Could you explain dual mass flywheels to me?

[Ari]

There was an interesting post a little while ago about swapping a dual mass flywheel for a single mass, and replies suggesting this was a bad idea.

I'd heard the term before but have never understood what it is or why the need. All I know is that they're typically fitted to diesels and they're expensive.

So what do they do, and how (if at all) should one modify their driving style to prolong their life?

[BorkFactor]

As far as I know, they are there to stop vibrations from the engine being felt at low revs. They are fitted to most modern cars I believe, my petrol Mondeo certainly had one.

Basically don't potter about using as little revs and in as high a gear as possible at low speeds - 5th gear at 30 mph up a hill will probably not be good for it.

Hopefully someone with a bit more technical knowledge will be along soon!

[Munter]

This should cover some of your questions.

http://www.youtube.com/watch?v=DbvP5EvpUbA

[Old Merc]

This is not engineers tech` speak?? As said its main purpose is to dampen and absorb vibrations,the same principle with crankshaft damper pulleys,not solid,"springy" with give in it.There was a long running topic here on DMF`s,a lot have problems,are very expensive to replace so why have them? fit solid kits??. My thinking is simple,the engine is designed around a DMF,by fitting a solid flywheel you have taken away something that "gives",so something else has to "give".I`ve heard horror stories about broken crankshafts,cracked bell housings,vibrations, on engines that have had the DMF replaced with a solid kit.

[Ari]

Certainly no intention of changing it for a solid one, just keen to know how best to treat it to get long life from it, bearing in mind the stories of expensive repair.

[2woody]

That's kinda right. It's best thought of as a way of splitting uip the necessary flywheel weight to provide some engine effect when necessary and some transmission effect when necessary.

OK - so how it works is to put some of the flywheel weight into a separate ring which is allowed to move a small distance in rotation against a spring. The clutch is bolted to this separate ring.

With the car in neutral, the engine sees the entire flywheel and all of the clutch ans gearbox input shaft as the flywheel weight. This means an ultra-stable idle with attending very good noise isolation characteristics.

With the car in gear, something very different is happening. The outer mass of the flywheel, the clutch and all of the geartrain are on the transmission side of the dual-mass spring element, whilst the engine sees only the inner mass. The result is that the transmission is very much less noisy and the engine can pick up speed more quickly.

In a single-mass vehicle, the clutch needs a sprung hub to separate the engine inertia from the drivetrain. You can also hear a "zizzle" noise from the transmission at very low revs ( the gears chattering) which is removed in a dual-mass set-up.

[Magic919]

2woody said:

With the car in gear, something very different is happening. The outer mass of the flywheel, the clutch and all of the geartrain are on the transmission side of the dual-mass spring element, whilst the engine sees only the inner mass. The result is that the transmission is very much less noisy and the engine can pick up speed more quickly.

This bit sounds impossible to me. How can the engine only 'see' part of the mass? The rest of the DMF is still coupled, albeit via springs and forms part of the rotational mass.

[SuperchargedVR6]

The way I understand DMFs is:-

Torque reaction reduction - Modern diesels and petrol turbos can kick out some very strong torque very abruptly and the DMF absorbs some of the initial 'shunt' on the crank shaft, if my non scientific blurb makes sense!

Nicer clutch pedal - SMFs can have quite an abrupt bite on the pedal, making the car easier to stall. DMFs again absorb some of the intial 'shock' of the clutch plates coming together, which creates that nice spongey and vague pedal feel people seem to like these days.

I could be wrong, but they are the benefits I can 'feel' them provide.

[maniac0796]

When you're engine is running, it isn't a smooth motion, especially a 4 cylinder diesel. This can be easily seen through the scope pattern of a crank sensor. As a cylinder fires, the waves will get closer together as the crank spins that bit faster and spread out again as it loses it's inertia. Coincedently, this is how an EMS spots a misfire, because the waves stay constant rather than contract and expand.

So we can translate this to the crank pulley and flywheel. These objects speed up and slow down along with the crank, as they're bolted to it. Early diesels used rubber mounted crank pulleys. This stopped excessive belt oscilation (watch an auxilary belt tensioner in slow motion) and acted a bit on the crank. A crank pulley isn't very big or heavy though, so it's force doesn't have much impact on the engine.

A flywheel however, is very heavy. It's also very wide, meaning it carrys and lot of rotation mass and torque. So when this starts oscilating with the crank, it creates a much larger resonation. Of course, you're thinking that flywheels are there to help an engine keep it's rotation and smoothness. In a nice smooth petrol engine, yes, but they don't have to deal with these massive oscilations.

So a dual mass flywheel, which is basically 2 flywheels which are attached by springs, helps by using the spring travel to damped those oscilations to the drivetrain. As the engine side spins, that quick accerlation of the pistion is taken up by the springs, and transfered to the gearbox side. This means the gearbox side spins a lot more smoothly, helping to keep the drivetrain intact and stop unwanted vibrations and resonations.

I believe it was originally invented by BMW and Valeo in the 80's, so it's hardly new tech. I think E30 318is' came with them.

This is my own interpretaion of how it works. I'm happy to learn more though.

[cmsapms]

So, what goes wrong with DMFs then? Broken springs? If the complete unit is so costly, why not replace the broken bit? Or is this another item whose obsolescence is built in merely to fleece the unsuspecting motorist?

My diseasel doesn't have a DMF and I don't notice any rattling/roughness/vibration. As it's a Defender TDi, it's impossible to seperate out all the different vibrations/rattles/roughness A lesson to be learnt there...

[Magic919]

I've heard of the rubber bits going, but I guess springs could break. I expect most want to minimise down-time, so it's easier to swap the unit out. The labour charges often exceed the parts, though this varies from car to car.

[Old Merc]

cmsapms said:

So, what goes wrong with DMFs then? Broken springs? If the complete unit is so costly, why not replace the broken bit? Or is this another item whose obsolescence is built in merely to fleece the unsuspecting motorist?

My diseasel doesn't have a DMF and I don't notice any rattling/roughness/vibration. As it's a Defender TDi, it's impossible to seperate out all the different vibrations/rattles/roughness A lesson to be learnt there...

In most cases faulty DMF`s just rattle and knock but I have seen cases where they blow up!! brake apart and damage the bell housing!! Our workshop "truck" is a 1994 Peugeot Diesel with 170K,transmission does not vibrate or judder and had its first clutch a few years ago.I`ve lost count how many DMF`s we have replaced on Peugeot 307`s & 407`s.Like the topic a while ago we will argue for ever on weather DMF`s are worth having.