Study of injection pressure couple with EGR on combustion performance and emissions of natural gas-diesel dual-fuel engine

Natural gas (NG) dual-fuel engines can attain a similar thermal efficiency as that of diesel engines while achieving lower emissions. However, the trade-off relationship between CH4 and NOX emissions limits the development of dual-fuel (DF) engines. In order to resolve this problem, the effects of injection pressure (IP) and exhaust gas recirculation (EGR) ratio on the combustion and emission of diesel/NG dual-fuel engines are investigated in this study. The results show that the diesel/NG dual fuel has a distinct three-stage heat release characteristic in the high-temperature combustion process. As the injection pressure increases, the flame propagation speed of methane and the indicated thermal efficiency (ITE) increase. However, the methane in the crevice region and cylinder wall cannot be ignited because of low temperature; these are the main sources of methane emissions. When the EGR rate increases, the indicated thermal efficiency first increases and there after decreases, and diesel is cleaved through the reaction chain, PC4H9 -> C2H4 -> CH4, to produce CH4; this becomes one of the reasons for CH4 emissions. When the EGR rate is small (< 10%), CO and CH4 emissions are not sensitive to IP; when the EGR rate is large (> 30%), CO and CH4 emissions can be significantly reduced by increasing the IP. Accordingly, when the injection pressure is 160 MPa and the EGR rate is 20%, the diesel/NG DF engine can achieve higher ITE and lower emissions.