Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Engine sounds again this time its all about diesels

  1. Jul 13, 2007 #1
    engine sounds again :) this time its all about diesels

    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:redface:) 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 :)
     
  2. jcsd
  3. Jul 14, 2007 #2

    Danger

    User Avatar
    Gold Member

    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.
     
  4. Jul 14, 2007 #3
    hmm not the clacking
    have you heard any new generation common rail direct injection engines?
    its particularly noticeable in those
     
  5. Jul 14, 2007 #4

    Danger

    User Avatar
    Gold Member

    I'm sure that I have—you can't turn around in these parts without bumping your ass 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.
     
  6. Jul 14, 2007 #5
    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?
     
  7. Jul 14, 2007 #6
    crdi and di

    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
     
  8. Jul 14, 2007 #7

    Danger

    User Avatar
    Gold Member

    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?
     
  9. Jul 15, 2007 #8
    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)
     
  10. Jul 15, 2007 #9
    hmm yeah i agree with you :)
     
  11. Jul 15, 2007 #10

    Danger

    User Avatar
    Gold Member

    Okay, that's what caused the confusion. I forgot that diesels combust immediately upon injection. :redface:
     
  12. Jul 15, 2007 #11
    Danger,
    Did you notice I didn't bust your chops for not using my full username? :rofl:
     
  13. Jul 16, 2007 #12

    Danger

    User Avatar
    Gold Member

    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. :biggrin:
     
  14. Jul 16, 2007 #13
    lolzzzzzzz
     
  15. Jul 16, 2007 #14

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    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.
     
  16. Jul 18, 2007 #15
    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.
     
  17. Jul 18, 2007 #16

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    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.... :grumpy:
     
  18. Jul 18, 2007 #17
    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
     
  19. Jul 27, 2007 #18
    The diesel "clatter" comes from the design and principles of diesel combustion. Diesel fuel is not ignited with a spark plug as gasoline is, it combusts on its own as it is injected into the cylinder. Also diesel fuel required extremely high temperatures to ignite. The combination of these two requirements makes it necessary to keep the combustion chambers very hot while the engine is running.

    Now automotive pistons are made from aluminum, and newer ones iron/maganese. When you heat up a metal it expands proportional to its coefficient of thermal expansion. Because diesel engines run hotter than gas, their pistons experience greater expansion. To seal the gap between the pistons side face and the cylinder walls, piston rings are used. In order to compensate for the greater expansion of pistons the piston rings in a diesel engine are run at a wider tolerance than a gas engine. This larger gap between the piston and the wall cause the loud clatter.

    Piston ring clearance in diesel engines have been getting closer in recent years (Compare an E320CDI to a 300SD), and thereby quieter.
     
  20. Jul 29, 2007 #19

    brewnog

    User Avatar
    Science Advisor
    Gold Member

    Got a source for this please?
     
  21. Jul 29, 2007 #20
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Engine sounds again this time its all about diesels
Loading...