In direct injection hydrogen engines, the Plume Ignition and Combustion Concept (PCC) developed by the authors has been applied to improve thermal efficiency and reduce NOx emissions under high-load operation. The PCC combustion method burns an optimized hydrogen jet plume that is ignited immediately upon completion of injection in the latter half of the compression stroke. In addition to optimizing the hydrogen jet configuration, this basic combustion concept was applied to burn a lean mixture and supercharging was used to recover the decline in power output due to combustion of a diluted mixture. As a result, a near-zero emission engine has been achieved that simultaneously provides high thermal efficiency, high power output and low NOx emissions at a single-digit ppm level. Various techniques have been applied to improve thermal efficiency further and achieve much lower NOx emissions close to a zero-emission target. These included the adoption of a re-entrant combustion chamber to reduce unburned hydrogen emissions, enhanced squish flow to promote better mixing of air and the hydrogen jet injected into the combustion chamber, and a narrower nozzle hole diameter to promote entrainment of air into the hydrogen jet. In this study, the effect of each of these measures was made clear experimentally and by analysis. This paper presents the results obtained.
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