Home Work 2 Engineering Thermodynamic 2, Sem II 2010/2011

Deadline submit on 26 February 2013 Lecturer: Dr. Ir. I Made Astina, M.Eng.

Solve all following problems by one group. Each group consists of 5 students. Each report of home work has to be signed by each member of the group. The sign is given in cover of the report. 1. A gasoline engine has a compression ratio of 8 and before compression has air at

300 K and 95 kPa. The input heat as resulted from combustion process is 750 kJ/kg. Find the peak temperature, maximum pressure, cycle efficiency, the exhaust temperature, and mean effective pressure in accordance with air-standard analysis.

2. A bio-Diesel engine has a total stroke volume of 3 L and consists of 4 cylinders.

The processes within each cylinder of the engine are modeled as a cold air-standard Diesel cycle with a cut-off ratio of 2.265 and compression ratio of 20. The state of the air at the beginning of compression is fixed by p1 = 95 kPa and T1 = 20oC. It is known that cv = 0,718 kJ/(kg K), cp = dan k =1,4. Determine the net work per cycle, in kJ, and the power developed by the engine, in kW, if the cycle is executed at 2000 rpm.

3. A plane with turbojet engine is analyzed for a circumstance flying condition. A

turbojet aircraft is flying with a velocity of 320 m/s at an altitude of 9150 m, where the ambient conditions are 32 kPa and 32°C. The pressure ratio across the compressor is 12, and the temperature at the turbine inlet is 1400 K. Air enters the compressor at a rate of 60 kg/s, and the jet fuel has a heating value of 43,000 kJ/kg. Assuming ideal operation for all components and constant specific heats for air at room temperature, determine (a) the velocity of the exhaust gases, (b) the propulsive power developed, and (c) the rate of fuel consumption.

4. Atkinson cycle consists of isentropic compression, constant volume heat addition,

isentropic expansion, and constant-pressure compression. For a particular Atkinson cycle, the compression ratio during isentropic compression is 8.5. At the beginning of this compression process, p1 = 100 kPa and T1 = 300 K. The constant volume

heat addition per unit mass of air is 1400 kPa. Determine (a) the net work, in kJ per kg of air, (b) the thermal efficiency of the cycle, and (c) the mean effective pressure, in kPa. Compare your answers with those obtained for the Otto cycle for the same main state of the processes.

5. Air enters the turbine of a gas turbine at 1200 kPa, 1200 K, and expands to 100 kPa in two stages. Between the stages, the air is reheated at a constant pressure of 350 kPa to 1200 K. The expansion through each turbine stage is isentropic. Determine, in kJ per kg of air flowing: (a) the work developed by each stage, (b) the heat transfer for the reheat process, (c) the increase in net work as compared to a single expansion with no reheat.

6. Two engines are to operate on Otto and Diesel cycles with the following data: Maximum temperature 1400 K, Exhaust temperature 700 K, State of air at the beginning of compreession 0.1 MPa, 300 K. Based on cold air standard analysis, estimate the compresssion ratio, the maximum pressure, efficiencies and rate of work outputs of the respectively cycles.

7. Air enters the compressor of a regenerative gas-turbine engine at 300 K and 100 kPa, where it is compressed to 800 kPa and 580 K. The regenerator has an effectiveness of 72 percent, and the air enters the turbine at 1200 K. For a turbine efficiency of 86 percent, determine (a) the amount of heat transfer in the regenerator and (b) the thermal efficiency. Assume variable specific heats for air.

8. A turbojet aircraft is flying with a velocity of 320 m/s at an altitude of 9150 m, where the ambient conditions are 32 kPa and 32°C. The pressure ratio across the compressor is 12, and the temperature at the turbine inlet is 1400 K. Air enters the compressor at a rate of 60 kg/s, and the jet fuel has a heating value of 42,700 kJ/kg. Assuming ideal operation for all components and constant specific heats for air at room temperature, determine (a) the velocity of the exhaust gases, (b) the propulsive power developed, and (c) the rate of fuel consumption.

9. An ideal Otto cycle has a compression ratio of 9.2 and uses air as the working fluid. At the beginning of the compression process, air is at 98 kPa and 27°C. The pressure is doubled during the constant-volume heat-addition process. Accounting for the variation of specific heats with temperature, determine (a) the amount of

heat transferred to the air, (b) the net work output, (c) the thermal efficiency, and (d) the mean effective pressure for the cycle.

10. An ideal Stirling engine using air as the working fluid operates between temperature limits of 300 and 1500 K and pressure limits of 150 kPa and 3 MPa. Assuming the mass of the air used in the cycle is 0.1 kg, determine (a) the thermal efficiency of the cycle, (b) the amount of heat transfer in the regenerator, and (c) the work output per cycle.

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