Air–fuel Ratio

on . Posted in Fluid Dynamics

Air-fuel ratio, abbreviated as AFR, a dimensionless number, is the ratio of the mass or volume of air to the mass or volume of fuel in a combustion process, typically in an internal combustion engine.  This ratio determines the mixture of air and fuel that is supplied to the engine's combustion chamber.  The appropriate air-fuel ratio is crucial for efficient and effective combustion, as it directly impacts factors such as power output, fuel efficiency, and emissions.

For gasoline engines, the stoichiometric air-fuel ratio is approximately 14.7:1, meaning that for every 14.7 parts of air, there is 1 part of fuel by mass or volume.  This ratio is considered ideal for achieving complete combustion of the fuel and producing the least amount of emissions, particularly carbon dioxide (CO2) and harmful pollutants like nitrogen oxides (NOx) and unburned hydrocarbons.

Engines can operate with air-fuel ratios richer (more fuel) or leaner (more air) than the stoichiometric ratio, depending on the specific requirements.  A richer mixture can provide more power but might lead to increased fuel consumption and emissions.  A leaner mixture might improve fuel efficiency but can also lead to higher nitrogen oxide emissions and potential engine overheating.  Modern engines often utilize sensors and engine control units (ECUs) to monitor and adjust the air-fuel ratio in real-time to optimize performance, efficiency, and emissions under varying conditions such as load, speed, and temperature.


Air–fuel ratio formula

\( AFR =   m_a \;/\; m_f  \)     (Air-fuel Ratio)

\( m_a =  AFR \; m_f \)

\( m_f =  m_a \;/\; AFE  \)

Symbol English Metric
\( AFR \) = air–fuel ratio \(dimensionless\)
\( m_a \) = air mixture \(lbm\) \(kg\)
\( m_f \) = fuel mixture \(lbm\) \(kg\)


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