1. Gas Power Cycles

2. Power Cycles Ideal Cycles, Internal Combustion
Otto cycle, spark ignition
Diesel cycle, compression ignition
Sterling & Ericsson cycles
Brayton cycles
Jet-propulsion cycle
Ideal Cycles, External Combustion
Rankine cycle

3. Modeling

4. Ideal Cycles Idealizations & Simplifications
Cycle does not involve any friction
All expansion and compression processes are quasi-equilibrium processes
Pipes connecting components have no heat loss
Neglecting changes in kinetic and potential energy (except in nozzles & diffusers)

5. Carnot Cycle

6. Carnot Cycle

7. Gas Power Cycles Working fluid remains a gas for the entire cycle
Examples:
Spark-ignition engines
Diesel engines
Gas turbines

8. Air-Standard Assumptions
Air is the working fluid, circulated in a closed loop, is an ideal gas
All cycles, processes are internally reversible
Combustion process replaced by heat-addition from external source
Exhaust is replaced by heat rejection process which restores working fluid to initial state

9. Cold-Air-Standard Assumption
Air has constant specific heats, values are for room temperature (25°C or 77°F)

10. Engine Terms
Top dead center
Bottom dead center
Bore
Stroke

11. Engine Terms
Clearance volume
Displacement volume
Compression ratio

12. Engine Terms
Mean effective pressure (MEP)

13. Otto Cycle Processes of Otto Cycle: Isentropic compression
Constant-volume heat addition
Isentropic expansion
Constant-volume heat rejection

14. Otto Cycle

15. Otto Cycle Ideal Otto Cycle Four internally reversible processes
1-2 Isentropic compression
2-3 Constant-volume heat addition
3-4 Isentropic expansion
4-1 Constant-volume heat rejection

16. Otto Cycle
Closed system, pe, ke ≈ 0
Energy balance (cold air std)

17. Otto Cycle Thermal efficiency of ideal Otto cycle:
Since V2= V3 and V4 = V1
Where r is compression ratio
k is ratio of specific heats

18. Otto Cycle

24. Spark or Compression Ignition
Spark (Otto), air-fuel mixture compressed (constant-volume heat addition)
Compression (Diesel), air compressed, then fuel added (constant-pressure heat addition)

25. Diesel Cycle

26. Diesel Cycle Processes of Diesel cycle: Isentropic compression
Constant-pressure heat addition
Isentropic expansion
Constant-volume heat rejection

27. Diesel Cycle
For ideal diesel cycle
With cold air assumptions

28. Diesel Cycle
Cut off ratio rc
Efficiency becomes

34. Brayton Cycle
Gas turbine cycle
Open vs closed system model

35. Brayton Cycle Four internally reversible processes
1-2 Isentropic Compression (compressor)
2-3 Constant-pressure heat addition
3-4 Isentropic expansion (turbine)
4-1 Constant-pressure heat rejection

36. Brayton Cycle Analyze as steady-flow process So
With cold-air-standard assumptions

37. Brayton Cycle
Since processes 1-2 and 3-4 are isentropic, P2 = P3 and P4 = P1
where

38. Brayton Cycle

39. Brayton Cycle Back work ratio Improvements in gas turbines
Combustion temp
Machinery component efficiencies
Adding modifications to basic cycle

44. Actual Gas-Turbine Cycles
For actual gas turbines, compressor and turbine are not isentropic

49. Regeneration

50. Regeneration Use heat exchanger called recuperator or regenerator
Counter flow

51. Regeneration
Effectiveness
For cold-air assumptions

54. Brayton with Intercooling, Reheat, & Regeneration

55. Brayton with Intercooling, Reheat, & Regeneration
For max performance

62. Ideal Jet-Propulsion Cycles

64. Ideal Jet-Propulsion Cycles
Propulsive power
Propulsive efficiency

72. Turbojet Engines
Turbofan: for same power, large volume of slower-moving air produces more thrust than a small volume of fast-moving air (bypass ratio 5-6)
Turboprop: by pass ratio of 100

75. Jets Afterburner: addition to turbojet
Ramjet: use diffusers and nozzles
Scramjet: supersonic ramjet
Rocket: carries own oxidizer

76. Second Law Issues
Ideal Otto, Diesel, and Brayton cycles are internally reversible
2nd Law analysis identifies where losses are so improvements can be made
Look at closed, steady-flow systems

77. Second Law Issues For exergy and exergy destruction for closed system:
For steady-flow system:

78. Second Law Issues
For a cycle that starts and end at the same state: