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This paper presents three different methods for obtaining equivalence ratio/air-fuel ratio from the internal combustion engines. Emphasis is placed on reviewing several important air-fuel ratio models based upon the analysis of gaseous emission constituents in the exhaust of the engines as alternate to that obtained conventionally from fuel and air flow rate consumptions of the engines. A thorough analysis of all the factors that may affect the accuracy of fuel-air equivalence ratio is conducted which include the ambient conditions such as temperature and humidity, the assumed water gas reaction constant and the inclusion of oxides of nitrogen constituent in the model. A comprehensive model based upon chemical reaction of air-fuel mixture is proposed with flexible inputs of fuel type and the above factors in addition to the volume concentration inputs of carbon monoxide, carbon dioxide, oxygen and unburnt hydrocarbons.

This paper claims that, instead of relying on direct fuel and air flow rate measurements, exhaust gaseous emissions analysis should be considered as the standard method for the determination of air-fuel ratio, in particular for petrol-powered engines. Various sensitivity parameters are defined and the maximum possible relative divergences of the fuel-air ratio are calculated to support the above claim. It is proved that to obtain the air-fuel ratio with the same accuracy for a petrol engine, the accuracy of the instruments used for fuel and air flow rate measurements should be at least 10 times more accurate than the emission analysers. Estimations of hydrogen and oxygen concentration based on other measurable emission concentrations in the exhaust gas stream are also conducted and validated satisfactorily with the experimental data.