Numerical Investigation of the Combustion Process and Emissions Formation in a Heavy-duty Diesel Engine Featured with Multi-pulse Fuel Injection

Combustion in conventional and advanced diesel engines is an intricate process that encompasses interaction among fuel injection, fuel-air mixing, combustion, heat transfer, and engine geometry. Manipulation of fuel injection strategies has been recognized as a promising approach for optimizing diesel engine combustion. Although numerous studies have investigated this topic, the underlying physics behind flame interactions from multiple fuel injections, spray-flame-wall interaction and their effects on reaction zones, and NOx/soot emissions are still not well understood. To this end, a computational fluid dynamics (CFD) study is performed to investigate the effects of pilot and post injections on in-cylinder combustion process and emissions (NOx and soot) formation in a heavy-duty (HD) diesel engine. A full-sector CFD model of the HD engine employing detailed chemistry is validated against experimental data for in-cylinder pressure, heat release rate, combustion phasing, and engine-out NOx/soot and carbon dioxide (CO2) emissions at five load points. The validated CFD model is further leveraged to gain insights into the complex pilot-main and main-post injection interactions at low load (20%) and mid load (60%) conditions, respectively. The 20% load point consists of four fuel injections (two pilots, one main and one post injection), whereas 60% load point has three injections (one pilot, one main and one post). It is observed that pilot injections significantly alter the main flame structure by shifting reaction zones contributing to heat release from combined rich premixed + non-premixed + lean premixed zones to primarily non-premixed zones. Presence of pilot injection decreases NOx concentration (while shifting the contribution of NO2 towards NOx from 50% to 14%) and increases soot concentration. The local consumption of oxygen and less time available for main fuel-air mixing due to reduction in ignition delay (ID) caused by the pilot injection are the major reasons behind increase in soot. The investigation on post injection reveals that although post injection increases soot formation, it also increases soot oxidation, with soot oxidation dominating soot formation. This results in an overall reduction in soot emissions. Hydroxyl (OH) radicals play an important role in enhancing the soot oxidation rate. Furthermore, as the post start-of-injection (SOI) timing is retarded, both soot formation and oxidation decrease, with an overall increase in net soot emissions.