Striving for sustainable transportation solutions, hydrogen is often identified as a promising energy carrier and internal combustion engines are seen as a cost effective consumer of hydrogen to facilitate the development of a large-scale hydrogen infrastructure. Driven by efficiency and emissions targets defined by the U.S. Department of Energy, a research team at Argonne National Laboratory has worked on optimizing a spark-ignited direct injection engine for hydrogen. Using direct injection improves volumetric efficiency and provides the opportunity to properly stratify the fuel-air mixture in-cylinder. Collaborative 3D-CFD and experimental efforts have focused on optimizing the mixture stratification and have demonstrated the potential for high engine efficiency with low NOx emissions. Performance of the hydrogen engine is evaluated in this paper over a speed range from 1000 to 3000 RPM and a load range from 1.7 to 14.3 bar BMEP.Engine maps show the hydrogen direct injection engine operating above 35% brake thermal efficiency (BTE) over approximately 80% of the tested operating range. A more detailed characterization of engine efficiency is done by quantifying the effects of different loss mechanisms in the engine at relevant points throughout the engine map. The dominant loss mechanism is heat loss to the combustion chamber walls and as a function of both engine speed and load there exists a trade-off between wall heat losses and other partial losses. As a result, the peak BTE is observed at 2000 RPM, 13.5 bar BMEP.A series of engine maps show efficiency improvements due to optimal injection timing and also show efficiency and NOx improvements due to injector nozzle design. The most promising engine configuration uses a 4-hole nozzle which shows improvement over the previous 5-hole nozzle. The final engine map shows a peak BTE of 45.5% and part-load BTE of 33.3%, demonstrating the ability of the hydrogen direct injection engine to exceed both U.S. DOE light-duty efficiency targets. The 4-hole nozzle also provides a mixture that is less potent for NOx than the 5-hole nozzle which correlates to a significant decrease in NOx emissions at the peak efficiency operating point. The corresponding map of NOx emissions is dominated by less than 0.10 g/kWh of NOx.