Gas Power Cycles Cengel & Boles, Chapter 8 ME 152

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

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

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

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

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

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

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

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

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

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

Cold-Air Standard Thermal Efficiency Otto Cycle Diesel Cycle ME 152

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

Air-Standard Brayton Cycle Analysis Compressor Combustor (heat addition) Turbine Heat Exchanger (heat rejection) ME 152

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

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

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

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