Air-Fuel Ratio

In a combustion process, the air–fuel ratio (AFR) is the mass ratio of air to a solid, liquid, or gaseous fuel. The combustion can be controlled, as in an internal combustion engine or an industrial furnace, or it can end in an explosion (e.g., a dust explosion, gas or vapour explosion or in a thermobaric weapon).

The air–fuel ratio controls whether a mixture is combustible, how much energy is released, and how much pollution is created during the reaction. In most cases, there is a range of fuel to air ratios beyond which ignition will not occur.

The air–fuel ratio is an important parameter for anti-pollution and performance optimization in an internal combustion engine or industrial furnace. The stoichiometric mixture, shortened to stoich, is created when exactly enough air is delivered to totally burn all of the fuel. Rich ratios are those that are less than stoichiometric. Rich combinations are inefficient, but they can produce more power and burn at a lower temperature. “Lean” ratios are those that are higher than stoichiometric. Lean mixtures are more efficient, but they also result in lower temperatures, which can lead to nitrogen oxide generation. Lean-burn capabilities are built into several engines. Because of differences in air density due to altitude or intake air temperature, probable dilution by ambient water vapour, or enrichment by oxygen additions, the oxygen content of combustion air should be given for exact air–fuel ratio calculations. 

Internal combustion engine:

A stoichiometric mixture, in theory, has just enough air to burn the available fuel completely. In practice, this is never exactly reached, owing to the limited time allowed for each combustion cycle in an internal combustion engine. At 6,000 revolutions per minute, the majority of the combustion process is finished in around 2 milliseconds. (100 revolutions per second; 10 milliseconds each crankshaft revolution, equating to 5 milliseconds for each piston stroke in a four-stroke engine). This is the time it takes from the ignition of the spark plug to the combustion of 90% of the fuel–air mixture, which usually takes about 80 degrees of crankshaft rotation. Catalytic converters perform best when the exhaust gases going through them are virtually perfect.

If the engine is put under heavy load at this fuel–air combination, a precisely stoichiometric mixture burns very hot and can destroy engine components. Detonation of the fuel-air mix while approaching or shortly after maximum cylinder pressure is possible under high load (referred to as banging or pinging), specifically a “pre-detonation” event in the context of a spark-ignition engine type, due to the high temperatures at this composition. Because the uncontrolled combustion of the fuel-air combination can create extremely high pressures in the cylinder, detonation can cause significant engine damage. As a result, stoichiometric mixes are only employed when the load is small to low-moderate. A richer mixture (lower air–fuel ratio) is utilized for acceleration and high-load circumstances to produce cooler combustion products (using evaporative cooling), avoiding overheating of the cylinder head and thereby preventing detonation. 

Mixture:

In the aviation world, the word “mixture” appears frequently in training books, operation manuals, and maintenance manuals.

At any given time, the air–fuel ratio is the ratio between the mass of air and the mass of fuel in the fuel–air mix. The mass refers to the total mass of all combustible and non-combustible ingredients that make up the fuel and air. In determining the value of mfuel, for example, a computation of the mass of natural gas—which commonly comprises carbon dioxide (CO2), nitrogen (N2), and various alkanes—includes the mass of carbon dioxide, nitrogen, and all alkanes.

The stoichiometric mixture for pure octane is roughly 15.1:1

Maximum power is usually achieved in naturally aspirated engines fuelled by octane with AFRs ranging from 12.5 to 13.3:1 or λ 0.850 to 0.901.

The 12:1 air-fuel ratio is regarded as the maximum output ratio, while the 16:1 air-fuel ratio is considered the maximum fuel economy ratio.

Conclusion:

For a gasoline engine, the stoichiometric mixture is the optimal air-to-fuel ratio that burns all of the fuel with no surplus air. The combustion can be controlled, as in an internal combustion engine or an industrial furnace, or it can end in an explosion. The air–fuel ratio controls whether a mixture is combustible, how much energy is released, and how much pollution is created during the reaction.

The air–fuel ratio is an important parameter for anti-pollution and performance optimization in an internal combustion engine or industrial furnace. Rich combinations are inefficient, but they can produce more power and burn at a lower temperature. “Lean” ratios are those that are higher than stoichiometric.

If the engine is put under heavy load at this fuel–air combination, a precisely stoichiometric mixture burns very hot and can destroy engine components. Detonation of the fuel-air mix while approaching or shortly after maximum cylinder pressure is possible under high load (referred to as banging or pinging), specifically a “pre-detonation” event in the context of a spark-ignition engine type, due to the high temperatures at this composition.

At any given time, the air–fuel ratio is the ratio between the mass of air and the mass of fuel in the fuel–air mix. The mass refers to the total mass of all combustible and non-combustible ingredients that make up the fuel and air.