Three slurry fuels containing different concentrations of coal have been tested in a small-bore 4-cylinder diesel engine equipped with modified injectors under various loads and speeds. The coal in the slurry was a finely pulverized coal having a mean particle diameter of 2.05 micrometers. Exhaust temperatures, smoke, blowby, and brake thermal efficiencies were determined under these conditions. The relative effectiveness of coal in saving diesel fuel was determined. In general, at higher speeds, exhaust temperatures and smoke increased with the concentration of coal while brake thermal efficiency decreased. The blowby increased significantly in a period of 14 hours at the end of which the fuel pump seized. However, no abnormal wear was observed on injectors and valves.From this preliminary study, it appears that two BTU’s of coal will replace about one BTU of diesel fuel in the energy utilization of a slurry fuel of this type, and that the use of slurry fuels will require major engine modifications.