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However, 2 questions still remain, and I hope I can still get some feedback:
1. In regards to ignition timing, I read (and anonman mentionned too) how if you increase RPM, ideally the spark would ignite earlier as the pistons are travelling faster; likewise, if you decrease RPM the pistons travel slower and you want the spark to occur a little later to ensure that the peak pressure point does not occur too close to TDC... however, Deakin says that as you lean your mixture, the combustion cycle speeds up, and this increases the risk of detonation because he says the peak pressure point is now closer to TDC.... this all seems contradictory to what I mentionned above about RPM (i.e. high RPM is associated with faster combustion cycles but not with detonation... operating at high MP and low RPM risks detonation, not the other way around).
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It is not correct to say that high RPM is associated with faster combustion cycles. The speed of combustion at a certain mixture setting will remain fairly constant. This is the reason we would like to advance the timing at high rpm. Since it takes awhile for the flame front to travel throughout the cylinder and pressure to rise, we 'start the fire' sooner. If we do not do this, then our engine will lose power.
The problem with almost all recip aviation engines is that, unlike modern automobile and motorcycle engines, ignition timing is fixed. In other words, in your car, your ignition timing is advanced at high rpm operations and retarded during low rpm cruising. This delivers optimum performance and economy. Most modern car engines also have 'knock sensors' mounted on the engine block that can sense detonation and retard the timing. Combined with the oxygen sensors in the exhaust system, these engines are continually adjusting the mixture and the spark timing for optimum performance.
In aviation engines, spark timing is fixed. It never changes. Ignition timing is optimized for full throttle, high rpm operations (high RPM is a relative term, since our engines have a low redline compared to car engines that can rev to 6-9,000rpm and motorcycles that redline at 16,000). This gives the most power during takeoff and helps avoid detonation during full power operation. When we cruise at a lower rpm, it would be nice to be able to retard the timing a little, but this is not possible.
Even though spark timing is fixed, we can still control the speed of combustion with the mixture. If we run very rich, the fuel/air mixture will burn fairly slowly. This has the same effect of retarding the spark timing. If we lean, the fuel/air mixture will burn faster. This is similar to the effect of advancing the timing. If we lean beyond a certain point, the fuel/air mixture burns slower again, effectively retarding the timing.
If we go out and take off at full power with a lean mixture (but not lean of peak EGT), the speed at which the fuel/air mix in the cylinders burns will increase. This can cause peak pressure to occur too early, when the piston is not sufficiently past TDC. Detonation or engine damage can result. This is why we run rich at takeoff.
If we try to cruise at an excessively high MAP, and low RPM we can again cause peak pressure to occur while the piston is too close to TDC, leading to engine killing detonation. If we run at high MAP, low RPM, with a lean mixture, we are even more likely to damage the engine, since the peak pressure will occur even closer to TDC. This is why we use a power chart to determine the correct MAP/RPM combination and carefully lean in cruise. (It is important to remember that below about 65% power, cylinder pressures will be low enough, that it becomes very difficult to cause engine damage with improper leaning.)
Confused yet? REMEMBER, NO MATTER WHAT RPM WE ARE OPERATING AT, WE ALWAYS WANT PEAK CYLINDER PRESSURE TO OCCUR AT ABOUT THE SAME NUMBER OF DEGREES AFTER TDC. Since our ignition timing is fixed, changing RPM or adjusting the mixture will both affect the rate this pressure rise occurs.