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In this chapter we shall consider the physical principles underlying the operations of heat engines because of the intrinsic importance of these principles and because of the part they have played in the development of fundamental physical ideas. Heat engines are designed and built to convert heat into work. In most cases the heat is obtained from the combustion of a common fuel such as coal, oil, gasoline, or natural gas. An important new source of heat that is just beginning to be used, and will be used more extensively in the future, is the mass which is converted into energy by means of a process called nuclear fission. Several power plants are now in operation which get the heat for their engines from the nuclear fission of the element uranium.
There are many different types of heat engines; we shall present brief descriptions of the operations of a few of them. In general, a heat engine utilizes a working substance, usually steam, or a mixture of fuel and air, or fuel and oxygen, through a series of operations known as a cycle. The working substance goes through a series of changes of state in this cycle, as a result of which some of the heat which has been supplied to it from a source at a high temperature is converted into work which is delivered to some external agency. Experience shows that not all of the heat supplied is converted into work; the heat which has not thus been utilized is delivered by the engine to some outside reservoir at a lower temperature.