This paper presents the results of a study on reasons for the occurrence of pre-ignition in highly supercharged spark ignition engines. During the study, the phenomena to be taken into account were foremost structured into a decision tree according to their physical working principles. Using this decision tree all conceivable single mechanisms to be considered as reasons for pre-ignition could be derived. In order to judge each of them with respect to their ability to promote pre-ignition in a test engine, experimental investigations as well as numerical simulations were carried out.
The interdependence between engine operating conditions and pre-ignition frequency was examined experimentally by varying specific parameters. Additionally, optical measurements using an UV sensitive high-speed camera system were performed to obtain information about the spatial distribution of pre-ignition origins and their progress.
The numerical simulations included the analysis of the general auto-ignition behavior of air/fuel mixtures depending on the thermodynamic conditions, mixture composition and other factors by taking into account detailed reaction kinetics. Furthermore, 3D CFD simulations were conducted in order to determine the predominant conditions within the combustion chamber.
By combining the results of the experimental and numerical investigations, many of the identified mechanisms could be ruled out. Others were classified to be very improbable. The release of lubricant oil droplets from the cylinder liner turned out to be the most probable explanation for the occurrence of pre-ignition. As a main factor influencing the amount of droplets released and hence the pre-ignition frequency, oil dilution caused by spray impingement due to the lateral injector position was identified.
