Syngas, an environmentally friendly alternative gaseous fuel for internal combustion engine operation, mainly consists of carbon monoxide (CO) and hydrogen (H2). It can substitute fossil diesel oil in a compression ignition diesel engine through dual fuel operation route. In the present investigation, experiments were conducted in a constant speed single cylinder direct injection diesel engine fuelled with syngas-diesel in a dual fuel operation mode. The main contribution of this study is to introduce the new synthetic gaseous fuel (syngas) including the possible use of CO gas, an alternative diesel engine fuel. In this work, four different H2 and CO compositions of syngas were chosen for dual fuel study under different engine loading levels. Keeping the same power output at the corresponding tested loads, the engine performance of dual fuel operations were compared to that of diesel mode for the entire load range. The maximum diesel replacement in the engine was found to be 72.3 for 100 H2 fuel. This amount replacement rate was reduced for the low energetic lower H2 content fuels. The brake thermal efficiency was always found highest (about 21) in the case of diesel mode operation. However, the 100 H2 syngas showed a comparative performance level with diesel mode at the expense of higher NO x emissions. At 80 engine load, the brake thermal efficiency was found to be 15.7 for 100 CO syngas. This value increased to 16.1, 18.3 and 19.8 when the 100 CO syngas composition was replaced by H2 contents of 50, 75 and 100, respectively. At part loads (i.e., at 20 and 40), dual fuel mode resulted a poor performance including higher emission levels. In contrast, at higher loads, syngas fuels showed a good competitive performance to diesel mode. At all the tested loads, the NOx emission was observed highest for 100 H2 syngas as compared to other fuel conditions, and a maximum of 240 ppm was found at 100 load. However, when the CO fractions of 25, 50 and 100, were substituted to hydrogen fuel, the emission levels got reduced to 175 ppm, 127 ppm, and 114 ppm, respectively. Further, higher CO and HC emission levels were recorded for 25, 50, and 100 CO fraction syngas fuels due to their CO content. Ignition delay was found to increase for the dual fuel operation as compared to diesel mode, and also it seemed to be still longer for higher H 2 content syngas fuels. The peak pressure and maximum rate of pressure rise were found to decrease for all the cases of dual fuel operation, except for 100 H2 syngas (beyond 60 load). The reduction in peak pressure resulted a rise in the exhaust gas temperature at all loads under dual fuel operation. The present investigation provides some useful experimental data which can be applied to the possible existing engine parameters modifications to produce a competitive syngas dual fuel performance at all the loading operations. © 2011 American Society of Mechanical Engineers.