what does dual mass flywheel do?

[sideways500]

Hi All,
I understand the concept of a flywheel, but what is the dual mass bit for? How are they constructed and what are the benefits?

Thanks.

[russell_ram]

A dual mass flywheel isolates the gearbox input from the torsional speed variations seen on the rear of the engine crank. The engine speeds up and slows down (around it's average indicated rpm) as each piston decelerates coming up to firing (due to compression) and then fires and accerelates away down the bore again. On a deisel these speed fluctuations are much larger due to the increased compression ratio and cylinder BMEP.

The dual mass flywheel is a 'tuned absorber' formed by effectively decoupling the rear of the crank from the flywheel by use of torsional springs. This 'tuned absorber' attentuates (decreases) the level of (input to output) vibration once you get above it's tuned frequency. On a DMF this usually around 5-600rpm. So, the level of torsional vibration out of the back of the crank is reduced at normal engine operating speeds compared to what would be there without a DMF.

Below its tuned frequency a DMF will amplify vibration which is why, in some applications, you can experience severe gear rattle on switch off as engine revs drop through the tuned frequency to zero.

This benefits both NVH (reduced gear rattle at idle, reduced driveline torsional vibration etc etc) and also durability (gearbox manufacturers normally have a max level of allowable TV into the front of their gearbox).

Historically only used on Diesels but now also on high end petrols and also in autos (particularly new ZFs).

Another interesting 'feature' is the high undesirability of running at below the tuned frequency of the DMF due to TV amplification as explained above. Anyone who has experienced the 'stop dead' stall characteristics of the newer Ford diesels is experiencing Ford not allowing you to do this by employing fuel cut-off to stop the engine running as you drop down close to the speed where the DMF goes into resonance.

Does that help?

Russ

[sideways500]

Very clear and concise explanation. Thanks very much.

[petclub]

In case you're looking for a replacement, I've just had a clutch fail on my MG ZS, a replacement is about £400 (fitted). However, on the Rover 75 type they can be dual mass and these are more like £800.