In order to comply with future CO2 targets for passenger cars with gasoline engines, engines with reduced displacement in combination with direct injection and turbocharging were pushed in recent years. This engine downsizing allows a significant reduction in fuel consumption compared to a corresponding larger engine.
However, the increased boost pressure values and thus increased specific performance of these downsize engines exacerbate the problem of engine knocking and lead to new combustion anomalies at low rotation speed and full load conditions, the so-called Low Speed Preignition (LSPI). Here there is a sporadic spontaneous combustion of the fuel prior to the opening of the ignition spark. The occurrence of Low Speed Preignitions, accompanied by the loss of control of the combustion process and greatly increased peak pressures, currently represents the limiting factor for further efficiency gains in gasoline engines.
According to the current state of research, engine oil which comes from the annular gap area in the combustion chamber is seen as one of the most likely causes for triggering Low Speed Preignition. This effect is enhanced by increasing lubrication oil dilution, caused by fuel condensation on the manifold walls.
In addition to the engine oil properties, fuel properties are considered an essential factor influencing the oil induced Low Speed Preignition at today Downsizing engines.
Within the project, this hypothesis was verified using extensive parameter variations at the beginning. The focus was on different engine oil specifications and their impact on the Low Speed Preignition behavior at the test bench.
Especially in the field of additives the significant influence of certain motor oil components could be identified. In addition, the effects of different ethanol fuels to the occurrence of Low Speed Preignition were examined in a further step. The increased enthalpy of vaporization allows a reduction in Low Speed Preignition until a certain degree of ethanol addition. Though, the increased degree of ethanol addition and the increased amount of fuel injection (due to a lower heating value) lead to problems with the processing of the fuel mixture. This means a higher risk of local oil dilution, caused by condensation on the manifold walls. This behavior must be taken into account, for example by multiple fuel injections. In addition, the respective fuel composition showed a significant influence on the frequency of Low Speed Preignition.
The project is an important contribution to the clarification of Low Speed Preignition. The generated knowledge is particularly valuable both for the development of new engines as well as for fuel and oil producers - especially with regard to the further reduction in fuel consumption and greenhouse gas emission.