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. I will go through these slides in two 90-minutes lectures. Zhigang Suo, Harvard University

How humans tell each other something? The thing itself Pictures Words Equations Language Books Movies The Internet 2

Thermodynamics = heat + motion Too many devices to classify neatly Fuel (input): biomass, fossil, solar thermal, geothermal, nuclear, electricity. Application (output): mobile power plant (transpiration in air, land, sea), stationary power plant (electricity generation), refrigerator, heat pump. Power cycle, refrigeration cycle. Working fluid: Gas cycle (air), vapor cycle (steam, phase change). Fluid-solid coupling: piston engine (reciprocating, crankshaft), turbine engine (jet, compressor). Site of burning: external combustion, internal combustion. 3

Plan Internal combustion engines Gas turbines Stirling and Ericsson engines Vapor power cycle Refrigeration cycle 4

5 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

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

7 US Navy Training Manual, Basic Machines 1 cycle 4 strokes 2 revolutions INTAKE STROKECOMPRESSION 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 Animated engines

8 Spark-ignition engine (gasoline engine, petrol engine, Otto engine)

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

10 Cold air-standard assumption Model air as an ideal gas of constant specific heat at room temperature (25°C). 2 independent variables to name all states of thermodynamic equilibrium 6 functions of state: PTvush 4 equations of state Gibbs equation

11 Thermal efficiency of Otto cycle Compression ratio: Conservation of energy: Isentropic processes: Thermal efficiency: w out w in

Otto cycle represented in planes of different variables 12 v s q in q out

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

14 Compression-ignition engine (Diesel engine) compression ratio: cut-off ratio: Conservation of energy: Isentropic processes Thermal efficiency:

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

16 Gas turbine (Brayton cycle) 4 steady-flow components: isobaric and isentropic s P q out q in

17 Thermal efficiency of Brayton cycle Definition of pressure ratio: Conservation of energy: Isentropic processes: Thermal efficiency:

18 Brayton cycle has large back work ratio w out w in

Intercooling, reheating, regeneration 19

Gas turbine for jet propulsion Thousands of years of history 20 Who invented this? Hero of Alexandria Frank Whittle (UK), Hans von Ohain (Germany) (first century AD) (during World War II)

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

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23 Air as an ideal gas of variable specific heat See section 7.9 for the use of this table

Plan Internal combustion engines Gas turbines Stirling and Ericsson engines Vapor power cycle Refrigeration cycle 24

Displacer-type Stirling engine 25

26 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 27 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 28 Specific work Specific gas constant GasFormulaR (kJ/kgK) Air SteamH2OH2O AmmoniaNH HydrogenH2H HeliumHe2.077

29 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 30

31 Coal power station coverts coal to electricity

Brayton Point Power Station Sommerset, Massachusetts 32 Mount Hope Bay

Nuclear power station converts uranium to electricity 33 Animation

Nine Mile Point Nuclear Power Plant, New York 34 Lake Ontario Cooling tower

35 Why water? Why steam? Water is cheap. Water flows! Water is a liquid at the temperature of heat sink (rivers, lakes,...). Vaporization changes specific volume greatly: a lot of work at relatively low pressure.

36 Rankine cycle 4 steady-flow components: isobaric and isentropic 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

37 Rankin cycle has small back work ratio

38 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

Cogeneration 39

Plan Internal combustion engines Gas turbines Stirling and Ericsson engines Vapor power cycle Refrigeration cycle 40

41 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. 42

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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). 45