Effect of Water Dilution and Homogenous Lean Operation on Combustion and Detailed Emissions in a Spark Ignited Gasoline Direct Injection Engine

Twenty-nine percent of the greenhouse gas emissions in the US are produced by the transportation sector according to the US Environmental Protection Agency. The combination of increasingly stringent regulations on emissions and fuel economy, along with the current practical limitations of electrification motivate continued development efforts for improving internal combustion engine efficiency and emissions. Ethanol, an extensive fuel additive or drop-in replacement for gasoline, is already recognized as a promising transition fuel in decarbonization efforts. Furthermore, lean combustion in spark-ignited (SI) engines has been pursued extensively for engine efficiency and emissions improvements. Lean combustion, however, faces the challenges of decreased combustion stability and strong increases to engine-out NO_x at conditions where conventional SI engines are stable (ϕ > 0.7). Water dilution, historically used as a knock inhibitor in performance engines, has shown potential for improving both emissions and efficiency in modern engines. This study evaluates the combined impact of homogeneous lean operation and water dilution on a 2.4L 4-cylinder naturally aspirated (NA) gasoline direct injection (GDI) engine fueled with a gasoline ethanol blend. The results indicate that efficiency during stable lean operation is maintained or improved, and NO_x emissions are reduced by 15–40% depending on the level of water dilution. The lean stability limit is slightly degraded with water dilution; however, the decrease in NO_x emissions allow for less lean operation while maintaining non-water-dilute efficiency improvement and emissions reduction. Detailed hydrocarbon emissions reveal that ethanol is nearly three times more sensitive to water dilution than gasoline, and hydrocarbon kinetic pathways associated with ethanol show a similar increased sensitivity to water dilution.