Effect of injection parameters on combustion and emission characteristics under catalyst heating operation in a direct-injection spark-ignition engine

Clean and stable operation during the catalyst heating mode in a direct-injection spark-ignition engine is crucial to meet increasingly stringent emission regulations. Although the injection strategy has a profound effect on both emission and combustion characteristics, the relative importance of various injection parameters is not well understood under catalyst heating operation. Consequently, a parametric sweep of injection timing, injection pressure, and a number of split injections was conducted to reveal the relative impact of each parameter on particulate emissions, unburned hydrocarbon emissions, and combustion characteristics. A single-cylinder research engine with a side-mounted multi-hole injector was used in the experiments. The fuel was injected during the early intake stroke with 2-4 split injections using 10 and 20 MPa of injection pressure. Retarding the injection timing had the most significant influence on the accumulation-mode particulate emission. However, combustion stability can deteriorate if the injection timing is retarded beyond a certain point, possibly because of unfavorable fuel distribution near the spark plug. The higher number of split injections and/or injection pressure also contributed to lower accumulation-mode particulates. However, the combination of higher injection pressure and split numbers led to increased combustion variability due to the injector operating in a ballistic regime. The unburned hydrocarbon emissions were found to be insensitive to the injection parameters. Utilizing the parametric sweep results, it was hypothesized that the curtailment of the amount of fuel injected while the piston is located closer to the injector tip contributes to a lower fuel film on the piston surface and hence reduces engine-out particulate emissions. The estimation of fuel films using three-dimensional computational fluid dynamics confirmed that the hypothesis is valid under the catalyst heating operation used in this study. In the last step, the split ratio was varied such that the injection duration per injection increased gradually. Such a split ratio enabled further reduction of particulate emissions without incurring a penalty in unburned hydrocarbon emissions and combustion stability.