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Abnormal combustion referred to as Low Speed Pre-Ignition (LSPI) may restrict low speed torque improvements in turbocharged Direct Injection (DI) - Spark Ignition (SI) Engines. Recent investigations have reported that the auto-ignition of an engine oil droplet from the piston crevice in the combustion chamber may cause unexpected and random LSPI. This study shows that engine oil formulations have significant effects on LSPI. We found that the spontaneous ignition temperature of engine oil, as determined using High-Pressure Differential Scanning Calorimetry (HP-DSC) correlates with LSPI frequency in a prototype turbocharged DI-SI engine. Based on these findings, we believe that the oxidation reaction of the oil is very important factor to the LSPI. Our test data, using a prototype engine, shows both preventative and contributory effects of base oil and metal-based engine oil additives.

As one of spark ignition (SI) engine solutions to improve fuel economy while maintaining drivability, concept of combing turbocharging and direct injection (DI) fuel injection system with engine down-sizing has increased its application in the market. Abnormal combustion phenomena referred to as Low Speed Pre-Ignition (LSPI) has been recognized as potential restriction to improve low speed engine torque that contributes fuel economy improvement. As reported in the part 1 [1], the study showed that engine oil composition had significant influence on the frequency of LSPI in both preventive and contributory effects. Further investigation was conducted to evaluate engine oil formulation variables and other factors that may have influences on the LSPI, such as engine oil degradation. Engine test that consisted of 2 phases was designed in order to confirm the correlation between LSPI frequency and engine oil degradation.