Minimizing the time needed to achieve light-off temperatures in diesel engine aftertreatment devices is key to mitigate pollutant emissions during the first minutes of operation. Catalyst heating operation typically includes one or multiple post-injections late during the expansion stroke aimed to increase the enthalpy of the exhaust gases. However, post-injection retardability is constrained by low combustion efficiency and the formation of CO and unburned hydrocarbons that cannot be oxidized by a still-inactive oxidation catalyst.
In this study, the effects of post-injection strategy on the performance and emissions of a medium duty diesel engine have been investigated experimentally, focusing on the impacts on post-injection retardability. A five injection strategy (two pilot, one main, two post) was implemented in the engine, and the injection duration ratio between the two post-injections has been varied systematically while performing post-injection timing sweeps to identify the trade-offs between exhaust enthalpy, emissions, fuel delivered by each post-injection, and post-injection timing. The exhaust enthalpy and the indicated specific CO and unburned hydrocarbon emissions increase as the post-injection retard for all the injection duration ratios. However, this effect is more dramatic for long first post injection durations and short second post injection durations (i.e., a high first-to-second post-injection duration ratio). For these cases, the fuel vaporization cooling effect of the first post-injection on the in-cylinder reactivity causes that both post injections burn together at lower in-cylinder temperatures than those of the other first-to-second injection duration ratios, leading to higher emissions with no significant advantages in exhaust enthalpy. In general, a fuel split ratio equal to 1 with half of the fuel injected in the second post-injection, which ignition is sustained by half of the fuel injected in the first post-injection provides better performance-emissions trade-off and higher post-injection retardability.