<div class="htmlview paragraph">One of the most attractive features of hybrid vehicles powered by SI-engines with three way catalysts is the potential of reaching extremely low emissions. In conventional drive trains, limitations in the air/fuel control result in lambda excursions during transients. These deviations from the ideal lambda result in increased emissions.</div> <div class="htmlview paragraph">In a hybrid vehicle, rapid load and speed changes of the SI-engine could be limited to an acceptable level as the battery acts as a power buffer. However, the efficiency of charging and discharging the battery is rather low, which means that excessive power buffering will increase the fuel consumption of the vehicle. Thus it is of great importance to know what degree of speed and load changes the air/fuel control system could cope with without an increase in emissions.</div> <div class="htmlview paragraph">Most SI-engine models for hybrid vehicles are quasi stationary, which means that data achieved at stationary load conditions are assumed to be representative for transient operation as well. In this paper a concept is presented in which stationary engine data is used, but corrected for the effects of lambda excursions.</div> <div class="htmlview paragraph">In the present paper the model is described in depth and some calculated experiments are presented. It is found that load changes in the range of 20% to 90% at frequencies higher than 0.5 Hz will result in lambda excursions that probably will result in unacceptable emissions.</div>