Gas Power Cycles

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Gas Power Cycles

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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

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Modeling
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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)

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Carnot Cycle
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Carnot Cycle
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Gas Power Cycles

• Working fluid remains a gas for the entire cycle
• Examples
• Spark-ignition engines
• Diesel engines
• Gas turbines

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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

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Cold-Air-Standard Assumption

• Air has constant specific heats, values are for
  room temperature (25C or 77F)

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Engine Terms

• Top dead center
• Bottom dead center
• Bore
• Stroke

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Engine Terms

• Clearance volume
• Displacement volume
• Compression ratio

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Engine Terms

• Mean effective pressure (MEP)

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Otto Cycle

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

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Otto Cycle
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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

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Otto Cycle

• Closed system, pe, ke 0
• Energy balance (cold air std)

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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

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Otto Cycle
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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)

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Diesel Cycle
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Diesel Cycle

• Processes of Diesel cycle
• Isentropic compression
• Constant-pressure heat addition
• Isentropic expansion
• Constant-volume heat rejection

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Diesel Cycle

• For ideal diesel cycle
• With cold air assumptions

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Diesel Cycle

• Cut off ratio rc
• Efficiency becomes

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Brayton Cycle

• Gas turbine cycle
• Open vs closed system model

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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

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Brayton Cycle

• Analyze as steady-flow process
• So
• With cold-air-standard assumptions

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Brayton Cycle

• Since processes 1-2 and 3-4 are isentropic, P2
  P3 and P4 P1
• where

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Brayton Cycle
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Brayton Cycle

• Back work ratio

• Improvements in gas turbines
• Combustion temp
• Machinery component efficiencies
• Adding modifications to basic cycle

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Actual Gas-Turbine Cycles

• For actual gas turbines, compressor and turbine
  are not isentropic

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Regeneration
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Regeneration

• Use heat exchanger called recuperator or
  regenerator
• Counter flow

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Regeneration

• Effectiveness
• For cold-air assumptions

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Brayton with Intercooling, Reheat, Regeneration
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Brayton with Intercooling, Reheat, Regeneration

• For max performance

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Ideal Jet-Propulsion Cycles
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Ideal Jet-Propulsion Cycles

• Propulsive power
• Propulsive efficiency

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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

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Jets

• Afterburner addition to turbojet
• Ramjet use diffusers and nozzles
• Scramjet supersonic ramjet
• Rocket carries own oxidizer

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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

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Second Law Issues

• For exergy and exergy destruction for closed
  system
• For steady-flow system

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Second Law Issues

• For a cycle that starts and end at the same state

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