In Figure 6.15, ideal P−V, actual P−V, and T−s diagrams are shown. There are some basic differences in ideal and actual indicator diagrams as valves do not open or close at sharp points practically. Valves start to open just before the point and close just after the points as shown in Figure 6.15b. The process 0−1 shows the suction of the charge which is compressed for 1−2. After compression head is added at constant volume for 2−3. 3−4 is isentropic expansion process, at the end of expansion exhaust valve opens and heat is rejected for 4−1.
Thermal efficiency of Otto Cycle, 
Figure 6.15 Indicator Diagrams for Otto Cycle
Example 6.1: In an air standard Otto cycle, the pressure, and temperature at the start of the compression stroke are 0.1 MPa and 300 K, respectively. The temperature at the end of the compression and at the end of the heat addition processes are 600 and 1,600 K, respectively. Calculate (i) thermal efficiency, (ii) heat added, (iii) net work per kg of air, and (iv) mean effective pressure.
Solution:
Example 6.2: An engine operation on an air standard Otto cycle has a compression ratio equal to 7. The conditions at the start of compression are 0.1 MPa and 300 K. The pressure at the end of heat addition is 4 MPa. Determine (i) thermal efficiency, (ii) net work done per kg of air where Cv = 0.718 kJ/kg, γair = 1.4, and (iii) mean effective pressure.
Solution:
Example 6.3: Derive the expression for compression ratio, in Otto cycle, for maximum work done in terms of maximum and minimum temperatures.
Solution:
Refer Figure 6.10c, maximum and minimum temperatures in the cycle are T3 and T1, respectively.
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