Thermodynamics assign

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Assignment 4: Due Date: Friday 23 rd

October 2015, 4:00 pm Please submit through the library

Weighting: 10% of the total marks for this course Submit full worked solutions to the problems and up to three sentences for the questions requiring an explanation.

Questions Related to Engine Laboratories (Please do these questions after completing Labs 1 and 2)

Q. 1 (1 mark) The diesel engine in the lab has a maximum volume of 243 cm

3 (@ bottom

dead centre) and a minimum volume of 11 cm 3 (@ top dead centre). It takes in air at 25 °C

and atmospheric pressure (101 kPa). Assuming an ideal diesel cycle and cold air-standard assumptions, calculate the maximum pressure in the cycle. How does this compare with the maximum pressure measured from the indicator diagram in the lab? Q. 2 (1 mark) The diesel engine in the lab has a maximum volume of 243 cm

3 (@ bottom

dead centre) and a minimum volume of 11 cm 3 (@ top dead centre). Assuming the cutoff ratio

rc = 2.0, determine the efficiency of the engine if it operates on an ideal Diesel cycle. What efficiencies did you measure for the diesel engine in the lab based on the indicated power (from the in-cylinder pressure measurement) and the brake power (i.e. power to dynamometer)? Comment on possible causes for the difference. Q. 3 (1 mark) The petrol engine in the lab has a maximum volume of 195 cm

3 (@ bottom

dead centre) and a minimum volume of 22.9 cm 3 (@ top dead centre). Using cold-air standard

assumptions for an Otto cycle, find the efficiency of the ideal cycle. What efficiencies did you measure for the petrol engine in the lab based on the indicated power and the brake power? Q. 4 (2 marks) Assuming the 4-stroke petrol engine in the lab is operating at 2700 rpm with a

power output of 1500 W and takes in air at 25 °C and atmospheric pressure (101 kPa) at bottom dead centre (V1 = 195 cm

3 ) determine:

I. The energy output from the engine per cycle II. The energy input per cycle from the fuel (assuming Otto-cycle efficiency)

III. The mass of the air in the cylinder assuming an ideal air-standard cycle IV. The maximum pressure during the ideal cycle. Make a note of how this compares with

the maximum pressure measured on the indicator diagram in the lab.

General Questions

Q. 5 (1 mark) Saturated refrigerant-134a at -10 °C enters an isentropic compressor at a rate of 0.1 kg/s and is compressed to 1200 kPa. Determine the power input to the compressor.

1200 kPa

Saturated R134a

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Q. 6 (1 mark) A 0.5 m 3 rigid, sealed container which contains air is heated and stirred in an

isochoric, adiabatic process. A 100 Ω resistance heater is powered by a 230 V supply. The

shaft power to the stirrer is 200 W. The initial temperature of the air is 25 °C and initial pressure is 100 kPa. Determine the new pressure and temperature after the heating and stirring has continued for 2.0 min.

Q. 7 (1 mark) An ideal Brayton cycle is shown in the figure below. The pressure ratio is 10.0 and the working fluid may be considered to have the properties air at room temperature (cold- air-standard assumption). The compressor and turbine are isentropic. Air enters the compressor at 295 K, 100 kPa and the turbine at 1600 K. Air flows through the turbine at 0.5 kg/s. Determine (a) the air temperature at the compressor exit, (b) the net power output, (c) the back work ratio and (d) the thermal efficiency.

Air

V = 0.5 m 3

T1 = 25 °C

P1 = 100 kPa (abs)

Adiabatic

230 V 100 Ω

TurbineCompressor

Heat exchanger

Heat exchanger

Qout

Q in

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Q. 8 (2 marks) A refrigerator uses refrigerant-134a as the working fluid and operates on an ideal vapor-compression refrigeration cycle between 0.1 MPa and 1.2 MPa. The power input to the compressor is 1.9 kW. Determine (a) The temperature at the exit to the compressor, (b) the mass flow rate of refrigerant, (c) the rate of heat removal from the refrigerated space (d) the heat transfer rate to the environment from the condenser and (e) the coefficient of performance for the refrigerator. (You may assume the refrigerant is saturated vapor at the entrance to the compressor).