This document provides an overview of gas power cycles and ideal cycles used to model internal combustion engines. It begins by outlining the objectives and basic considerations for analyzing power cycles. Idealizations used include assuming internally reversible, quasi-equilibrium processes and negligible heat transfer. The Carnot, Otto, Diesel, and dual cycles are described as ideal models for heat engines. Sample problems are provided for calculating values like temperature, work, efficiency and mean effective pressure for the Otto and Diesel cycles. Limitations of the ideal cycles are noted, along with characteristics of reciprocating engines.

1. Chapter 9
GAS POWER CYCLES
(Part 1a)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Thermodynamics: An Engineering Approach, 6th
Edition
Yunus A. Cengel, Michael A. Boles
McGraw-Hill, 2008

2. 2
Objectives
1. Evaluate the performance of gas power cycles.
2. Develop simplifying assumptions applicable to gas power cycles.
3. Review the operation of reciprocating engines.
4. Analyze both closed and open gas power cycles.
5. Solve problems based on the Otto and Diesel cycles.
6. Solve problems based on the Brayton cycle; Brayton cycle with regeneration;
and Brayton cycle with intercooling, reheating, and regeneration.
7. Identify simplifying assumptions and perform second-law analysis on gas
power cycles.

3. 3
Basic Considerations In Power Cycles Analysis
The analysis of many complex
processes can be reduced to a
manageable level by utilizing
some idealizations.
Most power-producing devices operate on cycles.
Ideal cycle: A cycle that resembles the actual cycle
closely but is made up totally of internally reversible
processes is called an ideal cycle.
Recall: Thermal efficiency of heat engines
Reversible cycles such as Carnot cycle have the
highest thermal efficiency of all heat engines operating
between the same temperature levels.
Unlike ideal cycles, they are totally reversible, and
unsuitable as a realistic model.

4. 4
1. The cycle does not involve any friction. Therefore,
the working fluid does not experience any pressure
drop as it flows in pipes or heat exchangers.
2. All expansion and compression processes take
place in a quasi-equilibrium manner.
3. The pipes connecting the various components of a
system are well insulated, so heat transfer
through them is negligible.
Care should be exercised in the
interpretation of the results from
ideal cycles.
On both P-v and T-s diagrams, the area enclosed by the
process curve represents the net work of the cycle.
On a T-s diagram, the ratio of the area
enclosed by the cyclic curve to the area
under the heat-addition process curve
represents the thermal efficiency of the
cycle.
Idealizations (simplifications) in the analysis of power cycles

5. 5
Carnot Cycle - Its Value In
Engineering
P-v and T-s diagrams of a Carnot cycle.
Example: A steady-flow Carnot engine.
The Carnot cycle is composed of 4 totally reversible
processes: isothermal heat addition, isentropic expansion,
isothermal heat rejection, and isentropic compression.
For both ideal and actual cycles: Thermal
efficiency increases with an increase in the
average temperature at which heat is supplied
to the system or with a decrease in the average
temperature at which heat is rejected from the
system.

6. 6
Air-standard Assumptions
The combustion process is replaced by a
heat-addition process in ideal cycles.
1. The working fluid is air, which continuously
circulates in a closed loop and always
behaves as an ideal gas.
2. All the processes that make up the cycle
are internally reversible.
3. The combustion process is replaced by a
heat-addition process from an external
source.
4. The exhaust process is replaced by a
heat-rejection process that restores the
working fluid to its initial state.
Cold-air-standard assumptions: When the working fluid is considered to be air
with constant specific heats at room temperature (25°C).
Air-standard cycle: A cycle for which the air-standard assumptions are
applicable.

7. Chapter 9
GAS POWER CYCLES
(Part 1b)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Thermodynamics: An Engineering Approach, 6th
Edition
Yunus A. Cengel, Michael A. Boles
McGraw-Hill, 2008

8. 8
Problem
Otto Cycle
9–34
An ideal Otto cycle has a compression ratio of 8. At the beginning of the
compression process, air is at 95 kPa and 27°C, and 750 kJ/kg of heat is
transferred to air during the constant-volume heat-addition process. Assuming
that the specific heats are constant with temperature, determine:
• the pressure & temperature at the end of heat addition process,
• the net work output,
• the thermal efficiency, and
• the mean effective pressure for the cycle.
Answers: (a) 3898 kPa, 1539 K, (b) 392.4 kJ/kg, (c) 52.3 percent, (d ) 495 kPa

9. 9
Overview of Reciprocating Engines
The reciprocating engine (basically a piston–cylinder device) is an invention that
has proved to be very versatile and has a wide range of applications.
Reciprocating engine is the
powerhouse of the vast majority
of automobiles, trucks, light
aircraft, ships, electric power
generators, and many other
devices.

10. 10
Basic Components
Compression ratio:
The piston reciprocates in the cylinder between two fixed positions called the top dead
centre (TDC) - the position that forms the smallest volume in the cylinder - and the bottom
dead centre (BDC) - position that forms the largest volume in the cylinder.
The distance between TDC and BDC is called the stroke of
the engine. The diameter of the piston is called the bore.

11. 11
Performance Characteristics
Classifications of IC Engines:
1. Spark-ignition (SI) or Petrol engines
2. Compression-ignition (CI) or Diesel
engines
Mean effective pressure (MEP):
A fictitious pressure that, if it is acted on the piston
during the entire power stroke, would produce the
same amount of net work as that produced during the
actual cycle.
Net work output per cycle:

12. 12
Otto Cycle: Ideal Spark-Ignition Engines Cycle
Actual and ideal cycles in spark-ignition engines on a P-v diagram.
The piston executes four complete strokes within the cylinder. The crankshaft
completes two revolutions for each thermodynamic cycle.
These engines are called four-stroke IC engines.

13. 13
T-s Diagram of Ideal Otto Cycle
IC Engines Classifications:
Four-stroke cycle
1 cycle = 4 stroke = 2 revolutions of crankshaft
Two-stroke cycle
1 cycle = 2 stroke = 1 revolution of crankshaft
Sequence of processes:

14. 14
In two-stroke engines, all four functions described earlier are executed in two
strokes: the power and compression stroke.
Generally less efficient, but are relatively simple and inexpensive. They have high
power-to-weight and power-to-volume ratios.
Two-Stroke IC Engines

15. 15
Thermal Efficiency of Otto Cycle
The heat supplied to the working fluid during
constant-volume heating (combustion),
The heat rejected from the working fluid during
constant-volume cooling (exhaust),
Thermal efficiency,
Temperature-volume relation,
Compression ratio,
Cold-air standard assumption.

16. 16
Problem
Otto Cycle
9–34B
Reconsider the ideal Otto cycle in Problem 9-34. Assuming that the specific
heats vary with temperature, determine:
• the pressure & temperature at the end of heat addition process,
• the net work output,
• the thermal efficiency, and
• the mean effective pressure for the cycle.
Answers: (a) 3898 kPa, 1539 K, (b) 392.4 kJ/kg, (c) 52.3 percent, (d ) 495 kPa

17. 17
Problem
Otto Cycle
9–37
The compression ratio of an air-standard Otto cycle is 9.5. Prior to the isentropic
compression process, the air is at 100 kPa, 35°C, and 600 cm3
. The temperature
at the end of the isentropic expansion process is 800 K. Using specific heat
values at room temperature, determine:
• the highest temperature and pressure in the cycle;
• the amount of heat transferred in, in kJ;
• the thermal efficiency; and
• the mean effective pressure.
Answers: (a) 1969 K, 6072 kPa, (b) 0.59 kJ, (c) 59.4 percent, (d) 652 kPa

18. 18
Problem
9–39E
An ideal Otto cycle with air as the working fluid has a compression ratio of 8.
The minimum and maximum temperatures in the cycle are 300 K and 1340 K.
Accounting for the variation of specific heats with temperature, determine:
• the amount of heat transferred to the air during heat-addition process,
• the thermal efficiency, and
• the thermal efficiency of a Carnot cycle operating between the same
temperature limits.
Otto Cycle
Class Exercise

19. 19
Premature ignition of the fuel produces audible noise called engine knock. It hurts
performance and causes engine damage.
Autoignition places upper limit on compression ratios that can be used in SI engines.
Specific heat ratio, k affects the thermal efficiency of the Otto cycle.
Engine Knock (Autoignition)

20. 20
Problem
Otto Cycle
9–41
A four-cylinder, four-stroke, 2.2-L gasoline engine operates on the Otto cycle with
a compression ratio of 10. The air is at 100 kPa and 60°C at the beginning of the
compression process, and the maximum pressure in the cycle is 8 MPa. The
compression and expansion processes may be modeled as polytropic with an
index of 1.3. Using constant specific heats at 850 K, determine:
• the temperature at the end of the expansion process,
• the net work output and the thermal efficiency,
• the mean effective pressure,
• the engine speed for a net power output of 70 kW, and
• the specific fuel consumption, in g/kWh, defined as the ratio of the mass
of the fuel consumed to the net work produced.
Note: The air–fuel ratio, defined as the amount of air divided by the amount of
fuel intake, is 16.

21. Chapter 9
GAS POWER CYCLES
(Part 1c)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Thermodynamics: An Engineering Approach, 6th
Edition
Yunus A. Cengel, Michael A. Boles
McGraw-Hill, 2008

22. 22
Problem
Diesel Cycle
9–47
An air-standard Diesel cycle has a compression ratio of 16 and a cutoff ratio of
2. At the beginning of the compression process, air is at 95 kPa and 27°C.
Accounting for the variation of specific heats with temperature, determine:
• the temperature after the heat-addition process,
• the thermal efficiency, and
• the mean effective pressure.
Answers: (a) 1724.8 K, (b) 56.3 percent, (c) 675.9 kPa

23. 23
Diesel Cycle: Ideal Cycle for CI Engines
The combustion process takes place over a
longer interval - fuel injection starts when
the piston approaches TDC and continues
during the first part of power stroke.
Hence, combustion process in the ideal
Diesel cycle is approximated as a constant-
pressure heat-addition process.
In diesel engines, only air is compressed during the compression stroke, eliminating
the possibility of autoignition. These engines can be designed to operate at higher
compression ratios, typically between 12 and 24.
Fuels that are less refined (thus less expensive) can be used in diesel engines.

24. 24
1-2 Isentropic compression
2-3 Constant-pressure heat addition
3-4 Isentropic expansion
4-1 Constant-volume heat rejection.
Sequence of processes:
Note:
Petrol and diesel engines differ only in the
manner the heat addition (or combustion)
process takes place.
It is approximated as a constant volume
process in the petrol engine cycle and as a
constant pressure process in the Diesel
engine cycle.

25. 25Cutoff ratio,
Thermal Efficiency of Diesel Cycle
Heat supplied to the working fluid during the
constant-pressure heating (combustion),
Heat rejected from the working fluid during the
constant-volume cooling (exhaust),
Thermal efficiency of Diesel cycle (general),
- constant specific heats

26. 26
Problem
Diesel Cycle
9–51
An ideal diesel engine has a compression ratio of 20 and uses air as the working
fluid. The state of air at the beginning of the compression process is 95 kPa and
20°C. If the maximum temperature in the cycle is not to exceed 2200 K,
determine:
• the thermal efficiency, and
• the mean effective pressure.
Assume constant specific heats for air at room temperature.
Answers: (a) 63.5 percent, (b) 933 kPa
Skip…

27. 27
Problem
Diesel Cycle
9–54
A four-cylinder two-stroke 2.4-L diesel engine that operates on an ideal Diesel
cycle has a compression ratio of 17 and a cutoff ratio of 2.2. Air is at 55°C and 97
kPa at the beginning of the compression process.
Using the cold-air standard assumptions, determine how much power the
engine will deliver at 1500 rpm.
Skip…

28. 28
For the same compression ratio, thermal efficiency of Otto cycle is greater than that
of the Diesel cycle.
As the cutoff ratio decreases, the thermal
efficiency of the Diesel cycle increases.
When rc =1, the efficiencies of the Otto
and Diesel cycles are identical.
Thermal efficiencies of large diesel engines
range from about 35 to 40 percent.
Higher efficiency and lower fuel costs
make diesel engines attractive in
applications such as in locomotive engines,
emergency power generation units, large
ships, and heavy trucks.

29. 29
Problem
Diesel Cycle
9-59
A six-cylinder, four-stroke, 4.5-L compression-ignition engine operates on the
ideal Diesel cycle with a compression ratio of 17. The air is at 95 kPa and 55°C
at the beginning of the compression process and the engine speed is 2000 rpm.
The engine uses light diesel fuel with a heating value of 42,500 kJ/kg, an air–fuel
ratio of 24, and a combustion efficiency of 98 percent. Using constant specific
heats at 850 K, determine:
a) the maximum temperature in the cycle and the cutoff ratio,
b) the net work output per cycle and the thermal efficiency,
c) the mean effective pressure,
d ) the net power output, and
e) the specific fuel consumption, in g/kWh, defined as the ratio of the
mass of the fuel consumed to the net work produced.
Answers: (a) 2383 K, 2.7 (b) 4.36 kJ, 0.543, (c) 969 kPa, (d ) 72.7 kW, (e) 159 g/kWh

30. 30
Approximating the combustion process as
a constant-volume or a constant-pressure
heat-addition process is overly simplistic
and not quite realistic.
A better approach would be to model the
combustion process in both SI and CI
engines as a combination of two heat-
transfer processes, one at constant volume
and the other at constant pressure.
The ideal cycle based on this concept is
called the dual cycle.
Dual Cycle: Realistic Ideal Cycle for CI Engines
Note: Both the Otto and the Diesel cycles can be obtained
as special cases of the dual cycle.