Can you lot help with a query?
While lying in bed this morning and having to listen to Mr next-door starting up and then revving his diesel van, I started wondering why a diesel engine makes more noise than a petrol one. I understand the basic principles of the two different engines types, but I can't figure out why one type would make more noise than the other.
Don't get me wrong I'm not anti diesel, being a driver of a '95 Passat tdi myself, I'd just like to know what makes this happen and having been a regular reader of the Backroom thought that the experts could help me with this one.
Thanks,
Steve
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I always believed it was effectively the detonation noise caused by the explosion following the compression. Perhaps its louder because of the higher compression? I'm sure one of the regular experts will know!
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And the diesel takes a full charge of air whatever the "throttle".
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Diesels rely on very high compression to increase the temperature of the oil /fuel mixture to the point where it ignites spontaneously. This causes a rather heavy detonation particularly when cold in older engine designs. As the engine warms up, the detonation becomes smoother and the noise level drops.
Modern 'Common rail ' type diesels are quieter all round as the fuel mixture is injected directly into the cylinder under very high pressure and ignites smoother.
You have my sympathy, as my local milkman parks his dreadful Ford 'bucket of bolts' directly under my bedroom window at anytime between 2.30 and 5.00 am everyday! Thats why my eyes look like saucers by 4 o'clock every day at work. Hope this explains.
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I'd just wish to twist you explanation a little. The older indirect engine design was used so much because the early direct injection versions were very noisy.
It is only with up to date designs and electronic control they have been able to smooth out the racket from direct injection.
David
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That noise is the fireman/stoker shovelling in some more nutty slack.(filthy coal burners)
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Tony,
As someone who spent months fitting a cast multifuel stove to the living room your images of the "solid fuel" diesel engine strengthen my liking for them.
All we need is a Franco-Belge engine transplant and a small trailer of bog oak towed behind....sheer bliss.
I must say the old Land Rover looks like its running on damp stove nuts.
David
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Brian
Your explanation of the way the noise is generated is right. Indirect injection engines were developed to try to improve refinement of the original direct injection units. They lost out on efficiency, as they were restricted on maximum compression ratio and rotational speeds, hence the reintroduction of the DI units, of which the Transit was a noisy example. Probably considered acceptable as a commercial vehicle.
The DI principle was moved to cars because of the efficiency gains, but needed improved refinement. Computer control and electrically controlled injection provies the solution. The quieter operation of the latest engines is brought about by using two stage injection, which uses a small initial pulse of fuel, followed by the main pulse so reducing the speed of combustion pressure increase and hence reducing noise. Some engines also have 'knock control' to further refine the injection timing. Ford have gone one further with the TDCi. Apparently they reckon two-stage injectors can wear and lose efficiency after a few 10's of thousands of miles, so they use the knock control on the initial pulse too, so, they claim, preserving the engine refinement for the life of the engine.
Regards
John
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Hi folks its good to be back after a rather terminal PC problem, but the new server is up and running and is quicker than ever but then onto the issue in hand.
Steve, Brian T is quite correct in what he says, diesels have a 20+ to one compression ratio. The air gets very hot when its compressed in less than a 1/10th of a second on starting and then the injection of the fuel in just a couple of degrees of crank turn raises pressures very quickly so you get the knock, knock as it burns. If you listen to more basic diesels such as in cement mixers or dumper trucks they will sound even worse, but modern diesels with all of the electronic controls manage to keep things much smoother. Larger medium speed diesels used for power use of say 1MW (1400 BHP) at upto 1000rpm are even more noisey, but (Sorry folks favorite subject again) large marine diesels at 90 rpm are almost silent. You can hear the fuel being injected and then you can hear the burn. Its hard to describe but once heard never forgotten. If anyone has a video of Robbie Coltranes Channel 4 series on engines there is a short piece on "Marine Cathederal" engines where they run up a FIAT marine diesel, its only a small one but does give an idea.
Bill
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Rover was one of the first to switch to direct injection diesels (Perkins)and they got round some of the combustion noise by using the then new Bosch two stage fuel injectors.
These work by injecting fuel slowly at first until burning takes place and then more rapidly to complete the combustion.
Lucas, you appear to have some fetish about coal which is a new one on me.
You remind me a of Les Dawson R.I.P. who was always obsessed with knickers. Still...takes all sorts.
alvin
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I have been driving a TDCi Focus over the last few weeks. It is very refined most of the time and has a decent amount of go - probably closest to a 1.6 petrol. It still sounds like a bag of spanners when cold and is pretty noisy at idle even when hot.
On an m-way it's fine but the fuel consumption goes up LOTS when shown a hill - according to Ford Fuel computer. Going up Beacon Hill near Amesbury on the A303 at a steady 70 gives figures of around 9mpg in 4th gear (it won't pull 5th up such a hill).
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