Flywheel and Governor are used to control the speed fluctuations of an internal combustion engine. But these fluctuations take place in two ways:
1) During a cycle (due to variation of torque on the crankshaft)
2) Over a no of cycle (due to the variation of load on the engine)
In first case, speed fluctuations occur due to the variation in the output torque on the crankshaft during a cycle. These fluctuations can be controlled by mounting a suitable flywheel on the crankshaft. The function of the flywheel is to act as a reservoir which will absorb energy during those periods of crank rotation when the turning moment is greater than the resisting moment. Absorption of energy is necessarily accompanied by an increase of speed and restoration of energy by a decrease of speed. The flywheel must be so proportioned that these changes of speed do not exceed the permissible limits.
So the flywheel serves to limit the inevitable fluctuations of speed during each cycle which arise from the fluctuations of turning moment on the crankshaft.
Speed fluctuations due to the fluctuations of turning moment on the crankshaft during each cycle can be controlled by mounting the suitable flywheel.
In the second case, speed fluctuations occur due to the variation of the load on the engine over a no. of cycle and require a governor to control the mean speed of the engine within the specified limit. The governor controls the mean speed of the engine (when there is a variation in load) over a no. of cycle by varying the fuel supply. The governor has no control over the cyclic fluctuations of speed. On the other hand, the flywheel has no effect on the mean speed of rotation.
If load on the engine is constant, the mean speed of the engine will be constant from cycle to cycle. But if the load changes, mean speed will also change, unless the output of the engine is adjusted to the new demand. It is the purpose of the governor to make this adjustment automatically.