0 . I am a student of automobile engin
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eering and am preparing a thesis on piston design. I have two questions. Can you confirm that a piston has been developed in which part of the load on the gudgeon pin is transferred to the piston crown? And has a variable compression ratio piston been applied to a spark ignition engine?
A In the spring 1968 issue of the "Hepolite
Bulletin" (published by Hepworth and Grandage Ltd, St John's Works, Bradford 4) mention is made of a piston/connecting rod assembly in which the rod is extended above the pin to form an arcuate upper end. This mates with an arcuate surface on the under side of the crown, a pad of self-lubricating or low friction material being located between the crown and the rod. As mentioned in the Bulletin, such a layout would be particularly suitable for pistons with a combustion chamber in the crown because it would assist in the dissipation of heat from the crown.
So far as is known, a variable compression ratio piston has not as yet been applied to a spark ignition engine. Increasing the compression automatically at low throttle opening would undoubtedly improve fuel consumption. the effective compression ratio of a petrol engine being normally reduced from the nominal ratio of, say, BIto 1 to as low as 21 to 1 at low road speed. But current types of VCR piston are heavy and might not be suitable for petrol engines because of the high inertia forces that would be produced at the relatively high operational speed. And immediate response to an increase in cylinder pressure would be essential.
In the case of a diesel the compression ratio can be reduced dyer a number of cycles when the load is increased because the unit can accommodate a transient increase in cylinder pressure above the normal operating pressure. Unless the compression ratio of a petrol engine were reduced over a single cycle when the engine was running under light load and throttle was opened fully, it would momentarily develop a severe knock.