h i g h l i g h t sCompression ignition engine fueled with manifold injected natural gas. Engine operated under dual fueling mode with diesel and RME examined as pilot fuels. Emissions maps presented for specific emissions of CO 2 , HC and NO X . Novel method of presentation for the comparison of data collected from dual fueled CI engine. Engine efficiency for a dual fueled engine examined across entire range of operation. a r t i c l e i n f o Keywords: Contours Performance maps Natural gas Diesel RME Combustion a b s t r a c t When natural gas is port/manifold injected into a compression ignition engine, the mixture of air and the natural gas is compressed during the compression stroke of the engine. Due to the difference in the values of specific heat capacity ratio between air and natural gas, the temperature and pressure at the time of pilot fuel injection are different when compared to a case where only air is compressed. Also, the presence of natural gas affects the peak in-cylinder (adiabatic flame) temperature. This significantly affects the performance as well as emissions characteristics of natural gas based dual fueling in CI engine. Natural gas has been extensively tested in a single cylinder compression ignition engine to obtain performance and emissions maps.Two pilot fuels, diesel and RME, have been used to pilot natural gas combustion. The performance of the two liquid fuels used as pilots has also been assessed and compared. Tests were conducted at 48 different operating conditions (six different speeds and eight different power output conditions for each speed) for single fueling cases. Both the diesel and RME based single fueling cases were used as baselines to compare the natural gas based dual fueling where data was collected at 36 operating conditions (six different speeds and six different power output conditions for each speed). Performance and emissions characteristics were mapped on speed vs brake power plots. The thermal efficiency values of the natural gas dual fueling were lower when compared to the respective pilot fuel based single fueling apart from the highest powers. The effect of engine speed on volumetric efficiency in case of the natural gas based dual fueling was significantly different from what was observed with the single fueling. Contours of specific NO X for diesel and RME based single fueling differ significantly when these fuels were used to pilot natural gas combustion. For both of the single fueling cases, maximum specific NO X were centered at the intersection of medium speeds and medium powers and they decrease in all directions from this region of maximum values. On the other hand, an opposite trend was observed with dual fueling cases where minimum specific NO X were observed at the center of the map and they increase in all direction from this region of minimum NO X . RME piloted specific NO X at the highest speeds were the only exception to this trend. Higher specific HC and lower specific CO 2 emissions were observed in case of natural gas based dual fueling...