Optical investigation of the reduction of unburned hydrocarbons using close-coupled post injections at LTC conditions in a heavy-duty diesel engine

Partially premixed low-temperature combustion (LTC) using exhaust-gas recirculation (EGR) has the potential to reduce engine-out NO_x and soot emissions, but increased unburned hydrocarbon (UHC) emissions need to be addressed. In this study, we investigate close-coupled post injections for reducing UHC emissions. By injecting small amounts of fuel soon after the end of the main injection, fuel-lean mixtures near the injector that suffer incomplete combustion can be enriched with post-injection fuel and burned to completion. The goal of this work is to understand the in-cylinder mechanisms affecting the post-injection efficacy and to quantify its sensitivity to operational parameters including postinjection duration, injection dwell, load, and ignition delay time of the post-injection mixture. Three optical diagnostics - planar laser induced fluorescence of OH radicals, planar laser induced fluorescence of formaldehyde, and high-speed imaging of natural combustion luminescence - complement measurements of engine-out UHC with parametric variations of main- and post-injection timing and duration. Across all conditions tested, each at 1200 RPM, the optimal post-injection command duration for UHC reduction was approximately 400 microseconds (2.9 °CA). Also, conditions with shorter (3.4 °CA) post-injection ignition delays were over twice as effective on a percentage basis at reducing engine-out UHC as those with longer (5.5 °CA) post-injection ignition delays. Optical data at the post-injection "sweet-spot," where UHC emissions are minimized, indicate that the post injection promotes transition to second-stage ignition in the near-injector region, most likely by enriching the overly fuel-lean mixtures in the wake of the main injection.