AbstractLow-Speed Pre-Ignition (LSPI) is a form of abnormal combustion that can occur in direct-injection gasoline engines when the air-fuel mixture auto-ignites before the spark timing. A sudden and extreme pressure rise, along with high amplitude pressure oscillations, can result in immediate and catastrophic mechanical damage to the engine. The exact mechanisms behind this abnormal combustion are not yet fully understood, however, due to the conditions under which it occurs, it is now seen as one of the key limiting factors to further downsizing, in the next generation of high efficiency engines. Although termed ‘low-speed’, this phenomenon has been observed to occur across a wide engine speed range.
A review of an extensive set of engine testing data was conducted in order to study LSPI phenomena observed in turbo-charged, direct injection gasoline engines. A wide range of turbocharged DI gasoline engine makes and sizes were tested, capturing several thousand LSPI events. The challenges faced when trying to accurately and consistently measure this phenomenon are outlined, resulting in the development of new methods to identify LSPI along with means of filtering false positive events. New methods for provoking LSPI have been successfully trialled helping lead to shorter test times during durability testing as well as a better understanding of the mechanisms behind LSPI.
A parametric study was carried out, investigating the influence of several factors on LSPI, which were quantified and compared against the wider literature. These factors included piston cooling jets, injection timing and Exhaust Gas Recirculation. A new phenomenon has been identified whereby an LSPI event in one cylinder is a direct cause of further LSPI events in other cylinders, and a hypothesis developed regarding its underlying mechanism based upon mechanical vibration.
Analysis of the test results, further in-house experiments and inferences drawn from the wider literature, were used to develop an extensive new test methodology for LSPI characterisation. The novel methodology describes a strategy for engine and vehicle testing, from initial engine development investigations through to final validation and production in a vehicle concluding
with recommendations for suitable control responses to LSPI events encountered in a vehicle application.
A summary of LSPI and abnormal combustion phenomena is proposed based upon the findings of this study for pre-ignition cases at low and high-speed (LSPI, HSPI), end gas auto-ignition and single cylinder and multi-cylinder events. This summary, along with the novel test procedures developed for calibration, validation, detection and mitigation strategies, can be readily employed by automotive combustion engineers in further research and development programmes.
|Date of Award||Dec 2020|
|Supervisor||Guillaume De Sercey (Supervisor), Steven Begg (Supervisor), Morgan Heikal (Supervisor) & Richard Osborne (Supervisor)|