Post-injection strategies aimed at reducing engine-out emissions of unburned hydrocarbons (UHC) were investigated in an optical heavy-duty diesel engine operating at a low-load, low-temperature combustion (LTC) condition with high dilution (12.7% intake oxygen) where UHC emissions are problematic. Exhaust gas measurements showed that a carefully selected post injection reduced engine-out load-specific UHC emissions by 20% compared to operation with a single injection in the same load range. High-speed in-cylinder chemiluminescence imaging revealed that without a post injection, most of the chemiluminescence emission occurs close to the bowl wall, with no significant chemiluminescence signal within 27 mm of the injector. Previous studies have shown that over-leaning in this near-injector region after the end of injection causes the local equivalence ratio to fall below the ignitability limit. With a carefully selected post-injection, mixtures close to the injector show significant chemiluminescence emission, indicating more complete combustion of those regions, likely due to increased local equivalence ratios.
Simultaneous planar laser-induced fluorescence (PLIF) of OH with 284-nm excitation and PLIF of combined formaldehyde and poly aromatic hydrocarbons (PAH) with 355-nm excitation were employed to identify the regions of first- and second-stage ignition, as well as providing some indication of local equivalence ratios. The laser diagnostics show that without a post injection, regions close to the injector show formaldehyde fluorescence late in the cycle without detectable OH fluorescence, indicating that these regions do not achieve second-stage ignition, and therefore likely contribute to UHC emissions. Persistence of formaldehyde fluorescence late in the cycle is also consistent with fuel-lean mixtures. With a carefully selected post injection, strong OH fluorescence appears in the near-injector regions, indicating that they are likely enriched by the post-injection such that they reach second-stage ignition and more complete oxidation. The reduction observed in the exhaust UHC emission is therefore attributed to the enrichment mechanism of the near-injector regions by the close-coupled post-injection.