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Slide 1:

Gas Power Cycles

P-v and T-s diagrams of a Carnot Cycle:

P-v and T-s diagrams of a Carnot Cycle 8-2

Nomenclature for Reciprocating Engines:

Nomenclature for Reciprocating Engines (Fig. 8-10) 8-3

Reciprocating Engine Displacement and Clearance Volumes:

Reciprocating Engine Displacement and Clearance Volumes (Fig. 8-11) 8-4

Actual and Ideal Cycles in Spark-Ignition Engines and Their P-v Diagram:

Actual and Ideal Cycles in Spark-Ignition Engines and Their P-v Diagram (Fig. 8-13) 8-6

T-s Diagram for the Ideal Otto Cycle:

T-s Diagram for the Ideal Otto Cycle (Fig. 8-15) 8-8

The Thermal Efficiency of the Otto Cycle:

The Thermal Efficiency of the Otto Cycle (Fig. 8-18) 8-9 The thermal efficiency of the Otto Cycle increases with the specific heat ratio k of the working fluid

T-s and P-v Diagrams for the Ideal Diesel Cycle:

T-s and P-v Diagrams for the Ideal Diesel Cycle (Fig. 8-21) 8-10

Thermal Efficiency of the Ideal Diesel Cycle:

Thermal Efficiency of the Ideal Diesel Cycle (Fig. 8-22) 8-11 The thermal efficiency of the ideal Diesel cycle as a function of compression and cutoff rates ( k =1.4)

P-v Diagram of an Ideal Dual Cycle:

P-v Diagram of an Ideal Dual Cycle (Fig. 8-23) 8-12

T-s and P-v Diagrams of Carnot, Stirling, and Ericsson Cycles:

T-s and P-v Diagrams of Carnot, Stirling, and Ericsson Cycles (Fig. 8-26) 8-13

An Open-Cycle Gas-Turbine Engine :

An Open-Cycle Gas-Turbine Engine (Fig. 8-29) 8-14

A Closed-Cycle Gas-Turbine Engine:

A Closed-Cycle Gas-Turbine Engine (Fig. 8-30) 8-15

T-s and P-v Diagrams for the Ideal Brayton Cycle:

T-s and P-v Diagrams for the Ideal Brayton Cycle (Fig. 8-31) 8-16

Thermal Efficiency of the Ideal Brayton Cycle as a Function of the Pressure Ratio:

Thermal Efficiency of the Ideal Brayton Cycle as a Function of the Pressure Ratio (Fig. 8-32) 8-17

The Net Work of the Brayton Cycle:

The Net Work of the Brayton Cycle 8-18 For fixed values of T min and T max , the net work of the Brayton cycle first increases with the pressure ratio, then reaches a maximum at r p =( T max / T min ) k/[2(k-1)] , and finally decreases

T-s Diagram of a Brayton Cycle with Regeneration:

T-s Diagram of a Brayton Cycle with Regeneration (Fig. 8-39) 8-22

Chapter Summary:

Chapter Summary A cycle during which a net amount of work is produced is called a power cycle , and a power cycle during which the working fluid remains a gas throughout is called a gas power cycle . 8-31

Chapter Summary:

Chapter Summary The most efficient cycle operating between a heat source at temperature T H and a sink at temperature T L is the Carnot cycle, and its thermal efficiency is given by 8-32

Chapter Summary:

Chapter Summary The actual gas cycles are rather complex. The approximations used to simplify the analysis are known as the air-standard assumptions . Under these assumptions, all the processes are assumed to be internally reversible; the working fluid is assumed to be air, which behaves as an ideal gas; and the combustion and exhaust processes are replaced by heat-addition and heat-rejection processes, respectively. 8-33

Chapter Summary:

Chapter Summary The air-standard assumptions are called cold-air-standard assumptions if, in addition, air is assumed to have constant specific heats at room temperature. 8-34

Chapter Summary:

Chapter Summary The Otto cycle is the ideal cycle for the spark-ignition reciprocating engines, and it consists of four internally reversible processes: isentropic compression, constant volume heat addition, isentropic expansion, and con-stant volume heat rejection. 8-36

Chapter Summary:

Chapter Summary Under cold-air-standard assumptions, the thermal efficiency of the ideal Otto cycle is where r is the compression ratio and k is the specific heat ratio C p / C v . 8-37

Chapter Summary:

Chapter Summary The Diesel cycle is the ideal cycle for the compression-ignition reciprocating engines. It is very similar to the Otto cycle, except that the constant volume heat-addition process is replaced by a constant pressure heat-addition process. 8-38

Chapter Summary:

Chapter Summary The Diesel cycle thermal efficiency under cold-air-standard assumptions is where r c is the cutoff ratio , defined as the ratio of the cylinder volumes after and before the combustion process. 8-39

Slide 26:

Stirling and Ericsson cycles are two totally reversible cycles that involve an isothermal heat-addition process at T H and an isothermal heat-rejection process at T L . They differ from the Carnot cycle in that the two isentropic processes are replaced by two constant volume regeneration processes in the Stirling cycle and by two constant pressure regeneration processes in the Ericsson cycle. Both cycles utilize regeneration , a process during which heat is transferred to a thermal energy storage device (called a regenerator ) during one part of the cycle that is then transferred back to the working fluid during another part of the cycle. 8-40

Chapter Summary:

Chapter Summary The ideal cycle for modern gas-turbine engines is the Brayton cycle , which is made up of four internally reversible processes: isentropic compression, constant pressure heat addition, isentropic expansion, and constant pressure heat rejection. 8-41

Chapter Summary:

Chapter Summary Under cold-air-standard assumptions, the Brayton cycle thermal efficiency is where r p = P max / P min is the pressure ratio and k is the specific heat ratio. The thermal efficiency of the simple Brayton cycle increases with the pressure ratio. 8-42