With the development of downsized spark ignition (SI) engines, low-speed pre-ignition (LSPI) has been observed more frequently as an abnormal combustion phenomenon, and there is a critical need to solve this issue. It has been acknowledged that LSPI is not directly triggered by autoignition of the fuel, but by some other material with a short ignition delay time. It was previously reported that LSPI can be caused by droplets of lubricant oil intermixed with the fuel. In this work, the ignition behavior of lubricant component containing fuel droplets was experimentally investigated by using a constant volume chamber (CVC) and a rapid compression and expansion machine (RCEM), which enable visualization of the combustion process in the cylinder. Various combinations of fuel compositions for the ambient fuel-air mixture and fractions of base oil/metallic additives/fuel for droplets were tested. CVC results confirmed the high ignitability of lubricant component containing fuel droplets in pre-heated air, which was responsible for the early timing of LSPI. RCEM results also revealed the high ignitability of lubricant component containing fuel droplets in the premixed fuel/air mixture. Stochastic occurrence of ignition induced by fuel droplets was also observed, particularly under a condition of a small lubricant oil fraction. Finally, the contribution of base oil and metallic additives to the ignitability (delay times and probabilities) of droplets was separately evaluated to understand the fundamental mechanism involved. Results confirmed that the base oil itself can promote the high ignitability of fuel droplets and that metallic additives can also additionally promote or inhibit the ignitability of droplets in the premixed fuel-air mixture.
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