Thermal Efficiency and Emissions Improved by Use of Combustion Modes Specific to Engine Operating Conditions: Researchers at Musashi Institute of Technology and the National Traffic Safety Environment Laboratory (NTSEL) studied DI hydrogen engine combustion and performance using a 1.054-liter, single-cylinder research engine for heavy-duty application. Hydrogen was injected at as high as 20 MPa with injection duration of 30 crank angle degrees at 3,000 rpm so that the engine can be operated with either premix combustion or diffusion combustion depending on engine speed and load. According to the test results, three combustion modes existed throughout the entire engine operating range. The combustion mode was defined by a combination of hydrogen injection timing and spark ignition timing, e.g., early hydrogen injection with spark timing near the top dead center and late injection with either spark ignition as injection starts or spark ignition later in the injection period. This chapter reports the characterization of combustion and NOx emissions. The chapter consists of 11 pages including 12 figures, and 2 tables.