ENGINES, REFRIGERATORS, AND HEAT PUMPS

1. 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

2. Thermodynamics =heat + motionToo 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).

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

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

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

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

7. Reciprocating engines of two types Spark-ignition engine (Otto, 1876) Compression-ignition engine (Diesel, 1892) https://ccrc.kaust.edu.sa/pages/HCCI.aspx

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

10. Cold air-standard assumption Model air as an ideal gas of constant specific heat at room temperature(25°C). 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 = 0.2870 kJ/kg K, cv = 0.718 kJ/kg K 4 equations of state

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

12. Cold air-standard Otto cycle s 3 4 qin qout 2 1 v Ideal gas of constant specific heat 2 independent variables to name all states of thermodynamic equilibrium 5 functions of state: PTvus 2 constants: R, cv 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:

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

16. Gas turbine (Brayton cycle) 4 steady-flow components Pressure ratio P qin 3 2 1 4 qout s

17. Gas turbine for jet propulsiontwo thousand plus years of history Who invented this? Hero of Alexandria Frank Whittle (UK), Hans von Ohain (Germany) (first century AD) (during World War II) http://www.techknow.org.uk/wiki/index.php?title=File:Hero_4.jpg

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

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

20. Displacer-type Stirling engine https://www.stirlingengine.com/faq/

21. Stirling engine and regenerator (1816) reversible cycle between two fixed temperatures, having the Carnot efficiency https://people.ok.ubc.ca/jbobowsk/Stirling/how.html

22. Stirling vs. Carnotfor given limits of volume, pressure, and temperature • 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.

23. Work out by Stirling cycle Specific work Specific gas constant

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

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

26. Carnot cycle is unsuitable as vapor power cycle 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.

27. Rankine cycle 4 steady-flow components wpump,in = h2 - h1 qboiler,in = h3 - h2 wturbine,out = h3 – h4 qcondenser,out = h4 – h1 P qboiler,in 2 3 wturbine,out wpumo,in 1 4 qcondenser, out s

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

29. Refrigerator and heat pump 4 steady-flow components

30. Selecting Refrigerant • Large enthalpy of vaporization • Sufficiently low freezing temperature • Sufficiently high critical temperature • Low condensing pressure • Do no harm: non-toxic, non-corrosive, non-flammable, environmentally-friendly • Low cost • R-717 (Ammonia, NH3) used in industrial and heavy-commercial sectors. Toxic. • R-12 (Freon 12, CCl2F2). Damage ozone layer. Banned. • R-134a (HFC 134a, CH2FCF3) used in domestic refrigerators, as well as automotive air conditioners.

33. 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).