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Diesel knock sensors ever posible? ALL

I was wondering if a knock sensor could posably work on a Diesel. (I realize this is just a theoritical question) I assume the knocking of a properly running diesel would be too strong to detect any preignition?

Bill D.








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    Re: Diesel knock sensors ever posible? ALL

    "Pinging"/"knocking" in a spark ignited engine occurs when the heat and pressure of the fuel mixture that was lit by the spark plug rises to a point which exceeds the flash point of the remaining fuel mixture, causing it to ignite in a secondary location, often at the far end where the accumulated pressure is "trapped", instead of a smooth burn across the cylinder. These two separate areas of combustion (they're not explosions) collide and make one helluva noise. It's the shock wave that you hear. The measure of octane is a fuel's resistance to doing EXACTLY this detonation under pressure...the more it can stand without lighting itself off, the higher the octane number. All the cylinder conditions at the instant of combustion have their effect, and changing ignition timing is one of the most effective ways to reduce it, so there's an easy remedy.

    A Diesel engine works by creating so much heat between the molecules of air by compressing it 2-3 times smaller than in a gasoline engine that combustion can occur, and then spraying a fine mist of fuel into the cylinder. The fuel is always being consumed (oxidized) at the injector as it is being metered. Unlike a gasoline engine, the fuel is not already in the cylinder when combustion starts so you CANNOT have "pinging" in a Diesel for the reasons you get it in a spark motor. That's not to say that Diesels don't make lotsa noise, we all know they do, but it's because the pressure rise is much higher and at a different rate.

    I know that was a long way around the block to simply say "no" to your question.








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      Re: Diesel knock sensors ever posible? ALL

      As a secondary question, though...

      If you crank the injection timing forward on a diesel engine, you end up with a very audible rapping noise, above and beyond the normal combustion noise, which should be crisp sounding but not a hammering sound. In this way, the fuel is being injected before the ideal time, and starting to ignite before the proper time.

      Similarly, when you back off on the injection timing, a diesel engine becomes very quiet, and the combustion clatter becomes almost muffled. The engine also doesn't run well, and smokes white (unburned).

      Improper (advanced) injection timing was what killed a good share of VW/AUdi (Volvo) diesels in the mid-eighties.








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        Re: Diesel knock sensors ever posible? ALL

        You also need to remember that it is only in the last 10 years or so that diesel technology has advanced since probably 1930. Listen to a new highway tractor, and watch the stacks compared to ten years ago. No smoke, quiet, powerful, economical- all due to the electronic controls added. The new GM pickup diesel uses "common rail" injectors, which I presume means electronic, and it would not surprise me if they use a knock sensor tuned to specific resonances- even a gas engine must use a specific knock sensor based on engine type, sensor location,etc.







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