Stochastic or Low-Speed Preignition (SPI or LSPI) is an undesirable abnormal combustion phenomenon encountered in 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 test and second-generation GM dexos® oil requirements. Engine design, calibration and fuels also 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. The fuel blends had varying levels of paraffins, olefins, aromatics and ethanol. Engine tests were performed on a 2-liter 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, the impact of fuel composition and properties on the severity of preignition events were examined by comparing the peak knock values for all abnormal combustion cycles. The frequency of multi-cycle events was also investigated. The paper suggests that not only the number of preignition events but also the severity of preignition should be considered when evaluating fuel impacts on SPI. Most importantly, for this work, is the determination that particulate forming tendency of the fuel correlates to its SPI tendency, although whether this is causal or correlational cannot be discerned.
