Numerical Investigations on Split Injection Strategies to Reduce CO and Soot Emissions of a Light-Duty Small-Bore Diesel Engine Operated in NADI-PCCI Mode

Premixed Charge Compression Ignition (PCCI) is a promising LTC strategy to reduce NO_x and soot emissions without relying on after-treatment devices. One major drawback of PCCI is high HC and CO emissions resulting from fuel-wall impingement due to early injection of diesel. Narrow-angle direct injection (NADI) helps reduce the wall wetting of fuel. But it is effective only at lower loads. At mid and higher loads, it increases soot and CO emissions in small-bore engines due to the formation of fuel-rich pockets in the piston bowl region. This problem is addressed using a split injection strategy in the present work. A 3-D CFD model is developed and validated with experimental data at two load conditions. Simulations are performed using CONVERGE CFD software. Split injection strategies are explored using wide (148 deg) and narrow (88 deg) spray included angles. The investigations concluded that a main injection of 20 deg bTDC and 30 deg bTDC were optimal for wide and narrow spray included angles, respectively. For both cases, a dwell time of 15 deg CA was optimal. Compared to single injection, split injection resulted in 2% and 4% improvement in indicated thermal efficiency for wide and narrow-angle, respectively. Split injection results in a reduction of 83% and 80% in CO, 56% and 64% in soot and 48% and 60% in HC emissions for wide and narrow included angle respectively when compared with single-injection NADI-PCCI combustion.