The combustion of 50 wt % coal slurries, using water, diesel fuel, and methanol as carrier liquids, was investigated in a single-cylinder research engine. Preliminary data on the engine performance of dry powder coal, injected at high pressure into the engine, were also obtained. High temperatures were achieved in the engine cylinder using low-heat rejection engine technology, electrically heated glow plugs, and heated inlet air. Five coal-derived liquids, representing various low-cost processing technologies, were also evaluated for potential use as fuels in a variable compression ratio single-cylinder engine. Comparisons of the different fuels and methods of providing additional cylinder heat were obtained from cylinder pressure data and heat release calculations. Conclusions are: Autoignition of coal/water and coal/diesel slurries are possible in a medium speed diesel engine. Burning rates of the autoignited slurries could be significantly enhanced by a pilot injection of diesel fuel. Thermal efficiencies of the coal/water slurry equaled the base diesel fuel under some conditions. Engines designed for coal slurry should have the prechamber volume maximized. The Great Plains Naphtha is the best selection as a diesel fuel blending stock of the fuels tested when the ease of blending into diesel fuel, and engine performance are considered. 27 refs., 80 figs., 21 tabs.