Question about dual mass flywheels...

[Mr Pies]

As I understand it, dmf's are used on most modern diesels to smooth out the power delivery from the engine because of the high torque produced.
I keep reading a lot about how a few car manufacturers are now going back to single mass flywheels due to how unreliable dmf's are proving to be.

If this is true, how are they now smoothing out the power delivery? Is it done via mapping?

[Astra Dan]

Maybe some sort of hydraulic engine mounts? I know the Vectra B had two or three of these, and I think they were pre-DMF and I've never noticed the take up being unduly rough.
Maybe they're just putting the springs in the clutch plate instead. You know, where they've always been.
There's a lot of people with petrol and diesel cars who've 'back tracked' and fitted solid flywheels from older models to eliminate DMF, might be worth a search of the forums.

[littleredrooster]

Mr Pies said:

As I understand it, dmf's are used on most modern diesels to smooth out the power delivery from the engine because of the high torque produced.

Not quite right. DMFs are used to smooth out torsional vibrations emanating from the crank. It's got nothing to do with power delivery or high torque.

A diesel engine crank almost stops-and-starts every half revolution as it encounters compression and power strokes from the pistons/rods. This can result in unwanted NVH (noise, vibration and harshness) being transmitted into the bodyshell of the car. The DMF is designed to absorb some of this vibration.

As an aside, the worst thing you can do to a DMF to shorten its life is to drive everywhere at 1200-1500rpm. This is where the vibration is at its most powerful and stresses the DMF the most.

[motco]

littleredrooster said:

As an aside, the worst thing you can do to a DMF to shorten its life is to drive everywhere at 1200-1500rpm. This is where the vibration is at its most powerful and stresses the DMF the most.

Just the way some driving instructors tell pupils to drive. Always change up at 2000 revs is the mantra. "It's green don't ya know..."

[zcacogp]

littleredrooster said:

DMFs are used to smooth out torsional vibrations emanating from the crank. It's got nothing to do with power delivery or high torque.

A diesel engine crank almost stops-and-starts every half revolution as it encounters compression and power strokes from the pistons/rods. This can result in unwanted NVH (noise, vibration and harshness) being transmitted into the bodyshell of the car. The DMF is designed to absorb some of this vibration.

As an aside, the worst thing you can do to a DMF to shorten its life is to drive everywhere at 1200-1500rpm. This is where the vibration is at its most powerful and stresses the DMF the most.

Very interesting post - thanks LRR. Never quite understood what a DMF was, or what it does; now I do!


Oli.

[McSam]

motco said:

Just the way some driving instructors tell pupils to drive. Always change up at 2000 revs is the mantra. "It's green don't ya know..."

You know there's actually a section on the test "scoring" sheet for eco driving?

Anyway, littleredrooster is exactly right. There will be a particular engine speed that a given engine will cause the biggest vibrations for the flywheel to deal with, you just have to hope no previous owners sat with it at that speed all the time! I should imagine manufacturers expected the reliability to be fine, having ensured a natural frequency that the engine shouldn't be sitting at for long periods, but they've probably been disappointed to find that the modern diesel owner doesn't really know how to drive their cars properly and so rattles the thing to death a lot sooner than expected.

[Willy Nilly]

do car diesel not have balancing shafts?

this shows how they work

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

[Astra Dan]

McSam said:

...you just have to hope no previous owners sat with it at that speed all the time! I should imagine manufacturers expected the reliability to be fine, having ensured a natural frequency that the engine shouldn't be sitting at for long periods, but they've probably been disappointed to find that the modern diesel owner doesn't really know how to drive their cars properly and so rattles the thing to death a lot sooner than expected.

On the other hand though, shouldn't you be able to get in any car and just drive it and not have to research the harmonics and resonant frequencies for an hour?

[Nick3point2]

littleredrooster said:

As an aside, the worst thing you can do to a DMF to shorten its life is to drive everywhere at 1200-1500rpm. This is where the vibration is at its most powerful and stresses the DMF the most.

That's why mine died!

[vrsmxtb]

Aren't they inherently flawed then, seeing as most torquier modern diesels seem perfectly happy trundling around at 1500rpm. I think my DMF in my Fabia is showing signs of wear, even though I try and keep revs at just over 2000rpm in the sweet spot regardless of speed.

[Apache]

Willy Nilly said:

do car diesel not have balancing shafts?

this shows how they work

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

Nice one Wills, I love these animations. Now, have you got one for an auto box?

[StottyZr]

littleredrooster said:

As an aside, the worst thing you can do to a DMF to shorten its life is to drive everywhere at 1200-1500rpm. This is where the vibration is at its most powerful and stresses the DMF the most.

I have no knowledge of DMF's, and I doubt this. I drive around below 1500rpm and the is as happy as a pig in st.

Common sense would dictate a manufacturer would make modifications to a modern diesel to stop this being the most damaging RPM's as the car is going to spend half its life in this range.

And who is it who constantly drives their VRS at over 2k :S why would you buy a diesel to drive to drive it like this?! It must gobble fuel

[Mr Pies]

Brilliant thanks. So how are they getting round reducing the torsional virbration when using a single mass flywheel?

[The Black Flash]

Willy Nilly said:

do car diesel not have balancing shafts?

this shows how they work

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

It's not about balance, it's about high instantaneous torque inputs to the crank from each power stroke. You'd still have the problem on a perfectly balanced engine. The DMF is designed to absorb these "hammer blows" and smooth it out.

[AnotherGareth]

StottyZr said:

And who is it who constantly drives their VRS at over 2k :S why would you buy a diesel to drive to drive it like this?! It must gobble fuel

Perhaps because they enjoy driving.

[motco]

The Black Flash said:

Willy Nilly said:

do car diesel not have balancing shafts?

this shows how they work

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

It's not about balance, it's about high instantaneous torque inputs to the crank from each power stroke. You'd still have the problem on a perfectly balanced engine. The DMF is designed to absorb these "hammer blows" and smooth it out.

Exactly, and a six cylinder engine of the same capacity would be better as the 'bangs' are fifty percent more frequent and thirty three percent less severe.

[StottyZr]

AnotherGareth said:

Perhaps because they enjoy driving.

:facepalm:

[littleredrooster]

StottyZr said:

littleredrooster said:

As an aside, the worst thing you can do to a DMF to shorten its life is to drive everywhere at 1200-1500rpm. This is where the vibration is at its most powerful and stresses the DMF the most.

I have no knowledge of DMF's, and I doubt this. I drive around below 1500rpm and the is as happy as a pig in st.

You can doubt it all you like, but it remains an engineering fact. The reason that it feels as if it is as happy as a pig in st is because the DMF is doing what it was designed to do. It will, however, shorten its life dramatically.

[McSam]

Astra Dan said:

On the other hand though, shouldn't you be able to get in any car and just drive it and not have to research the harmonics and resonant frequencies for an hour?

Absolutely you should, I didn't mean that the driver should have to have any clue about such things! But I meant that a manufacturer might deem 1200rpm a pretty handy natural frequency as the engine is very unlikely to sit there for long periods - well above idle, well below useful rev range, just a speed that the engine will quickly pass through in first and that's it. Except with all the silly buggers who think being in the highest gear possible all the time is economical, even if it means labouring the bejesus out of the poor engine up a steep hill in fifth, heavy load at 1200rpm is what you might end up with. I was just thinking about the dislocation between what an engineer might think is sensible, and what today's motorists will actually do to the car.

[wolf1]

Would help if they hadn't used crappy rubber bonded crank pulleys to attempt to eliminate the vibrations (which fail all the time) A decent encased crankshaft vibration damper running large slip rings in a shear fluid. Rolls Royce fitted one to The 27 litre CV12 engine fitted to challenger tanks and other applications and that used to purr like a kitty cat.