Does engine knock occur more easily at higher Or lower RPM?

Discussion in 'Engines' started by donkeyking, Jan 20, 2014.

  1. donkeyking

    donkeyking Member

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    Does engine knock occur more easily at low RPM? and why?

    thanks
     
  2. GregP

    GregP Well-Known Member

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    #2 GregP, Jan 21, 2014
    Last edited: Feb 4, 2014
    Engine knock occurs more easily at medium to higher rpm when you load the engine. It is usually caused by too lean a mixture or a fuel rated low in Octane or performance number. So if you cruise at a lean mixture and them decide to climb without adjusting the mixture, you might get detonation. If you fuel a petrol engine with kerosene instead of gasoline, and try to run it, the detonation will be catashrophic. Mostly in aircraft, if you use the right fuel and run by the book, knocking will never happen. Usually it is a car thing when an engine has gone way too long without a tune up.

    Piston slap, on the other hand, usually hapens mostly at idle and is cause usually when someone removes the bottom ring, such as the JRS piston mod for the Allison V-1710 where they remove the bottom oil ring, enlarge the third compression ring groove, and use that for the oil ring ... leaving about 4 - 5 inches of piston without the support of a ring. If you do that, expect a cracked cylinder liner at about 250 - 350 hours of operation no matter WHAT you do to make it run right.
     
  3. donkeyking

    donkeyking Member

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    thanks GregP. it is very useful
     
  4. camman

    camman New Member

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    I've run a gasoline engine on kerosene before, and there was only minor detonation. It still was not good for the engine, but it was not as catastrophic as you implied.
     
  5. donkeyking

    donkeyking Member

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    hi caman, do your know your engine's compression ratio? maybe it is not very high
     
  6. camman

    camman New Member

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    Hi donkeyking, the compression ratio is 8.5:1. However, I would attribute the low amount of detonation to several factors: 1. The engine was running with no load. 2. The engine has very little ignition advance which minimizes detonation. 3. The engine is rather small (187cc), which makes for a small combustion chamber with very little instability.
     
  7. GregP

    GregP Well-Known Member

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    Hi Donkeyking,

    It is very catastrophic in aero engines. If a gasoline engine is fueled with Jet fuel, there is a mixture in the tank od gasoline and jet fuel, which is mostly kerosene. The engine will start and you can taxi out and run up. But when you appliy full power, there is usually enough ability to run for the engines to get you airborne to about 100 - 300 feet before the tops of the piston melt out ot get blown out and you crash.

    That is a catactrophic result. The result will be the same ina lawn mower. You can stat it and it will idel for awhile seemingly just fine. But go mow a yard with it and you won;t get very far before it's new engine time. In the case of a lawn mower, it is usually very obvious since you will get an abnormal amount of smoke out of the exhaust.
     
  8. donkeyking

    donkeyking Member

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    Hi GregP,

    I still can't understand the following you told to me.
    Is the higher or medium RPM relative speed or absolute speed?

    For example, there two engines, Engine A and B. Their displacements are same, but rpm are different.

    Engine A, the highest RPM is 600. Engine B, the highest RPM is 800.

    Now we run two engines and adjust RPM to 600 for two engines.

    Because 600 is the highest speed for engine A, so at this stage, engine A is not easy to knock.
    For engine B, 600 is between medium speed (400) and the highest speed(800). So engine B is easier to knock than engine A in this case.

    Can you please correct me if I am wrong
     
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