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Which performance to push?

pushrodsAfter having written in my previous article about process control issues, something occurred to me about the differences between engine applications. In general, RET-Monitor and Race Engine Technology magazine put an emphasis on high-performance engines. This often means also high-revving engines, due to the simple assumption that the easiest way to make power is make use of high rpms.

What came to mind is what this statement means in regard to other engines. The reason I started thinking about this was a statement I came across in an article in Race Engine Technology concerning pushrod systems. The comment was that valvetrain systems using pushrods are at no disadvantage compared to overhead camshaft systems with direct valve actuation in those race series which, by their regulations, have intake restrictions, generally restricting rpm as consequence.

So, when one takes the high-revving part out of the equation, what would be the definition of high performance? In the case of a valve actuation system, high performance most definitively means stability of the system in relation to its loading. And this is where it becomes interesting to compare engines in entirely different corners of the automotive industry.

In issue 29 of Race Engine Technology (March/April 2008) an interesting article on pushrod system technology was written by Ian Bamsey and Wayne Scraba. The goal of the article was to show which parameters of a pushrod valvetrain system directly influence the system's functionality and stability. In the article, exhaust pushrod loading in an NHRA Top Fuel engine was mentioned, being in the area of 7000 lb force. From my experience with other performance engines - it will become clear below what that performance engine experience is - I very well know that having these kinds of loads means the valvetrain components need to be robust, and the system's behaviour very stable, for there not to be any collapse.

Which other engines in the industry also use pushrod valvetrain systems and show comparable loading of the valvetrain? For one thing, there are not many engine designs that can be compared here. A number of heavy duty (HD) truck engines still depend heavily on a pushrod systems due to its compactness in regard to engine size. Because of their required normal engine life of about a million miles, one might think these Kings of the Road are not as highly loaded as Top Fuel Engines, which by comparison are required to have only a marginal engine life. However, and this is where I try to translate my experience from HD truck engine development to the world of race engines, pushrod loading of the Top Fueller is at about the same level as the HD truck engine.

The major differences in the introduction of the loads between the two engine types and applications are combustion and rotational speed. Looking at rotational speeds, it can quickly be seen that the NHRA Top Fuel engine, running near to five-digit speeds, has clearly the more difficult load case. When considering the combustion loads, however, it's a different story. Based on the charging pressures, one can compare 5 bar for the Top Fuel engine with compressor with 3.5 bar for a HD Truck engine (single-stage charging, as is still the case in most of these engines).

Although from a relative factor the percentage of valve diameter to piston diameter of the NHRA engine is far larger, with 40% over 30% of a HD truck engine - not that strange, given the swept volume difference of 8.2 litre V8 versus a 13 litre-class inline six-cylinder engine - the absolute diameter is about the same, at 40mm. Assuming combustion pressure over the four-stroke cycle and exhaust valve timings, the pressure on the exhaust valve could be determined. I do not have pressure traces for a Top Fuel engine, so it is difficult to provide comparisons at the moment, but it would be interesting to follow up on this. If there is any data out there, let me know.

But, and this is where the comparison goes off a little, a HD truck engine has a engine compression brake functionality that relies on opening the exhaust valve at the moment of highest compression, in order not to get energy back when the piston goes on its way down again. The loads introduced exactly at the moment of start actuation are far higher than in normal engine operation, but equal to Top Fuel pushrod loads.

So, having explained the differences in application and load cases, the detailed environment and load cycle might not be fully comparable, but looking at the possible consequences of these loads, the required engineering solutions are exactly the same, mainly discussing cam follower roller robustness and pushrod dimensioning.

I would like to look a little further into the pushrod dimensions, just to realise where the differences are and what the reasons behind the design choices are.

The above comparison would mean that the pushrod valve actuation system of the Top Fuel engine would, dimensionally speaking, need to be more or less the same. However, if we look at the dimensions of the pushrod itself - Top Fuel , 14.3 mm x 4.8 mm (0,562 in x 0.188 in) and HD truck, 18 mm x 3 mm (about 0,687 in x 0.118 in) - then we can conclude that there are clear differences.

The Top Fuel engine has a smaller outer diameter and, as a consequence, a higher wall thickness. A principal look at the buckling resistance (Inertia goes to the fourth power), only on diameters, will show that the HD truck pushrods are significantly more robust than those on the Top Fuel. Assuming these components for both the Top Fuel and the HD truck need to be designed for infinite life, this would mean that the pushrod material specification must be significantly different.

Looking at the material specs confirms this. For example, heat-treated 4135 (1.7220 EU) is 1900 N/mm2 (275 kpsi); normal cold-drawn St37 (1.0037 EU) is about 300 N/mm2 (44kpsi) - a significant difference.
Looking at the dimensional difference of the two engines, 8.2 litres against 13 litres, and the bore size of roughly 100 mm to 130 mm, that would mean the available space for the pushrod would be, as rule of thumb, 100 / 130 x 18 = 14 mm.

This brings us back to the start of this article, where I started from comparable pushrod loads to be withstood within the maximum available clearance in the engine structure. Due to the smaller size of the Top Fuel engine, materials with higher material properties are an absolute must.

So, putting the statement 'high performance' in perspective, it is nice to see that, as with many other examples, current HD truck engines, running for enormous amount of hours, are almost race engines, being produced as cost-effectively as possible. Think of it as a Top Fuel car pulling a trailer over the highway.

Fig. 1 - Typical pushrod system (Image from Wikipedia UK, originally uploaded by IJB TA , GNU Free Documentation License)

Written by Dieter van der Put

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