Effectiveness of Exhaust Gas Recirculation on Low-Load Combustion Efficiency of a Reactivity Controlled Compression Ignition Engine

Reactivity controlled compression ignition (RCCI) is a potential low-temperature combustion (LTC) technique for running intrinsically efficient compression ignition engines while reducing the oxides of nitrogen (NO_x) and particulate matter (PM) emissions. However, poor low-load combustion efficiency is a major challenge in the RCCI strategy. In this work, a combination of injection strategy and cold and hot exhaust gas recirculation (EGR) strategies were investigated to improve the low-load combustion efficiency of a production light-duty compression ignition engine operating in the gasoline-diesel dual-fuel RCCI mode. The engine was operated at a low load of 3 bar gross indicated mean effective pressure and at an engine speed of 1500 rpm with wide ranges of single and multiple fuel injection strategies. Significant improvement in combustion efficiency was achieved by targeting the directly injected diesel fuel in the piston lip region. Multiple fuel injection strategy in which more than 50% of the diesel fuel was targeted in the squish region was beneficial in terms of NO_x, total hydrocarbon (THC), and soot emissions. RCCI operation with cold EGR, at the optimum injection timing, resulted in more than 96% reduction in engine-out NO_x emissions (<20 ppm, 0.4 g/kWh) with near-zero soot (0.001 g/kWh) emissions with indicated thermal efficiency (46%), similar to conventional diesel combustion (CDC). Increasing the reactivity of the gasoline-air mixture, with the optimum distribution of the diesel fuel between the piston bowl and squish regions, reduced THC emissions by 75% and carbon monoxide (CO) emissions by 30% and improved the combustion efficiency by ~25.3% points compared to the baseline dual-fuel operation.