This chapter examines internal combustion engines as energy conversion devices, transforming stored fuel energy into mechanical work. The operation of reciprocating four-stroke engines and means for analyzing their efficiency, emissions, and power output are reviewed. Attributes of various fuel options are enumerated. An examination of characteristics and standards related to gasoline and diesel fuel are reviewed. These are compared with some of the more practical alternative fuels that could replace fossil fuels. The technologies and analysis of engine control and emissions reduction are examined, starting with the ideal Otto cycle for four-stroke spark ignition engines. Calculations of carbon dioxide emissions, ideal air-fuel ratios, and power output are discussed for liquid and gaseous fuels. The effectiveness of modern feedback fuel control and three-way catalytic converters are covered in some detail, along with the positive impact they have had on air quality. References for more detailed study are provided.