I thought you N/A gurus might find this a bit interesting...the math behind compression ratio:
Let me define volume above the piston when it is down V1, and the volume above the piston when it is fully up V2.
Physical Work is defined as W= P*V = integral (P*dV), where you integrate from the start volume to end volume (V1 to V2), where P = pressure, V= volume.
Note: work is a measurement of energy, where Horse power is energy/second, so more work, generally means more power. The work of the pistons changing the volume of air is directly powering your car)

so W = integral (PdV),
and in theory we will look at only compression ratio so we will consider Temperature T, and n moles of gas to be constant in the combustion process, (which it is not).
so say we have that piston with V2 compressing to V1, to find work, we can not directly integrate that equation because P is NOT constant, not allowing us to pull it out of the integral...therefore, we substitute for P, P=nRT/V from ideal gas law, where n= number of moles of gas, T = temp, and R = 8.31 or another constant number giving us:
W = integral (nRT/V dV)
assuming that nRT are constants, we pull them out of the integral giving:
W = nRT int (1/V)dV
after integrating from V1 to V2:
W = nRT ln V2 - ln V1 (where ln = natural log)
so if you use the identity: ln x - ln y = ln (x/y);

W = nRT ln V2/V1....

as you can see, n moles of air/gas matter tremendously, thats why fuel when combusting not only releases energy in the form of heat, but also multiplies the number of moles of gas, gives so much energy, and notice that V2/V1 = compression ratio! power is directly related to the natural log of compression ratio, which is not as much as it would be if there weren't those damn natural logs there.

a little off topic, but diesel engines don't need spark plugs because they compress the gas to such a high temperature that it spontaneously combusts.

Hope you got all of that.
-homesauce

 

Can you simplify that a bit more...those letters mixed with numbers are kinda confusing ..hehe,Dam,reminds me of calculas from high school.So in other words,a diesel engine works kinda like a self detonating engine....correct???

 

ok, we're looking at the energy done by the engine;
W= work done by compression of the gas, P = pressure, V = volume, T = temperature, n= moles of gas, R = constant,
W = int (PdV)
W = int (nRT/V dV)
W = nRT int(1/V dV)
W = nRT ln(V2/V1)
and V2/V1 is compression ratio.

 

diesil engines don't need spark plugs, no, but they aren't self powered, the fuel doesn't spontaniously combust, lol. They use what's called Glo Plugs, they serve the same purpose as spark plugs, but don't emit a spark, rather they get REALLY hot, glowing hot, so when the fuel is compressed, it heats the fuel to ignite it.

 

once the temp is up to speed, you don't need the plugs anymore. The pressure is great enough with the warm engine to keep things going

 

doesnt T change w/ respect to V? thus how do u pull out T if its not a constant throughout the interval of V1 and V2?

 

heh heh

 

I wanted to just look at the compression ratio, and how it affects power, I wasn't concerned about T changing with the combustion of the gas, so notice I said, treat T as a constant, even though I know it's not...I'm not trying to sound smart, or like a hard ***...because I'm not, I just thought you guys might find it interesting how the natural log of compression ratio is directly related to power...That's why 1 point in compression ratio only increases HP by 10 or so, because it's the natural log. so take that smart *** "heh heh" outa here. One day, I'm gonna be the one designing the engine in the Honda cars that we all drive and love, so give me some respect.

 

Oh right, forgot about that, silly me