Exploring a diesel substitute for compression ignition engine is a crucial step toward achieving cleaner, healthier, and more sustainable transportation and energy systems. Hydroxy (HHO) gas is considered as a partial substitute to diesel fuel because of its combustion characteristics. Hydrogen-based fuel can be generated on-demand through water electrolysis for immediate use, eliminating the need for storage. As a potential substitute for or supplement to diesel fuel, HHO offers a promising solution for sustainable energy development. The depletion of fossil fuels and emissions from automobiles drive climate change, prompting a shift toward cleaner alternatives. Hydrogen (H₂), particularly as HHO-a mixture of hydrogen and oxygen, emerges as a renewable fuel that enhances combustion efficiency in engines. This chapter explores the properties and benefits of hydrogen and HHO, outlines various electrolyzer types, and highlights their role in sustainable energy solutions. Key characteristics of hydrogen are crucial for optimizing engine performance and preventing hazards. The chapter discusses flammability limits, combustion risks, and material challenges like hydrogen embrittlement. It also examines HHO gas production optimization, comparing wet and dry cell electrolyzers. Integrating HHO into diesel engines shows potential in reducing fuel consumption and improving thermal efficiency, offering a pathway for enhanced fuel efficiency and lower environmental impact.