In combustion physics, fuel mass fraction is the ratio of fuel mass flow to the total mass flow of a fuel mixture. If an air flow is fuel free, the fuel mass fraction is zero; in pure fuel without trapped gases, the ratio is unity. As fuel is burned in a combustion process, the fuel mass fraction is reduced. The definition reads as
Y F = m F m t o t {\displaystyle Y_{F}={\frac {m_{F}}{m_{\rm {tot}}}}}
where
m F {\displaystyle m_{F}} is the mass of the fuel in the mixture
m t o t {\displaystyle m_{\rm {tot}}} is the total mass of the mixture
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