Abstract:

Dual-fuel compression ignition, or reactivity controlled compression ignition (RCCI) is a promising strategy for engines to achieve high efficiency and clean combustion from low to mid-high load. To extend the operating range to high engine load, further examination of the in-cylinder fuel stratification and combustion processes is required. In this paper, fuel-tracer planar laser-induced fluorescence (PLIF) was used to quantify the fuel stratification of RCCI in an optical engine. Toluene was chosen as the tracer, which was mixed with isooctane and n-heptane. A laser of 266 nm was used to stimulate the toluene fluorescence. The engine was run at 1200 r·min^-1 under a load of 6.9×10⁵ Pa IMEP (indicated mean effective pressure). Iso-octane was delivered via the intake port and n-heptane was injected directly into the cylinder at-10° CA (crank angle) after top dead center (ATDC). A fuel-gas adiabatic mixing assumption was adopted to correct temperature non-uniformity of the PLIF images. As an example, the results obtained at 5° CA ATDC gave an overestimated maximum equivalence ratio in the diagnostic region before any correction of 15%. Based on the measurements, the effects of reactivity, concentration and temperature stratification on ignition delay of RCCI were evaluated using Chemkin software. The results indicated that the reactivity stratification and concentration stratification dominated the ignition delay of RCCI, with the reactivity stratification being more significant than the concentration stratification. The temperature stratification had only minor effects on the ignition delay. The high-speed imaging of the RCCI combustion showed that the initial ignition sites emerged at the edge of the combustion chamber where the local fuel reactivity or fuel concentration was high. The flames then progressed into the center of the combustion chamber where the fuel was leaner and less reactive. The soot emissions shown by the soot radiation images mainly formed in the high reactive region.

Key words: Laser-induced fluorescence, Dual-fuel compression ignition, Fuel stratification, Optical engine