What is meant by stoichiometric air-fuel ratio in combustion reactions?

The stoichiometric air-fuel ratio is the ideal proportion of air to fuel needed for complete combustion without any excess.

In more detail, the stoichiometric air-fuel ratio is a critical concept in combustion reactions, particularly in engines and industrial processes. It refers to the exact ratio of air (oxygen) to fuel that is required to ensure a complete combustion reaction, where all the fuel is burnt and no excess oxygen or fuel is left. This ratio is determined by the chemical composition of the fuel and is typically expressed in terms of weight or volume.

For example, for petrol (gasoline), which is primarily composed of octane (C8H18), the stoichiometric air-fuel ratio is approximately 14.7:1 by weight. This means that for every kilogram of petrol, 14.7 kilograms of air are required for complete combustion. This ratio ensures that all the carbon in the fuel is converted to carbon dioxide (CO2), all the hydrogen is converted to water (H2O), and no unburnt fuel or excess oxygen remains.

However, in practical applications, achieving the stoichiometric air-fuel ratio can be challenging. Factors such as temperature, pressure, and turbulence can affect the combustion process. Therefore, engines often operate with a slightly richer or leaner mixture. A richer mixture has more fuel than the stoichiometric ratio, which can increase power but also produce more pollutants. A leaner mixture has less fuel, which can reduce emissions but also decrease power and potentially cause engine damage.

Understanding and controlling the air-fuel ratio is crucial for optimising the efficiency and emissions of combustion processes. It is a key concept in fields such as automotive engineering, energy production, and environmental science.