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Thread: polini cooling

  1. #11
    Diamond geezer 500 Club
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    polini cooling

    Mick... also got me that we are talking about "pulse supercharged" engine...................

    Partial quote to promote more thoughts, from
    http://www.challengers101.com/ExhaustGasTemp.html
    (Actually a reprint of an article from the April 2003 issue of Ultra-Flight Magazine. )
    .........
    Even though, exhaust systems for aircraft engines are designed to provide power over a wide RPM range, there will be operating ranges where the reflected pressure pulses will not be arriving at the exhaust port at the right time. The high and low pressure waves will shift one way or the other inside the exhaust. This shift will cause a relocation in the hot exhaust gas pulses, as well, and can cause spikes in EGT readings at certain, rather narrow, engine RPM ranges.

    These spikes in EGT, either up or down, generally have nothing to do with fuel-air mixture. The temperature of the burning gasses in the combustion chamber may not have changed at all. But in the area of the exhaust system where the EGT sensing probe is located, the pressure pulses are moving the gasses in such a way as to create radical changes in EGT readings.

    The most common, and misunderstood, condition affecting EGT is engine loading. If the engine is not loaded sufficiently, it will run high EGTs. If the mixture is richened, the EGTs will go up, rather than down. A lightly loaded engine can be enriched to the point that it will barely run, and still have high EGTs. The explanation is found in one of Newton's laws. Simply stated, it says that energy cannot be created, nor destroyed.

    When we burn a fuel-air mixture inside our engine, we are converting chemical energy into heat energy. When applying the pressure created by the expansion of the burning gasses inside the combustion chamber, to a movable piston, we are converting a portion of the heat energy to mechanical energy. The heat energy not used in turning the crankshaft is given off through the exhaust and the engine cooling system. If we add all this up, it would equal the amount of energy present in the fuel before it was introduced into the engine.

    So it would be correct to assume that, for the same amount of fuel-air mixture being burned in the combustion chamber, if less heat energy is being utilized to turn the propeller, then more heat energy will be going out the exhaust. In other words, a lightly loaded engine, will throw considerably more heat out of the exhaust than the same engine, with the same throttle setting, carrying a heavier load. So, not only will a heavier loaded two-stroke engine have lower EGTs, but the engine will produce more power for the same amount of fuel burned. If a lightly loaded engine, with a high EGT, has the mixture enriched, the extra fuel will go out the exhaust, increasing the EGT even more.

    .................................................. ...........................

  2. #12
    Senior Member 500 Club
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    polini cooling

    HI Wally,

    Thanks for that

    Now that makes some sense

    Mick

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