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 the potential for simultaneously low NOx and soot emissions with diesel-like efficiency. However, these strategies are generally confined to low loads due to inadequate control of combustion phasing and heat-release rate. One PCI strategy, dual-fuel reactivity-controlled compression ignition (RCCI), has been developed to control combustion phasing and rate of heat release. The RCCI concept uses in-cylinder blending of two fuels with different auto-ignition characteristics to achieve controlled high-efficiency clean combustion.

This study explores fuel reactivity stratification as a method to control the rate of heat release for PCI combustion. To introduce fuel reactivity stratification, the research engine is equipped with two fuel systems. A low-pressure (100 bar) gasoline direct injector (GDI) delivers iso-octane, and a higher-pressure (600 bar) common-rail diesel direct-injector delivers n-heptane. A sweep of the common-rail injection timing creates a range of fuel reactivity stratification. A high-speed digital camera provides images of ignition and combustion luminosity, composed primarily of chemiluminescence. A quantitative laser-induced fuel-tracer fluorescence diagnostic also provides two-dimensional measurements of the mixture distribution prior to ignition. The injection timing sweep showed that the peak heat-release rate is highest for either early or late common-rail injections of n-heptane, and displays a minimum at mid-range injection timings near 50° BTDC. At very early injection timings, the optical data show that the charge is well-mixed and overall fuel lean, so that it ignites volumetrically, resulting in rapid energy release. Conversely, when the injection timing is late in the cycle (near TDC), the mixing time is relatively short and much of the fuel-air mixture in the n-heptane jet is fuel-rich. Such mixtures that are near stoichiometric or richer have similar ignition delays, so that the charge ignites nearly instantaneously throughout the n-heptane jets. For the mid-range injection timings, at the minimum in the peak energy release rate, ignition occurs in the downstream portion of the n-heptane jet in localized auto-ignition pockets generated by the common-rail injection of n-heptane. The subsequent combustion process then progresses upstream toward the centrally mounted common-rail injector at a slower rate than either the early or late injection timings. In agreement with the observed combustion zone progression from the bowl-wall toward the injector, the fuel concentration measurements show that the fuel reactivity generally decreases from the bowl-wall toward the common-rail injector.