Initial pre-ignition: Investigation of fundamental mechanism of initial low-speed pre-ignition

In the framework of this research project, the fundamental mechanism of the initial pre-ignition in highly charged gasoline engines was investigated at low engine speeds. In order to get insight to the charac-teristics of this stochastically occurring, irregular phenomenon, testbed experiments in combination with numerical simulations were conducted. Besides the thermodynamic characterization of the LSPI fre-quency (LSPI: Low-speed pre-ignition) under varying engine conditions, processes inside the combus-tion chamber are optically recorded. Additionally, the influence of the liquid fuel on the pre-ignition mech-anism is investigated by modifying the engine configuration to CNG operation. Furthermore, the possi-bility of oil droplet induced ignitions is investigated by supplying lubricating oil to the engine manifold.
In order to investigate the LSPI phenomenon numerically, the thermodynamic conditions inside the com-bustions chamber are calculated using a 3D-CFD engine model. On top of this, a detailed 11-component fuel surrogate approach was used to investigate the wetting of the combustion chamber walls by the fuel spray. Based upon this data, comprehensive numerical studies on oil droplet induced ignitions and hot particle induced ignitions are carried out.
The performed investigations showed that all initial pre-ignitions are induced by light-emitting surface deposits or flying objects. The areas of deposit formation and detachment events could be correlated with regions of intensive fuel wall wetting. Further on, there was no experimental or simulative evidence of oil droplet induced pre-ignitions. In the case of inert particles, numerical studies showed that they were not able to surpass the necessary minimum surface temperature to initiate a pre-ignition within two consecutive cycles.
Based on the results obtained, a fundamental formation mechanism could be synthesized, which sug-gests that accumulations of oil and fuel in the combustion chamber are processed through multiple combustion processes in such a way that highly reactive deposits can arise. These can be heated up in such a way that they can trigger premature ignition of the mixture directly at the wall or in the gas phase.