engine sounds again :) this time its all about diesels

enginelover

hmm hi
i just discovered this forum :)
and i just loved it
have been looking around for something like this for some time :)
i just went through the engine sound thread a while back
i have two questions.
1)
why do diesels make the pitter patter noise(for the lack of a better word) when they are cold(is this combustion noise or the noise of a pressure wave from the exhaust valve)?
and
has anyone heard an air engine ?
whats the difference in sound?
i am not too sure about how they work but i do know they run on compressed air and hence no combustion takes place in their cylinders.
so was wondering if this makes any sense to you guys :)

Danger

If you're referring to that raucous 'clacking', I've always been under the impression that it's mechanical noise from the valve train. It comes from the engine compartment, not the exhaust. I've never actually investigated it, though.

enginelover

hmm not the clacking
have you heard any new generation common rail direct injection engines?
its particularly noticeable in those

Danger

I'm sure that I have—you can't turn around in these parts without bumping your *** on a diesel pick-up—but I've never paid any attention. This is the wrong time of year to try hearing one that's cold, but I'll keep it in mind for winter.

Averagesupernova

I can tell you it is definitely NOT the valve train. I was working on a Ford Powerstroke 7.3L diesel that had one side (V8) that was not firing. Turned out that the wires that carry current to the injectors and the glow plugs plug into a harness which is part of the valve cover gasket. It is a way to get the the signals that fire each injector and carry current to the glow plugs under the valve cover. The connector part of the valve cover gasket had melted and was not firing the injectors on that side of the engine. There was NO diesel rattle coming from that side of the engine. The rattle appears to come when the fuel is injected. Diesel engines have rattled ever since I can remember no matter what scheme they use for fuel delivery. Some of the newer diesel engines are quieter than previously. However, the Powerstroke IS a common rail design and is just as noisy as any diesel I've been around. Also, aren't ALL diesel engines considered direct injection?

enginelover

the difference i guess in the sound is more due to the pilot injections as they call it
a couple of smaller injections of diesel followed by a big one so there is less noise
so its particularly noticeable there
and am sure it is not valve noise cause i have hard diesels with their tappet clearances screwed so i know what that is

Danger

That's interesting, Average. Thanks for (partially) straightening that out. I'm still confused, though. Is it the injector solenoids themselves making the racket, or what?

Averagesupernova

As near as I am able to tell, it is the actual combustion that gives a diesel its familiar rattle. As I said in a previous post, diesel engines have ALWAYS had a rattle no matter what fuel injection scheme they use. It has little to do with the solenoid at least on Ford Powerstroke. (Which is actually made by International)

enginelover

hmm yeah i agree with you :)

Danger

Okay, that's what caused the confusion. I forgot that diesels combust immediately upon injection.

Averagesupernova

Danger,
Did you notice I didn't bust your chops for not using my full username?

Danger

Yeah. Sorry, man... it's just laziness on my part. Maybe I'll just stash your full name in the clipboard and paste it in when needed.

ank_gl

lolzzzzzzz

brewnog

Diesels 'rattle' more because of the different combustion mechanism. When the fuel is injected, there's a higher initial heat release than with spark ignition engines. Common rail engines are even noisier because of the higher injection pressures allowing an even higher initial heat release spike.

gaming_addict

As Brewnog said it. However for turbo-diesels, and at high RPM, I barely notice that sound, they sound more like large air blowers or even like jet engines. They are unlike gasoline engines even high RPM, the sound pulses is neatly distinguished.

brewnog

Indeed, but if you even think about describing any real engine sounds like turbos, pumps, induction systems, injectors, valvetrains or fans, the OP is likely to tell you that you haven't got a clue....

ray b

my new to me older MB 300sd is much louder at idle then at speed

diesels knock as the fuel goes bang
gas motors burn more slowly and only knock if something is very wrong
some call the diesel sounds pennies in a can at lower speeds and idle

newer diesel cars use muliti injection pulses [computer controled]
to try to reduce noise and gain MPG too
but at a cost of complex controls