The use of hydrogen as an engine fuel has great potential for reducing exhaust emissions since, with the exception of a small amount of carbon containing emissions originating from the lubricating oil, oxides of nitrogen are the only pollutant emitted. There is long term interest in the use of hydrogen as a fuel in the event it proves necessary to take extreme measures to limit the emission of carbon dioxide because of the greenhouse effect. Conversion of an engine to operate on hydrogen is not a straightforward task. Hydrogen-fueled spark ignition engines suffer from high susceptability to surface ignition and subsequent backfiring. For this reason the feasibility of employing direct injection of hydrogen to fuel a diesel engine has been explored.A single-cylinder Lister ST-1 direct injection diesel engine has been converted to operate on hydrogen. Hydrogen gas, at 10.3 MPa (1500 psig) pressure, is admitted to the engine combustion chamber through a hydraulically-actuated injection valve which controls the timing and duration of the hydrogen injection. Ignition is provided by a continuously operating sheathed glow plug of the type employed in passenger car diesel engines to assist cold starting.Results indicate that the hydrogen-fueled diesel engine can produce higher power than an ordinary diesel engine due to the absence of smoke emissions. Another positive feature is reduced NOx emissions compared to the ordinary diesel engine. Indicated efficiency of the hydrogen-fueled diesel engine is about 90% of that of the original diesel at moderate loads. As full load is approached, the efficiency of the diesel falls off more rapidly and the two efficiencies therefore converge to about the same value. At very light loads, however, the efficiency of the hydrogen-fueled engine decreases compared to that at moderate loads.The project has demonstrated that the hydrogen-fueled diesel engine with glow plug ignition develops greater power with lower emissions than the same engine operated on diesel fuel. Further improvement in efficiency is desirable, especially at light loads, and improvement will most likely be obtained by refinement of the combustion chamber and injector nozzle designs.