Educational: Dual Mass Flywheels

[davelowe]

Courtesy of an HonestJohn post, and youtube.

Why are they fitted:

It's the two mass bit that's important. The springing between the two parts of the flywheel effectively isolates the transmission from the torsional oscillations of the crank. The main function of the DMF is to improve noise vibration and harshness. You could say that it's one of the additional parts which has made modern diesels more acceptable in terms of refinement to people who were used to petrols.

One of the problems of engine design, particularly diesel engine design is dealing with the torsional oscillations of the crank. These vibrations can actually build up in magnitude and twist the crank so far that the stress produced by the vibration is actually damaging to the crank. Left completely unchecked, a snapped crank is the likely outcome. Typically, the stress would be highest at the change in section at the edge of the main bearings, and if there's poor detail geometry, and/or poor (tensile) residual stress in the crank surface, then it's likely that a fatigue crack will initiate there, and then grow a little during each vibration cycle until the crank snaps.

Via the DMF, the crank will "see" an effective mass, and an effective stiffness looking into the transmission. If you do away with the DMF, the mass and stiffness which the crank drives will change, and the way the crank vibrates will change.

And here is a video showing the typical make-up and why it does a useful job (if not for long):

https://www.youtube.com/watch?v=YnaXB8q3uzQ

[direstraits]

Great post. Worth mentioning that DMF is likely to get broken if moment and power of engine increase (or it can slip).