An Experimental Study on the Effect of Exhaust Gas Recirculation on a Natural Gas-Diesel Dual-Fuel Engine

Natural gas (NG)-diesel dual-fuel combustion can be a suitable solution to reduce the overall CO₂ emissions of heavy-duty vehicles using diesel engines. One configuration of such a dual-fuel engine can be port injection of NG to form a combustible air-NG mixture in the cylinder. This mixture is then ignited by a direct injection of diesel. Other potential advantages of such an engine include the flexibility of switching back to diesel-only mode, reduced hardware development costs and lower soot emissions. However, the trade-off is lower brake thermal efficiency (BTE) and higher hydrocarbon emissions, especially methane, at low load and/or high engine speed conditions. Advancing the diesel injection timing tends to improve the BTE but may cause the NOx emissions to increase. In this study, exhaust gas recirculation (EGR) is used in combination with the diesel injection timing control to demonstrate the compromises between lowering NOx, soot, and methane emissions while maintaining diesel-like BTE. Determining such optimal operating conditions can not only reduce the consumption of diesel and NG but may also enhance the life of the exhaust after-treatment system components such as the diesel particulate filter (DPF). Tests are performed on a heavy-duty, four-stroke, NG-diesel dual-fuel single-cylinder research engine with independent and flexible air and fuel delivery systems. Two load levels corresponding to 50% and 75% of full load are investigated at a constant engine speed of 1000 rpm and NG-diesel energy ratio of 3:1. Results show that advancing the diesel injection timing at a low EGR ratio (~10% based on intake and exhaust CO₂) can reduce the soot and methane emissions but cause the NOx emissions to increase. Further increase of EGR to up to 18% can reduce the NOx emissions while limiting the soot emissions to the heavy-duty regulatory limits. In general, with the use of EGR, dual-fuel combustion can provide an improved NOx-soot trade-off compared to diesel-only combustion.