Re-optimisation of spark ignition engines
by C.R. Stone
International Journal of Vehicle Design (IJVD), Vol. 6, No. 2, 1985

Abstract: The power output and fuel economy of spark ignition engines is usually optimised for full throttle operation. The work described here is aimed at improving the part load performance of spark ignition engines, the normal operating regime in automotive applications. The experimental work was with a Ricardo E6 single cylinder variable compression ratio engine. Normally the highest useful compression ratio is limited by knock. In these experiments the engine was operated at a higher compression ratio, to benefit from the improved thermodynamic cycle efficiency. To avoid knock at full throttle the ignition had to be retarded, and the fuel/air mixture weakened. This caused a reduction in efficiency that would not occur if higher octane fuel were available. Knock is particularly troublesome at lower engine speeds, and if necessary the throttle opening could be restricted. These controls are now feasible with electronic engine management systems. The results are expressed as fuel consumption maps for different compression ratios, and the fuel economy benefits are estimated for vehicles with CVT and conventional transmissions. The results are reviewed in the context of the predicted fall in fuel octane rating, a fall exacerbated by the restrictions on the use of lead in fuels. One conclusion is that a fall in octane rating should not necessarily lead to a reduction in compression ratios.

Online publication date: Sun, 25-May-2014

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