ME 152 1

Gas Power Cycles

Cengel & Boles, Chapter 8

ME 152 2

Analysis of Power Cycles - Basics

• Power cycle = Heat engine• Recall thermal efficiency:

• Carnot heat engine:

• The Carnot cycle has the maximum possible efficiency, but is not a realistic model for a power cycle since it is so impractical

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ME 152 3

Analysis of Power Cycles - Basics, cont.• More practical models are called

ideal cycles - they are internally reversible but typically have external irreversibilities

• Ideal cycle assumptions include:– absence of friction

– quasi-equilibrium processes

– pipes and connections between various components are well-insulated, i.e., heat transfer is negligible

– negligible KE and PE effects (except in diffusers and nozzles)

– negligible pressure drop in HXers

ME 152 4

Gas Power Cycles

• Working fluid remains in gaseous phase throughout cycle

• Common gas cycles– Otto*: spark-ignition ICE engine, closed

system– Diesel*: compression-ignition ICE

engine, closed system– Dual: Otto/Diesel combo, closed system– Stirling: ext. combustion, closed system– Ericsson: ext. combustion, control

volume– Brayton*: gas turbine engine or power

plant, control volume

* covered in this course

ME 152 5

Internal Combustion Engine (ICE) terms• Bottom-dead center (BDC) – piston

position where volume is maximum

• Top-dead center (TDC) – piston position where volume is minimum

• Clearance volume – minimum cylinder volume (VTDC = V2)

• Compression ratio (r)

• Displacement volume

• Mean Effective Pressure (MEP)

•

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ME 152 6

ICE terms, cont.

• Spark-ignition (SI) engine - reciprocating engine where air-fuel combustion is initiated by a spark plug

• Compression-ignition (CI) engine - reciprocating engine where air-fuel combustion is initiated by compression

• Four-stroke engine - piston executes intake, compression, expansion, and exhaust in four strokes while crankshaft completes two revolutions

• Two-stroke engine - piston executes intake, compression, expansion, and exhaust in two strokes while crankshaft completes one revolution

ME 152 7

Analysis of Gas Power Cycles

• Air-standard assumptions:– working fluid is a fixed mass of air

which is modeled as a closed system and behaves as an ideal gas

– all processes are internally reversible unless stated otherwise

– combustion process is replaced by a heat addition process from an external source

– exhaust process is replaced by a heat rejection process that restores air to its initial state

ME 152 8

Analysis of Gas Power Cycles, cont.• Constant specific heat approach (aka

cold-air standard) - for approximate analysis only

where cv , cp are evaluated at 25°C, 1 atm

• Variable specific heat approach - for more accurate analysis

where u and h obtained from Table A-17

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ME 152 9

Analysis of Gas Power Cycles, cont.• Isentropic compression/expansion

– if compression ratio (v1/v2) is known, e.g., in Otto or Diesel cycles, use

(find u2 or h2 from vr2 in Table A-17)

– if pressure ratio (P2/P1) is known, e.g., in a Brayton cycle, use

(find u2 or h2 from Pr2 in Table A-17)

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ME 152 10

Otto Cycle Analysis

• Thermal efficiency

• Heat addition (process 2-3, v = const)

• Heat rejection (process 4-1, v = const)

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ME 152 11

Diesel Cycle Analysis

• Thermal efficiency

• Heat addition (process 2-3, P = const)

• Heat rejection (process 4-1, v = const)

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ME 152 12

Cold-Air Standard Thermal Efficiency

• Otto Cycle

• Diesel Cycle

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ME 152 13

The Brayton Cycle

• Ideal cycle for gas turbine engines and power plants

• The air-standard Brayton cycle has a closed-loop configuration, even though most applications are open-loop

• Basic components:– Compressor (increases pressure of gas)

– Heat exchanger or combustor (const P heat addition)

– Turbine (produces power)

– Heat exchanger (const P heat rejection)

ME 152 14

Air-Standard Brayton Cycle Analysis

• Compressor

• Combustor (heat addition)

• Turbine

• Heat Exchanger (heat rejection)

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ME 152 15

Air-Standard Brayton Cycle Analysis, cont.• Thermal Efficiency

• Back Work Ratio

– as discussed in Ch. 6, a gas compressor requires much greater work input per unit mass than a pump for a given pressure rise; thus the rbw for a gas power cycle (40-60%) is much greater than that for a vapor power cycle (1-2%)

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ME 152 16

Air-Standard Brayton Cycle Analysis, cont.

• Cold-air standard thermal efficiency

• High pressure ratios (rp =P2/P1) yield the highest thermal efficiency, however, moderate pressure ratios often yield a higher power-to-weight ratio

• Maximum turbine inlet temperature is around 1700 K, imposed by metallurgical properties

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ME 152 17

Improving Gas Turbine Cycle Performance

• Regeneration - utilizes turbine exhaust gas to preheat air entering the combustor; this reduces heat addition requirement and increases thermal efficiency

• Multistage turbine with reheat - similar to vapor power cycles; increases thermal efficiency

• Compressor intercooling - gas is cooled between compressor stages; decreases compressor work and bwr, increases thermal efficiency

ME 152 18

Gas Turbine Aircraft Propulsion• Gas turbines are ideal for aircraft propulsion

due to high power-to-weight ratio

• Basic turbojet engine - inlet diffuser, compressor, combustor, turbine, exit nozzle

• Turbofan engine - inlet fan brings in additional air which bypasses engine core and increases thrust from nozzle

• Turboprop engine - turbine powers a propeller, which provides primary thrust

• Ramjet - high-speed air is compressed by ram effect and then heated by combustor; thrust is developed by nozzle w/o need for compressor or turbine