What is the Stiochiometric Air-fuel Ratio and Air-Fuel Equivalence Ratio (λ)?

Internal combustion engines burn fuel to create kinetic energy. The burning of fuel is basically the reaction of fuel with oxygen in the air. The amount of oxygen present in the cylinder is the limiting factor for the amount of fuel that can be burnt. If there’s too much fuel present, not all fuel will be burnt and un-burnt fuel will be pushed out through the exhaust valve.

Let us have a look at the oxidation reaction of methane (natural gas) as a fuel.

CH4 + 2(O₂) → CO₂ + 2(H₂0)

If we look up the atomic weights of the atoms that make up octane and oxygen, we get the following numbers:

Carbon (C) = 12.01

Oxygen (O) = 16

Hydrogen (H) = 1.008

So 1 molecule of methane has a molecular weight of: 1*12.01 + 4*1.008 = 16.042 ,One oxygen molecule weighs: 2 * 16 = 32 .So, the oxygen &fuel mass ratio is then: (2 * 32) / (1 * 16.042) = 64 / 16.042 =3.99

So we need 3.99 kg of oxygen for every 1 kg of fuel (Methane)

Since 23.2 mass-percent of air is actually oxygen, we need : 3.99 * (100/23.2)       = 17.2 kg air for every 1 kg of methane.

So the stoichiometric air-fuel ratio of methane is 17.2.

So, If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture. A 'Stoichiometric' AFR has the correct amount of air and fuel to produce a chemically complete combustion event. For gasoline engines, the stoichiometric, A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel.

A highly accurate air-fuel ratio control scheme has been developed for reducing exhaust emissions from gas engines. So what is meant by a rich or lean AFR? For natural gas engine a lower AFR number contains less air than the 17.2:1 stoichiometric AFR, therefore it is a richer mixture. Conversely, a higher AFR number contains more air and therefore it is a leaner mixture.

For Example:(Source: http://en.wikipedia.org/wiki/Stoichiometric)

18:1 = Lean

17.2:1 = Stoichiometric

16:1 = Rich

Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated spark-ignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.

Air-Fuel Equivalence Ratio (λ)

Air-Fuel equivalence ratio, λ, is the ratio of actual AFR to stoichiometry for a given mixture. λ= 1.0 is at stoichiometry, rich mixtures λ < 1.0, and lean mixtures λ > 1.0.

There is a direct relationship between λ and AFR. To calculate AFR from a given λ, multiply the measured λ by the stoichiometric AFR for that fuel.