Thermal engines use fuel and oxygen (from air) to produce energy through combustion. To guarantee the combustion process, certain quantities of fuel and air need to be supplied in the combustion chamber. A complete combustion takes place when all the fuel is burned, in the exhaust gas there will be no quantities of unburned fuel. Air-fuel ratio (AF or AFR) is the ratio between the mass of air and mass fuel, used by the engine when running.
The ideal (theoretical) air-fuel ratio, for a complete combustion, is called stoichiometric air-fuel ratio. For a gasoline (petrol) engine, the stoichiometric air-fuel ratio is around 14.7:1. This means that, in order to burn completely 1 kg of fuel, we need 14.7 kg of air. The combustion is possible even is the AFR is different than stoichiometric. For the combustion process to take place in a gasoline engine, the minimum AFR is around 6:1 and the maximum can go up to 20:1.
When the air-fuel ratio is higher than the stoichiometric ratio, the air-fuel mixture is called lean. When the air-fuel ratio is lower than the stoichiometric ratio, the air-fuel mixture is called rich. For example, for a gasoline engine, an AFR of 16.5:1 is lean and 13.7:1 is rich.
Spark ignition (SI) engines usually run on gasoline (petrol) fuel. The AFR of the SI engines varies within the range 12:1 (rich) to 20:1 (lean), depending on the operating condition of the engine (temperature, speed, load, etc.). Modern internal combustion engines operate as much as possible around the stoichiometric AFR (mainly for gas after-treatment reasons).
Compression ignition (CI) engines usually run on diesel fuel. Due to the nature of the combustion process, CI engines always run on lean mixtures, with AFR between 18:1 and 70:1. The main difference, compared with SI engines, is that CI engines run on stratified (non homogeneous) air-fuel mixtures, while SI run on homogeneous mixtures (in case of port-injection engines).
The engine performance in terms of power and fuel consumption is highly dependent on the air-fuel ratio. For a petrol engine, the lowest fuel consumption is obtained at lean AFR. The main reason is that there is enough oxygen available to burn completely all the fuel which translates in mechanical work. On the other hand, the maximum power is obtained with rich air-fuel mixtures. As explained before, putting more fuel in the cylinder at high engine load and speed, cools down the combustion chamber (through fuel evaporation and heat absorption) which allows the engine to produce maximum engine torque thus maximum power.
#AirFuelRatio #RichMixture #LeanMixture
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