taken from honest john forum

The flywheel is effectively a weight which is fastened to the end of the crankshaft of the engine. The power from the pistons tends to be created in “pulses” and the weight of the flywheel smoothes out these pulses by providing inertia to the rotating engine. As well as providing a weight the flywheel has a gear around its circumference on which the starter motor operates and is a convenient means of attaching the clutch which provides a variable connection to the transmission.

Modern diesel engines generate high torque and as a result they need extra smoothing out or “damping”. To help with this process a DMF (Dual Mass Flywheel) is fitted. This is effectively two flywheels that transmit the drive through a number of springs which cushion the drive to the transmission