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To comply with increasingly stringent pollutant emissions regulations, diesel engine operation in a catalyst-heating mode is critical to achieve rapid light-off of exhaust aftertreatment catalysts during the first minutes of cold starting. Current approaches to catalyst-heating operation typically involve one or more late post injections to retard combustion phasing and increase exhaust temperatures. The ability to retard post injection timing(s) while maintaining acceptable pollutant emissions levels is pivotal for improved catalyst-heating calibrations. Higher fuel cetane number has been reported to enable later post injections with increased exhaust heat and decreased pollutant emissions, but the mechanism is not well understood. The purpose of this experimental and numerical simulation study is to provide further insight into the ways in which fuel cetane number affects combustion and pollutant formation in a medium-duty diesel engine.

To cope with regulatory standards, minimizing tailpipe emissions with rapid catalyst light-off during cold-start is critical. This requires catalyst-heating operation with increased exhaust enthalpy, typically by using late post injections for retarded combustion and, therefore, increased exhaust temperature. However, retardability of post injection(s) is constrained by acceptable pollutant emissions such as unburned hydrocarbon (UHC). This study provides further insight into the mechanisms that control the formation of UHC under catalyst-heating operation in a medium-duty diesel engine, and based on the understanding, develops combustion strategies to simultaneously improve exhaust enthalpy and reduce harmful emissions. Experiments were performed with a full boiling-range diesel fuel (cetane number of 45) using an optimized five-injections strategy (2 pilots, 1 main, and 2 posts) as baseline condition.