1. Announcements 1/28/11 Prayer HW 8: one with invalid CID Lab 2 due tomorrow Exam 1 starts in a week and a half

2. Special Processes: Review Constant volume (isovolumetric) Constant pressure (isobaric) Constant temperature (isothermal) No heat added (adiabatic) General process? a. a.  E int = ? b. b.W on gas = ? c. c.Q added = ? A B P V

3. Last Special Case: Cyclical Processes  E int = ? net W by gas or W on gas ? |W| = ? Q added = ? P V State 1

4. Engines: Energy Transformation Q in = work by gas + Q out Notation: Q h, Q c, T h, T c, |W| Q h = |W| + Q c engine heat in (higher T) work exhaust out (lower T)

5. (a) - piston at room temp and atmospheric pressure P V a Heat Engine Example

6. (a-b) - gas heated to keep volume constant as cart rolls onto piston P V a b Heat Engine Example

7. (b-c) - gas heated to increase volume and lift the cart P V a b c Heat Engine Example

8. (c-d) - gas cooled to keep volume constant at cart rolls off piston P V a b c d Heat Engine Example

9. (d-a) - gas cooled to reduce volume back to initial level P V a b c d Heat Engine Example

10. Efficiency How good is your engine? Definition

11. Worked Problem A car engine produces 5000 W of power, at 20 cycles/second. Its efficiency is 20%. What are |W|, Q h, and Q c per cycle? What do those quantities represent?

12. Reading Quiz (graded) What is the name we use for the cycle which models how a gas engine works? a. a.Carnot cycle b. b.Brayton cycle c. c.Diesel cycle d. d.Otto cycle e. e.Tri cycle

13. Otto Cycle: Gas Engines (Starting at 2) Piston is compressed quickly Heat is then added quickly by igniting fuel (const volume) Piston then expands quickly Heat is then expelled quickly (by getting rid of old air) → In our approxi- mation we pretend like same air is reused Image credit: http://www.grc.nasa.gov/WWW/K-12/airplane/otto.htmlhttp://www.grc.nasa.gov/WWW/K-12/airplane/otto.html “Compression ratio” r = V max /V min Derived in book:

14. Thought Question If I replaced all of the nitrogen (N 2 ) in the air with carbon dioxide (CO 2 ), what do you think would happen to the efficiency of car engines? a. a.They would get more efficient b. b.They would get less efficient c. c.The efficiency would not change

15. Diesel Cycle: Diesel Engines What’s the main difference between gas and diesel engines? Change to our PV-diagram model Diesel cycle details… done in HW problem

16. Worked problem: Class designed Make up a “three-legged cycle”. What is the efficiency of this cycle? Game plan: a. a.Calculate Q for each leg b. b.Calculate Q in, Q out, |W| c. c.e = |W|/Q in d. d.[Next class period: Test to make sure e < e max ]