What Is The Purpose Of Flywheel In Engines?

The main purpose of flywheel is to smooth the flow of energy between a power source and its load. In four stroke engine, the engine delivers energy mainly during power stoke. So, in one complete cycle there is lot of variation in output energy which results in non uniform speed of the output. Assuming no friction losses governing equation of flywheel is as

Change in KE  =  C . I . Square of average speed

where C is coefficient of speed fluctuation.

So more the inertia of flywheel less is the fluctuation in speed. But more increase in inertia decreases output power. So, it is designed for required smooth performance.

A Manuel Shift Vehicle has a flywheel bolted to the crankshaft at the rear of the engine where the transmission is bolted to the engine.It is pretty heavy about a inch and a half thick.It has a ring gear tach welded to it that gear is what the pinion gear in the starter engages with when you turn the key to start your Vehicle.As you turn the key the pinion engages  the  flywheel and with any luck or good care for your car  the flywheel spins and starts a whole bunch of things we Wont get into right now..Flywheels are pretty tough sometimes you can just turn the ring gear around resurface the face of the flywheel this is a way of saving yourself a lot of money. But that will depend on the way the car was driven. If your car is a automatic it has a flexplate not a flywheel big difference but the flex plate is usually replaced as a unit but the gear is replaceable. Owell thats the best I could do  hope it answers your ?

This site has a number of explanations that should help you form a complete picture of the value of a flywheel in an engine:

https://www.quora.com/What-is-the-purpose-of-a-flywheel-in-an-automobile-engine