What is Equivalence ratio?
How the actual fuel-to-air ratio compares with the stoichiometric ratio — the one that burns all fuel with exactly the oxygen available.
Flame temperature, \mathrm{NO}_x and \mathrm{CO} formation, and unburnt fuel all depend strongly on it. It summarizes a combustion condition in a single number.
\varphi = 1 is the exact stoichiometric mix; \varphi < 1 has extra air (lean); \varphi > 1 has extra fuel (rich).
Quick Calculate currently takes the composition directly. A stoichiometric methane–air mixture, for example, is CH4 1, O2 2, N2 7.52.
\varphi = \frac{(F/A)}{(F/A)_{\mathrm{st}}}
F and A are fuel and air amounts on the same basis; st marks the stoichiometric condition. With the air ratio \lambda:
\varphi = \frac{1}{\lambda}