Gas Power Cycles

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. Top dead center Bottom dead center Bore Stroke Engine Terms

11. Clearance volume Displacement volume Compression ratio Engine Terms

12. Mean effective pressure (MEP) Engine Terms

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

14. Otto Cycle

15. 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 Otto Cycle

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

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. Cut off ratio rc Efficiency becomes Diesel Cycle

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

35. 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 Brayton Cycle

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

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

49. Regeneration

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