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Old 09-17-2010, 04:38 PM
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nomocom nomocom is offline
Join Date: Jun 2009
Location: Caldwell ID
Posts: 264
Default Pelican's perch #43

Originally Posted by N941WR View Post

Under heavy load, such as at full power takeoff, most EI's limit the advance, 24 degrees in your example, to preclude pre-ignition.

At high altitudes and lower power settings, such as at cruise, most EI's will allow for greater advance settings.

The idea is that there is less fuel & air in the cylinders when operating at lower MAP's and it takes a longer time for the flame to propagate across the entire cylinder, so they can start it burning sooner.

At high power settings (MAP), the fuel/air mixture will burn faster because there is more fuel/air. Thus the EI's delay when to ignite the mixture until closer to TDC (Top Dead Center). If the mixture burns too fast or too far before TDC, the maximum pressure could be well before TDC, which you hear as engine destroying pinging.
I suspect you mean to preclude detonation? Easy to get the two confused, but if ignition wires are routed correctly and the timing chip not scrabbled, then its really detonation we are worried about, not so much pre-ignition.

IMHO, the best description of what goes on in our Lycoming cylinders is by Deakin, "Detonation Myths" Probably a good read in terms of understanding what is at stake with our choice of timing advance.

Deakin also points out that you won't hear pinging in an aviation big bore, as the frequencies produced are linked to the diameter of the chamber, thus the noises are lower frequency, and close to the usual noises coming from the engine.

I appreciate his detailed explanation of the different levels of detonation coupled with the timing and pressure maps of the cylinder. Even if you've got heavy detonation, don't expect to hear it, you might see really high CHT's if you are paying attention and have a probe in the offending cylinder. Otherwise, when you have the severe detonation long enough, the burnt valve or hole in the cylinder may be the first clue.
1990 RV-3 (now apart, upgrades in the works)
1959 C172 O-360
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