Investigation of engine oil effect on abnormal combustion in turbocharged direct injection – spark ignition engines

Satoshi Hirano
Toyota Motor Corporation

As one of the spark ignition (SI) engine solutions to improve fuel economy while maintaining drivability, the concept of combining turbocharging and direct injection (DI) fuel system with engine downsizing has increased its application in the market. The advantage comes from reduced mechanical friction thanks to the downsizing of engine displacement and down-speeding by higher transmission gear ratio based on high engine torque at low engine speed. However, an abnormal combustion phenomenon, which seems particular to turbocharged DI-SI engines has been observed and is being investigated.

Abnormal combustion, referred to as Low Speed Pre-Ignition (LSPI) may restrict low speed torque improvements in turbocharged DI-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 a 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. This study suggests that engine oil improvements can provide the LSPI prevention performance, which would allow for more extensive use of the benefits of turbocharged SI-DI engines while maintaining engine durability and reliability.

Biodata

1994-2005: Engine oil formulator in the additive industry
2006: Joined Toyota Motor Corp. Since 2007, he has been in charge of engine oil development and related technical support in the Engine Division of Toyota Motor Corp. He is a member of the JAMA Engine Oil Subcommittee and ILSAC member since 2008.