Improvements Using the Basic Theory of Squish

Looking at what is happening and the theory itself can lead to some very interesting conclusions.  Many 2-stroke head designs are just crap for racing, including many racing engines of yore, little and big sized, and even, unfortunately, some high performance engines today.  Squish bands are often narrow and very highly angled towards the combustion bowl, and often the lead-in to the bowl has a large radius.  All these things tend to promote poor running and bad air/fuel mixing. Sadly, engine designers are very conservative and why should they change the design of things that in the past appear to work in a tried and true formula.  There is a good reason, engines just work more efficiently if the head is optimised first.  GP engines figures prove this, “engine design on the edge” has gone from 50% thermal efficiency to the astounding figure of 75% in less than 30 years and initially two-stroke design led the charge.

If one take the standard design, wide squish with the combustion bowl plonked in the middle it gives us a simple and effective design to try our Squish Velocity theory.

According to Prof. Blair, we can improve the design just by simple inspection.

It is therefore no surprise that designs (B) and (C) have at least a 50% greater squish velocity when compared to the standard design but not a significantly higher squish kinetic energy.  The offset design (D) has a 100% higher squish velocity than the standard design and significantly higher Squish Kinetic Energy, (even at very large head clearances), 5 times the energy in fact at the same head setting as design (A).

These simple designs with advanced features of high average squish velocity and high squish kinetic energy are worthy of consideration.  None of these designs B, C and D would be beyond modern production techniques or even a home workshop with a lathe or mill to manufacture.  Considering their advanced design features and significant advantages over the standard head design, it is difficult to understand why more use has not been made of these cylinder head layouts in all two stroke engines.

If we want to go faster, have a more thermally efficient engine and employ higher compression ratios, which is quite logical, then the squish effect needs to be employed and applied to exploit squish velocity and squish kinetic energy.  These designs put forward over 15 years ago, based on empirical and scientific evidence, seem to fit the bill nicely.

References:

[1] Prof. G P Blair, Fundamentals and Empiricism in Engine Design, Back to Basics, (027) Race Engine Technology Magazine, Dec/Jan 2008.

[2] G.P. Blair, “Design and Simulation of Four-Stroke Engines”, Society of Automotive Engineers, 1998, SAE reference R-186.

[3] G.P. Blair, “Design and Simulation of Two-Stroke Engines”, Society of Automotive Engineers, 1996, SAE reference R-161.