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Why does the engine in my car shudder violently when the ignition is switched off? Surely it should just drift to a gentle stop?
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Diesels have very high compression ratios compared to petrol engines.
If they drifted to a gentle stop, it would be the signs of a badly worn engine. Old diesels of 20 years or more had heavy flywheels to overcome this, but then they were sluggish in their performance.
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mine dont.
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TourVanMan TM < Ex RF >
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As part of my job, driving various manufacturers of vehicles imported with or without diesels and they will shudder to a halt as already described, unless its a small diesel.
The other thing that wont help, if the engine mounts are faulty.
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One thing that is common to both petrol and diesels is that their engine mounting systems are designed so that the engine runs in what is known as the mass controlled regime (sometimes called floating power in the US), where the frequency of the forcing input is greater by a margin than the natural frequency (ies)* if the engine/gearbox on its mounts.
* In general, a compliantly mounted rigid body has six so-called rigid body resonant frequencies
As you turn a diesel off, it runs down through the resonant speed range, where the isolation doesn't work effectively, and you feel the vibration.
Diesels may spend longer in this range because of their increased mass moment of inertia (or flywheel effect).
While diesels do have a higher compression ratio, it's worth remembering that you do also get some engine acceleration from this on what would have been the power stroke - i.e., the compression pressure pushes down the piston, accelerating the engine.
Number_Cruncher
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Duel mass flywheels tend to reduce this effect. Mind you, they introduce other problems....
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I read that my engine has an anti-shudder or anti-run-on valve.
It closes the intake off when the engine is switched off.
It stops the engine without the usual wiggle.
Because diesels have no throttle there is far less pumping effort required, so when you switch it off it contributes to this phenomina.
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Fighting flywheels eh? That must introduce LOADS of problems.....
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i shudder to think how we would do without dual mass flywheels
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I wouldnt know which way to turn without mine
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TourVanMan TM < Ex RF >
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theres a mass of information if you know the right circle of people
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My Passat Tdi doesn't shudder when switched off.
--
\"Nothing less than 8 cylinders will do\"
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My various VAG Tdi's over the years haven't particularly shuddered, whereas the supposedly refined Peugeot 307SW pool car I had recently would have disgraced a cement mixer when starting and stopping...
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I'll second that. 130 TDI.
JH
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Have you all spent too long in the sun today?
Don't diesels have heavier flywheels that casue more of a lurch when they stop, as well as a more noticeable twist of the engine on the mounts when you rev abruptly?
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Have you all spent too long in the sun today? Don't diesels have heavier flywheels that casue more of a lurch when they stop, as well as a more noticeable twist of the engine on the mounts when you rev abruptly?
Shorely, the heavier the flywheel, the smoother the stop (against compression). The lighter the flywheel, then the more susceptible the engine would be to rocking back and forth against compression.
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i shudder to think how we would do without dual mass flywheels
I wouldn't have thought that dual mass flywheels would have any effect on the shudder phenomenon, as I thought that they were designed to cushion vibration spikes from being passed down the transmission thus giving a smoother drive ??
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I wouldn't have thought that dual mass flywheels would have any effect on the shudder phenomenon, as I thought that they were designed to cushion vibration spikes from being passed down the transmission thus giving a smoother drive ??
They do. But they have the biggest effect below the normal idle rpm for the engine (i.e. when starting and stopping).
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I think it depends how well the engine mountings cope with the issues mentioned above. My three latest diesels (Golf, A4, B-class) don't shudder when switched off, but SWMBO had 3 Peugeots which did; both 205s not oppressively but the 206 was awful and also had terrific driveline shunt when you released the accelerator. Horrid car in other ways too and not a patch on the 205s which we loved.
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The HDI in the C5 judders noticeably more when shutting down than the XUD did in the Xantia, particularly if it is not left to idle but switching off. When running, however, it is a very smooth engine, so I can't say I am concerned.
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The D5 engines that I had in the two Volvo V70s shook and shuddered all the time, which is why I am happily back driving a petrol-engined car.
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