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Lec 21: Isentropic efficiencies, air standard cycle, Carnot cycle, Otto cycle

2. For next time:Read: ? 8-6 to 8-7HW11 due Wednesday, November 12, 2003Outline:Isentropic efficiency Air standard cycleOtto cycleImportant points:Realize that we already know how to analyze all these new cycles, we just need to define what the cycle steps areKnow the difference between the air standard cycle and the cold air approximationsKnow how to solve cycles using variable specific heats and constant specific heats.

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Lec 21: Isentropic efficiencies, air standard cycle, Carnot cycle, Otto cycle

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    1. 1 Lec 21: Isentropic efficiencies, air standard cycle, Carnot cycle, Otto cycle

    2. 2 For next time: Read: 8-6 to 8-7 HW11 due Wednesday, November 12, 2003 Outline: Isentropic efficiency Air standard cycle Otto cycle Important points: Realize that we already know how to analyze all these new cycles, we just need to define what the cycle steps are Know the difference between the air standard cycle and the cold air approximations Know how to solve cycles using variable specific heats and constant specific heats

    3. 3 Isentropic Efficiencies

    4. 4 Compressor Isentropic Efficiency

    5. 5 Compressor Isentropic Efficiency

    6. 6 Compressor Isentropic Efficiency

    7. 7 Compressor Isentropic Efficiencies

    8. 8 Isentropic Efficiencies

    9. 9

    10. 10 Isentropic Efficiencies

    11. 11 TEAMPLAY Work problem 7-89

    12. 12 Chapter 8, Gas Cycles Carnot cycle is the most efficient cycle that can be executed between a heat source and a heat sink. However, isothermal heat transfer is difficult to obtain in reality--requires large heat exchangers and a lot of time.

    13. 13 Gas Cycles Therefore, the very important (reversible) Carnot cycle, composed of two reversible isothermal processes and two reversible adiabatic processes, is never realized as a practical matter. Its real value is as a standard of comparison for all other cycles.

    14. 14

    15. 15 Assumptions of air standard cycle Working fluid is air Air is ideal gas Combustion process is replaced by heat addition process Heat rejection is used to restore the fluid to its initial state and complete the cycle All processes are internally reversible Constant or variable specific heats can be used

    16. 16 Gas cycles have many engineering applications Internal combustion engine Otto cycle Diesel cycle Gas turbines Brayton cycle Refrigeration Reversed Brayton cycle

    17. 17 Some nomenclature before starting internal combustion engine cycles

    18. 18 More terminology

    19. 19 Terminology Bore = d Stroke = s Displacement volume =DV = Clearance volume = CV Compression ratio = r

    20. 20 Mean Effective Pressure

    21. 21 The net work output of a cycle is equivalent to the product of the mean effect pressure and the displacement volume

    22. 22 Real Otto cycle

    23. 23 Real and Idealized Cycle

    24. 24 Idealized Otto cycle

    25. 25 Idealized Otto cycle 1-2 - ADIABATIC COMPRESSION (ISENTROPIC) 2-3 - CONSTANT VOLUME HEAT ADDITION 3-4 - ADIABATIC EXPANSION (ISENTROPIC) 4-1 - CONSTANT VOLUME HEAT REJECTION

    26. 26 Performance of cycle

    27. 27 Cycle Performance

    28. 28 Cycle Performance

    29. 29 Teamplay

    30. 30 Cold air standard cycle

    31. 31 Cycle performance with cold air cycle assumptions

    32. 32 Cycle performance with cold air cycle assumptions

    33. 33 Cycle performance with cold air cycle assumptions

    34. 34 Differences between Otto and Carnot cycles

    35. 35 Effect of compression ratio on Otto cycle efficiency

    36. 36 Sample Problem

    37. 37 Draw cycle and label points

    38. 38 Major assumptions Kinetic and potential energies are zero Closed system 1 is start of compression Ideal cycle: 1-2 isentropic compression, 2-3 const. volume heat addition, etc. Cold cycle const. properties

    39. 39 Carry through with solution

    40. 40 Get T3 with first law:

    41. 41 Thermal Efficiency

    42. 42 Lets take a look at the Diesel cycle.

    43. 43

    44. 44

    45. 45 TEAMPLAY Work problem 8-16

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