Thermodynamic Cycles

1. Thermodynamic Cycles Chapter 9

2. For the rest of the semester.. • Look at different cycles that approximate real processes • You can categorize these processes several different ways • Power Cycles vs Refrigeration • Gas vs Vapor • Closed vs open • Internal Combustion vs External Combustion

3. Power Cycles • Subject of Chapters 9 and 10 • Otto Cycle • Spark Ignition • Diesel Cycle • Brayton Cycle • Gas Turbine • Rankine Cycle • Vapor

4. Refrigeration Cycles • Subject of Chapter 11

5. Brief Introduction • We will only be skimming these three chapters • Mechanical Engineers will start here when they take Thermo II • The content is interesting enough that every engineer should be exposed to it

6. Real vs. Ideal

7. Thermal Efficiency In chapter 9 and 10 all the cycles we’ll study model heat engines. They convert heat to work, so the efficiency is:

8. Ideal Cycles • We’ll be using ideal cycles to analyze real systems, so lets start with the only ideal cycle we’ve studied thus far Carnot Cycle

9. 1 2 Q Q Q=T dS 4 3 Ts diagram 1-2 Isothermal Heat Addition 2-3 Isentropic Expansion 3-4 Isothermal Heat Rejection 4-1 Isentropic Compression S T

10. Old McDonald Old McDonald had a farm iaiao

11. 1 Q Q 2 W=P dV 4 3 Pv Diagram 1-2 Isothermal Heat Addition 2-3 Isentropic Expansion 3-4 Isothermal Heat Rejection 4-1 Isentropic Compression v P

12. And… • Since it is a cycle • Q-W=0 • Q=W • In addition, we know that the efficiency for a Carnot Cycle is:

13. Q W Wnet = Qnet

14. The Carnot Cycle is not a good model for most things • For example • Internal combustion engine • Gas turbine • We need to develop a new model, that is still ideal

15. Air-Standard Assumptions • Air continuously circulates in a closed loop and behaves as an ideal gas • All the processes are internally reversible • Combustion is replaced by a heat-addition process from the outside • Heat rejection replaces the exhaust process

16. Cold Air Standard Assumptions • Also assume a constant value for Cp and Cv, evaluated at room temperature

17. Terminology for Reciprocating Devices

19. Compression Ratio

20. Mean Effective Pressure

21. Nikolaus Otto • A German engineer who worked with Etienne Lenoir to develop the first internal combustion engine in 1876 http://www.spartacus.schoolnet.co.uk/SCotto.htm

22. v v

23. v v v 1-2 Isentropic Compression 2-3 Constant Volume Heat Addition 3-4 Isentropic Expansion 4-1 Constant Volume Heat Rejection

24. Thermal Efficiency of the Otto Cycle

25. Apply First Law Closed System to Process 2-3, V = Constant

26. Apply First Law Closed System to Process 4-1, V = Constant

28. Recall processes 1-2 and 3-4 are isentropic, so But.. V3 = V2 and V4 = V1

29. 1

30. Otto Cycle Efficiency Is this the same as the Carnot efficiency? NO!!

31. Efficiency of the Otto Cycle vs Carnot Cycle • There are only two temperatures in the Carnot cycle • Heat is added at TH • Heat is rejected at TL • There are four temperatures in the Otto cycle!! • Heat is added over a range of temperatures • Heat is rejected over a range of temperatures

32. Comparison of the Otto and Carnot Cycle n=1-T1/T2 n=1-TL /TH Carnot Cycle Otto Cycle

33. One more simplication… Since process 1-2 is isentropic

34. Typical Compression Ratios for Gasoline Engines Increasing Compression RatioIncreases the Efficiency

35. Why not use higher compression Ratios? • Premature Ignition • Causes “Knock” • Reduces the Efficiency • Hard on the Engine

36. Next time • We’ll talk about the Diesel Cycle