Flywheels?
 

Can someone explain to me what a flywheel does exactly? What is a "dual mass flywheel" that Porsches use?

Hi,

The Flywheel attaches to the end of the Crankshaft and provides the physical interface which transmits the Engine's power and rotation to the Clutch and therefore Transmission. It mates with the Clutch which pushes against it using a Friction Material similar to that found in the brakes. You need a method of disengaging the transmission from the Engine in order to start it, idle, shift gears, etc. and the Clutch/Flywheel allows for this.

A conventional Flywheel consists of a single Disc Cast or Forged from steel or aluminum. It's mating Face is machined smooth in order that it mates with the friction material on the Clutch.

The engine‘s ignition-induced rotational speed irregularity causes Torsional Vibration(s) in the vehicle‘s driveline. At a given speed the ignition frequency is equal to the natural frequency of the driveline so that extremely high vibration amplitudes occur that cause transmission rattle and body boom.

This has the effect of causing the Clutch Disc to possibly skip or bounce against the mating surface of the Flywheel and also increasing wear to the various bearings, etc. in the Transmission. This also allows a great deal of vibration to pass through to the Driver and Occupants.

To minimize this in a conventional Flywheel/Clutch arrangement, the Clutch Disc contains a series of Marcel springs which act as a Harmonic Dampener reducing these vibrations. Also known as a Torsional Damper.

In a planetary Dual-Mass Flywheel, the planetary gear and the Torsional Damper are incorporated into the Flywheel. For this purpose, the Flywheel is divided into a primary and a secondary mass, hence the name Planetary Dual-Mass Flywheel.

That's because a Dual-Mass Flywheel is assembled from two pieces — an inner and outer ring — with a spring action between them. The idea is to provide a harmonic-dampener effect so Engine vibrations won't enter the Transmission and cause a gear rattle at idle or undue vibration. Dual-Mass Flywheels were first introduced on Diesel powered cars and trucks because a Diesel creates much more vibration than a Gas IC Engine does.

Therefore, the conventional Marcel springs in the clutch disc are redundant with a Dual-Mass Flywheel as the cushioning offered by the Clutch-Disc springs is already provided by the Dual-Mass Flywheel's springs.

Two goals are being met by these Marcel springs. In a conventional Clutch, the springs in the Disc cushion Clutch engagement, making it less abrupt, less On-Off. Dual-Mass Flywheels use springs for a completely different purpose — to dampen Harmonic Vibrations at idle, though they also provide the effect of cushioning Clutch engagement. When eliminating a Dual-Mass Flywheel and Clutch combination in favor of a conventional design, such as with a one-piece lightened Aluminum Flywheel, a sprung hub in the Clutch Disc will be required, so you must replace both the Flywheel and the Clutch.

On the downside, Dual-Mass Flywheels are more complex and due to the Harmonic Dampening, they cannot be refaced as a conventional one-piece Flywheel can, because a metal lathe would skip or chatter across the surface leaving it chipped and jagged rather than smooth as required. Hope this helps...

Happy Motoring!... Jim'99