1. 8.2 Automobiles

2. What is a car engine? Physicists call it a “heat engine” The specific design is an “internal combustion engine”

3. Today: Heat engines Car engines Knocking NOS

4. The Heat Engine cylinder Air outside the tail pipe Internal combustion engine

5. Heat Engines As heat flows naturally from hot to cold, a heat engine diverts some heat and converts it into useful work. Natural heat flow increases entropy Converting heat to work decreases entropy If more entropy is created than destroyed, the overall entropy doesn’t decrease and some heat can become work!

6. Heat Pump Heat Pump (last lecture) Heat Engine Heat Engine (this lecture)

7. Efficiency As the temperature difference between hot and cold increases: –A heat pump becomes less efficient –A heat engine becomes more efficient Heat engine Cold region Hot region work Heat engine Cold region Even hotter region work

8. It’s all about entropy “You down with entropy? Yeah, you know me!…” More entropy is created (by heat moving) when the temperature difference is increased That means more heat can be diverted into work without violating the 2 nd law!

9. Stirling Engines ’79 AMC spirit Hard to start quickly, poor acceleration response

10. Internal Combustion Engine Burns fuel and air in enclosed space Produces hot burned gases Allows heat to flow from hot engine to cold outside air Converts some of this heat into useful work Nikolaus Otto patented the 4- stroke engine when he was only 34!

11. The Four Stroke Engine

12. 1.Induction Stroke Engine pulls piston out of cylinder Low pressure inside cylinder Atmospheric pressure pushes fuel and air mixture into cylinder Engine does work on the gases during this stroke

13. Engine – Step 1 Fuel and air mixture after induction stroke: Pressure = Atmospheric Temperature = Ambient

14. 2.Compression Stroke Engine pushes piston into cylinder Mixture is compressed to high pressure and temperature Engine does work on the gases during this stroke

15. Engine – Step 2 Fuel and air mixture after compression stroke: Pressure = High Temperature = Hot

16. 3.Power Stroke Mixture burns to form hot gases Gases push piston out of cylinder Gases expand to lower pressure and temperature Gases do work on engine during this stroke

17. Engine – beginning of Step 3 Burned gases after ignition: Pressure = Very high Temperature = Very hot

18. Engine – end of Step 3 Burned gases after power stroke: Pressure = Moderate Temperature = High

19. 4.Exhaust Stroke Engine pushes piston into cylinder High pressure inside cylinder Pressure pushes burned gases out of cylinder Engine does work on the gases during this stroke

21. Clicker Question: What directly effects the efficiency of a car engine? A) Octane of the gas B) Weight of the car C) The temperature of the gas when it burns D) The number of cylinders

22. Efficiency is all about compression ratio Most cars: 8:1 – 12:1 High-performance cars: 15:1 Diesel: 20:1

23. Efficiency Limits An ideal engine is never perfect (never has 100% efficiency) –Not all the thermal energy can become work –Some heat must be ejected into the atmosphere However, ideal efficiency improves as –the burned gases become hotter –the outside air becomes colder Real engines never reach ideal efficiency

24. Knocking The 4-stroke cycle is a myth… -4° TDC +30 ° TDC

25. Engine is inefficient Pressure wave forms – knocks! Use higher octane fuel! Octane Ignition temperature 87 90 93 750 °C 800 °C 850 °C

26. Multiple cylinders At least one cylinder is always executing power stroke

27. Where does the spark come from? More later when we talk about magnetism!

28. Tricked-out cars

32. + Higher density of oxygen Can burn more fuel (more power) Joseph Priestly