1. Gas Cycles Carnot Cycle Heat Q 2 3 T2 Work W 1 T1 4 s1 s2
ADIABATIC COMPRESSION
(ISENTROPIC)
HEAT ADDITION
(ISOTHERMAL)
ADIABATIC EXPANSION
WORK
Heat Q 2 3 T2
Work W 1 T1 4 s1 s2

3. Carnot Cycle
Carnot cycle is the most efficient cycle that can be executed between a heat source and a heat sink.
However, isothermal heat transfer is difficult to obtain in reality--requires large heat exchangers and a lot of time.

4. Carnot Cycle
Therefore, the very important (reversible) Carnot cycle, composed of two reversible isothermal processes and two reversible adiabatic processes, is never realized as a practical matter.
Its real value is as a standard of comparison for all other cycles.

5. Gas cycles have many engineering applications
Internal combustion engine
Otto cycle
Diesel cycle
Gas turbines
Brayton cycle
Refrigeration
Reversed Brayton cycle

6. Some nomenclature before starting internal combustion engine cycles

7. More terminology

8. Terminology Bore = d Stroke = s Displacement volume =DV =
Clearance volume = CV
Compression ratio = r

9. Mean Effective Pressure
Mean Effective Pressure (MEP) is a fictitious pressure, such that if it acted on the piston during the entire power stroke, it would produce the same amount of net work.

10. The net work output of a cycle is equivalent to the product of the mean effect pressure and the displacement volume

11. Real Otto cycle

12. Real and Idealized Cycle

13. Otto Cycle P-V & T-s Diagrams
Pressure-Volume
Temperature-Entropy

14. Otto Cycle Derivation Thermal Efficiency:
For a constant volume heat addition (and rejection) process;
Assuming constant specific heat:

15. Otto Cycle Derivation
For an isentropic compression (and expansion) process:
where: γ = Cp/Cv
Then, by transposing,
Leading to

16. Differences between Otto and Carnot cycles

17. Otto Cycle Derivation
The compression ratio (rv) is a volume ratio and is equal to the expansion ratio in an otto cycle engine.
Compression Ratio
where Compression ratio is defined as

18. Otto Cycle Derivation Then by substitution,
The air standard thermal efficiency of the Otto cycle then becomes:

19. Otto Cycle Derivation
Summarizing
where
and
then
Isentropic behavior

20. Otto Cycle Derivation
Heat addition (Q) is accomplished through fuel combustion
Q = Lower Heat Value (LHV) BTU/lb, kJ/kg
also

21. Effect of compression ratio on Otto cycle efficiency

22. Sample Problem – 1
The air at the beginning of the compression stroke of an air-standard Otto cycle is at 95 kPa and 22C and the cylinder volume is 5600 cm3. The compression ratio is 9 and 8.6 kJ are added during the heat addition process. Calculate:
(a) the temperature and pressure after the compression and heat addition process
(b) the thermal efficiency of the cycle
Use cold air cycle assumptions.

23. Draw cycle and label points
r = V1 /V2 = V4 /V3 = 9
Q23 = 8.6 kJ
T1 = 295 K
P1 = 95 kPa

24. Carry through with solution
Calculate mass of air:
Compression occurs from 1 to 2:
But we need T3!

25. Get T3 with first law:
Solve for T3:

26. Thermal Efficiency

27. Sample Problem – 2

28. Solution

31. Diesel Cycle P-V & T-s Diagrams

34. Sample Problem – 3

37. Gasoline vs. Diesel Engine