Brent wrote: Aircraft engines have dual ignition systems
for increased detonation margins, NOT for reliability from redundancy.
Aircraft combustion chambers are so freaking big that by the time a flame
front has traversed the chamber the remaining gas has been compressed and
heated to the point of detonation. Starting the fire in two places
shortens the combustion time significantly and reduces the opportunity for
detonation. This is also the mechanism for the RPM drop during a mag
check. When you turn off one mag the combustion event takes longer which
has a similar effect as starting the process later, AKA "retarding the
timing".
Hi Brent,
From what little I know about the combustion process, the above paragraph
seems to contradict it's self. Retarding the spark timing helps to prevent
detonation. If turning off one mag has the same effect as retarding the timing
(as you stated above), wouldn't running on one mag help to prevent detonation
rather than contribute to it? After all, peak cylinder pressure will be lower
while running on one mag and thetaPP will move further away from TDC. Lower
pressure means lower CHT's which should also improve
the detonation margin. I think redundancy and improved fuel efficiency may
be the reason two mags are used on aircraft engines.
Ron
Laughlin
Legacy N44L (res)
Ron, in response to a good question. If the engine is tuned for dual
ignition, that is, the timing is set to produce best power and best economy when
both plugs are firing, then shutting off one plug will retard the effective
timing and reduce the engines tendency to detonate. If the engine is tuned
to run on one plug, the timing must be advanced relative to the dual ignition
timing to get back to best power and best economy. The engine will be more
likely to detonate at this more advanced timing even though it may be making
slightly less power. Therefore it is correct to say a dual ignition engine will
increase the detonation margin over a single ignition engine. In my opinion,
what is missing is, this is true when both engines have there timing set to
produce best power and economy.
Craig Berland