Diffusion Combustion of Hydrogen in Argon-Oxygen Mixture Improves Thermal Efficiency: Ultimately clean exhaust emissions with renewable energy were demonstrated in a hydrogen engine with an argon-oxygen mixture environment. If argon is reused in the engine system, the engine consumes hydrogen and oxygen to operate and emits only water because no nitrogen and no carbonaceous components are contained in the fuel and working fluid. Toyota engineers investigated its potential for thermal efficiency improvement and studied the experimental engine with a higher compression ratio, direct injection of hydrogen, and hydrogen jet orientations. Indicated thermal efficiency can be more than 50 percent. The chapter consists of 11 pages including 13 figures, and 1 table.