Investigation of Fuel Reactivity Stratification for Controlling PCI Heat-Release Rates Using High-Speed Chemiluminescence Imaging and Fuel Tracer Fluorescence

Premixed charge compression ignition (PCI) strategies offer thepotential for simultaneously low NOx and soot emissions withdiesel-like efficiency. However, these strategies are generallyconfined to low loads due to inadequate control of combustionphasing and heat-release rate. One PCI strategy, dual-fuelreactivity-controlled compression ignition (RCCI), has beendeveloped to control combustion phasing and rate of heat release.The RCCI concept uses in-cylinder blending of two fuels withdifferent auto-ignition characteristics to achieve controlledhigh-efficiency clean combustion. This study explores fuel reactivity stratification as a methodto control the rate of heat release for PCI combustion. Tointroduce fuel reactivity stratification, the research engine isequipped with two fuel systems. A low-pressure (100 bar) gasolinedirect injector (GDI) delivers iso-octane, and a higher-pressure(600 bar) common-rail diesel direct-injector delivers n-heptane. Asweep of the common-rail injection timing creates a range of fuelreactivity stratification. A high-speed digital camera providesimages of ignition and combustion luminosity, composed primarily ofchemiluminescence. A quantitative laser-induced fuel-tracerfluorescence diagnostic also provides two-dimensional measurementsof the mixture distribution prior to ignition. The injection timingsweep showed that the peak heat-release rate is highest for eitherearly or late common-rail injections of n-heptane, and displays aminimum at mid-range injection timings near 50° BTDC. At very earlyinjection timings, the optical data show that the charge iswell-mixed and overall fuel lean, so that it ignitesvolumetrically, resulting in rapid energy release. Conversely, whenthe injection timing is late in the cycle (near TDC), the mixingtime is relatively short and much of the fuel-air mixture in then-heptane jet is fuel-rich. Such mixtures that are nearstoichiometric or richer have similar ignition delays, so that thecharge ignites nearly instantaneously throughout the n-heptanejets. For the mid-range injection timings, at the minimum in thepeak energy release rate, ignition occurs in the downstream portionof the n-heptane jet in localized auto-ignition pockets generatedby the common-rail injection of n-heptane. The subsequentcombustion process then progresses upstream toward the centrallymounted common-rail injector at a slower rate than either the earlyor late injection timings. In agreement with the observedcombustion zone progression from the bowl-wall toward the injector,the fuel concentration measurements show that the fuel reactivitygenerally decreases from the bowl-wall toward the common-railinjector.