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Gas Power Cycles

Gas Power Cycles. 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. Modeling. Ideal Cycles.

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Gas Power Cycles

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  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

  19. 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

  20. Diesel Cycle

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

  22. Diesel Cycle • For ideal diesel cycle • With cold air assumptions

  23. Cut off ratio rc Efficiency becomes Diesel Cycle

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

  25. 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

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

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

  28. Brayton Cycle

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

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

  31. Regeneration

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

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