SOURCES OF HYDROCARBON EMISSIONS FROM LOW-TEMPERATURE PREMIXED COMPRESSION IGNITION COMBUSTION FROM A COMMON RAIL DIRECT INJECTION DIESEL ENGINE

摘要

Low-temperature premixed compression ignition combustion (PCI) discussed in this study is achieved via late injection timing (close to top dead center) and heavy exhaust gas recirculation (EGR) using ultra low sulfur Swedish diesel fuel (sulfur content less than 15 ppm). PCI obtains a simultaneous decrease in particulate matter (PM) and oxides of nitrogen (NO_x), as where injection timing is retarded, as opposed to conventional combustion, where a PM-NO_x trade-off is observed. In PCI, hydrocarbon (HC) and carbon monoxide (CO) are increased, and must be removed using aftertreatment. In order to understand the sources of HC from the PCI regime, gas chromatography with aflame ioni-zation detector is employed to perform exhaust HC speciation at three EGR rates and three injection timings. Volatile HC, semi-volatile HC, and CO increase as the injection timing is retarded or EGR is increased. Retarded injection timing or increased EGR reduces peak cylinder bulk temperature and thereby increases the yield of CO and volatile HC (mostly C_1-C_3). Retarded injection timing or increased EGR also increases ignition delay, which increases over-mixing and causes more mixture to become so lean that combustion ceases and the output of semi-volatile HC species (mostly unburned C_(10)-C_(12) fuel) is increased. Volatile HC species are increased more than semi-volatile HC species, which results in a shift to a lighter exhaust HC mixture as injection timing is retarded or EGR is increased.