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Engine sound

  1. Jul 5, 2007 #1
    hi to all, this is my first post here n quite a stupid one:shy::shy:
    last day i had a discussion with my friend over this..
    what exactly is the reason behind that sound of the engine???
    i mean is it the sound of the explosion of the charge or is the sound of the expanding gases comin out to the muffler from the exhaust of the engine????
  2. jcsd
  3. Jul 5, 2007 #2


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    A big mixture.

    Depending on what the engine is installed in, the biggest sources of noise are exhaust gases, induction roar, fan noise (and that of belts, pulleys and other rotating components), and combustion noise from the resonating engine parts.

    Big, 'flat' components like gearcases, sumps and rocker covers are prime resonating panels.
  4. Jul 5, 2007 #3
    nahh i dont mean that, i mean the typical sound of the repetetive strokes. i mean, the sound of those contineous kicks, is it bcoz of the explosion of charge or bcoz of its expansion when outlet valve opens???
  5. Jul 5, 2007 #4


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    Combustion noise, exhaust noise, and a bit of induction noise.
  6. Jul 5, 2007 #5
    nah dude... u r still not getting it
  7. Jul 5, 2007 #6


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    I thought Brews answered exactly what you were asking. It is tough to figure out what you are asking based on your descriptions.

    Combustion noise would be my number one guess to the noise signature of an auto engine.
  8. Jul 5, 2007 #7


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    What do you mean brewnog isn't getting it? He's answering the question you asked (though the question you asked might not be the the one you wanted the answer to). Care to try again?
  9. Jul 5, 2007 #8
    Yup, that's the loudest source of the sound. Specifically, the shockwave produced by the expansion in addition to other moving parts.. But in air-cooled engines, a little sound could be heard from the explosion of the mixture, adding to the frequency of the 'kicking' noise. It's hardly heard in a liquid-cooled engine because the water channels surrounding the cylinders muffle that sound.
    Last edited: Jul 5, 2007
  10. Jul 6, 2007 #9
    yes yes... thats what i m tryin to ask... is it bcoz of the shockwave of the outgoing gases or the explosion of charge??
    my initial guess was the shockwave thing
  11. Jul 6, 2007 #10
    i just had this debate with some friend.. thats why i asked u guys to get some clarification.. i m actually talkin abt the dak-dak-dak sound(i know i sound silly, but this is the only way left,hehe) of the strokes.
    ok imagine it this way, if an engine is running at very low rpm, u can hear to its stroke sounds very distinctly, that is what i m talkin abt...
    or imagine the machine the grandpa used in Mr Bean-The holidays(where the grandpa has installed an engine on his bycycle n which is ridiculously slow, hehe, there u can hear it clearly)
  12. Jul 6, 2007 #11


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    Source please.
  13. Jul 6, 2007 #12
    http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm and leave some to experience.

    It's the muffled sound of the exhaust pulses :) 'Exhaust Pulse'(formal term), 'Pressure waves', 'Shockwaves' are common terms among automotive people which is the explosive expansion of the gases when the exhaust valve opens.

    You're correct, keep in mind though that though may sound a lot different from one engine to the other (factors as exhaust system and engine design, RPM) or even resemble another sound like clapping sticks. It's basically the exhaust pulses which is caused by the explosive expansion of high pressure gases liberated by the rapidly opening valve during the exhaust stroke.

    However in situations that the exhaust system is doing an excellent job of absorbing the engine noise, you won't hear those pulses, the fan/motor noise could be even higher.

    For air-cooled engines, a small percentage comes from the ignited mixture. We once had an air-cooled VW beetle. When checking the exhaust system for leaks, we'd sometimes momentarily cover the exhaust pipe, and still hear 'clicking' sounds coming from the cylinder fins. On a liquid-cooled engine, covering the pipe, and you'll hear nothing =)
    Last edited: Jul 6, 2007
  14. Jul 6, 2007 #13
    nice link over there gaming addict.
  15. Jul 6, 2007 #14


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    There is nothing in that link that references expansion shocks as you stated.

    That source does talk about pressure pulses which are due to combustion, i.e. combustion noise.
  16. Jul 6, 2007 #15
    but combustion noise r the cause n pressure pulses r the effect..
    what i wanted to know was that the typical sound signature is bcoz of that explosion(combustion noise) or the pressure(exhaust) pulses..
    n i think it is the pressure pulses
  17. Jul 6, 2007 #16


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    I own a Mazda 6. The exhaust of the Mazda 6 was tuned, not for engine performance, but to provide a pleasing noise. And, of course, it sounds different if you are standing in front of it than if you are standing behind it! The sound a car makes is a complicated thing and I still don't think you are understanding the complexity in your own question.

    gaming_addict, I know you are just trying to help, but a lot of what you are saying (not just in this thread) is based on misconceptions and oversimplifications and isn't necessarily true. As a matter of procedure, we prefer technical information be sourced - and this would be helpful in your case, as it would enable you to verify that you have the answer correct before giving it.
  18. Jul 6, 2007 #17


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    Unless you are used to hearing engines running without mufflers and resonators, then the exhaust system plays a large role in that as well. The company my sister works for has developed a new system that includes a digital sound sampling system in the muffler to actually tune the exhaust noise to make your car sound like a different kind of car. It's pretty cool.


    Just to be nit picky again, the combustion process is a controlled burn/deflagration. It is not an explosion. If the mixture were to explode that is referred to as a detonation and that detonation and increases wear and decreases performance of the engine.
    Last edited: Jul 6, 2007
  19. Jul 6, 2007 #18
    But in a more specific manner, while the combustion is the main source of energy of the pulse, the pulse is not produced until the valve is quickly opened on the exhaust stroke (the fourth and last stroke in a 4 stroke engine).

    Combustion noise, actually, I haven't heard of that term yet, but if this is the noise from the combustion itself, happening at the same time. Then it's not the main source of the sound you described. On an air-cooled engine, that sound resembles a clicking sound(not from the valves) and is quite similar to the sound produced by a gun with silencer. It's an old VW, engine, so it might have a unique sound signature. The sound from the pulse, is deeper(lower tone or bassy) and louder.
    Last edited: Jul 6, 2007
  20. Jul 6, 2007 #19
    In layman's terms, that's called 'knocking' caused by using fuel with octane ratings below the recommended for a particular engine. :)
  21. Jul 7, 2007 #20


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    If you are at all familiar with high-performance engines, then you know that the whole point of headers is to equalize the distance that the exhaust pulses must travel. This puts them into a compatible frequency that minimizes reversion, and thus lowers the back-pressure. Same idea for a cross-over pipe, or 'H' pipe, or whatever you want to call it. Given two identical engines, you can definitely tell by the sound which one has stock exhaust and which one has headers.

    I must assume from this statement that you haven't had other than a Hollywood exposure to 'silencers'. To start with, there is no such thing. There are various efficiencies of 'suppressors'. Not one of them sounds anything like a movie 'silencer'. A really good one for a .45 will maybe make it sound like an unsuppressed .22, and it's about the size of a 5-cell flashlight.
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