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Hydrogen can be produced from various renewable energy sources, therefore it is predicted that hydrogen could play a greater role in meeting society's energy needs in the mid- to long-term. Conventional hydrogen engines have some disadvantages: higher cooling loss results in low thermal efficiency and abnormal combustion (backfire, pre-ignition, higher burning velocity) limits high load operation. Direct injection is an effective solution to overcome these disadvantages, but combustion methods that enable both high efficiency and low NOx have yet to be studied in enough detail. In this research, high-efficiency and low-NOx hydrogen combustion was investigated using a prototype high-pressure hydrogen injector (maximum 30 MPa). Experiments were carried out with a 2.2-liter 4-cylinder diesel engine equipped with a centrally mounted hydrogen injector, a toroidal shape combustion chamber, and a spark plug in the glow plug position.