Theroy discussion, how does compression make power?
03-14-2004, 04:00 PM
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Theroy discussion, how does compression make power? ok, lets jump right into it. power is made when air mixes with fuel and is burned. now, additional compression doesnt add any air or fuel so why does a motor with 11-1 compression make more power than a motor with 8-1 compression with all else being equal? it must leech some power off the crank to force the same amount of air/fuel into a smaller area but since you still have the same amount to burn, how do you gain power instead of losing it?
i understand how FI works because you are actually shoving MORE air into the cylinder which allows for more fuel to be injected thereby making more power. but, for the life of me, i can not figure out how engine compression can raise horsepower and fuel economy. i have talked to several "smart" people about this and none of them seem to know the answer. obviously the power gained is more than the power consumed to further compress the mixture but i just cant see it.
a couple theroys i have come up with:
1) the pistons are already moving so their natural innertia allows them to further compress the mixture. if this were true though that may actually slow the piston down more than a lower compression would so there would be less rod stretch as the rod tries to make the piston reverse direction and this seems to me like it would allow you to rev higher with a higher compression.
2) since everything is being compressed into a very tight little space all the air and fuel molecules are right near each other which would mean more of the air and fuel "come together" when ignited and you get a more efficient burn. then again, if this were true you could theoretically run leaner and make even more power.
anyway, just some idle thoughts in between races, what do you guys think?
i know we have some engineers on this site and i hope they chime in.
i understand how FI works because you are actually shoving MORE air into the cylinder which allows for more fuel to be injected thereby making more power. but, for the life of me, i can not figure out how engine compression can raise horsepower and fuel economy. i have talked to several "smart" people about this and none of them seem to know the answer. obviously the power gained is more than the power consumed to further compress the mixture but i just cant see it.
a couple theroys i have come up with:
1) the pistons are already moving so their natural innertia allows them to further compress the mixture. if this were true though that may actually slow the piston down more than a lower compression would so there would be less rod stretch as the rod tries to make the piston reverse direction and this seems to me like it would allow you to rev higher with a higher compression.
2) since everything is being compressed into a very tight little space all the air and fuel molecules are right near each other which would mean more of the air and fuel "come together" when ignited and you get a more efficient burn. then again, if this were true you could theoretically run leaner and make even more power.
anyway, just some idle thoughts in between races, what do you guys think?
i know we have some engineers on this site and i hope they chime in.
03-14-2004, 04:33 PM
#2
It's the same amount of air just burned in a smaller space. This will provide a more "violent" expulsion of gasses from the chamber which would in theory make the gases move faster from the chamber, thus improving power. If you compressed the same amount of air into a 3 litre coke bottle as a 20 oz. coke bottle, obviouslly you are going to get a grater explosion out of the small bottle. Same theory goes for a car.
03-14-2004, 04:45 PM
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It's really simple. To create a model it may be convenient to assume that combustion happens instantaneously. The power is created when products of combustion expand. So, right after combustion, you have certain amount of gas in the chamber. That amount is exerting pressure on the pistons (as wells as on the walls and roof of the chamber). Pressure * piston area = force. Force * distance travelled by the piston = energy (power).
A smaller chamber with the same amount of gas will have larger pressure in it, producing larger force on the pistons and ultimately greater energy in each cycle.
A smaller chamber with the same amount of gas will have larger pressure in it, producing larger force on the pistons and ultimately greater energy in each cycle.
03-14-2004, 04:58 PM
#4
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What he ^ said. Cylinder pressure makes power. When you compress the mixture further ( higher compression ) you start out with more pressure and continue to have more cylinder pressure thoughout the combustion cycle, hence more power.
A side note. Since you're NOT injecting more air/fuel to get this increased pressure, but rather just extracting more cylinder pressure from the same mixture, your engine becomes more efficient with higher compression. I.E. more power AND better gas mileage. It's one of the few win/win mods. If our gasoline was 110 octane we'd all be running around with 14 to 1 compression
A side note. Since you're NOT injecting more air/fuel to get this increased pressure, but rather just extracting more cylinder pressure from the same mixture, your engine becomes more efficient with higher compression. I.E. more power AND better gas mileage. It's one of the few win/win mods. If our gasoline was 110 octane we'd all be running around with 14 to 1 compression
