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.
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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
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For the current Fiesta 1.6 diesel Ford has done the job of the DMF with electronics.
HJ
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As BMW imply, running at lower revs enables lower fuel consumption and puts cars in the lower tax band.
Hence more vibration which the DMF dampens out. At a mechanical cost eventually.
The 'ooh my car costs only £35' per annum road tax etc. is vitally important for many people.
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For the current Fiesta 1.6 diesel Ford has done the job of the DMF with electronics. HJ
That's what I said ;-)
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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.
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Running at lower revs also prevents the Diesel Particulate Filter from regenerating unless it has some kind of additive system. Be interesting to see how BMW gets round that with the 320d Efficient Dynamics. This also happens to have a sophisticated DMF.:
21-8-2009: BMW 320d EfficientDynamics Saloon announced, combining 0-60 of 7.9 and top speed of 137 with 68.9mpg combined and 109g/km CO2. So costs just £35 a year to tax courtesy of a Vehicle Excise Duty band B rank. A 13 per cent benefit-in-kind rating also ensures it makes maximum economical sense. While for Corporation Tax purposes the entire cost of the car can be set against a single year's profits.
The single turbocharged 1,995cc diesel engine features an all-aluminium crankcase and third generation common-rail injection technology with piezo injectors to achieve its consumption and emissions figures without diluting driver enjoyment.
BMW 320d EfficientDynamics Saloon: 163PS, 360Nm, 0-60 7.9, 137mph, 68.9mpg combined, 109g/km CO2
The BMW 320d EfficientDynamics Saloon develops 163hp between 3,500rpm to 4,200rpm and 360Nm of torque from 1,750rpm to 3,000rpm. Acceleration in fifth gear from 50mph to 75mph takes just 9.6 seconds. The heightened performance has been made possible due to changes to the engine construction, a longer rear axle transmission ratio, lowered suspension and the use of specially designed 16" aerodynamic alloy wheels with 16" Michelin EnergySaver tyres.
The BMW 320d EfficientDynamics Saloon also comes with a dual mass flywheel with springs between the two parts to help smooth out any engine vibration through the drivetrain. This is enhanced with the fitting of new Centrifugal Pendulum Absorbers to improve smoothness further. This feature increases motoring comfort and should result in the driver using a higher gear more often. This combined with the longer transmission ratio thus serves to significantly reduce engine speed compared to a ?standard? 320d Saloon, benefiting fuel economy and emissions. The new car still comes fitted with the existing suite of BMW EfficientDynamics technologies, including Auto Start Stop, Brake Energy Regeneration and Electric Power Steering for optimum performance, that have featured on all four-cylinder BMW 3 Series models since September 2007.
The BMW 320d EfficientDynamics Saloon will be unveiled at the Frankfurt Motor Show and go on sale in January, 2010. The specification of the car will be on a par with a BMW 320d SE (with 17" alloys replaced by 16" alloys) and includes two-zone air-conditioning, rear Park Distance Control, BMW Professional radio, multi-function leather steering wheel and Extended lighting. Pricing is likely to mirror the existing ?standard? BMW 320d SE Saloon.
It's a mega tax beater, because at 109g/km a company can put the entire cost of the car against a single year's profits. That is, unless this gets changed in the November budget.
HJ
Edited by Honestjohn on 14/11/2009 at 11:59
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>>new Centrifugal Pendulum Absorbers
They are a very old idea - very clever and very effective too.
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>>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.
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>>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.
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>>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
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>>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
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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
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>>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.
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>>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.
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>>Exactly
That is not what you were saying.
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>>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.
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>>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.
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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.
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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.
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You've bored me into submission - whatever you say Cheddar.
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You've bored me into submission - >>
The pot calls the kettle black.
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You bored the rest of us ages ago!!!!!!!!!!!!!!!
Edited by jc2 on 15/11/2009 at 19:06
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>>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.
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