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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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Agenda for Today:

• Thermodyamics and Car Engines

LAWS OF THERMODYNAMICS: ZEROTH LAW

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

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

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

• 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

• 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.