This book deals with the combustion and exhaust emissions of gas engines fueled with green biogas. Biogas is a mixture of gases, primarily consisting of methane and carbon dioxide. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, food waste, etc. Biogas is considered to be a renewable source of energy. Therefore, it can contribute to the prevention of global warming. The biogas engine is used to co-generate electricity by operating engine and heat from hot exhaust gases. The energy source used very efficiently. Unlike other green energy sources such as wind and solar, biogas is readily available when needed. This book first describes the basics of biogas and its application to internal combustion engines. Next, it describes the engine system and the combustion phenomena in the engine cylinder. Engine technology continues to advance in spark ignition and dual-fuel engines to achieve higher thermal efficiency and lower harmful emissions. Several advanced combustion technologies are introduced to achieve higher thermal efficiency while avoiding knocking.