Page 30 - COPA_July2023

COPA U CORNER
Pedal geometry –The position where the best push should occur in the combustion event.
air inside the cylinder are correct, casually pulling the prop around is sufficient to ignite the fuel and start the engine.
A typical Cirrus cruise engine speed is 2,500 RPM (revolu- tions per minute). At that speed, it takes 0.024 seconds (24 milliseconds) to make one revolution. In those 360 degrees of revolution, the best time for the combustion peak pressure to occur is around 20 degrees past when the piston is at top center on compression stroke.
The combustion event takes about 0.006 seconds (6 millisec- onds), beginning with the spark plug fire. That’s one-fourth of one revolution at 2,500 RPM, which means the spark needs to occur sometime before the piston reaches top center. Magnetos on our engines are fixed timing at 22 or 24 degrees before top dead center (TDC) to get the maximum combus- tion pressure to occur near those 20 degrees past top center location, just where it needs to be at 2,500 RPM.
Magneto timing doesn’t change during aircraft engine opera- tion as is done on our car gasoline engines. If the engine data plate timing calls for 22 degrees, then that’s what the mechanic sets and that’s where it stays during all modes of engine operation. This means that our aircraft engines only have ideal timing at one engine RPM. This optimal RPM is the maximum red line RPM as shown in the aircraft’s operating handbook. Operation at typical cruise RPM isn’t optimal but works well enough.
This fixed timing becomes a problem at idle speeds and especially during the start event. The electric starter might only turn the engine at 200 RPM, a whopping 0.3 seconds
“ ... casually pulling the prop around is sufficient to ignite the fuel and start the engine.”
(300 milliseconds) per revolution. At that slow speed, the entire combustion event will happen before the piston gets anywhere near top center. If this were allowed to happen, the engine would turn backward. Certainly not conducive to successful engine starting.
The inside of the coupling showing the two pawls.
What we need is something that can delay the spark till after the piston passes top center but only during the starting process. Enter the impulse coupling – an ingenious device that halts the magneto’s rotation for about 25 to 30 degrees of crankshaft rotation (depending on the application) but disengages as the engine speed reaches about 375+ RPM. The impulse coupling is beautiful in its simplicity of design. Of course, that means only some engineering genius could create such a simple device to satisfy such complicated needs. As best I can tell, the original patent for the impulse coupling was filed on
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JULY 2023 COPA Pilot