It is a combination of all these factors that cause the combustion to occur, but pressure has a much greater effect on the running of these engines.
Remember from Chemistry (van der Waals equation),
(Pobs + a(n/V)^2) * (V-nb) = nRT
or,
PV=nRT to put it simple
The van der Waals equation is what is needed to understand what is happening in our engines. PV= nRT is not complex enough to correctly deal with the atms. produced in the combustion of our engines.
All this equation says (for non chemistry people) is:
The more you squeeze something, the hotter it becomes. Thus said, pressure causes excited atomic molecules which cause an increase in heat. With that in mind, if you inject a flammable fluid into an environment that is much hotter than the flash point of that fluid, the chemical properties of that fluid will change. The increase in pressure and change of environment give the compound, in this case the hydrocarbon cetane, energy required to push the atoms beyond their activated complex, or transition state.
Also, pressure affects the Kinetics of the chemical reactions. Typically, with increased pressure, the rate of chemical change increases, especially with the combustion of a hydrocarbon. In the process of the atoms realigning them selves to an alignment at a lower point in the electron energy well which produces some different chemical compound. In the process releasing a large amount of energy in the form of heat. The harnessed energy drives our engines. Also, it is the theories of Kinetics that allows man to separate crude oil into multiple grades of fuels. ie: the process of cracking.
This is what we think of when the C16H34 (Cetane) and O2 (Oxygen) produce CO2 and H2O
2 C16H34 + 49 O2 <=> 32 CO2 + 34 H2O
Emission standards are ever tougher because no chemical reaction is 100% complete, that is to say, not all the molecules will turn into CO2 and water. Though a chemical reaction occurs, the molecular orientation during these collisions is not always successful.
The incomplete burn, or bad emissions, can be calculated from the Steric factor, k = zpe^-(Ea/RT) and the frequency of this incomplete burn occurring is figured by the Arrhenius equation
ln(k) = -((Ea/R)T^-1) + ln (A)
That is why our engines have emissions.
This is a very long way to say that:
Pressure causes heat, which causes combustion and why engines produce other emissions.
Just my $0. 03
-Rich
P. S. I believe that my chemistry equations are correct, it has been a long time since I have been in a chem. lab, please check
P. P. S. van der Waals is one of many derivatives of this equation, this one is for extreme atmosphere correction with temperature.
correction with temperature.
Attention: TDR Forum Junkies 
Been thinking. Did some research on the 12 v verses the 24v. The 12 v has a compression ratio of 17. 5-1 and the 24 v has only a compression of 16. 5-1. Seems to make since to remove the head and have it to shaved to equal a compression ratio of 17. 5- or 18-1 and then install the oring head gasket with the metal to withstand the extra compression. The benefit would be more torque and better fuel mileage without of all of the mods I read about at a great cost. This would not need the extra fuel added or air added with boost. Yes this would void warranty but many are already their own warranty stations already. ANY FEEDBACK????? 
) is trying to run higher mechanical compression ratios with higher boost pressures. As either is increased, compression and peak firing pressures will increase as well. At some point, the design criteria used by the engine designers will be exceeded. Therefore, to get the engine to live, the rule of thumb is that as boost goes up, mechanical compression ratio should go down. If mechanical compression ratio goes down too much, however, the engine won't start when cold or at high altitudes - it can't build enough heat through compression to ignite the fuel. 
