Question

The compression ratio of an air-standard Otto cycle is 9.5. Prior to the isentropic compression process, the air is at 100 kPa, 35°C and 600 cm3 . The temperature at the end of the isentropic expansion process is 800 K. Using specific heat values at room temperature (25°C), determine

(a) the highest temperature and pressure in the cycle

(b) the amount of heat transferred in during the cycle (kJ)

(c) the thermal efficiency

(d) the mean effective pressure

Part (a) Draw P-V diagram of Otto cycle; see Figure 9-19. Highest temperature and pressure are at State 3; Need to find T3 and P3 Process 3?4 is isentropic expansion; T4 is given as 800 K; Use compression ratio = r = v4/v3 = 9.5 from problem statement Find T3 by using isentropic relationship between temperature & specific volume (Ans. 1970 K)

Process 1?2 is isentropic compression; Use compression ratio = r = v1/v2 = 9.5 from problem statement Find T2 by using isentropic relationship between temperature & specific volume (Ans. 758 K)

Then use ideal gas law to find P2 (Ans. 2340 kPa)

Then use ideal gas law to find P3 with the relationship v3 =v2 (Ans. 6070 kPa)

Part (b) Use ideal gas law to find mass of air Calculate heat transfer into the air from state 2 to state 3 (Ans. 0.59 kJ)

Part (c) Problem statement gave T1 and T4 In part (a) T3 and T2 were found Find the thermal efficiency using these four temperatures (Ans. 59%)

Part (d) Ans. 650 kPa

Answer 1