Air Fuel and Fuel Air ratios

In engine testing both the air mass flow rate m^._a and the fuel mass flow rate m_f^. are normally measured.
The ratio of these flow rates is useful in defining engine operating conditions:

                                                   m^._a
             Air/fuel ratio (A/F) = --------------------
                                                    m^._f

                                                   m^._f
             Fuel/air ratio (F/A) = --------------------
                                                    m^._a

ENGINE TYPE	Air to Fuel ratio A/F
SI Engine using gasoline fuel	12 < A/F < 18 (0.056 < F/A < 0.083)
CI Engines with diesel fuel	18 < A/F < 70 (0014 < F/A < 0.056)