Quote:
Originally Posted by bwilson4web
T... you might check the Wiki or ask Mr. Google. I'd have to reconstruct my college course notes (from 40 years ago!)
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So far I'm disappointed at what Google has to say on the subject.
From Wiki:
"A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency."
In other words, a high compression ratio is more efficient because it is more efficient.
From
answers.com:
"Engine efficiency is increased though compression ratio by allowing a more thermodynamic energy to be converted into mechanical energy. Energy transfer is the key to efficiency."
I believe all things can be distilled into a simple concept, but it's improper to define something by using the subject as the definition. Those that cannot explain something in lay terms often do not have a firm grasp of the concept themselves.
Now, I have a hard time understanding how a higher compression ratio equates to higher thermal efficiency in light of the 2nd law of thermal dynamics. In my mind, increasing pressure also increases the heat density of the fluid. Since energy always flows from higher concentrations to lower concentrations, this should mean that the highly compressed fluid would give up more of its heat energy to the surrounding cylinder walls, piston, and head. Fluid that is not compressed as much would not give up as much heat since the temperature differential is not as great.
So, something about the higher compression does cause greater thermal efficiency despite the heat that is lost due to the increased thermal density. The principle behind this is still a mystery to me.