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. Furthermore this engine was modified to allow the supply of hydrogen to a single cylinder. By controlling both injection and spark timing, homogeneous and stratified combustion was studied. In addition, diffusive combustion used in combination with SI (i.e., spark assisted diffusive combustion) was investigated.Result shows that stratified and diffusive combustion by high pressure direct injection greatly improves indicated thermal efficiency by approximately 3% compared with conventional homogeneous combustion. This advancement is derived from three reasons. 1) Stratified and diffusive combustion improves trade-off between cooling loss and degree of constant volume. 2) Pressure recovery effect by injection close to TDC has a large advantage on thermal efficiency. 3) EGR is effective when it is combined with stratified and diffusive combustion.Furthermore by suppressing jet penetration to reduce more cooling loss, a good indicated thermal efficiency for a small engine of 52% was achieved. Phenomenon was confirmed by CFD and visualization of in-cylinder combustion.In addition, some valuable knowledge was derived that dehumidifying EGR increases specific heat ratio of the working gas, and improves thermal efficiency while reducing NOx.