Lean mixture
Air–fuel ratio (AFR) is the mass ratio of air to fuel present in a combustion process. The fuel can be solid, liquid, or gaseous. In the first two cases, the mixture with air forms an aerosol; gas fuels form true mixtures. The combustions may cause intended or accidential explosions (dust explosion, thermobaric weapon, gas or vapour explosion) or it may be used in a controlled manner in an internal combustion engine or industrial furnace.
The air-fuel ratio (as well as the homogeneity of the mixture) determines, whether a mixture is combustible at all, how much energy is being released, and many unwanted pollutants are being produced in the reaction. Typically both a minimum and maximum ratio of fuel to air exists, below or above which ignition will not occur (lower and upper explosive limits).
In an internal combustion engine or industrial furnace, the AFR is an important measure for anti-pollution and performance-tuning reasons. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich. AFR numbers lower than stoichiometric are considered "rich". Rich mixtures are less efficient, but may produce more power and burn cooler, which is kinder on the engine. AFR numbers higher than stoichiometric are considered "lean." Lean mixtures are more efficient but may cause engine damage or premature wear and produce higher levels of nitrogen oxides. For precise AFR calculations, the oxygen content of combustion air should be specified because of possible dilution by ambient water vapor, or enrichment by oxygen additions.
In theory a stoichiometric mixture has just enough air to completely burn the available fuel. In practice this is never quite achieved, due primarily to the very short time available in an internal combustion engine for each combustion cycle. Most of the combustion process completes in approximately 4–5 milliseconds at an engine speed of 6,000 rpm. (100 revolutions per second; 10 milliseconds per revolution) This is the time that elapses from when the spark is fired until the burning of the fuel–air mix is essentially complete after some 80 degrees of crankshaft rotation. Catalytic converters are designed to work best when the exhaust gases passing through them are the result of nearly perfect combustion.
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