The combustion of highly boosted gasoline engines is limited by knocking combustion and pre-ignition. Therefore, a comprehensive modelling approach consisting of cycle-to-cycle simulation, reactor modelling with detailed chemistry and CFD-simulation was used to predict the knock initiation and to identify the source of pre-ignition. A 4-cylinder DISI test engine was set up and operated at low engine speeds and high boost pressures in order to verify the accuracy of the numerical approach.
The investigations showed that there is a correlation between the knocking combustion and the very first combustion phase. The onset of knock was simulated with a stochastic reactor model and detailed chemistry. In parallel, measurements with an optical spark plug were carried out in order to identify the location of knock onset. The simulation results were in good agreement with the measurements.
Deposits and oil/fuel-droplets are possible triggers of pre-ignition. A multi-component fuel approach was therefore introduced to predict the wall film formation with the CFD-simulation. Droplet-stripping from the wall film was evaluated. The simulation of the chemistry of the oil/fuel droplets confirmed the results from high-speed imaging that identified droplets and deposits as a possible source of pre-ignition.