The results of an experimental investigation of the use of low-Btu gas, such as would be obtained from the gasification of lignite, as a fuel for diesel engines is reported. The performance of a small (3.5 bhp) single cylinder diesel engine operating as a dual-fuel engine with low Btu gas as the primary fuel was determined, and compared to conventional diesel performance.Low-Btu gas (150 Btu/scf) was mixed with the intake air and inducted into the engine where the mixture was ignited with a “pilot” charge of diesel fuel. The effect of increasing the amount of gaseous fuel input on the power output, diesel fuel consumption, and efficiency was investigated at three speeds by conducting constant speed tests with the engine mounted on a hydraulic dynamometer test stand.Results are presented as plots of the performance variables as functions of the richness of the gas - air mixture that was inducted. At a speed of 2800 rpm, operating in the dual-fuel mode, the diesel pilot charge supplied only 30% of the total energy input to the engine (70% supplied by gas) at 95% of maximum power output. At higher speed (3100 rpm) maximum power output was limited by misfire and stalling to only 70% of that obtainable from conventional diesel operation. Advancing the injection timing resulted in realization of full power with a 65% reduction in diesel fuel consumption compared to that required to produce the same power operating as a conventional diesel. Brake thermal efficiency for dual-fuel operation was 5% to 10% lower than the values for conventional diesel operation. Engine knock was experienced at high power levels and advanced injection timings, and limited the output when operating in th dual-fuel mode.