You are correct I incorrectly called it the power stroke,it is the combustion cycle, however in the ideal (ideal in the since that it is completly constant pressure) Diesel cycle the combustion starts at or after TDC, it cannot be constant pressure if the volume is decreasing. In actual fact no real engine meets the ideal. Nor is the ideal the most efficient. In actual fact in a real Diesel engine the combustion starts before TDC and stops after. In older engines the injector started later and stopped sooner as the throttle simply limited the injector piston stroke. In modern engines the injectors are computer controlled, the timing at idle is set to reduce the Diesel rattle and set for max efficiency above idle.