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"Luckily, your plane won't be flying on the
dyno."
While you don't fly "on the dyno", you are flying
"a dyno". Albeit a difficult one to calibrate. For a given aircraft,
airspeed is horsepower. For a given RPM torque can be calculated from HP. If you
have a fuel flow meter, you can calculate BSFC.
"Can you
explain why spark advance is a good idea in auto engines and a bad one in
airplanes?"
It is not bad
idea, it is good idea...just like in cars. The difference is how the engines are
used and therefore the potential for improvement. Cars spend a lot of there
time at low RPM and low MAP (part throttle). For this reason, to get best
performance/best economy, they need a very elaborate spark timing control. For
example a car engine....WOT at 1500 RPM may require 18 deg BTDC, WOT at
6000 RPM may require 28 deg BTDC and 2500 RPM at closed throttle may require 40
deg BTDC and finally it will want to crank at 0 deg BTDC for best
performance. If an electronic ignition on an aircraft can give "best"
timing for all conditions and be reliable, it is better than a fixed timing
mag. However, the benefits are not as dramatic on airplanes as they are on
cars because most of our time in airplanes is spent at 2500 to 2700 RPM and near
WOT. The mags can be set at optimum timing for that condition. Non turbo engines
benefit more than turbo engines from a variable timing curve. On non turbo
engines, altitude has the same affect as closing the throttle. The other benefit
electronic ignition may have is more spark energy. When running real rich
or real lean, the ignition system is being tested as to whether or not it can
fire the plug or ignite the mixture. If you can deliver 3X the spark energy to
the plug....it will fire at more rich and more lean conditions as well as poorer
plugs will fire. I believe Walter pointed out, the higher spark energy
does increase the burn rate slightly, therefore the timing curve must be
appropriate for the ignition system. Otherwise you may have timing advance
that can be harmful to engine longevity.
The concern I
have with current electronic ignitions is reliability, not "potential"
performance. If I could pick the "best" ignition, it would have 12 coils
and very short secondary leads, controlled by a closed loop computer looking at
internal cylinder pressure. The computer would be programmed for my
specific application by a "good" engine development
engineer.
Sad to say....I
now have mags. After 15 years of developing car engines with computer
tables that included a 1000 variables that modified spark timing....mags do seem
like a step backwards.
Craig
Berland
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