Flywheel

What's the function of the big fly wheel? Engine cylinders are pushed out by pressure so how do they get themselves back in again? Some of the energy in the drive stroke is used to turn the flywheel. This has angular momentum which has to be conserved so it keeps turning. This then drives the cylinder in a compression stroke. Angular momentum is angular velocity x moment of inertia instead of velocity x mass. Moment of inertia depends on where the mass is. The further the mass is from the centre the harder it is to shift and so the greater the moment of inertia. So the mass of the flywheel is a long way from the centre and the flywheel is made big to increase the moment of inertia. This increases the angular momentum available.