Chapter 9 | 549maximum? At what pressure ratio does the thermal efficiency
become a maximum?
9–162 Repeat Problem 9–161 assuming isentropic
efficiencies of 85 percent for both the turbine
and the compressor.
9–163 Using EES (or other) software, determine the
effects of pressure ratio, maximum cycle tem-
perature, and compressor and turbine efficiencies on the net
work output per unit mass and the thermal efficiency of a
simple Brayton cycle with air as the working fluid. Air is at
100 kPa and 300 K at the compressor inlet. Also, assume
constant specific heats for air at room temperature. Deter-
mine the net work output and the thermal efficiency for all
combinations of the following parameters, and draw conclu-
sions from the results.
Pressure ratio: 5, 8, 14
Maximum cycle temperature: 800, 1200, 1600 K
Compressor isentropic efficiency: 80, 100 percent
Turbine isentropic efficiency: 80, 100 percent
9–164 Repeat Problem 9–163 by considering the vari-
ation of specific heats of air with temperature.
9–165 Repeat Problem 9–163 using helium as the
working fluid.
9–166 Using EES (or other) software, determine the
effects of pressure ratio, maximum cycle tem-
perature, regenerator effectiveness, and compressor and tur-
bine efficiencies on the net work output per unit mass and on
the thermal efficiency of a regenerative Brayton cycle with
air as the working fluid. Air is at 100 kPa and 300 K at the
compressor inlet. Also, assume constant specific heats for air
at room temperature. Determine the net work output and the
thermal efficiency for all combinations of the following
parameters.
Pressure ratio: 6, 10
Maximum cycle temperature: 1500, 2000 K
Compressor isentropic efficiency: 80, 100 percent
Turbine isentropic efficiency: 80, 100 percent
Regenerator effectiveness: 70, 90 percent
9–167 Repeat Problem 9–166 by considering the vari-
ation of specific heats of air with temperature.
9–168 Repeat Problem 9–166 using helium as the
working fluid.
9–169 Using EES (or other) software, determine the
effect of the number of compression and expan-
sion stages on the thermal efficiency of an ideal regenerative
Brayton cycle with multistage compression and expansion.
Assume that the overall pressure ratio of the cycle is 12, and
the air enters each stage of the compressor at 300 K and each
stage of the turbine at 1200 K. Using constant specific heats
for air at room temperature, determine the thermal efficiency
of the cycle by varying the number of stages from 1 to 22 in
increments of 3. Plot the thermal efficiency versus the number
of stages. Compare your results to the efficiency of an Erics-
son cycle operating between the same temperature limits.
9–170 Repeat Problem 9–169 using helium as the
working fluid.Fundamentals of Engineering (FE) Exam Problems
9–171 An Otto cycle with air as the working fluid has a
compression ratio of 8.2. Under cold-air-standard conditions,
the thermal efficiency of this cycle is
(a) 24 percent (b) 43 percent (c) 52 percent
(d) 57 percent (e) 75 percent
9–172 For specified limits for the maximum and minimum
temperatures, the ideal cycle with the lowest thermal effi-
ciency is
(a) Carnot (b) Stirling (c) Ericsson
(d) Otto (e) All are the same
9–173 A Carnot cycle operates between the temperature
limits of 300 and 2000 K, and produces 600 kW of net
power. The rate of entropy change of the working fluid dur-
ing the heat addition process is
(a)0 (b) 0.300 kW/K (c) 0.353 kW/K
(d) 0.261 kW/K (e) 2.0 kW/K
9–174 Air in an ideal Diesel cycle is compressed from 3 to
0.15 L, and then it expands during the constant pressure heat
addition process to 0.30 L. Under cold air standard condi-
tions, the thermal efficiency of this cycle is
(a) 35 percent (b) 44 percent (c) 65 percent
(d) 70 percent (e) 82 percent
9–175 Helium gas in an ideal Otto cycle is compressed
from 20°C and 2.5 to 0.25 L, and its temperature increases by
an additional 700°C during the heat addition process. The
temperature of helium before the expansion process is
(a) 1790°C (b) 2060°C (c) 1240°C
(d) 620°C (e) 820°C
9–176 In an ideal Otto cycle, air is compressed from 1.20
kg/m^3 and 2.2 to 0.26 L, and the net work output of the cycle
is 440 kJ/kg. The mean effective pressure (MEP) for this
cycle is
(a) 612 kPa (b) 599 kPa (c) 528 kPa
(d) 416 kPa (e) 367 kPa
9–177 In an ideal Brayton cycle, air is compressed from 95
kPa and 25°C to 800 kPa. Under cold-air-standard conditions,
the thermal efficiency of this cycle is
(a) 46 percent (b) 54 percent (c) 57 percent
(d) 39 percent (e) 61 percent
9–178 Consider an ideal Brayton cycle executed between
the pressure limits of 1200 and 100 kPa and temperature lim-
its of 20 and 1000°C with argon as the working fluid. The net
work output of the cycle is
(a) 68 kJ/kg (b) 93 kJ/kg (c) 158 kJ/kg
(d) 186 kJ/kg (e) 310 kJ/kg