Fuel estimation and air-to-fuel ratio control for Flexfuel spark-ignition engines

The Flexfuel engine is a spark-ignition (SI) engine designed to operate on any blend of gasoline and ethanol. The addition of ethanol in the fuel appears as an internal perturbation to the system, which has to be rejected in order to regulate the air-to-fuel (AFR) ratio at its desired value. One possible solution is to either measure or estimate the disturbance linked with the fuel change. The aim this paper is twofold. First, we attempt to assess the effect of fuel variability on a conventional AFR control structure based on feedforward and a P.I. controller. We then intend to adapt the proposed control scheme to the fuel changes. An estimation of the ethanol content in the fuel blend, relying on no additional sensor will be added to the control structure to adapt the feedforward term.