Detailed Analyses and Correlation of Fuel Effects on Stochastic Preignition 2020-01-0612
Stochastic or Low-Speed Preignition (SPI or LSPI) is an undesirable abnormal combustion phenomenon encountered in forced induction, direct injection, spark-ignition engines. It is characterized by very early heat release and high cylinder pressure and can cause knock, noise and ultimately engine damage. Much of the focus on mitigating SPI has been directed towards the engine oil formulation, leading to the emergence of the Sequence IX and second-generation GM dexos® oil requirements. Engine design, calibration and fuels all contribute to the prevalence of SPI. As part of a recently completed research consortium, a series of engine tests were completed to determine the impact of fuel composition on SPI frequency. Abnormal combustion events were identified by both high cylinder pressure and early heat release. The fuel blends had varying levels of paraffins, olefins, aromatics and ethanol. Engine tests were performed on a 2-litre turbocharged, direct-injection spark-ignition engine and comprised of multiple repeats of low-speed, high-load, steady-state test segments, interspersed with low load segments, with engine calibration and boundary conditions adjusted to amplify the appearance of SPI. Comprehensive analyses of the fuel properties and chemical composition revealed the key fuel characteristics, notably the Particulate Mass Index (PMI), that were best correlated with the appearance of SPI. In addition, it was possible to determine the impact of fuel composition on the severity of preignition events by comparing the knock intensity and peak-to-peak knock values for all abnormal combustion cycles. The abundance of multi-cycle events was also investigated. The paper suggests that not only the number of preignition events with but also the severity of preignition should be considered when evaluating fuel impacts on SPI.
Andre Swarts, Elana Chapman, Vincent Costanzo
Southwest Research Institute, General Motors LLC, Aramco Research Center