# ELI5; Why does higher compression equate to more power in an engine?

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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
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It’s not that higher compression ratios create more energy from the same fuel/air charge, it’s that they allow more of that energy to be converted into mechanical work by the engine.

By crushing the fuel/air charge to a higher compression, you can release more heat energy from the charge per cycle, which equates to more mechanical energy into the piston, which equates to more power from the engine.

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.

The compression ratio doesnt reduce the power as when you compress stuff in a closed cylinder it acts like an air spring. So its not a problem with power loss.

Well a given amount of fuel has a fixed amount of energy, and to burn it you need a corresponding amount of The compression ratio doesnt reduce the power as when you compress stuff in a closed cylinder it acts like an air spring. So its not a problem with power loss. spring. So its not a problem with power loss.

If you have a given cylinder it has a given volume, and the air that naturall fix in the volume limits how much fuel can be burned in it. And the force which it flings the cilinder depends on how much fuel is The compression ratio doesnt reduce the power as when you compress stuff in a closed cylinder it acts like an air spring. So its not a problem with power loss.

You get more power from higher compression because the tsarting “dead” volume in which the explosion starts is smaller, and the length on which the piston gets pushed is longer.

Also the starting pressure is higher, as the starting volume is smaller the gas has less volume to “spread” its energy over.

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Basically you get a higher % of time when pistons provide torque, and higher torque.