Well, the engine is actually the turbine that expands the steam to produce power. The boiler burns the fuel and generates the high pressure working fluid — akin to combustion of fuel and air in the ICE cylinder. The expansion in the Rankine cycle is done in a separate machine (the turbine) while in the Diesel cycle the combustion and expansion are in the same location.
Right. So it’s a bit unfair to equate it with that ship diesel that can drive a screw. Amazing machines, both, in any case!