Steam engines and electric motors make their highest torque at 0RPM. That gives zero HP as no motion involved. An ICE requires some RPM to create the airflow for the engine to run for combustion. In steam and electric, the chemical reaction known as combustion occurs outside the engine or motor. They use an independent energy source.
An ICE dyno graph of a well thought out set of components will start with a fairly high torque for the displacement at 3,000RPM and remain fairly flat until intake flow is restricting cylinder filling. As long as the torque does not drop like a rock the power curve will continue to climb for another 1,500 to 2,000RPM.
