On the Variation of the Effect of Natural Gas Fraction on Dual-Fuel Combustion of Diesel Engine Under Low-to-High Load Conditions

Natural gas-diesel dual fuel (NDDF) engine has the potential to significantly reduce carbon dioxide (CO2) and particulate matter (PM) emissions while retaining the diesel engine's high efficiency and reliability. The operation and performance of the NDDF engine depend on natural gas energy fraction (%NG), which ranges from low %NG, where diesel fuel provides most of the combustion energy, to high %NG, where diesel fuel is used only to initiate combustion. Although pertaining published studies demonstrated that the effect of %NG on the combustion performance and emissions of the NDDF engine depend upon the engine load, none of these studies discussed the reasons behind this dependence. The present study attempts to shed light on this issue by experimentally and numerically investigating the effect of different %NGs on the NDDF engine under different load conditions. Both experimental and numerical results revealed that the peak pressure rise rate (PPRR) first increases and then drops with increasing %NG under all load conditions. The calculated local equivalence ratio and charge temperature contours showed that increasing %NG to a certain limit increases local equivalence ratio inside the ignition kernel, which implies that more premixed natural gas-air mixture participates in reactions during the premixed combustion stage. This results in an increased flame temperature and higher PPRR right after ignition. However, increasing %NG beyond this range decreases local equivalence ratio inside the ignition kernel, which leads to lower PPRR. Increasing %NG retards the combustion phasing at low-to-medium load conditions. However, increasing %NG generally advances the combustion phasing under medium-to-high load conditions due to the fact that the flame propagation speed of natural gas-air mixture increases with %NG at medium-to-high load conditions. Methane emissions grow with increasing %NG under all examined engine load conditions. However, this growth becomes much smaller under medium-to-high engine load conditions. Increasing %NG significantly increases greenhouse gas (GHG) emissions under lower load conditions. However, using advanced diesel injection timing decreases GHG emissions of the NDDF engine under lower load conditions. Increasing %NG decreases GHG emissions by 6%-11% under medium-to-high load conditions.