Hydrogen-fueled homogeneous charge compression ignition (HCCI) engines have shown the ability to provide a cleaner and more efficient alternative to conventional fossil fuels. The use of hydrogen as a fuel has the potential to reduce greenhouse gas and promote sustainability.In this study, a modified single-cylinder Cooperative Fuel Research (CFR) engine was utilised to operate on hydrogen in a HCCI combustion mode under various compression ratio (CR) conditions. In the experiments, the amount of hydrogen injected was adjusted at each CR to maintain the crank angle at 50% mass fraction burned (CA50) combustion phasing at 3±1 crank angle degrees after top dead center or as lean as possible. The engine speed was fixed at 600 rpm, and the impact of different intake air temperatures was also investigated.The results indicated that as the compression ratio increases, the air-fuel ratio needs to be increased to maintain the desired CA50 value, i.e., the engine needs to operate leaner. The net indicated mean effective pressure of the engine reached a value of 2.9 bar at a compression ratio of 14 and an intake air temperature of 150OC. The effects of CR and intake temperature on engine performance metrics, such as power output and the rate of heat release, were also investigated. The experimental data showed that the intake air temperature did not have a significant effect on engine performance and power output. At a compression ratio of 16:1 and 600 rpm, the engine's indicated thermal efficiency was found to be approximately 33% across the range of intake temperatures investigated. Furthermore, the fact that the engine effectively produced zero NOx emissions under the various CR conditions tested further highlights the potential for hydrogen HCCI engines to be adopted as a cleaner and more efficient alternative to internal combustion engines using conventional fuels, provided the available range of operation is acceptable and can be made large enough for practical applications.