High efficiency, ultra-low emission combustion in a heavy-duty engine via fuel reactivity control

Engine experiments and multi-dimensional modeling were used to explore a dual-fuel concept to realize highly-efficiency premixed charge compression ignition (PCCI) combustion with near zero levels of NOx and soot. In-cylinder fuel blending using port-fuel-injection of gasoline and optimized, early-cycle direct-injection of diesel fuel was used to control combustion phasing and duration. The first part of the study used the KIVA-CHEMKIN code and a reduced primary reference fuel (PRF) mechanism to suggest optimized fuel blends and EGR combinations for premixed operation at three engine loads (6, 9, and 11 bar net IMEP). It was found that combustion phasing was easily controlled through optimized fuel reactivity. Furthermore, the results showed that the minimum fuel consumption could not be achieved using either neat diesel fuel or neat gasoline alone and that the optimal fuel reactivity required decreased with increasing load. A multi-objective genetic algorithm optimization was performed next to select the parameters of a split injection in order to achieve a spatially homogeneous fuel blend with minimum wall film using port-fuel-injection of gasoline and direct injection of diesel fuel. It was found that the most important parameters for reducing PRF inhomogeneity were the fuel split amounts and first pulse injection timing. Guided by the simulation findings, engine experiments were conducted on a heavy-duty test engine using the dual-fuel PCCI strategy with port-fuel-injection of gasoline and early cycle direct injection of diesel fuel. Multi-dimensional modeling was used to explain the observed trends. The experimental results confirmed that the optimal combustion phasing could be achieved using blends of gasoline and diesel. Additionally, it was found that in-cylinder reactivity gradients extended the combustion duration and reduced the rate of pressure rise compared to single fuel PCCI combustion. The improved control over combustion phasing and duration allowed an extension of the PCCI operating regime to higher engine loads while maintaining low rates of pressure rise (<10 bar/deg), low NOx and soot, and low fuel consumption. For example, at 11 bar IMEP and 1300 rev/min, controlled PCCI combustion was achieved with NOx and soot levels significantly below the US EPA 2010 heavy-duty limits while reaching a net indicated thermal efficiency of 50% (net ISFC of 169 g/kW-hr).