Numerical investigation of ducted fuel injection configurations in a downsized heavy-duty compression-ignition engine

Ducted fuel injection (DFI) is a novel concept to enhance fuel-air mixing and reduce soot emission for mixing-controlled compression ignition combustion, while its potential in heavy-duty compression-ignition (CI) engines is still not clear. In this study, a systematic numerical investigation is conducted to investigate the influence of DFI configurations on the combustion of a downsized heavy-duty CI engine equipped with a six-hole injector. Results show that, the duct diameter is critical to the combustion performance of a DFI system. Increasing the duct diameter increases the entrained air mass but reduces the mean inlet velocity. With increasing duct diameter, the soot emission exhibits a non-monotonic variation of ‘decrease-increase’ and an excessively large duct diameter with flame recession could result in an even higher soot emission compared to the free spray case. The duct length also largely affects the in-duct mixing and the downstream combustion characteristics, as an increased duct length reduces the equivalence ratio near the spray head and chamber wall, thus lowering the net soot emission. However, an excessively long duct would facilitate residual fuel in the duct after the end of injection, which weakens the late-cycle combustion and slightly reduces the combustion efficiency. The stand-off distance between the duct and the nozzle exhibits less prominent influence on the combustion performance, with its effect on the mixture formation being mainly manifested during the early injection stage. With increasing duct stand-off distance, the soot formation decreases while the late-cycle soot oxidation may be enhanced, leading to a slightly lower net soot emission. Compared to the benchmark free spray case, an optimized DFI configuration achieves a 71.8% reduction in the soot with a 0.4% increase in the indicated thermal efficiency. The results are helpful to the implementation of DFI in heavy-duty CI engines.