Air–fuel ratio

The air–fuel ratio determines whether a mixture is combustible at all, how much energy is being released, and how much unwanted pollutants are produced in the reaction.

In an internal combustion engine or industrial furnace, the air–fuel ratio is an important measure for anti-pollution and performance-tuning reasons.

Also common, narrow band meters in round housings with the standard mounting 52 and 67 mm (2+1⁄16 and 2+5⁄8 in) diameters, as other types of car 'gauges'.

This is the time that elapses from the spark plug firing until 90% of the fuel–air mix is combusted, typically some 80 degrees of crankshaft rotation later.

Catalytic converters are designed to work best when the exhaust gases passing through them are the result of nearly perfect combustion.

Due to the high temperatures at this mixture, the detonation of the fuel-air mix while approaching or shortly after maximum cylinder pressure is possible under high load (referred to as knocking or pinging), specifically a "pre-detonation" event in the context of a spark-ignition engine model.

Such detonation can cause serious engine damage as the uncontrolled burning of the fuel-air mix can create very high pressures in the cylinder.

For acceleration and high-load conditions, a richer mixture (lower air–fuel ratio) is used to produce cooler combustion products (thereby utilizing evaporative cooling), and so avoid overheating of the cylinder head, and thus prevent detonation.

In reality, most fuels consist of a combination of heptane, octane, a handful of other alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (methyl tert-butyl ether) or ethanol/methanol.

In the typical air to natural gas combustion burner, a double-cross limit strategy is employed to ensure ratio control.

[citation needed] The strategy involves adding the opposite flow feedback into the limiting control of the respective gas (air or fuel).

There are other terms commonly used when discussing the mixture of air and fuel in internal combustion engines.

Most practical AFR devices actually measure the amount of residual oxygen (for lean mixes) or unburnt hydrocarbons (for rich mixtures) in the exhaust gas.

The fuel–air equivalence ratio is related to the air–fuel equivalence ratio (defined previously) as follows: The relative amounts of oxygen enrichment and fuel dilution can be quantified by the mixture fraction, Z, defined as where YF,0 and YO,0 represent the fuel and oxidizer mass fractions at the inlet, WF and WO are the species molecular weights, and vF and vO are the fuel and oxygen stoichiometric coefficients, respectively.