Ideal and otto_cycle

1. Ideal Cycles,Ideal Cycles,
Air-Standard Assumptions,Air-Standard Assumptions,
andand
The Otto CycleThe Otto Cycle

2. Thermodynamic CyclesThermodynamic Cycles
• Power Cycles vs Refrigeration CyclesPower Cycles vs Refrigeration Cycles
• Power CyclesPower Cycles
– Gas vs vaporGas vs vapor
– Closed vs OpenClosed vs Open
– Internal Combustion vs ExternalInternal Combustion vs External
CombustionCombustion

4. Ideal cycles are simplified

5. Be careful how you interpret results from ideal cycles

6. Ideal CyclesIdeal Cycles
• More realistic than Carnot cycle.More realistic than Carnot cycle.
• Internally reversible but not totally.Internally reversible but not totally.
• Idealizations:Idealizations:
– No frictionNo friction
– Expansions and compressions – quasi-Expansions and compressions – quasi-
equilibriumequilibrium
– Heat transfer is negligibleHeat transfer is negligible

7. Area inside the cycle represents net work out for Ts or Pv
diagrams.

8. Diagrams for a Carnot Cycle

9. Equipment for a Carnot cycle. Changes have to be slow,
with very large heat exchangers, so not practical.

10. Example 8-1 Proof of thermal efficiency of a Carnot cycle
qin and qout are areas under lines
so TH(s2 – s1), etc.

11. Air Standard Assumptions

12. Air Standard AssumptionsAir Standard Assumptions
• Air in closed loop – ideal gasAir in closed loop – ideal gas
• Internally reversibleInternally reversible
• Combustion replaced by heatCombustion replaced by heat
additionaddition
• Exhaust replaced by heatExhaust replaced by heat
rejectionrejection
• Properties at room temp. – coldProperties at room temp. – cold
air standard assumptions.air standard assumptions.

13. Nomenclature

15. MEP = mean effective pressure, same amount of work
Compression
ratio?

16. Otto Cycle, ideal for spark ignition engines

17. Two-stroke engine

18. Ts plot of the Otto Cycle

19. Efficiency vs Compression Ratio
Higher ratios produce
autoignition and
knocking

20. Efficiency vs ratio of specific heats
Air
Combustion
mixture
He

21. Example 8-2 The Ideal Otto Cycle