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PHYSICS 103: Lecture 21

PHYSICS 103: Lecture 21. Agenda for Today:. Thermodyamics and Car Engines. HEAT. L AWS OF T HERMODYNAMICS: Z EROTH L AW. Heat flows from hot objects to cold objects until they have the same temperature. d. F. L AWS OF T HERMODYNAMICS: F IRST L AW. Conservation of Energy:.

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PHYSICS 103: Lecture 21

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  1. PHYSICS 103: Lecture 21 Agenda for Today: • Thermodyamics and Car Engines

  2. HEAT LAWS OF THERMODYNAMICS: ZEROTH LAW Heat flows from hot objects to cold objects until they have the same temperature.

  3. d F LAWS OF THERMODYNAMICS: FIRST LAW Conservation of Energy: All energy entering a system must be conserved HEAT: Q I N Change in internal energy DU WORK: W = Fd OUT Q = DU + W

  4. Example: D Internal Energy & Work Performed Example: D Internal Energy & No Work Performed HEAT: Q HEAT: Q I N I N DU: KE   T  WORK: W OUT LAWS OF THERMODYNAMICS: FIRST LAW DU: KE   T  Q = DU + W

  5. REAL ENGINES Internal Combustion Engine: 4-stroke Otto cycle Step 1: Intake of gas-air mixture from the carburetor • Engine pulls piston out of cylinder • Low pressure in cylinder • Atmoshperic pressure pushes fuel air mixture into cylinder • Engine does work on gases during this stroke

  6. LAWS OF THERMODYNAMICS: REAL ENGINES Internal Combustion Engine: 4-stroke Otto cycle Step 2: Compression of the air-gas mixture • Engine pushes piston into cylinder • High pressure and temperature in cylinder • Engine does work on gases during this stroke

  7. LAWS OF THERMODYNAMICS: REAL ENGINES Internal Combustion Engine: 4-stroke Otto cycle Step 3: Ignition and Expansion of the air-gas mixture • Mixture burns to form hot gases • Gases push piston out of cylinder • As gas expands, pressure and temperature is lowered • Gas does work on engine during this stroke

  8. LAWS OF THERMODYNAMICS: REAL ENGINES Internal Combustion Engine: 4-stroke Otto cycle Step 4: Exhaust of waste gas • Engine pushes piston back into cylinder • Pressure inside pushes burnt gas out of cylinder • As gas expands, pressure and temperature is lowered • Engine does work on gas during this stroke

  9. LAWS OF THERMODYNAMICS: REAL ENGINES Internal Combustion Engine: 4-stroke Otto cycle

  10. LAWS OF THERMODYNAMICS: REAL ENGINES Inline-4 Flat-4 V-6 Overhead Cam-4

  11. ENGINE EFFICIENCY • Increase compression ratio => Get more work out of engine • Limit to compression ratio: knocking - spontaneous ignition of fuel/air mixture before the engine is ready to extract work • High compression ratio cars use premium gasoline. Higher octane ignites at higher temperatures • On most normal cars, premium gas is a waste of money

  12. LAWS OF THERMODYNAMICS: DieselENGINES • Uses compression to ignite fuel • - squeezes air to high pressure and temperature (compression ratio of ~ 20:1 compared with 8:1 - 12:1) • - injects fuel air in between compression and power strokes • - fuel ignites spontaneously • High compression allows for high efficiency

  13. Test Your Understanding • How does the burned gas do more work on the piston during the power stroke than the piston does on the unburned fuel-air mixture during the compression stroke? • Why is a car more likely to knock on a hot day than a cold day? • High compression ratio cars use premium gasoline. Higher octane ignites at higher temperatures • On most normal cars, premium gas is a waste of money

  14. Main Points from Today’s Lecture • Engines You should understand that some of the heat (that flows from hot objects to cold objects) can be used to do work. Any device that does this is called a heat engine. You should have a basic understanding of the strokes of a car engine.

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