Gas Power Cycles

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 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) 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 air which circulates in a closed cycle 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 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 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) ME 152

10. Otto Cycle Analysis • Thermal efficiency • Heat addition (process 2-3, v = const) • Heat rejection (process 4-1, v = const) ME 152

11. Diesel Cycle Analysis • Thermal efficiency • Heat addition (process 2-3, P = const) • Heat rejection (process 4-1, v = const) ME 152

12. Cold-Air Standard Thermal Efficiency • Otto Cycle • Diesel Cycle 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) 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%) 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 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 ME 152