Air-Standard Otto Cycle

1. Air-Standard Otto Cycle Lecture – 33 11/19/08

2. Spark Ignition vs Compression Ignition Spark-ignition: mixture of fuel and air are ignited by a spark plug. Have advantages for applications requiring power up to about 225 kW (300 hp.). Relatively light and lower in cost, suited well to automobiles. Compression ignition engines: Air is compressed to high enough pressure and temperature that combustion occurs spontaneously when fuel is injected. Preferred for applications requiring large power and high fuel efficiency (trucks and buses, locomotives and ships). Recently diesels have become popular for automobiles. Require pollution controls for particles and NOX.

3. Fig09_01 Fig09_01

4. Fig09_02 Fig09_02

5. Fig09_03 Fig09_03

6. The Four Strokes Intake Compression Ignition/Power Stroke Exhaust Mean effective pressure = (Net work for one cycle) / (displacement volume)

7. Air Standard Analysis A fixed amount of air modeled as an ideal gas is the working fluid. The combustion process is replaced by a heat transfer from an external source There are no exhaust and intake processes as in an actual engine. The cycle is completed by a constant volume heat transfer process taking place while the piston is at the bottom dead center position All processes are internally reversible In a cold air standard cycle, the specific heats are assumed constant at their ambient temperature values.

8. Air Standard Otto Cycle The Otto cycle is shown on p-v and T-s diagrams. It consists of four internally reversible processes in series: Process 1-2 is an isentropic compression of the air as the piston moves from bottom dead center to top dead center Process 2-3 is a constant volume heat transfer to the air from an external source while the piston is at top dead center. Process 3-4 is an isentropic expansion (power stroke) Process 4-1 is a constant volume heat rejection process while the piston is at the bottom dead center

9. Cycle Analysis The air standard Otto cycle consists of two processes in which there is work but no heat transfer and two processes in which there is heat transfer and no work. Processes 1-2 and 3-4 have no heat transfer but only work. Processes 2-3 and 1-4 have only heat transfer and no work. Assuming no KE and PE effects, we can write the following balances:

10. Net Work and Efficiency The net work and efficiency of the cycle can be evaluated by the following relations:

11. Isentropic Compression and Expansion For the isentropic expansion and compression, the relations can be written as: When the Otto cycle is analyzed on a cold air standard basis, the following relations can be used.

12. Effect of Compression Ratio The efficiency of the Otto cycle depends on the compression ratio r. This can be seen by the following relations.

13. Fig09_04 Fig09_04

14. Diesel Cycle The air standard Diesel cycle is an ideal cycle that assumes heat addition occurs during a constant-pressure process that starts with the piston at top dead center. The cycle has four internally reversible processes. The four processes are: 1-2: isentropic compression 2-3: constant pressure heat addition and part of the power stroke; 3-4 : isentropic expansion, remainder of the power stroke; 4-1: heat is rejected from air while piston is at the bottom dead center.

15. Fig09_05 Fig09_05

16. Cycle Analysis Process 2-3 involves both work and heat addition

17. Cut-off ratio The cut-off ratio is defined as: V3/V2 For the constant pressure process 2-3, we can write

18. Isentropic Compression and Expansion For the isentropic expansion and compression, the relations can be written as: When the Diesel cycle is analyzed on a cold air standard basis, the following relations can be used.

19. Fig09_06 Fig09_06