Engines are machines designed to provide mechanical energy to a wide variety of systems by converting the chemical energy of a fuel (devices that are capable of producing a mechanical output starting from different types of energy, such as electric or hydraulic energy, are usually called motors). Internal combustion engines (ICEs) exploit the conversion of the chemical energy contained in suitable fuels—typically hydrocarbons (HCs)—into mechanical energy, owing to a combustion process. The heat generated by oxidation reactions of elements such as carbon or hydrogen produces a temperature increase in the fluid that acts as the working fluid in the power plant. The enthalpy increase is then exploited to generate mechanical work in suitable machines, thanks to the fluid expansion. If the combustion takes place within the working fluid itself, the machine is then called ICE; on the contrary, if the working fluid receives the heat from the combustion products remaining separated from them by a solid surface (for instance, in a heat exchanger), the machine is referred to as external combustion engine. Typical examples of external combustion engines are steam turbine plants or Stirling engines, whereas the ICEs category includes gas turbines and reciprocating and rotary ICEs. External combustion engines operate according to a thermodynamic closed cycle, as the working fluid undergoes thermodynamic transformations in a closed loop without any Handbook of Clean Energy Systems, Edited by Jinyue Yan. C2015 John Wiley & Sons, Ltd. ISBN: 978-1-118-38858-7. need of being replaced. On the contrary, the fluid operating in an ICE undergoes chemical transformations that require its periodical replacement with fresh fluid, thus performing a thermodynamic open cycle. Even though gas turbine plants can also be designed to operate through a closed cycle, they are typically operating in open cycle: consequently, strictly speaking, they should be included in the ICEs category. Nevertheless, only reciprocating and rotary engines are commonly designated as ICEs and will therefore be discussed in this chapter.