Chapter 9 | 5479–137 Repeat Problem 9–136 using constant specific heats
at room temperature.
9–138 A Carnot cycle is executed in a closed system and
uses 0.0025 kg of air as the working fluid. The cycle effi-
ciency is 60 percent, and the lowest temperature in the cycle
is 300 K. The pressure at the beginning of the isentropic
expansion is 700 kPa, and at the end of the isentropic com-
pression it is 1 MPa. Determine the net work output per cycle.
9–139 A four-cylinder spark-ignition engine has a
compression ratio of 8, and each cylinder has a
maximum volume of 0.6 L. At the beginning of the compres-
sion process, the air is at 98 kPa and 17°C, and the maximum
temperature in the cycle is 1800 K. Assuming the engine to
operate on the ideal Otto cycle, determine (a) the amount of
heat supplied per cylinder, (b) the thermal efficiency, and
(c) the number of revolutions per minute required for a net
power output of 60 kW. Assume variable specific heats for air.
9–140 Reconsider Problem 9–139. Using EES (or
other) software, study the effect of varying the
compression ratio from 5 to 11 on the net work done and
the efficiency of the cycle. Plot the P-vand T-sdiagrams for
the cycle, and discuss the results.
9–141 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 tempera-
ture, 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.
9–142 Repeat Problem 9–141 using constant specific heats
at room temperature.
9–143 Consider an engine operating on the ideal Diesel
cycle with air as the working fluid. The volume of the cylin-
der is 1200 cm^3 at the beginning of the compression process,
75 cm^3 at the end, and 150 cm^3 after the heat-addition
process. Air is at 17°C and 100 kPa at the beginning of the
compression process. Determine (a) the pressure at the begin-
ning of the heat-rejection process, (b) the net work per cycle,
in kJ, and (c) the mean effective pressure.
9–144 Repeat Problem 9–143 using argon as the working
fluid.
9–145E An ideal dual cycle has a compression ratio of 12
and uses air as the working fluid. At the beginning of the
compression process, air is at 14.7 psia and 90°F, and occu-
pies a volume of 75 in^3. During the heat-addition process,
0.3 Btu of heat is transferred to air at constant volume and
1.1 Btu at constant pressure. Using constant specific heats
evaluated at room temperature, determine the thermal effi-
ciency of the cycle.
9–146 Consider an ideal Stirling cycle using air as the work-
ing fluid. Air is at 350 K and 200 kPa at the beginning of the
isothermal compression process, and heat is supplied to air
from a source at 1800 K in the amount of 900 kJ/kg. Deter-
mine (a) the maximum pressure in the cycle and (b) the net
work output per unit mass of air. Answers: (a) 5873 kPa,
(b) 725 kJ/kg
9–147 Consider a simple ideal Brayton cycle with air as the
working fluid. The pressure ratio of the cycle is 6, and the
minimum and maximum temperatures are 300 and 1300 K,
respectively. Now the pressure ratio is doubled without chang-
ing the minimum and maximum temperatures in the cycle.
Determine the change in (a) the net work output per unit
mass and (b) the thermal efficiency of the cycle as a result of
this modification. Assume variable specific heats for air.
Answers: (a) 41.5 kJ/kg, (b) 10.6 percent
9–148 Repeat Problem 9–147 using constant specific heats
at room temperature.
9–149 Helium is used as the working fluid in a Brayton
cycle with regeneration. The pressure ratio of the cycle is 8,
the compressor inlet temperature is 300 K, and the turbine
inlet temperature is 1800 K. The effectiveness of the regener-
ator is 75 percent. Determine the thermal efficiency and the
required mass flow rate of helium for a net power output of
60 MW, assuming both the compressor and the turbine have
an isentropic efficiency of (a) 100 percent and (b) 80 percent.
9–150 A gas-turbine engine with regeneration operates with
two stages of compression and two stages of expansion. The
pressure ratio across each stage of the compressor and tur-
bine is 3.5. The air enters each stage of the compressor at
300 K and each stage of the turbine at 1200 K. The compres-
sor and turbine efficiencies are 78 and 86 percent, respec-
tively, and the effectiveness of the regenerator is 72 percent.
Determine the back work ratio and the thermal efficiency of
the cycle, assuming constant specific heats for air at room
temperature. Answers: 53.2 percent, 39.2 percent
9–151 Reconsider Problem 9–150. Using EES (or
other) software, study the effects of varying the
isentropic efficiencies for the compressor and turbine and
regenerator effectiveness on net work done and the heat sup-
plied to the cycle for the variable specific heat case. Let the
isentropic efficiencies and the effectiveness vary from 70 per-
cent to 90 percent. Plot the T-sdiagram for the cycle.
9–152 Repeat Problem 9–150 using helium as the working
fluid.
9–153 Consider the ideal regenerative Brayton cycle. Deter-
mine the pressure ratio that maximizes the thermal efficiency
of the cycle and compare this value with the pressure ratio
that maximizes the cycle net work. For the same maximum-
to-minimum temperature ratios, explain why the pressure
ratio for maximum efficiency is less than the pressure ratio
for maximum work.
9–154 Consider an ideal gas-turbine cycle with one stage of
compression and two stages of expansion and regeneration.