What should I use for my mixture settings (leaning)? Why should I stay away from 30 to 50 ROP? I run 50 ROP quite a bit. Hope I am not hurting anything. I thought running at peak was not a good thing to do.
Barry
GAMI has done a tremendous amount of research on this. They offer an excellent (but rather expensive) class that illustrates the issue on an actual running engine, in a test cell. Here are the key points, as I understand them:
EGT is not a limiting factor on non-turbocharged engines. In other words, you cannot hurt anything with “high” EGT on a normally aspirated engine. The EGT just can’t get high enough. The only EGT limitation is about 1,600 degrees at the entry to the turbine section of a turbocharger.
The EGT that you see on a gauge is only indirectly linked to the gas temperature that actually exists at the exhaust valve inside the cylinder. The gauge indication is dependent on probe placement, engine timing, cylinder size, mixture leaning, RPM level, throttle position, etc. For example, you can have much higher internal combustion chamber and valve temps with one set of parameters than another, while the displayed EGT is the same for both sets of parameters (because the resulting gas temp at the probe is the same in both cases).
At 30 to 40 degrees rich of peak, the mixture burn rate is the fastest it can get. That results in a more rapid pressure rise in the cylinder.
Because of the rapid rise, peak pressure occurs before the rod has reached the optimum angle to push on the crankshaft (greatest leverage). In turn, this creates an even higher pressure spike, because the gas is expanding rapidly but it can’t move the piston down fast enough to get more room for the gas bubble. The result is a distinct high-pressure spike within the cylinder, and higher temps on the head of the exhaust (and intake) valve; but since more of the burn has occurred before the exhaust valve opens, the displayed EGT is actually lower than “peak”. Despite the “lower than peak” displayed temp, the actual cylinder pressures and cylinder/valve temps are the highest that they can get for the circumstances. That’s why, despite the literature that constantly recommends “50 rich of peak”, it is almost the worst place to run an engine.
If you want absolute best economy, you want to run as far lean of peak as circumstances permit (carbureted versus fuel injected, headwind or tailwind, degree of engine roughness, etc.). You might do this for a maximum-range trip with a tailwind, that enables you to skip a fuel stop, which in turn can add an effective 20 to 40 knots to your door-to-door speed.
If you want the best compromise between economy, power, and speed, you want to run at indicated peak EGT. This is basically slightly leaner than the stoichometric mixture. The burn is most complete, because there is just slightly more air than fuel, with a resulting slightly slower burn rate. The cylinder pressure is better spread over the ideal rod angle, and the slowed burn means that the gases are still a bit hotter when they hit the EGT probe. Absolute peak cylinder pressures are not reached because of the slower burn and improved rod angle.
If you want best power without hitting the pressure spike with its high internal cylinder loads, you want to run at least 100 degrees rich of peak. This slows the burn enough to avoid the spike, while providing enough extra fuel to burn 100% of the available air as the cylinder continues down the power stroke. It doesn’t give you more power than the 50-rich point; it just gives you the same power without the internal cylinder pressure spike.
Their research is the basis for GAMI’s “core approach” to a more modern variable-timing approach to engine ignition. All the other systems (only the LASAR is certified so far) are using preprogrammed timing curves, along with throttle setting, manifold pressure and temperature, and RPM, to determine the ideal ignition timing. In stark contrast, GAMI’s approach is to equip every cylinder with a pressure transducer. Last I heard this was based on fiberoptics. Their ignition computer simply watches for the characteristic “signature” of the pressure spike, and retards timing as needed to prevent it. Consequently, this enables their system to use the optimum advance for any set of operating circumstances. It is conceptually similar to the “knock detection” sensors on modern auto engines. With the GAMI set-up, extremely lean mixtures that cause a very slow burn rate will automatically trigger significantly higher spark advance to compensate for the slower burn. Anything that causes a faster burn rate will generate reduced timing advance to prevent the pressure spike. Detonation is automatically prevented, as it is a subsequent result of the pressure spike situation. From what I can see, the GAMI product will be a quantum leap in aircraft ignition, if or when they get it to market.