Divergence analysis of an emissions-based air—fuel ratio model and exhaust oxygen estimation

Abstract

This paper claims that, instead of relying on direct fuel and air flowrate measurements, exhaust gaseous emissions analysis should be considered as the standard method for the determination of time-resolved air—fuel ratio (AFR), in particular for petrol powered engines.

Various sensitivity parameters are defined and the maximum possible relative divergences of the air—fuel ratio are calculated to support the above claim. It is proved that to obtain the air—fuel ratio with the same order of accuracy for a petrol engine near stoichiometric conditions, the resolution of the instruments used for fuel and air flowrate measurements should be at least ten times more accurate than the emission analysers.

Estimation of oxygen concentration is also conducted based upon other measurable emission constituents, viz. oxides of carbon (CO_x) and hydrocarbons. Two methods are proposed, one depends on known air—fuel ratio whereas the other one does not require that the air—fuel ratio is known. Estimation of oxygen concentration using the former method is not affected essentially by the effect of the presence of water vapour in the analyser on the measured emission constituents.