This section describes a model for a non-supercharged piston engine with a variable pitch propeller.
Piston engine manufacturers provide charts of engine power output for different flight conditions. To use the information in these charts in the flight simulator, data fitting techniques can yield closed form equations for the engine performance . Unfortunately, the charts give engine performance as a function of instrument readings (tachometer, manifold absolute pressure), while a flight simulator must calculate the engine performance as a function of the engine controls (throttle, propeller pitch, and mixture).
Thus, the engine speed and the manifold pressure appear as intermediate variables in the calculations of engine power. In fact, the simulator introduces manifold pressure and engine speed is state variables of the engine (which are numerically integrated in real time, like the airplane state variables).
Given the manifold pressure, engine speed, and outside air density, one can calculate the shaft power delivered by the engine.