Assignment No 06 Gas Power Cycles 1. What are the air-standard ...

ME 322 (Mechanical Power Engineering)
Assignment No 06
Gas Power Cycles
1. What are the air-standard assumptions and the cold-air-standard assumptions?
2. What is the difference between spark-ignition and compression-ignition engines?
3. How does a diesel engine differ from a gasoline engine?
4. What is the cutoff ratio? How does it affect the thermal efficiency of a Diesel cycle?
5. An air-standard cycle with variable specific heats is executed in a closed system and is composed
of these four processes:
1-2 Isentropic compression from 100 kPa and 27°C to 800 kPa
2-3 υ = constant heat addition to 1800 K
3-4 Isentropic expansion to 100 kPa
4-1 P = constant heat rejection to initial state
(a) Show the cycle on P-υ and T-s diagrams, (b) Calculate the net work output per unit mass, and
(c) Determine the thermal efficiency.
6. The compression ratio of an air-standard Otto cycle is 8. Prior to the isentropic compression
process, the air is at 100 kPa, 20°C, and 500 cm 3 . The temperature at the end of the isentropic
expansion process is 800 K. Using specific heat values at room temperature, determine (a) the
highest temperature and pressure in the cycle; (b) the amount of heat transferred in, in kJ; (c) the
thermal efficiency; and (d) the mean effective pressure.
7. An ideal diesel engine has a compression ratio of 22 and uses air as the working fluid. The state of
air at the beginning of the compression process is 95 kPa and 27°C. If the maximum temperature
in the cycle is not to exceed 2250 K, determine (a) the thermal efficiency and (b) the mean
effective pressure. Assume constant specific heats for air at room temperature.
8. Air is used as the working fluid in a simple ideal Brayton cycle that has a pressure ratio of 10, a
compressor inlet temperature of 300 K, and a turbine inlet temperature of 1000 K. Assume
constant specific heats at room temperature. Determine the required mass flow rate of air for a net
power output of 100 MW. What is the back work ratio for this cycle?
9. Consider a 320-MW steam power plant that operates on a simple ideal Rankine cycle. Steam enters
the turbine at 12 MPa and 600°C and is cooled in the condenser at a pressure of 10 kPa. Cooling
water enters the condenser at 20°C and leaves it at 32°C. Show the cycle on a T-s diagram with
respect to saturation lines, and determine (a) the quality of the steam at the turbine exit, (b) the
thermal efficiency of the cycle, (c) the mass flow rate of the steam, and (d) the mass flow rate of
the cooling water.
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ME 322 (Mechanical Power Engineering)
10. Consider a coal-fired steam power plant that produces 350 MW of electric power. The power plant
operates on a simple ideal Rankine cycle with turbine inlet conditions of 8 MPa and 550°C and a
condenser pressure of 20 kPa. The coal has a heating value (energy released when the fuel is
burned) of 30,000 kJ/kg. Assuming that 80 percent of this energy is transferred to the steam in the
boiler and that the electric generator has an efficiency of 94 percent, determine (a) the overall plant
efficiency (the ratio of net electric power output to the energy input as fuel) and (b) the required
rate of coal supply.
11. An air-standard cycle is executed in a closed system and is composed of five processes:
1-2 Isentropic compression from 100 kPa and 27°C to 800 kPa
2-3 υ = constant heat addition to 1000 °C
3-4 P = constant heat addition to 1400 °C
4-5 Isentropic expansion to original volume of starting (υ1)
5-1 υ = constant heat rejection to initial state
Considering specific heat values at room temperature
Show the cycle on P-υ or T-s diagrams,
Calculate the net work output per unit mass, and
Determine the thermal efficiency.
Solve all given problems in this assignment, however report out only all given
problems 10 and 11.
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