Air-fuel ratio (
AFR) is the mass ratio of air to fuel present during combustion. When all the fuel is combined with all the free oxygen, typically within a vehicle's combustion chamber, the mixture is chemically balanced and this AFR is called the
stoichiometric mixture (often abbreviated to
stoich). AFR is an important measure for anti-pollution and performance tuning reasons.
Lambda (λ) is an alternative way to represent AFR.
For
gasoline fuel, the stoichiometric air/fuel mixture is approximately 14.7 times the mass of air to fuel. 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
alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (
Methyl tertiary-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 boutique fuel mixes. Vehicles using
oxygen sensors enable the air-fuel ratio to be monitored by means of an
air fuel ratio meter.
Lean mixtures produce cooler combustion gases than does a stoichiometric mixture, primarily due to the excessive dilution by unconsumed oxygen and its associated nitrogen. Rich mixtures also produce cooler combustion gases than does a stoichiometric mixture, primarily due to the excessive amount of carbon which oxidises to form carbon monoxide, rather than carbon dioxide. The chemical reaction oxidising carbon to form carbon monoxide releases significantly less heat than the similar reaction to form carbon dioxide. (Carbon monoxide retains significant potential chemical energy. It is itself a fuel whereas carbon dioxide is not.) Lean mixtures and rich mixtures, when consumed in an internal combustion engine, both produce less power than does the stoichiometric mixture. Similarly, lean mixtures and rich mixtures return poorer fuel efficiency than the best mixture. (The mixture for the best fuel efficiency is slightly different to the stoichiometric mixture.)
