|
The job of the DMF is to handle low frequency spikes in torque. A lot of this occurs at low engine revs and so engine start and shutdown are some of the roughest times for a diesel engine. Bearing that in mind, it makes sense to have the clutch pedal pressed down for starting and shutdown. A lot of people do this for various reasons; reducing wear on starter, gears, box bearings and engine itself but most people do this for safety reasons anyway. TANGENT ALERT I once cranked my prelude without the clutch depressed, rather than lurch forward or jerk, the beasty just fired right up to 1500rpm instantly in 1st gear and drove off towards a wall ;) No harm done, passengers thought it was hillarious. Such a good car.
|
|
Years ago,when I worked in the motor industry,I was in the Big boss's office one day and on his wall,he had a list of the major concerns on our vehicles-I was suprised to see that the one I expected to be at the top of the list didn't even make it into the top ten!
|
|
The answer is simple. Boycot any car with a DMF fitted and add to that any car which also has a DPF fitted. Then the manufacturers wouldn't be able to sell these cars in the first place and would have to resolve the problem some other way.
I know why we need DPF's but why do we have to have DMF's too??
|
|
I think Gramar, because customers are forever demanding as little NVH (noise, vibration and harshness) as possible.
You can avoid DPF problems either by not having a diesel or, if you do have one, not using it entirely on short journeys. There seems to me no point in having a diesel if one's driving is done entirely in towns. In these situations hybrids like the Prius come into their own.
DMFs - as has been implied further up this thread, these give fewer problems on petrol engines, presumably because there is less vibration to countreract than in a diesel. (I'll stand to be corrected if one of the experts on here tels me that that's wrong.). But I expect the technology is improving and would expect less trouble on more recent engines than when DMFs first came in.
|
|
Avant,
Many thanks for a prompt answer. Currely I drive a petrol car with no DMF - It's a Suzuki Ignis Sport - it does not suffer from any NVH that a DMF would counter. Only tyre noise.
|
|
The Ignis is a useful and underrated car. It'll probably last you a long time.
|
|
My neighbour has an older Mercedes c class diesel 1998 I think I was suprised when he told me he had just had a new clutch fitted and it was a dual mass job the car has done 270k km and was changed not because it was faulty but because he is going on a long journey and he thought he had better get it changed as it had never been done.
|
|
So why are modern diesels having such problems. Either it is cheaper materials, poor design or a generic fault by manufacturers. Or is it deliberate to fleece the owners seeing as diesels are more popular than petrol.
|
|
Arise ye thread.
Now that HJ has made several comments about DMFs recently and not releasing the clutch to idle, I start to wonder how exactly I should be driving these things, now that I have thrown in the towel with Audi and trying to get some money back on a failed DMF (some other thread last year.)
Is it better to sit in traffic with the clutch in? I had heard that this puts undue pressure on the clutch plate springs.
Unfortunately, if I put the electronic handbrake on then blip the throttle to release the clutch, the ECU releases the handbrake again, as it thinks I am driving off. It is annoying to say the least. I can just about learn a new skill these days of switching off the engine on the drive before releasing the clutch, and the Audi won't let me start the engine unless the clutch is in, so those aspects are covered, but what about day to day driving?
Even changing gear on the move seems to lurch a bit, like a hard clutch dump even when I haven't done so. As said in the Audi goodwill thread, I have two other cars both on over 200,000 miles on original clutches, so I like to think I can be mechanically sympathetic, but it seems new technology is forcing me not to be.
Is this just bad design, or are the designers clever enough to screw over us careful drivers who paid very little for long term maintenance in the past?
Edited by Rob C on 30/04/2011 at 13:26
|
Is this just bad design, or are the designers clever enough to screw over us careful drivers who paid very little for long term maintenance in the past?
It's a design and engineering "Catch-22".
Modern turbo-diesels develop a lot of torque - if a traditional clutch/flywheel were used the shock loading on the transmission would be very high - which would cause premature transmission failure or demand the use of very heavy duty components, like HGVs use - but then owners would complain the clutch/gearchange is too slow and too heavy.
So the DMF evolved - but then it fails prematurely.
The engineering answer is to use a torque converter automatic with high torque diesels but that goes against environmental issues.
Perhaps more manufacturers should electronically restrict the torque whenever the clutch is being operated, as a few do in 1st gear - but this will reduce performance and increase emissions.
How do DSG transmissions fare when matched with a high-torque diesel?
|
Perhaps more manufacturers should electronically restrict the torque whenever the clutch is being operated, as a few do in 1st gear - but this will reduce performance and increase emissions.
Audi had a software frig on the 2.0 TDI 170's for a while I believe which backed of the torque on initiall pull-away, but people hated it and got the software put back to normal.
All of this said, the last 2 cars I've had have done around 60K each with no peep out of the DMF in any way. I think people are over paranoid about this, due to the old forum syndrome of , 'you dont get many people writing on a forum to say how well something is working', only the ones that have problems !
|
Now that HJ has made several comments about DMFs recently and not releasing the clutch to idle, I start to wonder how exactly I should be driving these things, now that I have thrown in the towel with Audi and trying to get some money back on a failed DMF (some other thread last year.)
That was one of the main things I liked about diesels. I can see how it might not be good because you are relying on flywheel inertia to prevent stalling. Still, it never seemed to be an issue with older TDs which I guess did not have DMFs (maybe because engines were bigger, as todays 1.6 TD is like an old 1.9TD). And what about transit vans..? You could certainly get a lot of shunt in the driveline by hammering through the gears in older TDs, also when hammering it 2nd gear came up very quickly with a seemingly huge amount of rotational inertia (you got the feeling the engine might "take off").
|
|
Be reminded that there are a number of petrol engines with DMF but less likely to fail because of lower torque-particularly in the off-idle area.
|
Be reminded that there are a number of petrol engines with DMF but less likely to fail because of lower torque-particularly in the off-idle area.
Do diesels really have higher torque from idle..? I thought torque falls off a cliff as you go from 1500 rpm to idle rpm because you are off boost. I can see they have higher stalling torque, but I always presumed that was due to higher reciprocating masses (i.e. flywheel effect). Once the turbo kicks in then of course they have much higher torque (from relatively low revs), hence the need to beef up many of the parts in the drivetrain.
|
|
The actual time averaged torque (ie the one you see on the performance chart isn't the problem. The problem is the massive torque fluctuations which occur each time a piston passes TDC. These torque fluctuations are heavily dependent on effective compression ratio. Since the effective compression ratio of a diesel is ~15 and the effective compression ratio of a petrol is ~9, the torque fluctuations in a diesel are over twice as large as that of a petrol. Actually the problem is worse than that since since diesels are unthrottled so even at low load, the gas pressure in the cylinder is always high. The time averaged torque is, in itself, fairly insignificant relative to the peaks and troughs.
|
|
So should I leave my left foot in when stationary in traffic or at lights?
|
|
When I see how many drivers park - with excessive revs mounting kerbs, or stop and start with lots of jerks, or brake , stop, and let the clutch out stalling the engine, it's a wonder any DMF survives....
|
|
No. Clutches should only be disengaged (pedal down) for short periods of time. Extended periods of clutch disengagement can cause premature wear of the throw-out bearing.
The kindest method of operation is to remember: the higher the load, the higher the RPM of the gear change. This is the pattern that automatic transmissions follow, yields the minimum stress on the engine and drivetrain and gives the most control to the driver.
This is intuitively correct and learner drivers automatically follow this pattern. Unfortunately this is often displaced by 'select-a-gear-for-the-day' laziness or a mistaken belief that lugging the engine down to 1200 RPM will always yield the best economy. 1200 RPM is great for pottering-but not for acceleration.
|
|
Thing is, I can tell when I'm stressing a vehicle with incorrect revs or poor clutch use. I can feel it through the car like any moderately accomplishe and sensitive driver, but now, according to HJ, just sitting in neutral at idle and lifting the clutch pedal so I can take my feet off the pedals is causing damage to my car?
Can this be true? The car makes no noise as I do it, the revs don't alter at all, there is no judder or cough. Its not an unreasonable thing for any motorist to do but not doing it is counter-intuitive and in my case means I have to learn a new way of juggling the elec handbrake, clutch, brake and throttle.
|
Thing is, I can tell when I'm stressing a vehicle with incorrect revs
With the greatest of respect you can't! You can tell when engine mounts and insulation are least effective and the most noise and vibration is transmitted to the cabin; driver perception doesn't necessarily corrospond to engine stress.
according to HJ, just sitting in neutral at idle and lifting the clutch pedal so I can take my feet off the pedals is causing damage to my car?
A dose of optimax should make the engine idle so smoothly that the DMF will last much longer eh, HJ? Nonsense! The torque fluctuations are minimised at idle. if it doesn't survive at idle, it won't urvive under load.
Edited by unthrottled on 23/05/2011 at 15:00
|
|
I remember my driving instructor telling me that the only time your foot goes on the clutch is to change gear.
|
I remember my driving instructor telling me that the only time your foot goes on the clutch is to change gear.
There's a generation of us grew up with BMC graphite clutch release bearings. Self-adjusting, but if you made a habit of touching the clutch when it wasn't strictly necessary you paid for it! Out of gear, handbrake on, clutch up when idling was de rigeur on all vehicles so fitted. The alternative was sub 20k clutch life.
|
|
|
TANGENT ALERT I once cranked my prelude without the clutch depressed, rather than lurch forward or jerk, the beasty just fired right up to 1500rpm instantly in 1st gear and drove off towards a wall ;) No harm done, passengers thought it was hillarious. Such a good car.
Had the same thing on my wifes Nissan Sunny, started it up from outside the car, and it was such a good starter it started and smashed into the step outside the house with moderate damage to the front valance !
|
|
|
|
