I'm trying to understand the difference between actual and theoretical indicator diagram for diesel engines. On a theoretical diagram, the compression stroke is a smooth curve due to adiabatic compression. However on the actual indicator diagram it sharply increases in gradient before the fuel is injected. This would make sense if there was a spark, but I know that there is no ignition until the end of the stroke when the fuel is injected. Could anyone explain the difference between the two curves?
Thank you