Here is a youtube video talking about diesel combustion pressure.
Gasoline engines use constant volume heat addition, meaning idealy your fuel is to be burned, all your heat added when the piston reaches top dead center. Hence the volume of your working fluid remains constant as heat is added.
Since this can not happen for a number of reasons we use spark advance.
A diesel engine uses constant pressure heat addition. Which means as the volume of the working fluid changes the pressure remains during heat addition. The fuel usually does not ignite in a diesel engine until right around TDC, the fuel injects, lights off and the constant pressure of fuel being injected/burned ATDC drives the piston down the bore.
The constant pressure thing is what limits the diesel engines speed and at the same time gives the diesel its legendary efficiency.
Well that video wont load for me with my slow internet, but I believe they say that pressure peaks some where between 10* and 20* ATDC.