Gasoline compression ignition using a single gasoline-type fuel for direct/port injection has been shown as a method to achieve low-temperature combustion with low engine-out NOx and soot emissions and high indicated thermal efficiency. However, key technical barriers to achieving low-temperature combustion on multi-cylinder engines include the air handling system (limited amount of exhaust gas recirculation) as well as mechanical engine limitations (e.g. peak pressure rise rate). In light of these limitations, high-temperature combustion with reduced amounts of exhaust gas recirculation appears more practical. Furthermore, for high-temperature gasoline compression ignition, an effective aftertreatment system allows high thermal efficiency with low tailpipe-out emissions. In this work, experimental testing was conducted on a 12.4 L multi-cylinder heavy-duty diesel engine operating with high-temperature gasoline compression ignition combustion with port and direct injection. Engine testing was conducted at an engine speed of 1038 r/min and brake mean effective pressure of 1.4 MPa for three injection strategies, late pilot injection, early pilot injection, and port/direct fuel injection. The impact on engine performance and emissions with respect to varying the combustion phasing were quantified within this study. At the same combustion phasing, early pilot injection and port/direct fuel injection had an earlier start of combustion and higher maximum pressure rise rates than late pilot injection attributable to more premixed fuel from pilot or port injection; however, brake thermal efficiencies were higher with late pilot injection due to reduced heat transfer. Early pilot injection also exhibited the highest cylinder-to-cylinder variations due to differences in injector behavior as well as the spray/wall interactions affecting mixing and evaporation process. Overall, peak brake thermal efficiency of 46.1% and 46% for late pilot injection and port/direct fuel injection was achieved comparable to diesel baseline (45.9%), while early pilot injection showed the lowest brake thermal efficiency (45.3%).
Steady-state experiments were conducted on a 12.4L, six-cylinder heavy-duty engine to investigate the influence of port-injected water and dilution via exhaust gas recirculation (EGR) on combustion and emissions for diesel and gasoline operation. Adding a diluent to the combustion process reduces peak combustion temperatures and can reduce the reactivity of the charge, thereby increasing the ignition-delay and, allowing for more time to premix air and fuel. Experiments spanned water/fuel mass ratios up to 140mass% and exhaust gas recirculation ratios up to 20vol% for gasoline and diesel operation with different injection strategies. Diluting the combustion process with either water or EGR resulted in a significant reduction in nitrogen oxide emissions along with a reduction in brake thermal efficiency. The sensitivity of brake thermal efficiency to water and EGR varied among the fuels and injection strategies investigated. An efficiency breakdown revealed that water injection considerably reduced the wall heat transfer; however, a substantial increase in exhaust enthalpy offset the reduction in wall heat transfer and led to a reduction in brake thermal efficiency. Regular diesel operation with main and post injection exhibited a brake thermal efficiency of 45.8% and a 0.3% reduction at a water/fuel ratio of 120%. The engine operation with gasoline, early pilot, and main injection strategy showed a brake thermal efficiency of 45.0% at 0% water/fuel ratio, and a 1.2% decrease in brake thermal efficiency for a water/fuel ratio of 140%. Using EGR as a diluent reduced the brake thermal efficiency by 0.3% for diesel operation, comparing ratios of 0% and 20% EGR. However, a higher impact on brake thermal efficiency was seen for gasoline operation with early pilot and main injection strategy, with a reduction of about 0.8% comparing 0% and 20% EGR. Dilution by means of EGR exhibited a reduction in nitrogen oxide emissions up to 15 g/kWh; water injection showed only up to 10 g/kWh reduction for the EGR rates and water/fuel ratio investigated.
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