A primer on fuel mixture theory
By Richard W. Kamm
I am constantly surprised how unknowledgeable many pilots and aircraft mechanics are concerning fuel air mixtures used in reciprocating aircraft engines. Many fables that never applied continue to be perpetuated by the uninformed. One of my least favorite is that "a lean mixture burns hotter than a rich mixture." As you should learn by the end of this article it all depends on the starting mixture, it can burn hotter, but it also can burn cooler.
To start with we must first define a few terms commonly used when discussing fuel mixtures.
Air-fuel ratio The air-fuel (expressed air to fuel) ratio is a weight relationship and can be expressed as a number or a decimal fraction. If given as a volume relationship, the number for air would be very large and would vary with every change in air density and therefore become meaningless.
Stoichiometric air-fuel ratio This is a chemical term referring to the exact mixture of air and fuel to completely combust a fuel to water and carbon dioxide. During the combustion process all the fuel and oxygen will be consumed. The stoichiometric mixture only refers to the burning characteristics of the fuel; it has nothing to do with the type of fuel metering system or engine. It varies with the type of fuel used. For gasoline and diesel fuels it is an air-fuel ratio of approximately 14.7 to 1 (fuel to air ratio of 0.067). Engine design can change how much of the available heat is converted to useful energy but it cannot change the chemical characteristics of the fuel. Modern automobiles often run in the stoichiometric range for environmental reasons, but it is of little use in aircraft because it does not result in the greatest power or the greatest economy, however it gives the highest cylinder head temperature.
Using a typical air-cooled aircraft engine running at normal power, with the throttle in a fixed position, if fuel is slowly added to the stoichiometric mixture (0.067) the added fuel will have a cooling effect and the combustion gas and cylinder head temperature will decrease. If the engine is equipped with a test club, a fixed pitch propeller or a propeller that can be put into the fixed pitch position, the propeller will act as a dynamometer and engine rpm will be an indication of power. As the mixture is enriched, the power (rpm) will increase until an F/A ratio of approximately 0.074 is reached. From 0.074 to 0.080, the power will remain relatively constant, although combustion temperatures will continue to decrease. As the mixture is further enriched above 0.080, power and combustion temperatures will both decrease. Mixtures from 0.074 to 0.080 are called best power mixtures, as their use results in the greatest power for a given airflow or manifold pressure. For further definition, 0.074 is called lean best power while 0.080 is rich best power. Do not be fooled by the designation lean best power. This is a rich mixture as it is rich of stoichiometric and the mixture is fuel cooled. Because of different induction system or combustion chamber designs the best power ratios can change slightly between engines but they always retain the same relationship rich of stoichiometric.
Unfortunately, under sea level conditions, best power mixture cannot be used at the highest engine powers. As manifold pressures are increased above the cruise power range, the combination of the heat of combustion and burning time would cause the mixture to overheat, and detonation would occur. To combat these effects, we enrich the mixture to an air/fuel ratio of approximately 0.100. Although this takes the mixture out of the best power range, it allows the use of higher manifold pressures and an increase in power results.
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