One of the functions of the flywheel, along with all the other moving components of the engine (crank, connecting rods, pistons) is to store enough energy to be able to compress the gas charge as required to enable the next piston that is to fire to get to the top dead center position. If there is inadequate moving inertia, so that too little kinetic energy is stored, the engine will be unable to compress the charge and consequently cannot run. This is particularly a problem in engines with few cylinders operating at low speed. In this event, then engine simply stalls.
The sizing of the flywheel also affects the torsional vibration characteristics of the entire drive train.