text: what is the mechanical efficiency of diesel and petrol engine.?   the theoretical maximum efficiency of any heat engine (petrol, diesel, jet, rocket, steam, stirling etc.) is (input temperature minus output temperature)/ output temperature where these are absolute temperatures; that is celsius plus 273 degrees.   input temperature is the hot part of the engine (the cylinder) and output temperature is the exhaust manifold temperature.   so the most efficient petrol engines have efficiencies of about thirty something percent.