Experimental study on the effects of injection pressure and injection timing on combustion and emissions in a direct-injection hydrogen engine

Hydrogen internal combustion engines are pivotal components of the power industry for achieving zero-carbon emissions. However, the development of hydrogen engines is still in its infancy, and experimental research on their injection strategies lacks systematization. In this study, the individual impacts of hydrogen injection pressure (within low-pressure ranges) and injection timing, as well as their coupling effects, on combustion characteristics, engine efficiency, and exhaust emissions were experimentally investigated. Results show that, under fixed timing, an injection pressure of 25–27.5 bar yields the highest and earliest peak in-cylinder pressures, whereas at 15 bar, the ignition delay increases to 14.7°CA, the flame development duration extends to 8.57°CA, and the late combustion duration shortens to 41.37°CA; the exhaust gas temperature peaks at 628 K at 20 bar, and NO X peaks at 537 ppm at 25 bar. BTE (brake thermal efficiency) exhibits a U-shaped relationship with pressure, with the minimum efficiency occurring near 25 bar when timing is held constant; advancing start of injection from 130° BTDC to 170° BTDC reduces both NO X and exhaust gas temperature, with the optimal fuel economy at 140° BTDC, and a peak in-cylinder pressure that is approximately 7 % higher and occurs 2–3°CA earlier at 130–140° BTDC. In the pressure–timing maps, IMEP (indicated mean effective pressure) is maximized at 30 bar and 90° BTDC; BTE reaches 33.5 % at 25 bar and 100° BTDC; NO X attains a minimum at 25 bar and 110° BTDC, while the exhaust gas temperature is lowest at 25 bar and 120° BTDC. Injection pressure is the primary lever for regulating fuel economy and emissions, while injection timing mainly adjusts combustion phasing and IMEP. The results provide clear guidance for calibrating low-pressure hydrogen injection systems, supply benchmark data for model validation, and support the development of practical control strategies for hydrogen engines.