Location: Outside U.S.
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.
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
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.
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Update: June 2012