Split Injection Spray Development, Mixture Formation, and Combustion Processes in a Diesel Engine Piston Cavity: Rig Test and Real Engine Results

The objectives of this study are to investigate the effects of premixed charge compression ignition (PCCI) strategies with split injection on soot emission characteristics. The split injection conditions included three injection intervals (1.1 ms, 1.3 ms, and 1.5 ms) and three injection quantity fraction ratios (Q1/Q2 = 10.0/14.6 mm³/st, 15.2/9.4 mm³/st, and 20.0/4.6 mm³/st). The results in real engine tests showed that shorter injection intervals, and the 1^st injection quantity contributes to reduced soot emissions.

A rig test with high-pressure and high-temperature constant-volume vessel (CVV) and a two-dimensional (2D) model piston cavity were used to determine correlations between injection conditions and soot emissions. During the rig test, fuel was injected into the CVV by a single-hole nozzle under split injection strategies. The injection strategies include the same injection intervals and quantity fraction ratios as in the real engine test. The 2D piston cavity model took the same shape as that used in a small-bore diesel engine to investigate spray development, mixture formation, and combustion process. Tracer laser absorption scattering (LAS) was used to observe the spray development and mixture formation processes without combustion. The spatial distributions of the vapor and liquid phases and spray mixture formation characteristics in the 2D piston cavity were investigated. Spray combustion and soot formation processes were studied using a high-speed video camera. The flame structure and soot formation process were examined using two-color pyrometry. The experimental results revealed that the split injection interval and mass fraction ratio influence the characteristics of the mixture formation and soot formation processes in the 2D piston cavity. The rig test results show that the correlation between soot emission characteristics and injection strategies is similar to that observed in the rig test.