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Name: Cormac
Status: Other
Grade: Other
Location: Outside U.S.
Country: Ireland
Date: Summer 2009

What is the thermodynamic/chemical reason for compressing a mixture (or air in a diesel engine) prior to ignition? How does the action of compression increase the power stroke? Is more heat somehow produced? I read that super/turbo charging crams more air into the engine but even this charge is further compressed before subsequent ignition.

Hi Cormac,

High compression in a diesel motor (around 20:1) is of course necessary to cause sufficient heating of the compressed charge to ignite the injected diesel fuel. But your question, I believe, was mainly to try to understand why compression rations of 10:1 or more in a gasoline motor, produce more power than lower ratios (say 8:1 or lower).

Higher compression ratios are simply more thermodynamically efficient and allow more energy to be extracted from the air-fuel mixture. There are several reason for this. Oxygen and fuel are squeezed together more intimately, resulting in more complete combustion. High compression also causes better mixing of fuel droplets and air. Another important benefit of high compression is that the mixture is compressed to a smaller volume, that is therefore exposed to less surface area of the relatively cold cylinder head and piston. The hot gasses that result after ignition are therefore able to retain their heat better. More heat means more ability to do more mechanical work.


Bob Wilson

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