PHYSICS 103: Lecture 21
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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

Agenda for Today:

  • Thermodyamics and Car Engines


HEAT

LAWS OF THERMODYNAMICS: ZEROTH LAW

Heat flows from hot objects to cold objects until they have the same temperature.


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


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


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


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


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


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


LAWS OF THERMODYNAMICS: REAL ENGINES

Internal Combustion Engine: 4-stroke Otto cycle


LAWS OF THERMODYNAMICS: REAL ENGINES

Inline-4

Flat-4

V-6

Overhead Cam-4


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


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


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


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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