In the bigger diesel engines, which can tolerate large amounts of excess air, if matched correctly, no further control systems may be necessary. In smaller engines however, particularly spark ignition units, additional controls may have to be introduced. In this latter group therefore, turbocharger controls are a necessity, and fall into two categories – those that protect the turbocharger by limiting its speed, and those that protect the engine by restricting the compressor outlet pressure and hence the engine boost. Transferring all this a typical compressor map the operating envelope is therefore the compressor ‘surge’ line to the left of a typical compressor map, the maximum safe wheel speed to the top,  and the compressor ‘choke’ area to the right.

For reasons of reliability and durability, it is always preferable to restrict turbocharger controls to the cold, engine air intake side of the application, if at all possible.

Perhaps the most obvious control is some form of blow-off valve. This is designed to re-route the compressor outlet air before it enters the intake manifold, and in most applications limits surge forces in the compressor wheel and bearing assembly when the throttle is snapped shut, as in the case of a gear change, for instance. Under such transitory conditions, interruption to the airflow would otherwise result in high-pressure spikes upstream of the throttle. A safety feature perhaps, but any bleed-off of the post-compressor air is wasted work by the turbine applying a back-pressure on the engine to compress the engine intake air in the first place, and therefore inefficient.

For maximum effectiveness and to increase overall engine efficiency, the boost condition is best controlled by limiting the hot, exhaust gas passing through the turbine wheel. The most common of these methods uses a wastegate or bypass valve, and takes its signal to open (limiting the flow of exhaust gas to the turbine), from a pressure tapping located on the compressor or a similar tapping from the exhaust manifold.

More usually these days, this bypass valve is integrated into the turbine housing next to the turbine wheel. These valves must be large enough to flow all of the anticipated bleed-off gas to prevent a phenomenon known as ‘boost creep’, which is the condition when exhaust gas flow completely overwhelms the bypass valve when fully open and the exhaust gas pressure consequently continues to rise in the upstream (exhaust) manifold. Under such conditions the intake manifold pressure will also continue to rise, creating even more exhaust gas and eventually producing dangerously high boost pressures with potentially catastrophic results for the engine.

From an efficiency viewpoint, however, the optimal method of controlling boost pressure is by using a variable geometry turbine. Sometimes also referred to as a variable area turbine nozzle, the most common of these consist of a circular array of pivoted aerofoil blades situated inside the turbine housing where the exhaust gas enters the turbine wheel. Designed to alter the angle of exhaust gas flow to the wheel according to its rotational speed, turbines such as these give much better low-speed torque characteristics as well as shorter spool-up times. As ever though, challenges to be overcome are the extremely hostile environment of high temperature gradients in the area and the corrosive nature of the exhaust gas.

With the increasing emphasis on fuel economy, not only in the auto industry but also the race track, surely it can’t be long until all engines – however small – are fitted with boost-controlled turbochargers.

Fig. 1 - Turbo wastegate valve

Written by John Coxon

Although rare back in 2005, variable geometry intake manifold systems for Formula One engines were relatively simple and generally fell into one of two formats. The first was a two-position device that moved in a straight line up and down inside the intake plenum to give two effectively differing lengths of intake tract. Comparatively simple in its concept, its limitations were always the point at which the switchover was to take place and the speed with which this would happen.

The second system was a development of the first, and consisted of a pair of concentric telescopic tubes running inside one another. In response to a speed signal the tubes either expanded or collapsed to give an infinitely variable length trumpet. Potentially more controllable since instantaneous switching wasn’t the issue, the downside here was that in spraying fuel down the intake ports (engines were port injected in those days), as the engine speed increased and the telescopic tubes collapsed then any fuel attached to the wall of the intake would be scraped off and be drawn into the engine, creating a richer fuel-air mixture than the one mapped; a momentary fall in power could result. With modern engine control systems this could be combatted to a certain extent in software but back then, according to reports, the effect on outright performance was noticeable.

Modern Formula One engines are of course directly injected, so the problem of stray fuel altering the carefully metered fuelling is now non-existent. Freed up from this need though, what might the variable geometry intake manifold of 2015 look like?

Of course we could go back to a development of the first system above from about ten years ago. This would work quite well but might increase the height of the engine installation to ensure that, as the intake runner length increased the proximity of the top of the airbox would not inadvertently restrict the air flow Another way might be to introduce a system of two or even three stages using an arrangement of flap valves inside a compact V6 intake manifold, similar to those used in a number of production vehicles from about 15 years or so ago. With an array of one or two sets of flap valves, two or three different intake runner lengths could be accommodated within the confines of the engine vee.

For my part, I thought I might want to examine a design based on the combination of the two – a simple continuous collapsible tube which in the shape of an arc could be moved by either electrical or hydraulic means. Not of much interest to the roadcar business perhaps, and therefore possibly not strictly roadcar-relevant as Formula One is supposed to be these days, but such a system could minimise the height of the airbox if this was indeed and considered an issue. It might be fanciful, and it might be difficult to make – and perhaps take a bit of development – but isn’t that what Formula One is all about?

The real question though is that with the availability of turbocharging and no restrictions on the level of manifold boost, in terms of outright performance is there any significant advantage to a variable intake system in place of simply more boost?

Fig. 1 - Ideas on variable intake manifolds

Written by John Coxon

These days though it’s not so easy to impress people, since opening the bonnet or hood to reveal the engine in all its glory displays only a mass of pipes and actuators. But for those in the know, the equivalent discussion to that of 50 years ago might be one of a single large throttle body against an array of port throttles.

Single large throttle plates are generally used in the original equipment installations of vehicle manufacturers. Comparatively cheap to manufacturer, and easy to set up on the bench away from the vehicle, they tend to be far more than just a throttle plate. With a couple of throttle angle potentiometers and an electric dc motor to open or shut the throttle in response to a signal from the engine’s ECU, these devices are no longer attached directly to the throttle pedal.

Used mainly where the vehicle needs to pass regulatory exhaust gas emission tests or to control the engine torque more precisely during transient conditions, in response to rapid opening (or closing) of the throttle, the response is relatively tardy as the manifold steadily fills up (or empties). Alongside this, the transient airflow has to be modelled so that the fuel demanded can be corrected to avoid rich or lean spikes in the exhaust gas, which could cause exhaust after-treatment issues downstream.

Port throttles on the other hand tend to be far more responsive to the throttle pedal, as the volume of air downstream of the throttle plate is rapidly consumed. This gives the driver a more urgent feel to the car during driving, particularly with the initial pull away from rest. The car may have the same overall power of the single throttle but the transient ‘feel’ of the vehicle is generally much better. The downside tends to be one of poorer control of exhaust emissions, despite the potential to improve fuel economy brought about by the reduced negative work on the piston at part-throttle.

Another advantage of port throttle applications is their ability to tolerate camshafts with a wider inlet and exhaust valve overlap at engine idle speeds. The close proximity of the closed throttle plate acts as a barrier to any residual exhaust gas finding its way back into the inlet port at intake valve opening, which in turn enables the engine to idle less erratically.

Perhaps the biggest advantage of using port throttle systems though is in the ability to design a suitable intake system to maximise outright engine performance. Using a number of simple intake runners (each with a throttle plate close to the intake valve) leading into a common plenum, while the length of the runner can be optimised for maximum performance, the plenum volume can be made as large as the space practically available, removing any performance-limiting design constraints away from the immediate vicinity of the engine and transferring it to the intake of the plenum. In the large single-throttle application, engine performance will inevitably be limited by the distance between the back of the intake valve and the throttle. Too large, and transient engine response will suffer; too small and the engine may suffer from inter-cylinder distribution problems.

But as an enthusiast appreciating the finer points of engine design I just like the throttle system detail under the bonnet.

Fig. 1 - Multi-throttle intake system – something to impress your friends?

Written by John Coxon

Engineers have grappled with this problem for many years, and while the currently favoured approach (given a healthy if not quite unlimited budget) would be to use computational fluid dynamics (CFD) and powerful computers, such facilities are rarely available to the average engine tuner or workshop. On the less virtual side of the industry, where funding is often hard to find, the task is downgraded to the physical: that of testing the airflow in the cylinder head. The CFD output of pictures of meshed intake ports and coloured streamlines flowing through the intake port may look ‘sexy’ (if sometimes a little confusing) but believe it or not similar results can be obtained using instrumented equipment measuring local flows on the airflow rig.

The most obvious way is to use a pitot tube inserted into the port. Handheld and used correctly, this can be moved around inside the port to give an idea of the distribution of the air flowing around the valve. However, introducing the tube to measure the velocity of the airflow in this way also has the effect of altering the local airflow around it, so what you think may be happening in the port and around the valve may not actually be the case when the tube is not there.

In the past of course, many engineers have grappled with introducing pressure tappings drilled into the port wall around the outside of the port throat, and while that can give an indication of the static pressure in this zone, it is difficult to do and destroys the cylinder head in the process (in drilling through water jackets an so on). Also, the data generated is relevant only to the static pressures around the wall of the port, as it makes no link with the bulk flow of the air away from the wall. Essentially, all we need is some kind of probe that extends into the critical airstream without altering the flow, and in a sense we already have that in every port – the valve!

By taking the pressure tapping off the seat of the valve and routing back through the valve stem, we can estimate the pressure of the air as it passes the restriction caused by the valve seat. Like the port wall tappings referred to above, since these seat tappings are normal to the direction of airflow then this measurement would be one of static pressure and not include the dynamic element of the flow, but by indexing the valve in a number of positions (different valve lifts and rotational position of the valve, for example) a good idea of how the air flows through the valve seat curtain area is obtained.  

This method may not impress your boss as much as CFD, or produce a pretty picture of how the air flows down and around the port, but in terms of speed and the fact that you will be testing the actual components to be used, the technique has much to recommend itself.

Fig. 1 - Airflow testing at the valve seat

Written by John Coxon

The great issue with the turbocharger, if you have ever tried to install one, is its sheer bulk. Consisting of an aluminium compressor at one end and cast-iron turbine at the other, separated by a cast-iron, oil pressure fed bearing housing that needs drain facilities to take this oil away, packaging the device under the engine cover of a typical Formula One car is going to be more than just a problem. Furthermore, with the 2014 Formula One technical regulations specifying that the single-stage compressor be linked to the sole single-stage exhaust turbine “by a common shaft parallel to the engine crankshaft and within 25 mm of its centre line”, your design options are strictly limited.

But innovation is the lifeblood of motorsport, and making each part work to its absolute best is the racecar designer’s brief. And so adapting turbocharger technology to the pencil-thin bodies shrink-wrapped around driver and engine components requires just a little more than finding a space big enough.

In one application I am aware of, the separate entities of the turbo unit have been split, with the compressor positioned at the front of the engine and the turbine towards the rear, linked together by a long shaft travelling through the vee of the V6 engine. Supported by at least two roller bearing cartridges included in the vee will be an electric motor/generator linked not only to the exhaust turbine but also to the compressor. When not scavenging excess power out of the exhaust as a generator and delivering it to the onboard energy store (battery or supercapacitor system), as a motor it can be spinning the compressor to minimise transient effects, maintaining an optimum boost pressure to the engine. With the bearings fed by gallery oil pressure with the flow biased perhaps towards the much hotter turbine side, this oil would drain straight back into the crankcase below the centre of the vee.

Apart from simple packaging, another advantage of this layout is that the compressor remains towards the cold part of the engine, at the front just down from the air intake with the turbine towards the rear. At the front, the compressor outlet air is kept only marginally cooler than it otherwise would be – let’s not forget that the compressor outlet air temperature, even for the most efficient of designs, can be 200 C or more. The turbine at the rear will be marginally hotter, thereby being slightly more efficient. But while these are marginal improvements, the big benefit must be that of packaging the intake and exhaust plumbing, which will also be less bulky and therefore weigh less. Furthermore, the reduced volumes of the intake or exhaust system will assist engine transient control, making engines slightly more responsive.

But is this approach truly innovative, as some are claiming? Designed to assist the automotive industry with research and development into new designs and materials, the layout of the Formula One powertrain was more or less set out in the regulations, and in my opinion splitting the traditional turbo unit in such a fashion leans more to the packaging requirements of Formula One than the auto industry that these regulations are supposed to help.

Spurred on by draconian fuel economy and emissions targets, the auto industry has a different set of issues to solve, which in many cases are well ahead of those created for Formula One. Indeed, I am sure that on hearing the initial regulations, the first port of call of any team will have been to talk to their trail-blazing roadcar cousins. 

So, in the white heat of such true innovation, is it fair or even correct to expect the sport of motor racing to keep up?

Fig. 1 - Alfred Buch’s patent application drawing

Written by John Coxon

A sort of cross between the traditional blower and a turbocharger, the unit is effectively a mechanically driven turbocharger compressor wheel driven directly from the crankshaft, and instead of generating the engine boost by forcing too much air into the engine intake ports (therefore raising the back-pressure in the manifold) – as in the case of, say, the Roots blower – the centrifugal compressor actually compresses the intake charge as it passes through. Drawing air into and through the centre of the spinning impeller, the centrifugal compressor accelerates the air radially, thereby increasing its energy, before entering the diffuser which, by slowing it down again, increases its pressure.

In terms of compression efficiency of the air, the centrifugal compressor is therefore likely to be far more efficient than other types of superchargers, and values of adiabatic efficiency of more than 80% can easily be realised. More important though, efficient compression minimises the increase in temperature of the engine intake charge, and with low charge temperatures the chances of combustion detonation is reduced. And if there is one thing that supercharged spark-ignition engines like, it is lots of cool, dense incoming air.

The problem with the centrifugal supercharger, however, is that the air flowing through it is proportional to the square of the rotational speed of the impeller. Compare that with the air flowing through an internal combustion engine, which is proportional only to the speed, and immediately you can see there is a mismatch. This is never a problem when centrifugal compressors are powered by an inward flow radial turbine, as in the case of a normal turbocharger. Here, the mass flow of exhaust gas powering the turbine effectively balances out the work done by the compressor and the system, if designed well, will find its own equilibrium. But when this compressor is mechanically driven from the crankshaft, compromises have to be made.

If the unit is geared to supply sensible boost pressures at low to medium speeds, at higher speeds excessive intake pressures will be generated. To combat this, some kind of blow-off valve limiting the maximum boost pressure could be provided, but this is highly inefficient and inevitably noisy too. On larger installations, where weight and or size are not an issue, some kind of constantly variable transmission could be incorporated but at this level of complexity, weight might also be an issue.  

The most sensible way therefore is to gear the compressor wheel speed to produce maximum boost at maximum engine speed. That should produce knock-free motoring when pressing on hard but little or no boost effort when it simply isn’t required, as for instance when cruising on the street. Furthermore, since there will be little or no boost at typical road driving speeds, when used as daily transport, fuel economy and tailpipe emissions should not increase beyond legal requirements.

Not perhaps the best supercharging solution for competition purposes but for applications where compactness of design and thermal efficiency are important – for example on street machines – the mechanically driven centrifugal blower could be a wise choice.

Fig. 1 - The V16 BRM used a compact two-stage centrifugal supercharger

Written by John Coxon

These days of course we are all too familiar with the concept of multi-point injection and a single runner per cylinder feeding out of a much larger plenum chamber, but believe it or not there were times when this was simply not practical. Designs of this type would invariably need multiple carburettors or, when electronic systems were in their infancy, a single injector per cylinder. Either way, parts were very expensive and therefore mostly unaffordable. All the engine intake air would invariably need to flow through a single fuelling system, and the technology at the time tended to limit the options. So as well as ensuring that the air was evenly distributed across the cylinders, you also had to ensure that the fuel was equally spread as well.

In those days, a single downdraft fixed-jet carburettor would have been placed high up in the engine bay, and the fuel drawn in through the venturi would impinge on the throttle plate below. I say ‘impinge’ because the fuel didn’t so much spray out of the venturi and into the airstream so much as dribble out and, on hitting the throttle plate, disperse into the intake air from this. In more modern times, anyone viewing this process with high-speed video will understand what I mean.

Once more or less in the airstream, the trick was to keep it there, and by a combination of turbulent flow (produced by the often torturous shape of the manifold) and wetting and subsequent evaporation of the fuel from the manifold walls, a charge approaching some form of homogeneity would result.

This was all very good at part-throttle when the air-to-fuel ratio across the cylinders was often fully acceptable. At full throttle, however, the combination of the internal flows in the manifold and the tendency of the fuel to migrate towards the inner two cylinders (of a four-cylinder engine) meant the spread of air-to-fuel ratios across the cylinders could sometimes be up to four or more. Viewed in terms of actual numbers, this would mean that at certain wide-open throttle conditions, when the nominal stoichiometric condition was 14.7:1, at least one cylinder was running at or near 12.7:1 (rich) and another at 16.7:1 (very lean). The tendency for the engine to run into detonation at low speed on the weaker cylinder was therefore very great.

At the same time though, another thing was noticeable – when clear of ‘knock’, and as the throttle was progressively closed, the power of the engine actually increased! Eventually this was put down to a combination of improved mixture preparation and a re-biasing of the air and fuel flows by the single throttle plate, but it just goes to show that you don’t always get maximum power at wide-open throttle.

Fig. 1 - An interesting arrangement on this Delage – an updraught carburettor!

Written by John Coxon

However, what is left will inevitably become either trapped between the moving parts of the engine, creating wear, or hopefully be flushed away by the lubricating oil. In the former, engine performance will deteriorate; in the latter, such debris may be transferred to other parts of the engine to wreak havoc elsewhere. Given that studies have shown that of that debris left behind after combustion, 30% passes out through the exhaust and 50% goes into the oil, a worrying 20% remains unaccounted for. So whichever way you look at it, the presence of any particulate matter in the engine’s intake air is undesirable.

As we know though, dust and dirt comes in all shapes and sizes, and to develop a filter to take out all of these contaminants is simply not feasible. To minimise engine wear therefore it is necessary to target those particles that are most likely do damage based on their size, shape and/or roughness and hardness.

You might be surprised to know that, according to studies in the US, the average amount of airborne dust and dirt in clean highway conditions is something like 0.1 mg per cubic metre of air. Computed to a 2.0 litre engine over 100,000 miles, that amounts to about 30 g of dust or dirt over the period. In the much dustier conditions at track or offroad venues, with concentrations near 5 mg per cubic metre all the way up to 5000 mg per cubic metre, clearly the importance of an air filter cannot be understated.

The particle size distribution of this dust and dirt is also highly revealing. Described as ‘of a bi-modal distribution’, highway samples have shown that the fine mode lies in the 0.1-2.5 µm range, while the coarse range is something nearer 2.5-30 µm. When you move to off-highway air contaminants then, as you might expect, the particle size increases enormously, with 100 µm being not that uncommon.

The inevitable conclusion to any work on correlating engine wear with particle size is that the situation is highly complex. The larger the particle then clearly the greater the risk, but since the dynamic running clearance between adjacent components throughout the engine will be in the 0-20 µm range, anything that can fall into this region is of concern, as indeed also is its relative hardness. With oil film thicknesses of the order of 1 µm (and getting thinner in the search for higher engine efficiency), any particle thicker than this has the potential to cause damage.  

But despite all this there are no universal air intake filter standards in the automotive sector – at least not that I am aware of. Individual vehicle/engine manufacturers will have their own standards, and these will have been developed over time and shared with their OE suppliers. Tests will have been carried out using various ISO-standard dusts, and the filtration efficiency reported. But since the relationships with their component suppliers will often run well into the service life of their vehicles, these standards are not widely publicised. And since engine issues attributable to poor air filtration are rarely if ever reported, the attitude taken tends to be, ‘If it ain’t broke then we won’t fix it’.   

And so, based on all this, the only recommendation I can give is this: use a filter and change it or (if of the re-useable type) clean it regularly. Only then can you be confident of getting clean air into your engine.

Fig. 1 - A used air filter showing the dirt that would have otherwise passed into the engine

Written by John Coxon

It was all to do with the introduction of the turbocharger and the efforts to maintain or even increase the levels of intake manifold pressure when the engine was off-throttle. At the time, turbocharger anti-lag systems were in their infancy. Sometimes referred to as ‘bang-bang’ systems, these were designed to keep the turbocharger spinning and producing boost, even when the throttle was closed. However, being strangled by the regulation 34 mm air restrictor into the compressor under WRC rules, the performance of the engine was restricted even at wide-open throttle.

At the time, the Ford rally team realised that while it couldn’t flow more air when the engine was on full throttle, at part-throttle – which is a large part of the time in a rally car – an opportunity was being lost to get the air through the restrictor for use later. If this extra air could somehow be stored temporarily in a useful form and then released again when the engine was able to use it, a modest increase in engine power could result.

Ingenious or not but certainly within the rules, the 2003 Ford Focus rally car featured some interesting additions. Hidden behind the rear bumper was a carefully fabricated, 45 litre, 2 mm thick titanium tank, linked to the intake manifold by 4 m of 30 mm diameter titanium piping running the entire length of the car. Weighing in at somewhere near 20 kg, the system was connected to the rest of the engine system by something that was euphemistically called the ‘idle control valve’. This electronically controlled butterfly valve would flow large amounts of air when required yet still operate at the low flows of idle air demand when required, as specified in the homologation papers of the time.

With all air passing through the 34 mm restrictor in all its various aspects, the system was deemed fully legal. But using a highly aggressive anti-lag ‘bang-bang’ system as soon as the post-aftercooler air pressure was greater than the tank pressure (and the throttle closed), air would pass into the tank. When these pressures equalised, the valve would close again, only to be re-opened when full throttle was demanded to deliver a higher intake manifold boost pressure and an approximate 5% increase in power over the more usual arrangement.

Suitable only for asphalt rally stages where maximum grip and driver commitment were needed, the effectiveness of the arrangement saw increased engine performance which gradually fell on longer special stages. Although not the dominant effect the team had hoped it might be, to avoid all other teams developing similar – and, let’s be honest, rather fanciful – systems, after three rallies it is little wonder the system was banned from WRC.

It was an interesting and innovative approach to the use of the airbox, but even though a part of me understands the reason it was banned, the other half can’t help worrying that the decision to actually do so undermines one of the major strengths of the sport.

Fig. 1 - Titanium surge tank mounted across and behind the rear bumper

Written by John Coxon

The least restrictive intake system is clearly that with the straightest runner within the packaging constraints of the engine design. Either straight or slightly converging along its length to accelerate the flow gently towards the engine, at the end nearest the valve great attention is made to the intake port dimensions to encourage air to flow past the valves when open, and flow with ease into the cylinder. At the other end, facing out to the atmosphere, the intention of maximising the flow is much the same, but the way it is achieved is slightly different.

In the simplest case, that of a straight, square-edged pipe leading out into the atmosphere, the resistance to the flow of incoming air will create a small pressure drop just after the entrance, which will recover slightly as we move down the tube. Creating a ‘vena contractor’ or the effect of a smaller diameter pipe [Fig. 1], the overall flow into this square-edged pipe will be reduced. To help guide the air from a position of rest and coax it into the intake pipe, engineers have introduced the concept of the air trumpet, sometimes also known as the air horn, stack or bellmouth [Fig. 2]. Whichever description you prefer, the design of this feature is generally divided into two zones – the ‘flare’, which is that part closest to stagnant intake air; and the ‘tube’, which guides it progressively into the rest of the intake runner.

The principal job of the flare is to encourage the incoming air to remain attached to the sides of the trumpet and avoid introducing eddies or vortices into the flow should it separate from the wall. Distributing the low-pressure zone created by the ‘suck’ of the engine over a wider area reduces losses to the system, increasing the flow rate into the engine. Shaped to encourage air from the sides but not the rear of the bellmouth (where air temperatures will be slightly higher), current practice is either to finish the flare with a sharp edge at either 90º to the trumpet or, preferably, extend the flare further back another 90º.

Once inside the horn, the tube will accelerate the flow cleanly and efficiently into the main body of the intake runner. Consisting of a simple polynomial profile that increases in radius of curvature until it is coincident with the next part of the intake, increased  airflows (and therefore engine power) of the order of 5-6% are reported possible over a standard square-edged tube.

With such artistry it is a pity that we have to hide them away inside an airbox.

Fig. 1 -  Profiles

Fig. 2 - Typical bellmouth

Written by John Coxon

Famous – or perhaps infamous – for its use on many race engines in the 1960s and ’70s, the slide throttle has much to recommend itself, although at the same time much to suggest you shouldn’t even think about using them at all. As a rule they are difficult to package, prone to jamming – normally wide open – and almost impossible to produce a smooth progression from fully open to fully closed without some kind of ‘trick’ mechanical linkage. Put against this, when fully open there is no throttle spindle to impede airflow, which is perhaps the reason why many inventors keep coming back to this design when inventing new and apparently innovative uses of controlling engines.

One of these innovations I discovered recently was a device to introduce swirl into the intake air immediately before it passes into the cylinder. Of 1998 vintage, the idea was that instead of sliding the throttle plate back and forth, the plate moved up and down. By biasing the flow relative to the centreline of the intake port, swirl could be introduced in varying amounts according to the demand of the engine – high swirl at low flows gradually reducing as full power was demanded. Novel but I’m not sure of the real benefit. Nevertheless, the manufacturer saw a benefit in preventing anyone else from using the idea commercially.

Another idea I saw recently linked to the use of slide throttles and one that is said to be fully patented is the device shown in Fig. 2. Perhaps the biggest problem encountered by drivers using slide throttles is the amount of pedal load required to open the throttles in the first place. This is a result of the return spring tension necessary to close the throttle and the high friction between plate and its guide when used for long periods of time which can be tiring to the driver.

What this particular invention tries to do is to use the vacuum behind the throttle plate at part-load when connected to a cylinder as a kind of servo device to open the throttle at much lighter throttle pedal loads that would otherwise be the case. It’s clearly an interesting idea and one that I can see would appeal to many a classic engine owner-racers with a slide throttle system.

For the rest of us who prefer the more progressive nature of the traditional butterfly system, however, mere memories of sticking throttles is enough to send shivers down our spines.

Fig. 1 - 1998 patent application 

Fig. 2 - Slide throttle servo system

Written by John Coxon

But the technology of separating engine intake air from, well, just about everything else hasn’t changed much over the years. One option is the physical barrier – a sieve of some description or other consisting of a series of holes through which only the smaller particles can pass. Simple and therefore popular, air filters of this type need to have much larger surface areas to avoid high pressure losses across the filter as the amount of dirt captured increases. 

The other option, which is popular in much older vehicles and finding traction again in the high-performance aftermarket, is to exploit the properties of certain types of oils, and in association with the barrier method it can deliver some of the most efficient types of filter. Years ago, after changing his engine oil, your grandfather would have then changed the oil in his air filter. Cleaning the metal gauze inside the air filter body with gasoline and coating it again with one of the vegetable-based oils available at the time, this oil – being surface active and positively charged – would stick to the metal gauze and attract dirt. The labyrinth of gauze would ensure that the larger grit particles would have dropped out and been captured by the oil in the base of the filter, letting the ‘clean’ air pass through into the engine.

Now of course we have much more efficient barrier systems and, when used in conjunction with the properties of modern types of oil designed purely for the purpose, air filters can be produced that are not only highly efficient but are cleanable – and, or so the manufacturers maintain, can last the lifetime of the vehicle. The barrier part of the filter consists of a couple of layers of woven cotton material, the first having a much looser weave than the second. While the first layer traps all the large particles, the smaller particles pass through and are trapped by the second layer.

To give strength to the assembly, the cotton filter material is encased in steel gauze which, when coated with this special oil, attracts dirt even before it gets to the filter medium. Pleated to create a large surface area and working together as a system, the assembly can therefore hold much larger quantities of dirt before the pressure drop across it demands cleaning. Not only does the oil attract the dirt, when diluted in something like isopropanol and sprayed onto the cotton gauze, the action on the cotton is to tighten the weave such than particles greater than 5 microns will not pass.

Something old, something new, but together they produce a formidable combination.

Fig. 1 - Just a cotton-picking filter

Written by John Coxon

The accepted limits on the operation of a turbocharger are compressor ‘surge’, where for a given airflow the pressure ratio is too high, compressor ‘choke’ – the point where maximum flow is achieved (at reduced compression efficiency) – and maximum turbo speed. Whereas compressor surge can be determined aurally, and choke by the compressor outlet temperature (where the limit is around 60% adiabatic efficiency), turbocharger speed has to be measured. For a shaft travelling at anywhere between 60,000 and 300,000 rpm, depending on the diameter of the compressor wheel, this can be a challenge.

There’s a story about a philosopher who claimed he could tell exactly how many horses were in a field just by looking at them. Dumbfounded and yet intrigued, his companion pointed towards a coral where 29 of his best stallions were simply milling around awaiting sale. Upon guessing correctly and being asked how he came up with the correct number the philosopher said, “It’s simple really – I counted the legs and divided by four” which of course is exactly the same way we go about measuring the speed of a turbocharger. Simply count the number of times a blade passes a certain point in a given period of time and then divide by the number of blades per revolution passing. Simple or is it?

The first thing to remember is that any electrical instrumentation anywhere near the turbine wheel will simply not be durable. Electronic sensors rarely work reliably at much over 80 C. And even though some sensors these days claim to work at temperatures well above this, it is simply not possible to measure the speed of the turbocharger at the turbine wheel when the turbine casing is glowing red hot.

At the other end, that of the much cooler compressor, the wheel is almost invariably made from a non-magnetic aluminium alloy. That means a magnetic pick-up fixed into the compressor casing to detect the compressor blades as they pass is out of the question. The only way to calculate turbo speed would seem to be to use either a magnetic nut (not entirely practical for a permanent installation) or some kind of eddy current/optical device measuring the passing of individual blades. The output of such a sensor, when connected to some kind of signal conditioning and fed into a data logger, will give an average speed accurate enough for confirming the engine airflow data on, say, a fuelling map.

However, to determine the instantaneous speed of the turbine, for instance in the cast of a burst test, this requires a slightly different technique. Here the signal output from the sensor (or magnetic nut retaining the compressor wheel on the shaft) is recorded and fed back across the Y-Y terminals of an X-Y oscilloscope. With a known frequency applied across the X-X axis, the resulting output produces the phenomenon known as Lissajous figures. By altering the frequency and phase of the X-X input, a point will be reached when a perfect circle is produced on the screen, and at this point the known frequency will be the same as that on the Y-Y plates but 90º out of phase, and hence the precise burst speed will be known, compared with only the average over the timing interval by the other method.

Convenient it probably isn’t, but a burst speed of say, 267,532 ±1 rpm rather says it all.

Fig. 1 - Turbocharger

Written by John Coxon

For simple power density in terms of kilowatts per kilogramme of engine weight, a gas turbine unit would still probably be the preferred option, and it is therefore little wonder that the 1960s were littered with automotive applications of this type. These engines relied on sheer throughput of fuel and air, but with the high rotational speeds pressure changing was not required. However, if you want power density with fuel economy then a turbocharged power unit is increasingly the way forwards in automotive circles.

Despite the engineer’s cool, calm logic in choosing one or other of these alternatives though, surely the most evocative form of power generation used in the automotive world is that of the Roots-type mechanical supercharger geared directly to the engine’s crankshaft. Delivering more or less a constant boost pressure irrespective of engine revs, supercharged engines of this type have a reputation for delivering almost instant high torques from very low engine speeds, giving the effect and driver feel of a much larger capacity engine. From the pure driving aesthetics therefore, little can beat a supercharged unit. But efficient is one thing they are certainly not.

In the case of a Roots supercharger the clearances between the lobes and the casing generally leave a lot to be desired. Large enough to avoid contact between the rotors yet small enough to stem any major flow losses, supercharger systems of this type will always have very low compression efficiencies. Not having any internal compression in the unit itself, manifold pressure arises from the fact that the unit will pump more air than the engine will consume, and at the point where the reverse flow losses equal the difference between the gross airflow pumped and that consumed by the engine, the boost pressure will be maintained.   

But it must be remembered that an engine running at low speed and high intake pressure is highly susceptible to detonation when the end gas in advance of the flame front ignites before its time. High-octane fuel blends can reduce this tendency, but if ordinary pump fuels are mandated (or they are all that is available), the boost pressure of the engine throughout the whole speed range – including that at peak power speed – is effectively dictated by this low-speed condition. In reality the inefficient compression will heat up the charge air temperature, which will require the ignition to be retarded away from optimum, increasing the exhaust gas temperature (risking damage to the exhaust valves) and reducing the torque of the engine.

That may sound like bad news on the engine dynamometer, but fortunately when installed into a vehicle the situation tends to be only transient. Although plenty of low-speed torque might be desirable, in practice the vehicle may be traction-limited, which in a way adds to the driveability of the vehicle. In modern road-going cars this retarded ignition can have additional benefits in reducing catalyst light-off time, albeit at the penalty of high fuel consumption. 

But hey, what price that self-satisfied grin as you launch away from the lights? 

Fig. 1 - An Alfa P3 under restoration. You can just see the supercharger(s) nestling down alongside the cylinder block

Written by John Coxon

However, add the complexities of a multi-cylinder arrangement at a wide range of engine speeds, feeding through a single throttle plate, and the degree of difficulty begins to increase. Now add a single-point fuelling system – a carburettor – delivering fuel proportional to the pulsating flow of air travelling either in towards the cylinder or out away from it, and you have what was commonly known as a classic race engine, the sort that engine ‘tuners’ at the time would attempt to develop on a day-to-day basis. 

A major problem with these engines though would be the distribution of the air-to-fuel ratio across the cylinders. As many of you will know, a gasoline fuel burns the exact amount of air – no more, no less – at an air-to-fuel ratio by mass of around 14.5-14.7:1. This is known as the stoichiometric ratio, and for maximum power it should be increased to nearer 13.0:1. Beyond this the cylinder will run excessively ‘rich’ and the power will fall away. Likewise, running ‘lean’, at 16:1 or greater, and the power falls away but perhaps not as quickly. To generate the maximum performance from a given amount of fuel therefore, each cylinder needs to run at or very near its optimum of 13:1.

Formula regulations in the past could often specify a single carburettor ‘choke’ feeding across all cylinders, and even in some of the best engines the spread of air-to-fuel ratios across the cylinders could be as much as 4:1, so while one cylinder could be running at 11:1, another one next to it could be at 15:1. That may mean an average of 13:1 but the engine is nowhere near delivering its maximum power.

The answer of course lay in the intake manifold, but not just in the distribution of the air – which was never that good anyway – rather mainly in the distribution of the fuel, which was crude to say the least.

In anything other than a straight intake pipe, fuel in the form of droplets will be easily centrifuged out of the airflow as soon as the air meets the first bend in its path. Attaching itself to the outside wall of the bend, the mixture will weaken off momentarily, only to evaporate back into the air stream a short time later. This continual wall wetting and evaporation helped atomisation of the fuel into the air, but at the same time it was difficult to control the amount of fuel eventually travelling down each branch and into the cylinders.

The advent of port fuel injection solved the issue of fuel distribution, leaving the task of air distribution to be solved by other means.

Fig. 1 - Classic intake manifold

Written by John Coxon

Grand Prix racing is one thing, and one assumes that the organisers do their utmost to sweep the tracks clean afterwards. But for the rest of us with less exotic machinery, the real enemy is not necessarily tyre debris but other forms of unwanted matter - the dirt thrown up by the tyres themselves, dust and general organic matter from the world around us.

The technology of air filtration is outwardly very simple; the sieving or straining of the air to remove unwanted media. However, investigating the process in more detail, the mechanisms can actually be quite complex. And while paper or woven nylon/polyester filters may be common in automotive applications, in motorsport the most commonly found varieties are made from polyester foam.

The great issue with the more traditional type of paper technologies is that in trapping the dust or dirt, once trapped on its surface, the flow capacity of filter is reduced. For a given airflow, the pressure drop across the filter will be greater, resulting in a lower manifold pressure and lower ultimate power. Particles smaller than, say, 5 microns will pass through the filter and will either be find their way into the engine or end up in the lube oil. Particles larger than this will end up on the surface of the filter.

For foam filters, particularly those of the ‘wet’ or oiled type, the labyrinth of cells (which will be larger than that of the paper element) not only prevent the larger particles from passing through, but many of the smaller particles that will pass through this surface layer will be trapped within the body of the filter by the stickiness of the oil coating (on the foam). The structure of the foam and the mechanism that traps the dirt is therefore such as not to restrict the flow of air, ensuring minimal restriction of the flow of air for longer. For a given volume of dirt removed, this type of filter is therefore more efficient. Furthermore, by using several layers - perhaps as many as three - of progressively smaller pore size, dirt can be effectively ‘moved around’ within the filter to hold it more within the body of the foam.

Changing the number of pores per inch and the type of oil to wet it with can also change the characteristics of the unit, and so filters of this nature can be ‘tuned’ to be more effective depending on the environment.

But here’s the bit that most self-funded racers will love. Unlike paper-based systems, such filters can be washed and re-oiled on a regular basis, thus maintaining the maximum cleaning efficiency, so once purchased, the ongoing cost will be minimal.

Maximum filtering efficiency with the minimum of pressure drop into the intake manifold. Isn’t that all we need from a filter?

Fig. 1 - Foam air filter

Written by John Coxon

The advantages of this are not just in supplying a denser charge to the engine. In directing the open intake of the ducting away from the outside of the vehicle, the inevitable roar of induction can be reduced or directed such that the external or drive-by noise will be below that of legislated levels. Like it or not, few racing circuits or motorsport venues these days allow unrestricted noise levels, and any method of reducing the pulsation so necessary further up the intake tract (but without affecting performance too much) has to be taken. Even the seemingly smallest of airboxes can be surprisingly effective at times, and pointing the entry away from the external boundary of the track can sometimes mean the difference between passing the inevitable noise test or not. A perfect example of this is in the world of karting, which mandates the use of an airbox - not, as you might think, to provide cold-ish air to the engine but to redirect/attenuate the high-frequency, eardrum-piercing noise of their high-speed two-stroke engines.

But not only can this airbox help to supply cold air to the engine, it can also be used to capture the forward motion of the vehicle and convert the (albeit small) velocity head thus generated into useful 'static' pressure energy. But in doing so the truly tricky task is to ensure that the air supplied is as evenly distributed as possible across all cylinders throughout the engine speed range and at all vehicle speeds. If an engine is installed in an east-west configuration - as in the case of, say, an ex-motorcycle engine in a single-seater racer - then if the intake tract is brought into the box centrally to the cylinders of a four-cylinder engine, the airflow will biased towards the centre two cylinders, with the outside pots being the losers. Baffles or turning vanes at the entrance to the box can be used to redress the situation, but a better solution may be to split the intake into two along the centre line of the car and introduce the split airstream centrally between cylinders 1 and 2, and 3 and 4.

When an engine is installed north-south the task is altogether more difficult. The air coming in from an overhead intake will invariably be biased towards the rearmost cylinders, and although baffles or turning vanes will be used to redistribute the flow, the complexity of the situation is often left to either trial and error or virtual analysis using CFD techniques.

As winter approaches, cold air might see the death of some of us mere humans but to the gasoline engine it is the elixir of life.

Fig. 1 - The cold air intake needs careful design

Written by John Coxon

At the time, the reason given for this practice was to even out the torque curve such that each cylinder would hit its resonant cylinder-filling frequency at a slightly differing speed and, by manipulating the torque curve, enable the driver to cope with the huge torques generated by the monstrous engine. How effective all this was is never recorded, but it is interesting to note that to my knowledge no-one ever copied this approach.

These days of course we have aerodynamics and downforce on the cars, and therefore more grip than the CAN-AM cars of the period, and are better placed to manipulate the torque curve by altering the ignition timing and fuelling by electronic means. Controlling the rate at which the torque comes in at the rear wheels is therefore not such a tribulation as it once was, and in the final analysis, if the regulations allow, we always have traction control.

But simply altering the fuelling and ignition timing to change the engine performance curve always somehow seems wrong, and is a last resort to many a power unit engineer, since moving away from the optimum efficiency is alien to our way of thinking. As a better way to change the shape of the torque curve and keep the cylinders working at their optimum, the use a variable intake manifold technology might be a better approach.

This can take many forms. In engines with plenum chambers fed by a single throttle plate/airflow meter, this can be simply a system of flap valves opening and closing either an aperture to vary the effective length of the intake runner or altering the resonant behaviour of the intake system, much like a Helmholtz resonator. These are in general not infinitely variable but simply two- or three-position devices that optimise the system at different speeds. While highly effective though, they are still compromised away from those speeds.

In competition engines with open intakes, the most effective method is the telescopic intake port, which not only alters the length of the intake runner but does so in a continuous way so that the engine can be optimised for wide-open throttle performance at all engine speeds. I once saw a system (now banned in Formula One) that amounted to a tube sliding within another operated by some form of stepper motor controlled by the engine ECU. An ingenious approach, not necessarily unique but one that was never to see the light of day, since the authorities outlawed such systems the following year.

I remember talking to the system's designer, who told me candidly that the method worked well at constant speed but suffered from a transient issue, although not in the speed of the motor and its ability to respond to the rapidly changing rpm of the engine. No, the major issue was to do with fuelling. Injected centrally into the intake port at the entrance to the intake runner, some of the fuel would attach itself to the runner wall. However, as the engine speed rose and the port collapsed, that fuel was effectively scraped off and introduced back into the airstream, upsetting the carefully calculated air-fuel ratio. Not an insurmountable problem but one that needed time to model and compensate for in the fuelling system.

As an engineer, this was all very interesting but somehow not as evocative as the sight of that CAN-AM engine over 40 years ago.

Fig. 1 - Such sweet music

Written by John Coxon

In essence, the options available over this time have been barrel, butterfly or slide, and although slide throttles were very popular at one time their propensity to stick wide open and the pedal efforts sometimes required to overcome the high return spring loads have made them more or less obsolete in anything other than historic racing. For most modern applications the battle is between the barrel and the butterfly as a means of controlling the airflow.

The big advantage of the slide throttle is that in the fully open position there is no spindle to restrict the airflow. Having much the same effect when fully open, these days the barrel throttle has its attractions, and when carefully designed has many of the attributes of the butterfly at part-throttle conditions. It is progressive, easily packaged and can be incorporated into the port, getting closer to the inlet valve without compromising the airflow to the engine. But as engine peak power speeds have risen and optimum runner lengths get shorter it has become increasingly difficult to incorporate barrel throttles into the intake port without affecting mixture preparation at part-throttle with port-injected fuelling systems.

Barrel throttles are also much more difficult to make than the other types. Requiring very close fitting tolerances between the port enclosure and rotating barrel to seal when the engine is idling, and despite their apparent advantages, they can be quite bulky and comparatively heavy compared with the other options.

Of course though, engines don't run at wide-open throttle all the time, and even at a circuit like Le Mans, famed for its Mulsanne Straight and high average speeds, the degree of full throttle accounts for no more than 70% of the lap for LM P1 contenders. For the rest of the lap, driveability is the key, and a system that gives good throttle progression and excellent mixture preparation, controlling the engine to a finer level, is of greater importance. After all, under race conditions the ability to extract the maximum speed from the vehicle through the corner, giving a greater terminal velocity at the end of the following straight, is what wins races.

Under these part-throttle conditions the humble butterfly throttle would seem to have the advantage. For although peak power speeds have risen and the overall length of the intake runner been reduced, in order to ensure complete evaporation of the fuel the optimum place for the butterfly will move up and away from the intake valve, towards the bellmouth. Also, with tapered intake runners, larger throttle plates will be required, proportionately reducing the 'blockage' factor of any spindle, and using the latest in profiled throttle plates aerodynamic losses to the intake flow can be minimised.

With its lack of a spindle, the barrel throttle may at first seem to be the best solution. However, for high-speed engines and those operating at part-throttle for long periods, the humble butterfly without its attendant spindle might still be winning the war.

Fig. 1 - The latest in spindleless butterfly throttles

Fig. 2 - Slide throttles on an historic engine

Written by John Coxon

For much of the time since then, turbochargers seemed to be the almost exclusive domain of the diesel engine, and since transient response isn't a diesel engine's greatest attribute, turbo lag didn't feature high up in the priorities. These days, however, the pressure on fuel economy is forcing gasoline engine manufacturers to downsize their products, so to retain performance, the spectra of turbocharger lag in gasoline engines is once again concentrating minds.

The traditional approach is to use multiple turbocharger units. Operating in parallel or (more likely) some kind of sequential arrangement, such systems are bulky and difficult to package, not to mention extremely complex and therefore potentially unreliable. The requirement for complex lubrication systems in addition to water-cooled turbine housings also adds further expense.

However, developments in electric motor technology and onboard electricity storage are coming to the rescue, and instead of having the compressor and turbine of a turbocharger together on the same shaft, in this latest application the turbocharger is effectively split into two, with each part - compressor and turbine - attached to a high-speed electric machine. So while the turbine powers the generator to create electrical energy, which is then stored, the electric motor subsequently takes that energy and powers the compressor, delivering boost to the engine when it most needs it. In this way, and by using some kind of buffer electrical storage (batteries or supercapacitor), the traditional turbocharger failing of turbocharger lag can be avoided.

But that isn't all. The physical separation of compressor and turbine gives the option to run the compressor at its most efficient operating point for longer, and the technology of the wastegate will perhaps become a thing of the past. Also, to the engineer responsible for packaging the unit in the engine bay, the ability to split the turbo, placing the compressor towards the intake side and the turbine next to the hot exhaust, must have obvious benefits.

Another benefit might be the move away from the fully floating bearings. Replaced by, at this stage of development, ceramic roller bearings, gone is the need for the lube oil feed and along with it all the affects of 'key-off' heat soak on the engine oil.

But after more than three years in development, at long last interest in the technology is being shown by the racing world but not in the way you might think. With the loss of 'Kinetic' from the KERS of Formula One, under the new regulations energy can be scavenged directly from the exhaust and at least one programme using this approach is being developed in time for 2014. Operating at just under 500 V and with currents of the order of 150-plus A, officially this is for Le Mans, but once the control module is condensed to a more reasonable 6 x 4 in (150 x 100 mm) many more manufacturers will begin to take note.

And when installed, once again together with its compressor, turbo lag will become a thing of the past.

Fig. 1 - Turbocharger compressor and electric motor

Written by John Coxon

Consisting of two internal rotors in the shape of lobes rotating together, and phased using gears to prevent contact, when geared to the crankshaft the air delivery per revolution is fixed, so as the engine speed increases so does the air delivery of the pump. Unlike the turbocharger, no fancy pop-off valves or bypass systems to avoid excessive boost pressures have to be incorporated. And since air delivery is broadly linear with the engine speed, the manifold or boost pressure will remain more or less unchanged.

Not fully appreciated by many, supercharged engines simply add to the driving experience. Opening the throttle produces instant power and the feeling of driving a much larger engine than the one under the bonnet, and because of this (and unlike turbocharged machines) driveability - that characteristic that often gets lost in the search for power - is vastly improved; ask anyone who has conducted back-to-back tests with a turbocharged vehicle. With a supercharged engine you quickly forget (if you ever knew) the presence of the supercharger, unlike a turbocharged unit where the delay in response, however slight, is always there, even with the best of systems.

Strictly speaking, a Roots-type supercharger is a pump rather than an air compressor, and while more modern mechanical units may include a small amount of internal compression, generally speaking these machines have no internal compression inside them; the increased intake plenum or boost pressure is created solely by the restriction to flow in the engine.

When running at a part-load, off-throttle condition - which let's face it, even competition vehicles do for quite a lot of the time - there is therefore little parasitic drag, unlike say that of a turbine of a turbocharger in the exhaust stream, which will always impose some level of back-pressure against the engine. On some newer units these twin lobes have been replaced by a three-lobe design, which when twisted to form a helix along the length of the rotor introduce an element of compression to the intake charge. More important for road-based applications, this reduces the intake port pulsation and therefore intake noise.

Since Roots-type machines have no contacting parts, friction within the device is low, and the design of the intermeshing rotors is such that no out-of-balance forces are generated. Roots blowers are therefore safe up to quite high speeds, with 4000 rpm being quoted by some manufacturers. At these speeds, however, designers need to take heed of the increased inertia of the rotor and design the drive system to cope.

One particular issue with Roots superchargers is the clearance between the rotors. Machined and coated to minimise this at all times but never to make contact, at low speeds pumping efficiency can be impaired and lead to a certain amount of leakage. In modern designs this has been all but eliminated, and reliability has been vastly improved such that when specified on OE vehicles, units can be expected to last the vehicle's life.

So while modern trends seem to be leaning towards complex turbocharger systems, for raw performance and simplicity of installation - as well the sheer driving delight - you simply can't beat a good blower.

Fig. 1 - A Roots-type supercharger

Written by John Coxon