This paper reports on research activities aiming to improve the efficiency of direct injected, hydrogen powered internal combustion engines. In a recent major change in the experimental setup the hydrogen single cylinder research engine at Argonne National Laboratory was upgraded to a new engine geometry providing increased compression ratio and a longer piston stroke compared to its predecessor. The higher compression ratio and the more advantageous volume to surface ratio of the combustion chamber are both intended to improve the overall efficiency of the experimental setup. Additionally, a new series of faster acting, piezo-activated injectors is used with the new engine providing increased flexibility for the optimization of DI injection strategies. This study focuses on the comparison of experimental data of the baseline versus the improved single cylinder research engine for similar engine operating conditions. A comparison between the two setups revealed an up to 6 % increase in indicated thermal efficiency with the new engine configuration. A detailed analysis of individual losses further suggests that the efficiency gain is mainly due to the higher theoretical efficiency as well as reduced wall heat losses. Subsequently the impact of combustion phasing, engine load and start of injection on engine efficiency and NOx emissions of the improved engine is analyzed. Numerical simulations are performed in order to understand the underlying physics and to further explain the experimental results. Initial test results of the improved engine configuration with increased intake and exhaust pressure, imitating turbocharged conditions, show that an estimated brake thermal efficiency of 43.4 % can be achieved, with further room for improvement through optimization of the injection strategy.