Catalyst-heating operation in compression-ignition engines: A comprehensive understanding using large eddy simulations

Catalyst-heating operation in compression-ignition engines is critical to ensure rapid light-off of exhaust catalysts during cold-start. This is typically achieved by using late post injections for increased exhaust enthalpy, where retardability is constrained mainly by emissions due to inactivity of the oxidation catalyst at these conditions. Comprehensive understanding of formation mechanism of pollutant emissions is needed to optimize engine performance and minimize tailpipe harmful emissions. In this study, a computational fluid dynamics model of a medium-duty compression-ignition engine is developed and validated against with catalyst-heating operation experimental data using large eddy simulations. The engine is fueled with a full boiling-range diesel fuel and uses an optimized five-injections strategy that consists of two pilots, one main, and two post injections. Results show that, significant amounts of unburned hydrocarbons (UHCs) and oxygenated UHCs (OUHC) are formed by the pilot injections, which may persist until exhaust valve opening. UHCs accumulate mainly in the outer-upper part of the cylinder guided by the piston lip, in the inner-bowl due to the bowl geometry, near the injector nozzle by the fuel from the end-phase of injection, and in the space between spray plumes transported by the swirl motion. The main injection exhibits a short ignition delay and rapidly consumes most of UHC and OUHC, except for the central part of the chamber near the injector nozzle. The 1^st post injection counteracts the expansion effect on temperature, plays a key role in increasing the exhaust enthalpy and reducing the harmful emissions by promoting combustion associated with the 2^nd post injection. The fuel delivered by the 2^nd post injection penetrates through the flame of the 1^st post injection, creating cool flame clouds beyond the flame that eventually transition to a diffusion flame. Finally, a unique phenomenological model is proposed to better visualize the interactions of post injections.