Actually, on a diesel, the air is compressed by itself; the air hasn't been mixed with anything unless you have a propane injection setup installed, or have a bad turbo seal and have oil getting into the intake
. The fuel is introduced into the system, through the injector that actually sticks into the combustion chamber (or, in our case, the pre-combustion chamber that also houses the part of the glow plug that actually heats up), at the point in the cycle where you want the cylinder to "fire". Theoretically, at the point that the fuel's injected into the combustion chamber, the air will be hot enough to ignite the fuel right then and there. Actually, Rudolf Diesel's original concept was a "slow-burn" where the fuel was slowly injected into the cylinder beginning at TDC and ending right before the exhaust valve opened. This didn't work very well, and they switched to the brief fine-mist injection of fuel that they use today, shortly after he died.