groove which houses this then needs an appropriate amount of material between it and adjacent features such as valve pockets or to other external surfaces. The piston ring thus has a large influence on the design of the piston which affects the piston mass accordingly. Therefore it is no surprise to learn that some manufacturers have experimented with alternative designs and one of the manufacturers has successfully raced this technology in Formula One in recent years. The company in question has a much narrower ring than we would consider conventional, and the attendant weight saving in the ring and the piston is considerable. These rings are also made from an unconventional material (for piston rings at least) and offer a weight saving which is worthy of consideration. These rings are coated with what we would consider to be conventional ring coating materials and run in bores which are of conventional materials. This technology would be compatible with most existing Formula One engines. Moreover, they have a very unusual approach to the design of the oil control rings which again is in an unconventional material.We can say therefore that, whilst piston rings appear to be simple components, they could provide a lot of scope for further development, thereby offering reduced mass and friction. These are aims that Formula One in particular should embrace if we believe that we need to develop road-relevant technologies to survive.