ENGINES, REFRIGERATORS, AND HEAT PUMPS This lecture highlights aspects in Chapters 9,10,11 of Cengel and Boles. Every thermodynamic device has moving parts. To understand these movements, it is important that you watch some videos on the Internet. Zhigang Suo, Harvard University

Thermodynamics = heat + motion Too many devices to classify neatly Application: mobile power plant (transpiration in air, land, sea), stationary power plant (electricity generation), refrigerator, heat pump. Fuel. biomass, fossil, solar thermal, geothermal, nuclear, electricity. Site of burning: external combustion, internal combustion. Working fluid: gas (air), vapor (steam). Fluid-solid coupling: piston (reciprocating, crankshaft), turbine (jet, compressor). 2

Plan Internal combustion engines Gas turbines Stirling and Ericsson engines Vapor power cycle Refrigeration cycle 3

4 External combustion engine Internal combustion engine (ICE) Fayette Internal Combustion Engiine I US Navy Training Manual, Basic Machines Combustion engine burns to move Otto (gasoline) engine Diesel engine Gas turbine Jet propulsion Steam engine Stirling engine Ericsson engine PISTON COMBUSTION CHAMBER WATER STEAM BOILER

5 US Navy Training Manual, Basic Machines Reciprocating engine also known as piston engine, converts linear motion to rotation PISTON CONNECTING ROD CRANKSHAFT CYLINDER

6 US Navy Training Manual, Basic Machines 1 cycle 4 strokes 2 revolutions INTAKE STROKECOPRESSION STROKE POWER STROKEEXHAUST STROKE fuel-air mixture entering cylinder exhaust valve closed piston moving down cam lobe lifting valve tappet intake valve open valve tappet lifting valve Fuel discharging from nozzle air enteringfuel-air mixture being compressed both valves closed piston moving up spark igniting mixture both valves closed exhaust valve open intake valve closed piston moving up piston moving down valve tappet lifting valve cam lobe lifting valve tappet

7 Spark-ignition engine (Otto, 1876) Compression-ignition engine (Diesel, 1892) Reciprocating engines of two types

Ideal cycle for analysis 8 No friction No pressure drop when unintended No heat transfer when unintended Internally reversible. Quasi-equilibrium cycle. Externally irreversible. Heat transfer between the engine and surroundings of finite difference in temperature.

9 1.Model the engine as a closed system, and the working fluid as air (an ideal gas). 2.The cycle is internally reversible. 3.Model combustion by adding heat from an external source 4.Model exhaust by rejecting heat to an external sink Air-standard assumptions Quick review: air as an ideal gas of variable specific heat See section 7.9 for the use of this table

10 Model air as an ideal gas of constant specific heat at room temperature (25°C). Cold air-standard assumption Quick review: Ideal gas of constant specific heat 2 independent variables to name all states of thermodynamic equilibrium 6 functions of state: PTvush 2 constants: R = kJ/kg K, c v = kJ/kg K 4 equations of state

11 Spark-ignition engine (gasoline engine, Otto engine)

Cold air-standard Otto cycle 12 v s q in q out Ideal gas of constant specific heat 2 independent variables to name all states of thermodynamic equilibrium 5 functions of state: PTvus 2 constants: R, c v 3 equations of state

13 Thermal efficiency of Otto cycle Definition of compression ratio: Conservation of energy: Isentropic processes: Definition of thermal efficiency: Algebra:

14 Compression-ignition engine (Diesel engine) compression ratio: cut-off ratio:

Plan Internal combustion engines Gas turbines Stirling and Ericsson engines Vapor power cycle Refrigeration 15

16 Pressure ratio Gas turbine (Brayton cycle) 4 steady-flow components s P q out q in

Gas turbine for jet propulsion two thousand plus years of history 17 Who invented this? Hero of Alexandria Frank Whittle (UK), Hans von Ohain (Germany) (first century AD) (during World War II)

18 Propulsive force: Propulsive power: Propulsive efficiency: Gas turbine for jet propulsion 6 steady-flow components

Plan Internal combustion engines Gas turbines Stirling and Ericsson engines Vapor power cycle Refrigeration cycle 19

Displacer-type Stirling engine 20

21 Stirling engine and regenerator (1816) reversible cycle between two fixed temperatures, having the Carnot efficiency

Stirling vs. Carnot for given limits of volume, pressure, and temperature 22 On PV plane, the black area represents the Carnot cycle, and shaded areas represent addition work done by the Stirling cycle. On TS plane, the black area represents the Carnot cycle, and the shaded areas represent additional heat taken in by the Stirling cycle. The Stirling cycle and the Carnot cycle have the same thermal efficiency. The Stirling cycle take in more heat and give more work than the Carnot cycle. Walker, Stirling Engine, 1980.

Work out by Stirling cycle 23 Specific work Specific gas constant GasFormulaR (kJ/kgK) Air SteamH2OH2O AmmoniaNH HydrogenH2H HeliumHe2.077

24 Ericsson engine with regenerator (1853) reversible cycle between two fixed temperatures, having the Carnot efficiency

Plan Internal combustion engines Gas turbines Stirling and Ericsson engines Vapor power cycle Refrigeration cycle 25

26 Issues with the in-dome Carnot cycle Process 1-2 limits the maximum temperature below the critical point (374°C for water) Process 2-3. The turbine cannot handle steam with a high moisture content because of the impingement of liquid droplets on the turbine blades causing erosion and wear. Process 4-1. It is not practical to design a compressor that handles two phases. Issues with supercritical Carnot cycle Process 1-2 requires isothermal heat transfer at variable pressures. Process 4-1 requires isentropic compression to extremely high pressures. Carnot cycle is unsuitable as vapor power cycle

27 Rankine cycle 4 steady-flow components s P q boiler,in q condenser, out w pump,in = h 2 - h 1 q boiler,in = h 3 - h 2 w turbine,out = h 3 – h 4 q condenser,out = h 4 – h 1 w turbine,out w pumo,in

Plan Internal combustion engines Gas turbines Stirling and Ericsson engines Vapor power cycle Refrigeration cycle 28

29 Refrigerator and heat pump 4 steady-flow components

Selecting Refrigerant 1.Large enthalpy of vaporization 2.Sufficiently low freezing temperature 3.Sufficiently high critical temperature 4.Low condensing pressure 5.Do no harm: non-toxic, non-corrosive, non-flammable, environmentally- friendly 6.Low cost R-717 (Ammonia, NH 3 ) used in industrial and heavy-commercial sectors. Toxic. R-12 (Freon 12, CCl 2 F 2 ). Damage ozone layer. Banned. R-134a (HFC 134a, CH 2 FCF 3 ) used in domestic refrigerators, as well as automotive air conditioners. 30

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Summary Engine converts fuel to motion. Refrigerator and heat pump use work to pump heat from a place of low temperature to a place of high temperature. Many ideal cycles are internally reversible, but externally irreversible. Stirling and Ericsson cycles are internally and externally reversible, so they have the same thermal efficiency as the Carnot cycle. Use ideal-gas model to analyze gas as working fluid. Use property table to analyze vapor as working fluid. Model piston engine as a closed system (Otto, Diesel, Stirling, Ericsson). Model turbine (or compressor) device as steady-flow components in series (Brayton cycle, Rankine cycle, refrigeration cycle). 33