Prepared by Pr. Noureddine
Ait Messaoudene
University of Hail
Based on
Yunus A. Cengel and Michael
A. Boles
Thermodynamics: An
Engineering Approach
6th Edition, McGraw Hill,
2007.
Faculty of Enginering
DEPARTMENT OF MECHANICAL ENGINEERING
Chapter 9
GAS POWER CYCLES
Lecture 5
9-1 BASIC CONSIDERATIONS
9-2 THE CARNOT CYCLE
9-3 AIR-STANDARD ASSUMPTIONS
9-4 AN OVERVIEW OF RECIPROCATING ENGINES
9–5 OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-IGNITION ENGINES
Two important areas of application for thermodynamics are power generation and refrigeration.
Both are usually accomplished by systems that operate on a thermodynamic cycle.
Thermodynamic cycles can be categorized as:
Depending on the function
Power cycles: The devices or systems used to produce a net power output are often called engines.
Refrigeration cycles: The devices or systems used to produce a refrigeration effect are called
refrigerators, air conditioners, or heat pumps.
Depending on the phase of the working fluid
Gas cycles: the working fluid remains in the gaseous phase throughout the entire cycle.
Vapor cycles: the working fluid exists in the vapor phase during one part of the cycle and in the liquid
phase during another part.
Depending on the nature of the cycle
Closed cycles: the working fluid is returned to the initial state at the end of the cycle and is
recirculated.
Open cycles: the working fluid is renewed at the end of each cycle instead of being recirculated.
Heat engines are categorized, depending on how the heat is supplied to the working fluid, as :
Internal combustion engines: (such as steam power plants), heat is supplied to the working fluid from an
external source such as a furnace, a geothermal well, a nuclear reactor, or even the sun .
External combustion engines: (such as automobile engines), heat is supplied to the working fluid by burning
the fuel within the system boundaries.
9–1 BASIC CONSIDERATIONS IN THE ANALYSIS OF POWER CYCLES
Most power-producing devices operate on cycles.
The cycles encountered in actual devices are difficult to analyze
Must use some idealizations (simplifying assumptions).
ideal cycle : simplified cycle, but it still retains the general characteristics of
the actual cycles it represents.
This allows to :
•study the effects of the major parameters that dominate the cycle without taking into
account all the details.
•may also serve as the starting point for a more in-depth study.
Heat engines are designed for the purpose of converting thermal energy to work, and
their performance is expressed in terms of the thermal efficiency ηth, which is the ratio
of the net work produced by the engine to the total heat input:
Why not use the Carnot cycle as the model cycle for all the heat engines instead of
bothering with several so-called ideal cycles?
No actual design is able to function according to this cycle
simplifications commonly employed :
1. The cycle does not involve any friction.
2. All expansion and compression processes take place in a quasi-equilibrium manner.
3. Negligible heat transfer through connecting pipes .
Neglect the changes in kinetic and potential energies (cycles (the only devices where
the changes in kinetic energy are significant are the nozzles and diffusers,
Diagrams commonly used
Wnet = Qnet
Direction of Qin
Direction of Qout
9–2 THE CARNOT CYCLE AND ITS VALUE IN ENGINEERING
The Carnot cycle is composed of four
totally reversible processes:
•isothermal heat addition,
•isentropic expansion,
•isothermal heat rejection,
•isentropic compression.
The real value of the Carnot cycle comes from its being a standard against which
the actual or the ideal cycles can be compared
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.
No limits to TH and
TL
Cooling
medium
Materials
9–3 AIR-STANDARD ASSUMPTIONS
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
air-standard cycle
air has constant specific heats whose
values are determined at room
temperature (25°C, or ≈300 K)
9–4 AN OVERVIEW OF RECIPROCATING ENGINES
Clearance
volume
compression ratio
top dead
center
Bottom
dead center
Displacement
volume
Dividing by
m
mean effective pressure
can be used as a parameter to compare the performances
of reciprocating engines of equal size
Reciprocating engines are classified as
spark-ignition (SI) engines: the
combustion of the air–fuel
mixture is initiated by a spark
plug; representative ideal cycle
= Otto cycle
compression-ignition (CI)
engines, the air–fuel mixture
is self-ignited as a result of
compressing the mixture
above its self-ignition
temperature; representative
ideal cycle= Diesel cycle
9–5 OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-IGNITION ENGINES
The Otto cycle is the ideal cycle for spark-ignition reciprocating engines. It is named
after Nikolaus A. Otto, who built a successful four-stroke engine in 1876 in Germany
using the cycle proposed by Frenchman Beau de Rochas in 1862
four-stroke internal
combustion engines
two mechanical
cycles
In two-stroke engines, all four functions described above are executed in just two strokes:
the power stroke and the compression stroke. In these engines, the crankcase is sealed, and
the intake and exhaust valves are replaced by openings in the lower portion of the cylinder
wall.
The thermodynamic analysis of the actual four-stroke or two-stroke cycles is complicated
air-standard assumptions
ideal Otto cycle
Intake
and
exhaust
9–5 OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-IGNITION ENGINES
closed system + disregarding the changes in k.e and p.e
the energy balance for any of the processes is expressed, on a
unit-mass basis, as
(9–5)
No work during
2-3 and 4-1
and
cold air standard
assumptions
thermal efficiency
of the ideal Otto
cycle
Processes 1-2 and 3-4 are
isentropic, and v2 = v3 and v4 = v1
where
and k = cp /cv
not much improvement
as r
But at high values of r
autoignition
engine knock
The other parameter is k
for a given r
as k
k depends on the fluid
monatomic gas (such as argon
or helium)
air (at room temperature)
CO2 (at room temperature)
and k decreases with temperature
The thermal efficiencies of actual spark-ignition engines range from about 25 % to 30 %