Gas Power CyclesGas Power Cycles
Power CyclesPower Cycles
Ideal Cycles, Internal CombustionIdeal Cycles, Internal Combustion Otto cycle, spark ignitionOtto cycle, spark ignition Diesel cycle, compression ignitionDiesel cycle, compression ignition Sterling & Ericsson cyclesSterling & Ericsson cycles Brayton cyclesBrayton cycles Jet-propulsion cycleJet-propulsion cycle
Ideal Cycles, External CombustionIdeal Cycles, External Combustion Rankine cycleRankine cycle
ModelingModeling
Ideal CyclesIdeal Cycles Idealizations & SimplificationsIdealizations & Simplifications
Cycle does not involve any frictionCycle does not involve any friction All expansion and compression All expansion and compression
processes are quasi-equilibrium processes are quasi-equilibrium processesprocesses
Pipes connecting components have no Pipes connecting components have no heat lossheat loss
Neglecting changes in kinetic and Neglecting changes in kinetic and potential energy (except in nozzles & potential energy (except in nozzles & diffusers)diffusers)
Carnot CycleCarnot Cycle
Carnot CycleCarnot Cycle
Gas Power CyclesGas Power Cycles
Working fluid remains a gas for the Working fluid remains a gas for the entire cycleentire cycle
Examples:Examples: Spark-ignition enginesSpark-ignition engines Diesel enginesDiesel engines Gas turbinesGas turbines
Air-Standard Assumptions Air-Standard Assumptions
Air is the working fluid, circulated in a Air is the working fluid, circulated in a closed loop, is an ideal gasclosed loop, is an ideal gas
All cycles, processes are internally All cycles, processes are internally reversiblereversible
Combustion process replaced by heat-Combustion process replaced by heat-addition from external sourceaddition from external source
Exhaust is replaced by heat rejection Exhaust is replaced by heat rejection process which restores working fluid to process which restores working fluid to initial stateinitial state
Cold-Air-Standard AssumptionCold-Air-Standard Assumption
Air has constant specific heats, Air has constant specific heats, values are for room temperature values are for room temperature (25°C or 77(25°C or 77°F)°F)
Engine TermsEngine Terms
Top dead centerTop dead center Bottom dead Bottom dead
centercenter BoreBore StrokeStroke
Engine TermsEngine Terms
Clearance volumeClearance volume Displacement Displacement
volumevolume Compression ratioCompression ratio
Engine TermsEngine Terms
Mean effective Mean effective pressure (MEP)pressure (MEP)
Otto CycleOtto Cycle
Processes of Otto Cycle:Processes of Otto Cycle: Isentropic compressionIsentropic compression Constant-Constant-volumevolume heat addition heat addition Isentropic expansionIsentropic expansion Constant-Constant-volumevolume heat rejection heat rejection
Otto CycleOtto Cycle
Otto CycleOtto Cycle
IdealIdeal Otto Cycle Otto Cycle Four internally Four internally
reversible processesreversible processes 1-2 Isentropic 1-2 Isentropic
compressioncompression 2-3 Constant-volume 2-3 Constant-volume
heat additionheat addition 3-4 Isentropic 3-4 Isentropic
expansionexpansion 4-1 Constant-volume 4-1 Constant-volume
heat rejectionheat rejection
Otto CycleOtto Cycle
Closed system, pe, ke ≈ 0 Closed system, pe, ke ≈ 0 Energy balance (cold air std)Energy balance (cold air std)
Otto CycleOtto Cycle
Thermal efficiency of ideal Otto cycle:Thermal efficiency of ideal Otto cycle:
Since Since VV22= V= V33 and and VV44 = V = V11
Where r is compression ratioWhere r is compression ratio
k is ratio of specific heatsk is ratio of specific heats
Otto CycleOtto Cycle
Spark or Compression IgnitionSpark or Compression Ignition
Spark (Otto), air-fuel Spark (Otto), air-fuel mixture compressed mixture compressed (constant-volume heat (constant-volume heat addition)addition)
Compression (Diesel), Compression (Diesel), air compressed, then air compressed, then fuel added (constant-fuel added (constant-pressure heat pressure heat addition)addition)
Diesel CycleDiesel Cycle
Diesel CycleDiesel Cycle
Processes of Diesel cycle:Processes of Diesel cycle: Isentropic compressionIsentropic compression Constant-Constant-pressurepressure heat addition heat addition Isentropic expansionIsentropic expansion Constant-Constant-volumevolume heat rejection heat rejection
Diesel CycleDiesel Cycle
For ideal diesel cycleFor ideal diesel cycle
With cold air assumptionsWith cold air assumptions
Diesel CycleDiesel Cycle
Cut off ratio rCut off ratio rcc
Efficiency becomesEfficiency becomes
Brayton CycleBrayton Cycle
Gas turbine cycleGas turbine cycle Open vs closed Open vs closed
system modelsystem model
Brayton CycleBrayton Cycle
Four internally Four internally reversible processesreversible processes 1-2 Isentropic 1-2 Isentropic
Compression Compression (compressor)(compressor)
2-3 Constant-pressure 2-3 Constant-pressure heat additionheat addition
3-4 Isentropic 3-4 Isentropic expansion (turbine)expansion (turbine)
4-1 Constant-pressure 4-1 Constant-pressure heat rejectionheat rejection
Brayton CycleBrayton Cycle
Analyze as steady-flow processAnalyze as steady-flow process
SoSo
With cold-air-standard assumptionsWith cold-air-standard assumptions
Brayton CycleBrayton Cycle
Since processes 1-2 and 3-4 are Since processes 1-2 and 3-4 are isentropic, Pisentropic, P22 = P = P33 and P and P44 = P = P11
wherewhere
Brayton CycleBrayton Cycle
Brayton CycleBrayton Cycle
Back work ratioBack work ratio Improvements in Improvements in gas turbinesgas turbines Combustion tempCombustion temp Machinery Machinery
component component efficienciesefficiencies
Adding Adding modifications to modifications to basic cyclebasic cycle
Actual Gas-Turbine CyclesActual Gas-Turbine Cycles
For actual gas For actual gas turbines, turbines, compressor and compressor and turbine are not turbine are not isentropic isentropic
RegenerationRegeneration
RegenerationRegeneration
Use heat Use heat exchanger called exchanger called recuperator or recuperator or regeneratorregenerator
Counter flowCounter flow
RegenerationRegeneration
EffectivenessEffectiveness
For cold-air For cold-air assumptionsassumptions
Brayton with Intercooling, Brayton with Intercooling, Reheat, & RegenerationReheat, & Regeneration
Brayton with Intercooling, Brayton with Intercooling, Reheat, & RegenerationReheat, & Regeneration
For max For max performance performance
Ideal Jet-Propulsion CyclesIdeal Jet-Propulsion Cycles
Ideal Jet-Propulsion CyclesIdeal Jet-Propulsion Cycles
Propulsive powerPropulsive power
Propulsive efficiencyPropulsive efficiency
Turbojet EnginesTurbojet Engines
Turbofan: for same power, large Turbofan: for same power, large volume of slower-moving air volume of slower-moving air produces more thrust than a small produces more thrust than a small volume of fast-moving air (bypass volume of fast-moving air (bypass ratio 5-6)ratio 5-6)
Turboprop: by pass ratio of 100Turboprop: by pass ratio of 100
JetsJets
Afterburner: addition to turbojetAfterburner: addition to turbojet Ramjet: use diffusers and nozzlesRamjet: use diffusers and nozzles Scramjet: supersonic ramjet Scramjet: supersonic ramjet Rocket: carries own oxidizerRocket: carries own oxidizer
Second Law IssuesSecond Law Issues
Ideal Otto, Diesel, and Brayton cycles Ideal Otto, Diesel, and Brayton cycles are internally reversibleare internally reversible
22ndnd Law analysis identifies where Law analysis identifies where losses are so improvements can be losses are so improvements can be mademade
Look at closed, steady-flow systemsLook at closed, steady-flow systems
Second Law IssuesSecond Law Issues
For exergy and exergy destruction For exergy and exergy destruction for closed for closed systemsystem::
For steady-flow For steady-flow systemsystem::
Second Law IssuesSecond Law Issues
For a cycle that starts and end at the For a cycle that starts and end at the same state:same state:
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