The objective of this study is to explore an Adaptive Injection Strategy (AIS) that employs multiple injections at successively low and high pressures in the same engine cycle to reduce spray-wall impingement, control combustion phasing, and limit pressure rise rates in a Premixed Compression Ignition (PCI) engine. This research focuses on understanding the performance and emissions benefits of low and high pressure split injections through experimental parametric sweeps and simulations of a 0.48 L single cylinder test engine operating at 2000 rev/min and 5.5 bar nominal IMEP. This study examines the effects of 1st and 2nd injection timing, 2nd injection pressure, EGR, nozzle angle, and swirl ratio for both single peak heat release (SHR) and two-peak high temperature heat release cases. For reference, two-peak heat release is referred to as Two-Stage Combustion (TSC) in this work. Heat release analysis of the measured cylinder pressure along with CFD modeling using the KIVA-CHEMKIN code are used to gain insight into the engine performance trends.
In order to investigate the AIS concept experimentally, a Variable Injection Pressure (VIP) system was developed. The VIP system is capable of both low and high pressure injections (~30 MPa and ~120 MPa respectively) through one injector in the same cycle by switching between low and high pressure fuel rails. For both the single heat release and TSC experiments, optimal operating conditions were found. An optimal emissions point for SHR was achieved using 55% EGR, 2.2 swirl ratio, 1.62 bar boost, and 20% early fuel fraction resulting in ISFC Gross of 183 g/kW-hr, indicated specific soot of 0.05 g/kW-hr, maximum PRR of 6 bar/deg, and indicated specific NOx of 0.10 g/kW-hr at 2000 rev/min and 5.5 bar nominal IMEP. Typically TSC resulted in maximum PRR of less than 3 bar/deg but worse fuel economy and emissions than SHR. An optimal emissions point was located at 44% EGR, 2.2 swirl ratio, 1.62 bar boost, and 50% first fuel split resulting in ISFC Gross of 195 g/kW-hr, indicated specific soot of 0.05 g/kW-hr, and indicated specific NOx of 0.25 g/kW-hr at 2000 rev/min and 5.5 bar nominal IMEP. The single heat release cases tend to have better fuel economy and lower emissions than the TSC cases. Further, for the single heat release cases, it was found that high EGR rates sufficiently suppressed the first stage of combustion allowing the combustion phasing to be controlled by the second injection. The TSC combustion phasing of both heat releases could be controlled with injection timing and EGR. Emission and engine performance trade-offs were observed over the injection timing ranges for all cases.