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.
