# What does a flywheel do and what is it connected to?

I understand roughly how an engine works and that there is a crankshaft which is able to turn the flywheel using internal combustion.

I have just realised that I have got absolutely no idea how the kinetic (movement) energy of the crankshaft is able to make the wheels go round and also vary the speed of the wheels based on the drivers' input. I know that somewhere there is a clutch, a gearbox and a "diff" that allows the wheels to turn at different speeds (going round a corner for example).

• Do all these things run off of the flywheel?

• What exactly does the flywheel do and what is it connected to other than the crankshaft?

• How is it connected to these other things?

A flywheel serves four main purposes (in most vehicles):

• It provides mass for rotational inertia to keep the engine in motion
• It is specifically weighted to provide balance for the crankshaft
• It provides a means to get the engine started (starter ring)
• It provides a connection for power transfer between the engine and transmission (along with the clutch it also provides a means to interrupt the power flow)

Another such item, like unto a flywheel is a flexplate. This is a thin plate which connects the engine to a torque converter in automatic transmissions. While it provides the starting, connection, and balance of a flywheel, it alone does not have enough mass to provide the rotational inertia. In this case, the torque converter provides this for the engine.

• It is that last bullet point that I'm still not really sure about. Can you explain how the engine connects to the transmission and/or clutch (diagrams are always helpful)? Commented Jan 12, 2016 at 8:09
• @MaxGoodridge The last point is not the purpose of the flywheel. It is the purpose of the clutch. It just makes sense to club the two together as on unit for engineering optimisation Commented Jan 12, 2016 at 9:28
• Replace all "weight"s with "mass". "Kinetic energy" --> "rotational inertia". Commented Jan 12, 2016 at 16:03
• pics or it didnt happen
– Josh
Commented Jan 12, 2016 at 18:40
• @Paulster2 - haha, yes . My comment was meant to make the distinction between the what a flywheel is and what additional purposes it can serve. Commented Jan 14, 2016 at 11:45

This is addition to Paulster's answer. Here's an animation

The disc in Grey is the flywheel

Wikipedia provides a good general outline -

A flywheel is a rotating mechanical device that is used to store rotational energy. ...
- Providing continuous energy when the energy source is discontinuous. For example, flywheels are used in reciprocating engines because the energy source, torque from the engine, is intermittent.
- Delivering energy at rates beyond the ability of a continuous energy source. This is achieved by collecting energy in the flywheel over time and then releasing the energy quickly, at rates that exceed the abilities of the energy source. - Controlling the orientation of a mechanical system. In such applications, the angular momentum of a flywheel is purposely transferred as a torque to the attaching mechanical system when energy is transferred to or from the flywheel, thereby causing the attaching system to rotate into some desired position.

In our case , point 1 serves more to make the engine operation smooth by minimizing crankshaft acceleration/decelerations between cylinder firings when in NEUTRAL.
Point 2. is utilized in Mechanical Kinetic Energy Recovery Systems

Consequences of high MI

1. Slower Engine Response- when disconnected from the transmission. (slower for shifting gears for professional cases)
2. Is an additional rotational mass to accelerate when connected to the transmission. The specification of the flywheel depend on all sorts of things like engine design, number of cylinders (with more, the power stroke is more evenly divided), RPM of operation etc.
It also depends on use cases, eg , like in racing, it could virtually be done away with as minimal time is spent with the vehicle stationary and in between gears.

A flywheel is a rotating mechanical device that is used to store rotational energy. ... - Providing continuous energy when the energy source is discontinuous. For example, flywheels are used in reciprocating engines because the energy source, torque from the engine, is intermittent.