A New Approach for Modeling Cycle-to-Cycle Variations within the Framework of a Real Working-Process Simulation

For a reliable and accurate simulation of SI engines reproduction of their operation limits (misfiring and knock limit) and in this context the knowledge of cyclic combustion variations and their influence on knock simulation are mandatory. For this purpose in this paper a real working-process simulation approach for the ability to predict cycle-to-cycle variations (ccv) of gasoline engines is proposed. An extensive measurement data base of four different test engines applying various operation strategies was provided in order to gain a better understanding of the physical background of the cyclic variations. So the ccv initiated by dilution strategies (internal EGR, lean operation), the ccv at full load and at the knock limit could be investigated in detail. Finally, the model was validated on the basis of three further engines which were not part of the actual development process. In order to obtain a thorough understanding of the ccv phenomenology additionally the so-called residual gas feedback effect had to be examined. Therefore a one dimensional CFD simulation was implemented since in this case gas dynamic processes are essential. Basing on the final ccv model it was possible to evaluate the influence of a fluctuating combustion on knock simulation. A zero-dimensional knock model approach was implemented within the ccv model in order to ascertain the benefit maintained by single cycle resolved knock simulations. However, in correspondence with the measurement data analysis of knocking single cycles, it comes to a conclusion that an accurate prediction of knock phenomena requires the consideration of further influences beyond the mere influence of cyclic variations.