# Air–fuel Ratio

on . Posted in Fluid Dynamics

Air–fuel ratio, abbreviated as AFR, a dimensionless number, is the mass ratio of air to fuel mixture present in an internal combustion engines.  Because it is a ratio, it is unitless. Fuel doesn't burn on its own. It has to be mixed with air. AFR tells you how many parts of air are mixed with each part of fuel. For example, a 12:1 AFR (or just 12) means the mixture is 12 parts air to one part fuel. Gasoline engines typically run around 14.7 AFR.

## Air–fuel ratio formula

$$\large{ AFR = \frac {m_a} {m_f} }$$
Symbol English Metric
$$\large{ AFR }$$ = air–fuel ratio $$\large{dimensionless}$$
$$\large{ m_a }$$ = air mixture $$\large{lbm}$$ $$\large{kg}$$
$$\large{ m_f }$$ = fuel mixture $$\large{lbm}$$ $$\large{kg}$$