Sorry to be a bit dim. It seems that almost every day on the forum there is some reference to dual mass flywheels and their problems.
Now it seems that I possess one of these and so far it has remained in good health, I'm delighted to say.
However, out of curiosity, could I just ask a mechanically learned member to explain why they are fitted in favour of a standard flywheel and what advantages they are supposed to confer. Also, how do they physically differ, and finally why are they more prone to failure?
Thanks in advance, looking forward to learning something.
Best Rgds
SS
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The flywheel is effectively a weight which is fastened to the end of the crankshaft of the engine. The power from the pistons tends to be created in “pulses” and the weight of the flywheel smoothes out these pulses by providing inertia to the rotating engine. As well as providing a weight the flywheel has a gear around its circumference on which the starter motor operates and is a convenient means of attaching the clutch which provides a variable connection to the transmission.
Modern diesel engines generate high torque and as a result they need extra smoothing out or “damping”. To help with this process a DMF (Dual Mass Flywheel) is fitted. This is effectively two flywheels that transmit the drive through a number of springs which cushion the drive to the transmission
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Did the old Yankee V8's have these then talking of torque?
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Didn't need them as the V8 has a much smoother torque delivery with twice as many firing impulses per revolution.
JS
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Most US V8s had an autobox on the back; so they dispensed with the flywheel completely in favour of a bit of tin.
The archive has a good desciption of a DMF - it's too long an explanation to write out again.
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Thanks everyone. I knew what a "normal" flywheel does but have to admit I had no concept of what a "DMF" might look like or indeed why they are used. ( See... into jargon now......."DMF" just trips off the keyboard ! )
Thanks for pointer to archive SL. Does anyone remember the thread title or approx date please ? I'd like to read that sometime.
Thanks again,
SS
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Beat me to it Number Cruncher - Thanks !
SS
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Thanks movilogo - I begin to understand why they are more fragile.
Last edited by: shoespy on Tue 25 Mar 08 at 21:17
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>>Modern diesel engines generate high torque
It's the frequency domain view of a dual mass flywheel as a mechanical low pass filter that is important here. It isn't the level of torque output of modern diesels that's the problem - at least not in a direct sense, it's actually how suddenly this torque is built up during a cylinder firing event.
The sudden pressure rise in the cylinder, and hence the sudden change in torque at the crank means that there is a lot more torsional vibration content spread across a broader range of frequencies. Mathematically, this translation between the time and frequency domains is expressed by the Fourier transform - without delving into the maths, you can say that something sudden and rapid in the time domain is spread out across a broad range of frequencies.
The dual mass flywheel allows the low frequency part of the torque [the very low frequency component of torque is actually the part of the spectrum which drives the car along!] to pass to the driveline, while it prevents the high frequency noise and vibration passing into the driveline, preventing it from being transmitted to the car.
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Triple mass ones are beginning to appear(supposed to be more relable).
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Amazing animation. If a picture paints 1,000 words, then a video --- just says it all!
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"Amazing animation"
Absolutely. Loved the translation of what it does to a moving trace on a graph at the end, too. Very elegant.
Seems a lot of effort to make diesels more acceptable, though! :-)
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Diesels were perfectly all right without this development. All that expense and potential fragility just to improve refinement a bit... Typical modern automotive decadence, like headlamp bulbs you can't replace without taking the engine out or wipers that turn themselves on and off in case the driver is too blind and deaf to notice when it's raining.
The fact that you can get ordinary flywheels to replace DM ones for some cars says it all.
Tchah!
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>>Tchah!
It would be a bold manufacturer who decided to try to sell an unrefined diesel in today's market.
>>The fact that you can get ordinary flywheels to replace DM ones for some cars says it all.
But, they ruin the NVH design. It's a bit like those who replace the fibre camshaft gear with steel of the Ford Essex engines that they ruin. They sound really awful!
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Is the difference in refinement really that marked, NC?
Non-turbo pre-cr diesels could be acceptably refined, and I imagine turbos would improve refinement rather than degrading it.
I don't know why people replace the fibre camshaft gear - do you mean distributor drive? - of the Essex engine, but probably for strength or reliability. Perhaps it makes a nastier noise, but no one would mind that if they thought the engine was going to stay together after being tweaked, would they?
I'm afraid the modern motorist expects RR refinement at Ford prices. Can't be done reliably and cheaply.
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>>probably for strength or reliability.
Yes, that's right, but they still sound nasty! It's the main camshaft drive gear - the learge fibre camshaft gear mates directly with the gear on the nose of the crank - there's no belt or chain on these engines.
>>I'm afraid the modern motorist expects RR refinement at Ford prices. Can't be done reliably and cheaply.
Yes, but isn't it amazing what can be done reliably and cheaply? The cost is more design analysis effort up front, to optimise things like component masses, and attachement stiffnesses, and acoustic coupling, but then, the payback is a refined design that works without the required addition of NVH treatments, damping materials, tuned absorbers, and, yes, perhaps, dual mass flywheels.
Although I can see that you aren't a great fan of NVH Lud, but I think it's actually one of the few ways that you can tell any modern cars apart - aside from the truly tedious soft touch, fit and finish obsessive dashboard plastics afficianados. They all work with exceptional reliability, with at least acceptable efficiency, and cost very littlle.
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What do you mean? I like hushed silence, rocket-like acceleration and great economy as much as the next driver... But I am not now and never have been either a subsidised business motorist or rich enough not to care about expensive surprises.
Yes, modern cars are a great improvement on those of just a couple of decades ago. My own Ford chavmobile, now about 15, included.
I was very impressed by a Citroen C4 I drove a year or so back: silent, rapid, comfortable and a diesel. The thing that worried me about it (or would if I had the money to buy one) wasn't the DMF that it probably has, or the CR technology with its foibles, but the LCD dashboard. I am a simple fellow and worry first about what I can see and hear.
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Didn't know the Essex V6 had a shaft-driven camshaft. Fibre gear not sensible in that role, a bit Essex I am tempted to say. Failure must often have resulted in bent valves, holed pistons and so on.
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There were a few failures, but not IIRC of epidemic proportions. Fibre was, in my opinion, the right material choice - the steel gears really are unacceptably noisy.
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>> It would be a bold manufacturer who decided to try to sell an unrefined diesel
>> in today's market.
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I had the van MOT'd a couple of weeks ago and was leant an old Vectra (as a 'courtesy' car). It was worn beyond belief, but to be perfectly honest it was very fit for purpose. It was as I say basically knackered, but it was not noisy, it steered straight, the brakes worked, it went very well and it was an Isuzu 1.7 lump. I am led to believe that these are quite 'rough' compared to today's offerings, but I have travelled in newer vehicles that were no better overall. There is a market out there for a reliable work horse.
MD
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>>There is a market out there for a reliable work horse.
I don't doubt that at all Martin, I think you're quite right. However, the gulf between the existence of a market, and there being a sufficient case for tooling up to make the vehicle, which must be sold in large numbers, because there would only be a tiny margin in each unit, is wider than most manufacturers are prepared to jump (in the UK at least, where for numerous reasons, the market is rather distorted)
For my part, although I frequently side with the defence of, explanation of, even apologist for high technology, I really prefer a simple solution. One of the [all too rare] delights I enjoy is seeing a simple mechanism or solution performing a task adequately.
For example, I think the door "hinges" on 2CVs are among the more elegant expressions of this design minimalism, while I find the soft closing tailgate on my W124 estate anathema.
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>> Typical modern automotive decadence
The Mercedes W124 was fitted with a dual mass flywheel. At least those few that left the factory with a manual transmission were. This included the petrol models as well as the diesels.
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