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My (former) mechanic had trouble keeping this
straight. The spark event is referenced to top dead center, so when the
timing is set to 22 degrees before top dead center on the compression stroke,
the plug fires while the piston is still traveling upward on the compression
stroke. By the time the fuel starts burning, the piston has passed top
dead center and is moving back down the barrel. If the timing is set to 28
degrees, that means the plug fires earlier, when the crank has another 28
degrees to turn before top dead center, and that's why the spark is said to be
more advanced.
When you advance the spark that much, the fire
starts earlier, possibly even before the piston gets to top dead center.
If the peak cylinder pressure occurs at top dead center, the expanding gas
pushes on a piston that has nowhere to go. The piston is directly above
the connecting rod which is directly above the crankshaft, and the effective
lever on which the piston pushes has a length of zero inches. Since work
equals force times distance, at that instant in time no work is done, so all the
energy in the burning fuel is imparted to the cylinder, and none to mechanical
work. That's why cylinder head temperatures rise and EGT and power
fall if the spark is advanced too much.
If the spark is excessively retarded, the moment of
peak cylinder pressure occurs late, when the piston is well along its path to
the bottom of the cylinder. Although the effective lever arm of the
connecting rod is much longer (it is of course at its longest at 90 degrees
of crank travel), the distance over which the force of the expanding gas can do
work on the piston is less, so less work is done. This leads to increased
EGT (the fire is still burning when the exhaust valve opens), much lower
cylinder head temperature, and less power.
Optimal spark advance will thus vary with the speed
of flame front propagation (which depends on lots of secondary factors such as
compression ratio, induction temperature, temperature of the combustion
chamber) and angular velocity of the crank.
When George Braly talks about theta PP, he's
referring to the point at which peak cylinder pressure occurs during the
combustion event. Controlling theta PP is how to optimize fuel efficiency
and engine temperatures, and this can be done in our fixed timing aircraft
engines only by adjusting the speed of flame front propagation, which is done
with the mixture control. Non-stoichiometric mixtures burn more slowly
than stoichiometric mixtures. When PRISM is available, theta PP will be
optimized by varying spark advance instead of by changing the speed of
flame-front propagation with the red knob. That's also why these engines
will be able to burn lower octane (ie, faster burning) fuel. The
controller will retard the spark to give the same theta PP with a faster burning
fuel.
Jonathan Fuller
N1538G
TNIO-550B A36
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