Optical investigation of multiple injections for unburned hydrocarbon emissions reduction with low-temperature combustion in a heavy-duty diesel engine

Low-temperature combustion (LTC) achieved by using exhaust-gas recirculation (EGR) is an operating strategy of current interest for heavy-duty and other compression-ignition (diesel) engines because it offers low nitrogen oxides (NOx) and soot emissions compared to conventional diesel combustion. While the long ignition-delay of EGR-LTC helps increase pre-combustion mixing to reduce soot formation, other emissions, including unburned hydrocarbons (UHC), can be problematic. Particularly an issue at low-load conditions, a considerable portion of UHC emissions in large-bore diesels is often due to overly-lean fuel/air mixtures formed near the injector during the long ignition delay. In this study, we explore the use of multiple post-injection strategies, which have a large main injection and one or two smaller post injections, to help reduce engine-out UHC emissions. The short post-injections closely timed after the end of the main injection help to enrich the overly-lean region near the injector, allowing for more complete combustion of a greater portion of the fuel/air mixture. Optical results from formaldehyde and OH planar laser-induced fluorescence provide evidence of the in-cylinder spatial and temporal progression toward complete combustion.