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Unformatted text preview: Learning Objectives 1. Evaluate the performance of gas power cycles for which the working fluid remains a gas throughout the entire cycle. 2. Develop simplifying assumptions applicable to gas power cycles. 3. Review the operation of reciprocating engines. 4. Analyze both closed and open gas power cycles. 5. Solve problems based on the Otto, Diesel, and Brayton cycles. 91 Basic Considerations in the Analysis of Power Cycles Overview • Most power producing devices operate on cycles • In general, these cycles are difficult to analyze • To make the analysis feasible, idealizations are utilized Ideal Cycle • A cycle that resembles an actual cycle closely but is made up totally of internally reversible processes 91 Basic Considerations in the Analysis of Power Cycles Thermal Efficiency • The ratio of net work produced by the engine to the total heat input • The Carnot cycle has the highest efficiency of all heat engines operating between the same temperature levels, i.e., no cycle can be more efficient than the Carnot cycle • Ideal cycles are internally reversible, but unlike the Carnot cycle, they are not necessarily externally reversible • As a result, ideal cycles may involve heat transfer through a finite temperature difference 91 Basic Considerations in the Analysis of Power Cycles Common Idealizations and Simplifications • The following idealizations and simplifications are commonly employed in the analysis of power cycles 1. The cycle does not involve any friction. Therefore, the working fluid does not experience any pressure drop as it flows in pipes or devices 2. All expansion and compression processes take place in a quasiequilibrium manner 3. The pipes connecting the various components of a system are all well insulated, and heat transfer through them is negligible 91 Basic Considerations in the Analysis of Power Cycles Property Diagrams • In the analysis of cycles, property diagrams such as the Pv and Ts diagrams serve as valuable aids • On both the Pv and Ts diagrams, the area enclosed by the process curves of a cycle represents the net work produced during the cycle, which is equivalent to the net heat transfer for that cycle • On a Ts diagram: o Heat addition → proceeds in the direction of increasing entropy o Heat rejection → proceeds in the direction of decreasing entropy o Isentropic → constant entropy 92 The Carnot Cycle and its Value in Engineering Carnot Cycle • The Carnot cycle is composed of four totally reversible processes 1. Isothermal heat addition 2. Isentropic expansion 3. Isothermal heat rejection 4. Isentropic compression Thermal Efficiency • The Carnot cycle is the most efficient cycle that can be operated between a heat source at temperature T H and a sink at temperature T L • The thermal efficiency of the Carnot cycle is 92 The Carnot Cycle and its Value in Engineering Value of Carnot Cycle • Reversible isothermal heat transfer is very difficult to achieve in reality •...
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 Spring '08
 DEINERT
 Thermodynamics, Internal combustion engine, Diesel engine, Heat engine, Gas turbine, Brayton cycle

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