Nanostructure and Reactivity of Soot Produced from Partially Premixed Charge Compression Ignition (PCCI) Combustion and Post Injection

Abstract

Researchers have invested significant effort on optimizing the engine operation mode while cutting down the emissions due to increasingly strict emissions regulations. This study explores Partially-Premixed Charge Compression Ignition (PCCI) combustion and post injection in a light duty multicylinder turbodiesel engine to reduce particulate matter (PM) and NOx emissions without sacrificing the engine performance. Three different fuels are tested in this PCCI combustion research: Ultra Low Sulfur Diesel (ULSD), diesel fuel produced via a low temperature Fischer-Tropsch process (LTFT) and a Renewable Diesel (RD). Late injection PCCI combustion can reduce NOx emissions by 76-78% and reduce soot emissions by 25-35%. High cetane number (CN), high ignition quality fuels LTFT and RD only increase CO emissions by 40-45% and THC emissions by 11-16% under late injection PCCI combustion compared to conventional combustion, while ULSD increases CO emissions by 78% and THC emissions by 24% under late injection PCCI combustion. The reaction rate constants of soot produced from late injection PCCI combustion are 1.2-2.2 times higher than soot from the conventional combustion conditions. The reaction rate constants of soot from LTFT and RD fuels are 47-66% lower than soot produced from ULSD. Soots produced from PCCI combustion have smaller graphene layers, higher surface oxygen concentration and higher portion of amorphous carbon. In addition, the primary particle and particle aggregate sizes are around 25nm and 400 nm for conventional combustion soot, while 10 nm and 150 nm for late injection PCCI combustion soot. Soots produced from LTFT and RD fuel under conventional combustion, show internal burning during oxidation. However, soots produced from late injection PCCI combustion and ULSD show shrinking core oxidation, likely because of their overall amorphous structure. Post injection is another method to reduce engine-out soot emissions while maintaining efficiency, potentially to reduce or eliminate exhaust aftertreatment. Close-coupled post injections reduce soot emissions by 11-21%, THC emissions by 14-28%, and CO emissions by 7-8%. However, NOx emissions increase by 3-5%. For long-dwell post injection condition, soot emissions are reduced by 28-33% and NOx emissions are reduced by 7-8%. CO and THC emissions do not vary much under long dwell post injection conditions. The reaction rate constants of soot from close-coupled post injection conditions increase by 10-13% compared to baseline condition, while the reaction rate constants of soot from long dwell post injection conditions decrease by 37-39% compared to baseline condition. Moreover, with the increase of injection dwell and post injection size, soot surface oxygen content and amorphous carbon content increase. This explains the change in reactivity of soot from different injection dwells. Primary soot particle and particle aggregate sizes do not vary much with post injection. Soot from post injection conditions all show shrinking core type oxidation without graphene layer rearrangement.