Energy can be converted from one form to another in a device or in a system. For example, batteries convert chemical energy to electrical energy and operate mobile electronic equipment. A dam converts gravitational potential energy to kinetic energy of moving water used on the blades of a turbine, which ultimately converts the kinetic energy to electric energy through an electric generator. Heat engines are devices converting heat into work. With each energy conversion, part of the energy becomes dispersed and less useful and hence unavailable for further use. This chapter discusses energy conversion and thermal efficiency of turbines, compressors, and heat engines. Heat engines include Carnot, Rankine, Brayton, Otto, and Diesel cycles.