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| [GMCnet] High EGT [message #264268] |
Wed, 15 October 2014 06:41  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
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Senior Member |
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Just ran across a possible explanation for my Cad 500's high EGT's in an
article by Roger Zerkle in Ultra-Flight magazine. He's specifically
addressing 2-stroke engine, but the principle seems applicable:
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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.
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I have noticed that I don't get higher EGT's when climbing hills, though I
don't recall seeing lower readings. I've always thought that I could
re-install a 3.07 final drive with the Cad 500; now I think I SHOULD -- at
least for a trial. This also plays well with a comment Doc Frohmader made
to me about engine loading and wasted energy out the exhaust.
Ken H.
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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