Direct cylinder injection for hydrogen-fueled piston engines was studied experimentally with an ASTM-CFR engine. An injection scheme was devised for which the combustion occurs during the period of hydrogen injection so that little unburned hydrogen accumulates in the cylinder. This scheme should preclude flashback and preignition, and lowers the rate of cylinder pressure rise to acceptable levels. The potential of hydrogen as a low pollution fuel was investigated by comparing the NOx emissions from the same engine operated on hydrogen and hydrocarbon fuel. NOx emissions for hydrogen are very low provided the equivalence ratio is less than 0.5, and provided the hydrogen and air are well mixed. For equivalence ratios greater than 0.5, NOx emissions for hydrogen fuel were higher than for hydrocarbon fuel. With hydrogen injection, indicated mean effective pressure was varied between 0.07 MPa and 0.78 MPa without intake air throttling. Indicated efficiencies higher than 40% were achieved for part-load equivalence ratios, near 0.3. The timing of hydrogen injection has a significant effect on NOx emissions.