Heat Engine Working Cycles

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HEAT ENGINE WORKING CYCLES

An engine is a device which transforms one form of energy into another form. However, while transforming energy from one form to another form, the efficiency of conversion plays an important role. Normally, most of the engines convert thermal energy to mechanical work and therefore they are called ‘heat engines’. Heat engine is a device which transforms the chemical energy of a fuel into thermal energy and utilizes this thermal energy to perform useful work. Depending upon whether the working substance is a gas or a liquid-vapour, there are two kinds of cycles, the non-phase change cycle and the phase change cycle. The non-phase change cycle employs a gas which remains in the same phase throughout the working cycle. The phase change cycle employs a substance that is usually a liquid to start with but which becomes a vapour after energy intake as heat, and may even be superheated during part of the cycle. This vapour is later condensed to repeat the cycle. In all present day phase change cycles, energy addition to the working fluid occurs outside the device, where work is done. In most non-phase change cycles, energy addition occurs in the cylinder where work is done.

Apart from the difference in cycles due to the nature of the working substance, working cycles may also be classified as open and closed cycles. In an open cycle the working fluid does not go through a cyclic process, though the heat engine must operate in a cycle. The working substance in such a case may finally be in a different state, from that at the start and may even change its composition during operation. Practical internal combus¬tion engine cycles and some gas turbine cycles are examples of open cycles. In a closed cycle, the working substance undergoes a series of processes in¬volving state changes, but finally returns to its original state. For an under¬standing of both the open and closed cycles it is advantageous to analyze the performance of an...
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