A question about detonation

Discussion in 'Engines' started by donkeyking, Sep 2, 2010.

  1. donkeyking

    donkeyking Member

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    #1 donkeyking, Sep 2, 2010
    Last edited: Sep 2, 2010
    Regarding detonation, why are there more chances to occur detonation in lean mixture than rich mixture when the engine uses the same fuel?
    For example Avgas 100/130, it is obvious this kind of fuel has more detonation chances in lean mixture than rich mixture.

    Many thanks

    Donkeyking
     
  2. Colin1

    Colin1 Active Member

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    A rich mixture is 'wetter' (ie a greater fuel bias in the fuel:air mixture) than a lean mixture and will cool the cylinders by evaporation.
     
  3. tail end charlie

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    There are many causes of detonation apart from the fuel, ignition timing, compression ratio shape of combustion chamber and amount of carbon build up to name a few. The change of a fuel from liquid to gas has a cooling effect so a rich mixture runs cooler than a lean mixture (as colin said). An engine uses heat to create motion so the bigger the explosion the more power it produces however with detonation it is getting out of control which can result in the engine completely "running away" and burning the pistons as well as the fuel...like it used to do on my crappy suzuki GT185.

    I used avgas 100 in my racing motorcycle not because it had more or less chance of detonation but because it had better fuel quality control, it was easier to set the jetting and in 2 yrs racing I never burned a piston.
     
  4. Aaron Brooks Wolters

    Aaron Brooks Wolters Well-Known Member

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    A lean fuel mixture causes causes cylinder temperature to go up. Fuel helps keep the cylinder cool. If you have ever notice a lean engine when it is running the header tubes will glow. I'm not sure about aircraft engines but most automotive engines like about a 14 to one air to fuel ratio.
     
  5. mikewint

    mikewint Well-Known Member

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    let's try this. liquid gasoline will not burn. if you are really brave a lit match can be plunged QUICKLY into a pool of gasoline and the match will extinguish. pure vapor will also not burn the same for hydrogen gas. combustion requires fuel PLUS oxygen. rich fuel mistures burn poorly, no oxygen. lean have more oxygen and burn quickly
     
  6. tail end charlie

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    Mike that sounds like great science, can I watch you do a demo lol


    seriously what you say is theoretically correct only you put the match in the gasoline eh?
     
  7. Aaron Brooks Wolters

    Aaron Brooks Wolters Well-Known Member

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    It is true. As long as the flame does not ignite the fumes the match will go out in the liquid. I have seen this done. I'm still not sure that I will ever try it because Murphy and I are on first name basis.:lol:
     
  8. mikewint

    mikewint Well-Known Member

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    #8 mikewint, Sep 2, 2010
    Last edited: Sep 2, 2010
    i have actually done this, the container is capped and cooled to keep vapors condensed. remove the cap quickly and plunge the match quickly. it is only the vapors which are explosive
    flooded engine will not start and in the cold it's much worse
     
  9. Aaron Brooks Wolters

    Aaron Brooks Wolters Well-Known Member

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    When you flood an engine the same thing happens. The spark will not light off the dense fuel mixture. And I believe you Mike. I wasn't joking when I said that I had seen it done. I'm just a little gun shy when it comes to open flames and gasoline. I've been caught on fire with it so I tend to keep my distance.:lol:
     
  10. tail end charlie

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    The opposite in a way is flour and wood dust/shavings they are notoriously explosive in a warehouse wheras few people would consider them dangerous.

    an empty petrol tanker in a harbour is much more dangerous than a full one, full it burns empty it explodes.
     
  11. mikewint

    mikewint Well-Known Member

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    all are just ways of saying that combustion requires both fuel and oxygen. any combustable substance surrounded by oxygen will burn. a steel bar will not burn but fine grade 0000 steel wool burns quite nicely
     
  12. donkeyking

    donkeyking Member

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    I have another question.

    Does turbofan or turbojet engine has detonation problem?

    thanks
     
  13. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
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    No - the combustion cycle of a turbine engine is entirely different from a reciprocating engine.
     
  14. mikewint

    mikewint Well-Known Member

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    FB, i may be wrong but i would say that there is no detontion at all just very rapid continous combustion. would that be correct?
     
  15. engguy

    engguy Member

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    Super lean ratios do not have the tendancy to detonate because of the cooling effect of the excess air.
    Aircraft engines tend to have ineffcient combustion chambers so that doesn't help either.
     
  16. Messy1

    Messy1 Well-Known Member

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    #16 Messy1, Sep 9, 2010
    Last edited: Sep 9, 2010
    Fuel quality and compression ratios are a huge factor in preventing detonation. A octane rating is a measure of a fuels resistance to burning. That is why high compression engines require high octane fuel. Higher compression makes more heat. Typical pump gas will only support on average 10:1, or 10.5:1 compression safely. Anything over that and you can can have trouble with engines knocking, pinging, or run-on after you turn off the ignition. Our open modified dirt track engines will be in the 13-14:1 compression ratio range with over 600hp, and racers will typically run 110 racing fuel, or straight methanol. It is actually better to have too much fuel than too little as too much fuel will flood your engine, but typically not cause any sever problems. A lean engine with detonation problems can destroy a new engine in a few minutes if the detonation is severe enough. We see racers all the time who have trouble with detonation either do to poor fuel quality, not enough fuel, timing problems, etc burning up $700 sets of pistons. There are many reasons detonation can occur, TEC's explanation in the #3 post is a good explanation.
     
  17. Messy1

    Messy1 Well-Known Member

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    I guess technically that could be true, only if your engine has so little fuel in the mixture that it will not ignite during the combustion cycle. We see more engine problems and damage due to detonation from lack of fuel, timing, etc than any other problem. Reading spark plugs will tell you if there is a detonation problem. White electrodes are caused by a lean mixture. Wet, dark plugs by too much fuel
     
  18. Colin1

    Colin1 Active Member

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    I was having a bit of trouble with that too
     
  19. tail end charlie

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    The main problem with an internal combustion engine is that it is controlling an explosion. The mixture explodes and as the flame expands it pushes the piston. However when the spark ignites the mixture the volume of the burning gas is small and then increases exponentially. For this reason the spark ignites before the piston reaches the top of the cycle (Top Dead Centre) so when the flame front develops it exerts the maximum pressure on the piston on its way down. Power is improved by increasing the compression ratio or by supercharging/turbocharging, by improving the mixing multivalve or swirl inlets, by increasing the flame fronts (twin spark), or increasing the number of explosions (higher revs). Every method comes up against limits of physics either metallurgy fluid dynamics even harmonics.

    Detonation is when the mixture starts to explode before you want it to, the biggest problem with detonation with a lean mixture is that the engine starts to run hotter and hotter and there is more oxygen than fuel, if it becomes hot enough it turns the pistons or valves into fuel and melts them . For the few seconds you have before the piston burns through the engine it seems really powerful (I learnt from expensive experience). Given enough compression a fuel will explode automatically like in a diesel so fuels were developed to prevent this.

    The knocking "pinking" associated with pre ignition/detonation is the flame front hitting the piston when it is at or near TDC.
     
  20. Messy1

    Messy1 Well-Known Member

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    Very good post charlie.
     
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