|
Why are dual mass flywheels necessary on some cars?
Edited by L'escargot on 14/11/2009 at 08:05
|
It's necessary to smooth out the power delivery of small capacity high torque producing engines without compromising drivability. Mostly it's only 4 cylinder engines that are so equipped because 6 and 8 cylinder engines have their own internal balance which is excellent in the case of a V8 and almost perfect in the case of the Inline-6. A V6 needs balancer shafts to balance but it's harmonics do balance so a dual mass flywheel shouldn't be needed.
In a 4 cylinder engine you have a lot of balance issues because of the vibrations created. Horsepower is a function of Torque and RPM - when we have an engine with lots of Torque at low speed the feeling we enjoy as drivers is having a lot of power low down the rev range. If you think of this as a "big bang being delivered slowly" then you have a very different situation to where the same engine is revving higher - a "smaller bang but delivered much faster". An engine that has a small enough flywheel to rev and slow quickly will have enough mass to damp out the "smaller but faster" vibrations but won't smooth out the "big and slow" ones. In order to smooth out the lower rev range a huge flywheel would be needed in comparison but this would prevent the engine revving - much like a truck engine. A dual mass flywheel allows a compromise of both, being essentially able to switch between the two modes of operation an maintain NVH characteristics that allow the car to be saleable outside the commerical market - think old Transit Engine or Perkins Prima in a Maestro vs the smoothness of the BMW 2.0 TD today.
|
With a 4 cyl engine the whole reciprocating mass (all pistons and conrods) stop and start at the same time, two at the top of the stroke and two at the bottom with one firing at the same time. The causes an inherent vibration that is not as apparent in 5 cyl + engines. A diesel engine has a high compression ratio which emphasises this and produces, as said, a bigger punch per stroke at low revs than a petrol engine of the same capacity.
A DMF helps to counter these characteristics.
CR diesel engines are able to vary the injection timing to the extent that these vibes are countered to a large extent. Ford have refined this in the new Fiesta to the point that they dont need to fit a DMF.
However a DMF is fitted to some petrol engines and not just 4 cyl, the Focus ST has a DMF so I guess Volvos with versions of the 5 cyl engine do likewise. In the Focus ST I guess a DMF is fitted because it produces TD like torque at very low revs.
|
A diesel engine has a high compression ratio ......... which produces ..........a bigger punch per stroke at low revs than a petrol engine of the same capacity.
Since you brought that up, don't forget that diesels have higher overall gearing so the torque at the driving wheels is a lower proportion of the engine output torque than it is with a petrol engine!
Edited by L'escargot on 14/11/2009 at 10:27
|
|
|
However a DMF is fitted to some petrol engines and not just 4 cyl the Focus ST has a DMF so I guess Volvos with versions of the 5 cyl engine do likewise. In the Focus ST I guess a DMF is fitted because it produces TD like torque at very low revs.
It's not unusual for Volvo T5 owners to replace the clutch kit with that of the diesel D5 model when required, especially when tuning the T5 engine.
|
>>new Centrifugal Pendulum Absorbers
They are a very old idea - very clever and very effective too.
|
|
|
|
>>stop and start at the same time, two at the top of the stroke and two at the bottom with one firing at the same time.
This doesn't dominate torque fluctuation, and is not the reason for DMF.
|
>>stop and start at the same time two at the top of the stroke and two at the bottom with one firing at the same time. This doesn't dominate torque fluctuation and is not the reason for DMF.
It does dominate torque fluctuation in that one power stroke is fully expended before another fires, hence the cross plane crank useage (aka Yamaha R1) though that is another subject we have already discussed at length.
|
>>It does dominate torque fluctuation
If you're seriously trying to say that the pistons stopping and starting is the reason why DMFs are fitted rather than the sudden peak in torque from the diesel's combustion process, then, you're mistaken.
If your view were correct, all 4cylinder engines would have been fitted with them, not just those that produce high torque peaks from their combustion processes.
Edited by Number_Cruncher on 14/11/2009 at 17:16
|
>>It does dominate torque fluctuation If you're seriously trying to say that the pistons stopping and starting is the reason why DMFs are fitted rather than the sudden peak in torque from the diesel's combustion process then you're mistaken. If your view were correct all 4cylinder engines would have been fitted with them not just those that produce high torque peaks from their combustion processes.
NC I said "It does dominate torque fluctuation in that one power stroke is fully expended before another fires" and on a TD each cyl, as you say, produces high torque as they fire.
A DMF is less necessary on a 6cyl etc because the torque generated by the firing stroke on one cyl in damped by the relative position of other pistons.
Edited by cheddar on 14/11/2009 at 17:27
|
In order to help you guys to slug it out, here is a technical paper from one of the guys responsible for the development of the modern DMF, explaining why and how it was developed. In German I'm afraid, because that's where the DMF was developed (by LuK, now part of the Schaeffler empire).
tinyurl.com/yj6rv9f
|
|
|
>>damped by the relative position of other pistons.
Forget the position of other pistons - it's a red herring.
Once you increase the number of cylinders, torque pulses begin to overlap, and so the high frequency variation in torque reduces.
|
>>damped by the relative position of other pistons. Forget the position of other pistons - it's a red herring.
Not a red herring.
Once you increase the number of cylinders torque pulses begin to overlap and so the high frequency variation in torque reduces.
Exactly so not a red herring.
|
>>Exactly
That is not what you were saying.
|
>>Exactly That is not what you were saying.
NC, sometimes trying to prove others wrong seems just as important to you as answering the question.
From the way that you explain it I beleive that we are talking about the same thing. We had this with cross plane cranks, it turned out to be a matter of terminology.
|
>>NC, sometimes trying to prove others wrong seems just as important to you as answering the question.
The pot calls the kettle black.
>>We had this with cross plane cranks, it turned out to be a matter of terminology.
No, we are talking about completey seperate things - the situation in a car engine and drivetrain means that the argument which supports the use use of cross plane cranks in bikes is simply inapplicable.
The starting and stopping of pistons simply isn't a big issue in this case - the torque fluctuations from the combustion is dominant.
|
No we are talking about completey seperate things - the situation in a car engine and drivetrain means that the argument which supports the use use of cross plane cranks in bikes is simply inapplicable.
>>
That's not the point I was making rather when discussing cross plane cranks we argued from different perspectives and seemed in the end to mean the same thing.
The starting and stopping of pistons simply isn't a big issue in this case - the torque fluctuations from the combustion is dominant. >>
NC, I repeat I said "in that one power stroke is fully expended before another fires" that is the point, with 5 or more cyls that is not the case.
|
|
You've bored me into submission - whatever you say Cheddar.
|
You've bored me into submission - >>
The pot calls the kettle black.
|
|
You bored the rest of us ages ago!!!!!!!!!!!!!!!
Edited by jc2 on 15/11/2009 at 19:06
|
|
|
|
|
>>It's necessary to smooth out the power delivery of small capacity high torque producing engines without compromising drivability. Mostly it's only 4 cylinder engines that are so equipped because 6 and 8 cylinder engines have their own internal balance which is excellent in the case of a V8 and almost perfect in the case of the Inline-6. A V6 needs balancer shafts to balance but it's harmonics do balance so a dual mass flywheel shouldn't be needed.
You're confusing balance with torque fluctuation. They are not related.
|
|
|
|
>>Why are dual mass flywheels necessary on some cars?
I think necessary is an odd way to phrase the question.
DMFs are not necessary in the same way that flexible engine mountings, acoustic insulation material, carpets, cushioned seats, and compliant suspension mountings are not necessary. The car will run perfectly well without them.
At a time when most cars are quite reliable, quite smart, quite well fitted out internally, levels of noise and vibration are one thing that may allow customers to differentiate between cars, and it would be a bold manufacturer who did not avail themselves of this technology while their competitors were making good use of it.
|
>>>>>it would be a bold manufacturer who did not avail themselves ofthis technology while their competitors were making good use of it.
It will be an even more bold manufacturer who discards all this "must have" technology in favour of simplicity and longevity for simple convenient transport for the masses.
------------------K--I--S--S--------------------
|
If you read this forum too long you will become convinced that every DMF will fail every 20,000 miles, every cambelt will snap, every modern diesel will eat its own engine etc.
DMF's are fitted to loads of cars - only a very few and in most cases on a few particular models give any trouble. There are a lot of cars which have them which people are not aware of because they never go wrong.
In the case of the Volvo mentioned I've seen several Volvo DMF's at 250,000 which are perfect and are retained at a clutch change.
To be honest, far more early clutch removals are necessitated by leaking slave cylinders than DMF failures.
|
To be honest far more early clutch removals are necessitated by leaking slave cylinders than DMF failures.
That was my personal experience with the Scenic when the concentric slave cylinder literally exploded at 62,000 miles. The DMF was "in perfect condition" according to the mechanic, and was left in situ while a new clutch kit was fitted. The car was just driven normally with no thought given to DMF welfare. Properly designed, they seem to be OK, and you hardly ever hear of Renault DMFs failing.
In our case it was the concentric slave cylinder design which turned what used to be a fairly quick and painless job into into major, expensive surgery. The traditional slave cylinders could be changed with the transmission in situ, but the concentric ones need the gearbox to be removed. This then brings the dilemma of having a clutch kit installed while it's all out (which we did), and the cost skyrockets.
Edited by DP on 15/11/2009 at 08:34
|
|
|
|
Dual Mass Flywheels first appeared around 1985 at a time when cars where generally getting quieter due to less wind, engine and suspension noise. Also, higher gearing and better aerodynamics meant that engines were starting to run at lower rpm than before so drivetrain resonances were becoming more noticeable.
The principle of a DMF is simple. It moves the resonant frequency of the drivetrain to below the idle speed of the engine and thus achieves a higher level of smoothness across the usable rev band.
Mercedes started the trend by fitting DMFs in their 6 cylinder petrol models (W124), followed by the 5 and 6 cylinder diesels. DMFs for 4 cylinder engines followed later, around 1989, due to technical difficulties.
These days DMFs are widely used in both petrol and diesel models. VW use them in their 4 wheel drive products. As far as I know, all Mercedes and BMW models with manual transmissions are fitted with them. My neighbour's lovely old E30 model BMW 318iS recently had the DMF replaced after almost 20 years.
|
If DMFs are being fitted to petrol cars as well these days I can't say I am all that surprised, as I have known quite a few fours that were rough though serviceable. As NC points out they do reduce NVH and are therefore irresistible to manufacturers; as he and others point out most of these devices give long and reliable service.
But: like a gearbox, a DMF isn't something you would want to have to replace or repair; like a gearbox, it can fail through some manufacturing flaw or (perhaps more often) from driver abuse. It's another damn expensive thing to worry about.
I wonder how many saloon car racers insist on a DMF as part of their driving package.
:o}
|
|
These days?? DMF's were fitted to petrol engines before being fitted to diesels.
|
|
|
|
My clutch failed at 35k and wasn't abused. I think some people think "abuse" is normal slipping of the clutch. The problem with DMF/clutch is that they cannot take over heating. In a normal solid flywheel/clutch combination the clutch will fail after normal wear whereas a DMF/clutch will fail if you slip the clutch for more than a few seconds causing overheating. So in a 6 speed gearbox with a highish reverse gear, you will have to slip the clutch under any load. Mine failed reversing up a steep hill with just two people and no luggage. It failed after just 2 metres. I know someone on a long trip to Italy, failed on a hot day in severe traffic congestion. 25K only. Another failure on a transit just with normal driving. I'm not surpised Ford came up with a different solution for the ultra-reliable Fiesta diesel. DMF has been a disaster for all manufacturers yet we have to pay heavily for their mistake ( X type replacement is £1200). It is a fault in the design so don't blame driver abuse as the same treatment on a solid flywheel would not cause a premature fail.
|
|
But that then begs the question as to why some Ford ( maybe most of them ? ) DMFs don't fail despite high and hard mileages doesn't it ?. No simple answer I suspect.
|
being in the trade on the side that comes to lose if a dmf fails
ive yet to meet anyone who is in their favour
and ive met quite a few that weld them up
caveat umpar
|
|
|
|
>>they cannot take over heating.
Yes. I know of at least one application where the manufacturer hasn't got the thermal analysis right and the DMF has suffered early failure as a result.
I don't deny that DMFs are problematic at the moment, but, I think they will become better matched to the application, and reliability will improve.
The problem for the manufacturer is in putting sensible bounds on their assumptions at design time. If, say, they assumed serious clutch abuse, clutches and their flywheels whether DMF or solid would end up being massively oversized with cost and performance penalties, wheras if they assume that all drivers will treat the clutch well, they will have lots of early failures - choosing the design point between these extremes, and from there deriving design rules and development testing guidelines is not a trivial task.
|
A personal rather than representative experience; My car - 9 years old, 2.5 V6. 90k+miles and original DMF. Smooth as silk changes and acceleration, would rather have it than not.........
Regards
|
|
|
|
|
|
|
|
