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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Twice as much?
When the writer says that eight cylinder engines are ‘twice as much’ let me explain.
Consider a single cylinder engine operating on the Four Stoke Cycle.
Stoke 1 Induction) Piston moves down the bore on the induction stroke (a petrol and air mixture is inducted via the inlet manifold)
Stroke 2 Compression) Piston moves up the bore and compresses the mixture of petrol and air – towards the end of the stroke the mixture is ignited by a spark from the spark plug and the rise in temperature, due to the rapidly burning gasses, and the resulting pressure forces the piston down the cylinder bore on to the next stroke - the start of the power stroke,
Stroke 3 Power) Piston moves down the cylinder bore due to the high gas pressure. ( It is only this one stroke that delivers power ) the power is delivered to the crankshaft via the connecting (con) rod
Stroke 3 Exhaust) Piston moves up the cylinder and the spent gas is evacuated to atmosphere via the exhaust valve, exhaust manifold and exhaust system.
So, a stroke is the name given to the piston moving either up or down the cylinder bore. As can be see the piston travels up the bore twice and down the bore twice. (Down, Up, Down, Up) and then the cycle repeats. The crankshaft move through two complete revolutions for each power stroke.
So, it takes 720 degrees of crank rotation to get one power stroke. In order to keep the crankshaft rotating during the induction, compression and exhaust stroke a heavy flywheel if fitted to the crankshaft. The heavy mass of the flywheel stores some of the energy from the power stroke and its this stored energy that helps the crankshaft to move the piston up and down the bore one the induction, compression and exhaust strokes. The power stroke of a single cylinder engine is more of a thump and only occurs every two crank / flywheel revolutions.
If the number of cylinders / pistons is increased the engine becomes smoother as there are more firing stokes. For example 720 degrees divided by the number of cylinders = the number of degrees between power strokes. So in a single cylinder engine there is one power stroke in 720 degrees.
In a four cylinder engine there will be a power stroke every 180 degrees (half of on revolution before the next power stroke and so on)
In a six cylinder engine there will be a power stoke every 120 degrees and in an eight cylinder engine there will be a power stroke every 90 degrees of crank / flywheel rotation. Therefore the more cylinders the smoother the engine will run AND the flywheel does not need to be so heavy. In fact many eight cylinder engines have, or did have, a very light flywheel simple to mount the starter ring gear on and to provide a friction surface for the clutch. So 720 degrees divided by the number of cylinders equals the distance in degrees between firing strokes – hope this helps??
Edited by Blakey57 on 13/02/2013 at 14:01
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Welcome to the forum and thank you for this explanation, although the thread dates from 5 years ago.
When you started by referring to the "Four Stoke Cycle" I thought you were talking about a steam engine....
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Not sure why you would think that ? And a steam engine is an External combustion engine of course. I think that efficient steam engines have / had two power srokes per revolution as steam acts on both sides of the piston, not at the same time of course :) . A sliding valve diverts the the steam to each end of the stroked cylinder...bit before my time though.
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Thie thing about the steam engine though, Blakey, is that you have to stoke the external combustion source to get those power strokes.
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err it wasn't me that was getting confused ;)
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Sorry, Blakey - we were having a gentle laugh at your typo in line 2.
Of course, if it wasn't a typo, then it gets interesting....
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Ha ha no offence taken - I must check my spelling befour (see what I did there??) I post. I have only just joined this forum and I look forward to reading the many different posts.
Thanks & Regards
Paul
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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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A 3D animation of DMF is here
youtube.com/watch?v=YnaXB8q3uzQ
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Thanks movilogo - I begin to understand why they are more fragile.
Edited by shoespy on 25/03/2008 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.
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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I had a 2001 c class petrol with a dual mass fly wheel sold it in 2009 with 100kmiles on the clock and its still running on the same clutch with 200kmiles on the clock.Here in Germany you will see many old DB manuals with the original DMF with 300 400kms on the clock.
Edited by Collos25 on 23/02/2013 at 09:15
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