Load Limit Extension in Pre-Mixed Compression Ignition Using a 2-Zone Combustion System

A novel 2-zone combustion system was examined at medium load operation consistent with loads in the light duty vehicle drive cycle (7.6 bar BMEP and 2600 rev/min). Pressure rise rate and noise can limit the part of the engine map where pre-mixed combustion strategies such as HCCI or RCCI can be used. The present 2-zone pistons have an axial projection that divides the near TDC volume into two regions (inner and outer) joined by a narrow communication channel defined by the squish height. Dividing the near TDC volume provides a means to prepare two fuel-air mixtures with different ignition characteristics. Depending on the fuel injection timing, the reactivity of the inner or outer volume can be raised to provide an ignition source for the fuel-air mixture in the other, less reactive volume. Multi-dimensional CFD modeling was used to design the 2-zone piston geometry examined in this study. For experimental evaluation of the design, the geometry was applied to a GM 1.9L ZDTH in-line 4-cylinder engine equipped for dual fuel RCCI operation. The intake system was modified for port injection of gasoline, and diesel was direct injected. Engine tests showed the load could be increased from 9.3 bar IMEP to 12.2 bar IMEP at equivalent noise level when compared to an open geometry using early injections. CFD modeling using KIVA-3V was used to explore the operation of the novel concept. Using a validated numerical model, the effects of changes in boundary conditions and load extension were examined. The numerical model was used to provide insight into cyclic variability seen in the experiments.