I understand engines and have rebuilt several, but don’t get this. It seems that for a given displacement, the engine sucks in the same amount of gas/air mix which would have a fixed BTU amount of energy. So why would higher compression create more power? And why isn’t this
“extra” negated by the extra effort it requires to create the higher compression?
In: Physics
When the gas/air mixture goes boom inside the engine, it does indeed generate the same amount of heat no matter what the compression. Now when a hot gas expands (in this case by pushing the piston down) it cools down. The more it expands, the more it cools down. This also means that at the end of the power stroke, the burnt gas in the cylinder is cooler in a high compression engine. This means we have effectively extracted more energy from the gas than in a lower expression engine and used it to push the piston down.
So in short higher compression engines are more efficient because more of the BOOM of the explosion is converted into motion of the piston and less of it escapes into the exhaust as heat.
So why isn’t every engine high compression then?
Two reasons. You do generate higher pressures so the engine needs to be sturdier. And since the temperatures get higher due to compressing the initial mixture more, you have the risk of knocking (ie. the gas-fuel mixture self igniting before the spark) so you need better quality, higher octane gasoline.
A fun detail with this… The most challenging regularity race for oldtimer cars runs all the way from Beijing to Paris. The quality of of fuel that can be had in Mongolia and some parts of Russia they pass through is so bad, that they actually enforce a maximum compression ratio upon the cars that are allowed to participate, so the cars’ engines don’t self-destruct going through Mongolia because of this.
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