What exactly is Detonation

Discussion in 'Motorized Bicycle General Discussion' started by Moto, Sep 11, 2012.

  1. Moto

    Moto Member

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    Ok so I am a bit embarrassed to ask such a noob question when Ive built a few of these bikes now, but what is detonation? I hear people talking about it all the time and I have gathered it has to do with heat and the head? Fill me in... MOTOdnut
     
  2. The_Aleman

    The_Aleman New Member

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    Detonation is abnormal combustion. Basically, aftermarket heads and low-octane fuel can cause preignition (early combustion), which is very bad for the piston.

    Running too lean and/or running too hot can also cause detonation. To avoid it, you must tune your engine correctly.
     
  3. Ludwig II

    Ludwig II Well-Known Member

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    What happens is that as the flame front spreads out from the plug, it forces unburnt fuel and air mixture ahead of it until it reaches a physical and thermal density sufficient for self ignition to occur, much like a diesel engine; but far too hot for the metal surrounding it. This ends up causing localised melting of the piston and the head, which is why it is sometimes also called pinking in the UK, or knocking. This is the sound of the micro explosions trying to eat your engine.
     
  4. Bikeguy Joe

    Bikeguy Joe Godfather of Motorized Bicycles

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  5. bairdco

    bairdco a guy who makes cool bikes

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    Detonation is a process of combustion in which a supersonic shock wave is propagated through a fluid due to an energy release in a reaction zone. It is the more powerful of the two general classes of combustion, the other one being deflagration. In a detonation, the shock compresses the material thus increasing the temperature to the point of ignition. The ignited material burns behind the shock and releases energy that supports the shock propagation. This self-sustained detonation wave is different from a deflagration, which propagates at a subsonic rate (i.e., slower than the sound speed in the material itself). Because detonations generate high pressures, they are usually much more destructive than deflagrations.

    Detonations can be produced by explosives, reactive gaseous mixtures, certain dusts and aerosols.
     
  6. Ludwig II

    Ludwig II Well-Known Member

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    That sounds bad. Remind me not to go near a naked flame after eating baked beans.
     
  7. ram50v8

    ram50v8 New Member

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    Moto, don't be embarrassed about this! Detonation and Pre-Ignition are 2 of the most mis-understood events that happen in a cylinder. In simple terms I will explain what detonation is. Detonations direct cause is when the spark fires and as the flame front moves across the cylinder the pressure rises causing a rise in temperature which at the hottest spot where un-burned Air/Fuel mixture resides causes the A/F mixture to ignite on its own. The resulting flame moves across the cylinder and meets the original flame front and that is detonation aka spark knock.

    The Other is pre-ignition where the flame front starts before it is desired (before the spark plug fires) and is caused by the combustion chamber temperature exceeding the ignition temp of the F/A mixture. This can be due to overheating of the engine or by a localized hot spot, or carbon build up that has started to glow in the cylinder and becomes a "glow plug"

    There are many overlaying factors that cloud these problems and cause the confusion. Overheating, fuel quality, Air/Fuel ratio, carbon build up (glowing carbon) and ignition timing. If you let it go and allow them to do their damage you can see the differences in damage. Pre-ignition tends to damage the center of the piston whereas detonation does its damage at the rings/piston edges. However with today's fuels and leaner mixtures the two often are in concert with each other.

    Now before some one starts jumping in, remember this was the layman's version. There are many many factors involved that can make it one way over the other. Some are mechanical like static compression. Others are timing related verses temp and fuel quality. And then there is the fuel itself and it's ability to perform with maximum efficiency without igniting too soon or burning too fast. So many things involved but hope this answers your question. With today's modern engines (not these lol) we want to run right at the edge for maximum power and efficiency.

    Darren
     
  8. Saddletramp1200

    Saddletramp1200 Custom MB Buiilder

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    Welcome to the forum All! Wow Darren You ever work on drag bikes, Real ones? My # 3 piston is acting up and I don't know why. I am a Motorcycle mechanic. I am sometimes wrong, but Not often. (c)
     
  9. Moto

    Moto Member

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    Thanks so much for the great info! What exactly does it sound like?
     
  10. ram50v8

    ram50v8 New Member

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    Unfortunately the exact sound varies with the severity of the knock, engine RPM, engine design and other factors. They can also be called a silent killer of engines because you can get just to the point where the knock cannot be heard but it destroys the engine. We only hear it when it is severe and unfortunately, the smaller engines it is just about impossible to hear until it is too late. The best recommendation I can make is pay carefully attention to your spark plug for aluminum deposits and look into the bore for signs of piston damage.

    Oh as for the sound, it can sound like the metal end of a pocket screwdriver tapping on a hard surface and gets louder from there.
     
  11. Ludwig II

    Ludwig II Well-Known Member

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    At one race meeting, we thought we were over jetted because of losing power at the top end of the revs. Checked the plug, blacker than midnight in a coal shed. Went down 2 jet sizes, took the bike out again, it stopped dead at the furthest point on the track from the pits.

    But then, they always do.
     

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