Isolation of the parametric effects of pre-blended fuel on low load gasoline compression ignition (GCI)

Gasoline compression ignition (GCI) combustion is a promising solution to address increasingly stringent efficiency and emissions regulations imposed on the internal combustion engine. However, the high resistance to auto-ignition of modern market gasoline makes low load compression ignition operation difficult. The most comprehensive work focused on low load GCI operation has been performed on multi-cylinder research engines where it is difficult to decouple effects of the combustion event from air-handling and system level parameters (e.g., intake pressurization and exhaust gas recirculation (EGR)). Further, most research has focused on technology applications (e.g., use of variable valve actuation or supercharging) rather than fundamental effects, making identification of influential factors difficult. Accordingly, there is a need for detailed investigations focused on isolating the critical parameters that can be used to enable low load GCI operation. A full factorial parametric study was completed to isolate the effects of intake temperature, EGR rate, and fuel reactivity on low load performance. A minimum intake pressure metric was used to compare these parameters. This allowed for combustion phasing and load to be held constant while isolating the experiment from fuel injection effects. The effort showed that increasing intake temperature yields a linear reduction in the minimum intake pressure required for stable operation. Adding a small amount of diesel fuel to gasoline improved combustion stability with minimal need for energy addition through intake pressurization. The minimum intake pressure requirement also showed very good correlation with the measured research octane number of the fuel. However, increasing the fuel reactivity with diesel fuel, caused NOx emissions to increase. Response model analysis was used to determine the intake conditions required to maintain NOx levels that may not require lean NOx after treatment. The combination of diesel fuel blending and EGR allowed NOx levels to be reduced to near zero values with the minimum intake pressurization required. A detailed investigation into the effects of EGR showed that, for a given fuel, there is a maximum EGR rate that allows for stable operation, which effectively constrains the minimum NOx prior to aftertreatment.