Auto-ignition and Detonation Induced by Density Gradient of Surrogate Lubricant under Boosted-Gasoline-Engine-Like Condition

摘要

Low-Speed Pre-Ignition (LSPI) events occur in highly boosted direct-injected gasoline engines when operating at a low-speed and high-load region. The LSPI event appears once per several thousand cycles; once happening, it could last for a few cycles and suddenly returns to normal combustion. These features are coincident with intermittent lubricating oil piston crown scattering behavior, which experiences accumulation and heavy scattering. In this work, the theory originally proposed by Bradley to classify the auto-ignition propagation modes triggered by hot spots is developed to be capable of analyzing the reaction front propagation generated from the lubricating oil clouds, where the auto-ignition is induced by a reactivity gradient. A critical condition related to the interaction between the reaction and acoustic waves is defined with respect to the density gradient that characterizes the oil clouds. Considering the initial conditions and the realistic gasoline fuel used in the SI engine, a detonation peninsula specific for the initial conditions is plotted after a large number of calculations to determine the coupling conditions between the reaction and pressure waves. The auto-ignition and detonation induced by surrogate lubricating oil in stoichiometric iso-Octane/air mixture are studied numerically. And the oils scattered from different parts of the cylinder (piston crown and piston crevice) are considered. Our calculation shows that the auto-ignition can be triggered by the oils due to chemically reactive and easily ignitable properties of the lubricant, but only the one induced by a sufficient amount of oil is likely to develop into detonation waves. This indicates that a small amount of oil cannot trigger an LSPI event, there must be accumulation in the oil before this is released into the combustion chamber to provide the desired energy transfer from the reaction wave to the pressure wave to compress the unburned mixture in front of the auto-ignition wave and develop into a detonation. Considering the similar intermittent features between LSPI and the piston crown scattering, of all the possible pre-ignition sources (such as hot spots, oil droplets), the oil released from the piston crown could have the largest possibility to trigger an LSPI event. Therefore, to eliminate the LSPI tendency in a boosted spark-ignition (SI) engine, the oil accumulation on the piston crown should always be avoided.