Direct-Injected Hydrogen Improves Indicated Thermal Efficiency 52 Percent: When hydrogen is discussed as a vehicle fuel, use of a fuel cell is generally considered. Fuel-cell technology has made significant advancement in recent years, yet there are still technological issues that need to be resolved including, for example, cost, durability, and application. In the meantime, operating a reciprocating engine on hydrogen seems more practical and feasible for the manufacture of such an engine at lower cost with proven durability if hydrogen infrastructure is available. In fact, in the early 1970s, hydrogen engines were developed at various sites and demonstrated viability for automotive application. Resuming that effort now along with current efforts in fuel-cell vehicle development potentially makes the hydrogen-fueled vehicle available sooner. High-pressure injection of hydrogen directly into the cylinder with high EGR has the potential to solve inherent problems of backfire and high NOx in a hydrogen-fueled engine. Toyota engineers report indicated thermal efficiency of 52 percent achieved by the direct-injected hydrogen engine and describe technologies to further increase thermal efficiency. This chapter reports test results of a high-pressure, direct-injected hydrogen engine and optimization of the parameters to achieve high thermal efficiency and low NOx emissions.