Air-fuel ratio

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In internal combustion engines, the air-fuel ratio refers to the proportion of air and fuel present during combustion. The chemically optimal point at which this happens is the stoichiometric ratio (sometimes referred to as stoich), where all the fuel and all the oxygen content in the air of the combustion chamber will perfectly balance each other out during combustion.

For gasoline fuel, the stoichiometric air/fuel mixture is approximately 14.7 times the mass of air to fuel. This is the mixture that modern engine management systems employing fuel injection attempt to achieve in light load cruise situations. Any mixture less than 14.7 to 1 is considered to be a rich mixture, any more than 14.7 to 1 is a lean mixture -- given perfect (ideal) "test" fuel (gasoline consisting of solely n-heptane and iso-octane). In reality, most fuels consist of a combination of heptane, octane, a handful of other -tanes, plus additives including detergents, and possibly oxygenators such as MTBE (Methyl tert-butyl ether) or ethanol/methanol. These compounds all alter the stoichiometric ratio, with most of the additives pushing the ratio downward (oxygenators bring extra oxygen to the combustion event in liquid form that is released at time of combustions; for MTBE-laden fuel, a stoichiometric ratio can be as low as 14.1:1). Vehicles using an oxygen sensor(s) or other feedback-loop to control fuel to air ratios (usually by controlling fuel volume) will usually compensate automatically for this change in the fuel's stoichiometric rate by measuring the exhaust gas composition, while vehicles without such controls (such as most motorcycles, and cars predating the mid-1970's) may have difficulties running certain boutique blends of fuels (esp. winter fuels used in some areas) and may need to be rejetted (or otherwise have the fueling ratios altered) to compensate for special botique fuel mixes. Vehicles using oxygen sensors enable the air-fuel ratio to be monitored by means of an air fuel ratio meter.

In industrial fired heaters, power plant steam generators, and large gas-fired turbines, the more common term is percent excess combustion air. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air is being used.

1 Synopsis
2 Other terms used
3 See also
4 External links

[edit] Synopsis

In theory a stoich 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 6000 rpm. 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 show nearly perfect combustion has taken place.

A stoichiometric mixture unfortunately burns very hot and can damage engine components if the engine is placed under high load at this fuel air mixture. Due to the high temperatures at this mixture, detonation of the fuel air mix shortly after maximum cylinder pressure is possible under high load (referred to as knocking or pinging). Detonation can cause serious engine damage as the uncontrolled burning of the fuel air mix can create very high pressures in the cylinder. As a consequence stoichiometric mixtures are only used under light load conditions with more fuel added for acceleration and high load condition to prevent detonation and cool down the combustion gasses.

In the typical air to natural gas combustion burner, a double cross limit strategy is employed to insure ratio control. This strategy involves adding the opposide flow feedback into the limiting contol of the respective gases (air or gas). This assures ratio contol within an acceptable margin.

[edit] Other terms used

There are other terms commonly used when discussion the mixture of air and fuel in internal combustion engines. However, “Stoich” is also known as a slang word.

[edit] AFR

The Air fuel ratio is the most common reference term used for mixtures in internal combustion engines. It is the ratio between the mass of air and the mass of fuel in the fuel-air mix at any given moment.

For octane the stochiometric mixture is 14.7:1 at sea level

In Naturally Aspirated engines powered by octane, maximum power is frequently reached at AFR's ranging from 12.5 - 13.3:1

[edit] FAR

Fuel Air ratio is frequently used in government studies of internal combustion engine and refers to the ratio of fuel to the air, it is 1/AFR.

[edit] Lambda

Is the common term used when there may be more than one fuel involved. It standardizes the AFR to the stochiometric mixture for the specific combination of fuels in use. Any fuel at stoichiometric mixture gives a Lambda of 1.

Lambda values greater than 1.0 indicate excess air and are called lean mixtures. Lambda values less than 1.0 indicate excess fuel for complete combustion, and are called rich mixtures.

[edit] Equivalence ratio

Is the common term used when there may be more than one fuel involved. It standardizes the FAR to stochiometric mixture for the specific combination of fuels in use. Any stoichiometric fuel air mixture gives an Equivalence ratio of 1.

Equivalence numbers greater than one represent excess fuel in the fuel air mix. Equivalence numbers less than 1 represent a deficiency of fuel in the fuel air mix for complete combustion.