Effect of Post Injections on In-Cylinder and Exhaust Soot for Low-Temperature Combustion in a Heavy-Duty Diesel Engine

Multiple fuel-injections during a single engine cycle can reduce combustion noise and improve pollutant emissions tradeoffs. Various hypotheses have been proposed in the literature regarding the in-cylinder processes responsible for the pollutant emissions improvements. This paper provides a brief overview of these hypotheses along with an investigation exploring which of these mechanisms are important for post injections under low-temperature combustion (LTC) conditions in a heavy-duty diesel engine. In-cylinder soot and exhaust smoke are measured by 2-color soot thermometry and filter paper blackening, respectively. The evolution and interaction of soot regions from each of the injections is visualized using high-speed imaging of soot luminosity, both in the piston bowl and in the squish regions. Results for early main-injection (-18° after top dead center command start of injection) conditions show an abrupt reduction in exhaust soot as the post injection is delayed to timings where the post-injection jet starts to enter the squish volume. At these conditions, the post injection in the squish volume produces no soot of its own, and it does not interact with soot from the main injection, which remains in the piston bowl. Reduction in soot for this case is primarily due to fuel in the post injection burning soot-free in the squish volume. In contrast, for a late main-injection (2° after top dead center), the exhaust soot decreases to levels below "main-injection-only" values as the post injection is retarded. With late main-injection, significant soot from the main injection remains in the squish volume, so that strong interaction between the main and post jets is evident. The retarded post injection conditions have higher late-cycle soot temperature and faster oxidation of main-injection soot remaining in the squish volume. Finally, for either main-injection timing, the optimal post-injection timing is at roughly the same crank angle position. Hence, for these LTC post-injection conditions, the absolute crank angle position of the post injection appears more important for soot reduction than the dwell between injections, though the dwell also could play a role in soot reduction, especially when using a late main injection.