In an internal combustion engine, the engine speed is determined by the rate of combustion of the fuel-air mixture. As the fuel-air mixture burns, it expands rapidly and exerts a force on the piston. This force is then transmitted to the crankshaft, causing it to rotate. The engine speed is directly proportional to the rate of combustion, and thus, to the amount of power produced by the engine.
Engine speed can be adjusted by controlling the amount of fuel injected into the cylinder, or in the case of a naturally aspirated engine, how much is drawn in. At a higher engine speed, more fuel and air are needed to maintain that engine speed, which also creates a higher amount of work. Higher rotational speeds are therefore more fuel efficient, but at a point the engine will reach its maximum engine speed.
When the engine reaches a certain speed, the inertia of the piston assembly and the resistance of the components such as the valvetrain and oil pump will become too great for the crankshaft to overcome. At this point, the engine will either seize or experience a significant drop in power.
