The Effects of Piston Crevices and Injection Strategy on Low-Speed Pre-Ignition in Boosted SI Engines

The spark ignition (SI) engine has been known to exhibit several different abnormal combustion phenomena, such as knock or pre-ignition, which have been addressed with improved engine design or control schemes. However, in highly boosted SI engines, Low-Speed Pre-Ignition (LSPI), a pre-ignition event typically followed by heavy knock, has developed into a topic of major interest due to its potential for engine damage. Previous experiments associated increases in hydrocarbon emissions with the blowdown event of an LSPI cycle [1]. Also, the same experiments showed that there was a dependency of the LSPI activity on fuel and/or lubricant compositions [1]. Based on these findings it was hypothesized that accumulated hydrocarbons play a role in LSPI and are consumed during LSPI events. A potential source for accumulated HC is the top land piston crevice. Since the top land crevice is a quench zone, a series of experiments was undertaken to increase the clearance in the top land region to promote flame propagation into that region. The results indicate that increasing the size of the crevice volume reduced the incidence of LSPI significantly. Further experiments were conducted to determine the sensitivity of the effect of the shape and size of the crevice volume on the LSPI rate, yielding significant changes in the LSPI frequency. In addition to the piston hardware study, it was found that the LSPI behavior is strongly dependent on the mixture formation, particularly the injection strategy and type of injection system used (PFI versus GDI). It was hypothesized that by varying the amount of liquid fuel that impinges with the cylinder wall, the oil and fuel mixture is influenced, which in turn affects the top land accumulation rate as well as the release timing.