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. Time-resolved UHC measurements in the exhaust port with varying injection timings were used to analyze effects of each injection on UHC emissions. Results show that reducing the ignition delay of the second post injection while keeping its injection timing consistent is very effective to improve UHC emissions without penalizing the exhaust enthalpy. The two pilot and main injections were block-shifted to increasingly retarded timings, resulting in improved exhaust enthalpy, CO and NOx emissions without penalty in UHC emissions, but with significantly higher soot emissions and lower thermal efficiency. To overcome this, an enhanced combustion strategy adopting additional injection(s) is proposed, which consists of a main combustion event near top dead center to increase the in-cylinder temperature, a series of short injections to counteract the expansion effect on temperature, and a large post injection to maximize the exhaust heat. As the result, the proposed strategy successfully improves the exhaust enthalpy and thermal efficiency while suppressing UHC, CO and NOx emissions, therefore, enabling more retarded timings of post injection(s).
