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E6BAV8R · May 8th, 2007, 22:55

I was reading http://www.avweb.com/news/pelican/182084-1.html "Mixture Magic" over on Avweb and he does some very interesting tests with mixtures and provides some very good information. However, one thing I came across shocked me:

"...where I mention how much difference mixture makes in the speed of combustion. At takeoff power, if we bring the mixture control back a bit from full rich, the rate of combustion speeds up, and puts that pressure pulse closer to TDC (Top Dead Center). Again, a very bad thing. (Combustion speed reaches maximum around 50° F to 75° F rich of peak EGT, and further leaning causes it to slow down again.)

"Our POHs instruct us to use full-rich for takeoff. The extraordinarily rich mixture is required to assure that detonation does not occur. The conventional wisdom is that the purpose of the "excess" fuel is to cool the engine, but in fact its primary purpose is to slow the combustion rate and delay the PPP, which eliminates the risk of detonation by reducing the pressure peak. This does, in fact, result in cooler operation, but that's actually a second-order effect of the delayed PPP. (If we could just retard the ignition timing for takeoff, we wouldn't need to throw all that extra fuel at the problem.)"

PPP = Point of Peak Pressure

Now, what I bolded is the part that made me think - Does the mixture really increase the rate of combustion of the spark plug? I could see that the leaner the mixture, the less fuel there would be to ignite and would need to delay the spark slightly to achieve its 'normal' peak combustion at TDC.

Nonetheless, he does provide some very good information and graphs to the points he makes and does provide some interesting views towards what GA pilots have always been taught to do with leaning and setting standard MP and RPM settings during flight.

May 8th, 2007, 22:55	#1
E6BAV8R Senior Member Join Date: Nov 2006 Location: DFW Posts: 799	The Red Knob.... I was reading http://www.avweb.com/news/pelican/182084-1.html "Mixture Magic" over on Avweb and he does some very interesting tests with mixtures and provides some very good information. However, one thing I came across shocked me: "...where I mention how much difference mixture makes in the speed of combustion. At takeoff power, if we bring the mixture control back a bit from full rich, the rate of combustion speeds up, and puts that pressure pulse closer to TDC (Top Dead Center). Again, a very bad thing. (Combustion speed reaches maximum around 50° F to 75° F rich of peak EGT, and further leaning causes it to slow down again.) "Our POHs instruct us to use full-rich for takeoff. The extraordinarily rich mixture is required to assure that detonation does not occur. The conventional wisdom is that the purpose of the "excess" fuel is to cool the engine, but in fact its primary purpose is to slow the combustion rate and delay the PPP, which eliminates the risk of detonation by reducing the pressure peak. This does, in fact, result in cooler operation, but that's actually a second-order effect of the delayed PPP. (If we could just retard the ignition timing for takeoff, we wouldn't need to throw all that extra fuel at the problem.)" PPP = Point of Peak Pressure Now, what I bolded is the part that made me think - Does the mixture really increase the rate of combustion of the spark plug? I could see that the leaner the mixture, the less fuel there would be to ignite and would need to delay the spark slightly to achieve its 'normal' peak combustion at TDC. Nonetheless, he does provide some very good information and graphs to the points he makes and does provide some interesting views towards what GA pilots have always been taught to do with leaning and setting standard MP and RPM settings during flight.