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information at your own risk.
ROYMECH

Home
Thermos Index
Thermodynamics Cycles

Introduction
Various internal combustion engine types have been devised and represented by various idealised cycles
(otto cycle for four stroke, diesel cycle etc.. These idealised cycles are useful for determining the practical
limitations and efficiencies possible. They do not however provide the answer to the question..

"What is the greatest fraction of the heat transfer from a energy source is it possible to convert into work ?
i.e. what is the limiting efficiency of conversion ?"

Carnot introduced a theoretical gas cycle based on ideal reversible process which provides this information

Carnot Cycle
Carnot in 1824 arrived at the "carnot cycle" which is an idealised gas cycle that obtains the maximum
amount of work from an engine working in a thermodynamically reversible manner. This cycle provides a
maximum efficiency for any thermodynamic heat engine

The Carnot cycle for perfect gases is an idealised cycle composed of four reversible processes
1. an isothermal expansion of fixed mass of gas (say in a cylinder- causing a piston to move out)
2. an adiabetic expansion the gas (say in a cylinder- causing a piston to move out)
3. an isothermal contraction of the gas (say in a cylinder- causing a piston to move in)
4. an adiabetic contraction of the gas (say in a cylinder- causing a piston to move in)
This imaginary working fluid is contained in the (cylinder)closed system and simply receives and rejects
energy to a source and sink using perfect heat transfer (with no temperature difference ). As a result of
receiving and rejecting energy it expands and contracts during four ideal reversible "no-flow"
processes. The fluid is an ideal gas following the ideal gas laws.
Page 2

Disclaimer: The information on this page has not been checked by an independent person. Use this
information at your own risk.
ROYMECH

Home
Thermos Index
Thermodynamics Cycles

"What is the greatest fraction of the heat transfer from a energy source is it possible to convert into work ?
i.e. what is the limiting efficiency of conversion ?"

Carnot introduced a theoretical gas cycle based on ideal reversible process which provides this information

The work done through during a complete cycle is determined using the relationships identified on
webpage Polytropic processes....

From the general relationship for adiabatic polytropic processes the following relationship is
identified. Relationships

Reversible
Process
Heat Transfer
at T1
to Working
Fluid
From Hot
Source
Heat
Rejected at
T2 from
Working Fluid
From Sink
Work done by
working fluid
Change in
Internal
Energy of
Fluid
Isothermal
Expansion
RmT 1log er 0 RmT 1log er 0
Adiabatic
Expansion
0 0
Rm(T 1 - T 2 )/
(1- ? )
-Rm(T 1 -
T 2 )/ (1- ? )
Isothermal
Compression
0 RmT 2log er -RmT 2log er 0
Adiabatic
Compression
0 0
-Rm(T 1 - T 2 )/
(1- ? )
Rm(T 1 - T 2 )/
(1- ? )
Totals
RmT 1log er =
Q 1
RmT 2log er =
Q 2
Rm(T 1-
T 2 )log er= W
0

From the table it can clearly be seen that the total work done by the carnot cycle is Rm(T1 - T1 )loge r = Q1 -
Q2.

Page 3

Disclaimer: The information on this page has not been checked by an independent person. Use this
information at your own risk.
ROYMECH

Home
Thermos Index
Thermodynamics Cycles

"What is the greatest fraction of the heat transfer from a energy source is it possible to convert into work ?
i.e. what is the limiting efficiency of conversion ?"

Carnot introduced a theoretical gas cycle based on ideal reversible process which provides this information

The work done through during a complete cycle is determined using the relationships identified on
webpage Polytropic processes....

From the general relationship for adiabatic polytropic processes the following relationship is
identified. Relationships

From the table it can clearly be seen that the total work done by the carnot cycle is Rm(T1 - T1 )loge r = Q1 -
Q2.

The energy supplied = RmT1 loge r = Q1. Therefore

This is the maximum efficiency achievable by an reversible thermodynamic cycle working with a ideal perfect gas.

The following relationship results from the above....

Air Standard cycles
Although the Carnot cycle is theoretically the most efficient it is in no way a practical device. Also the
energy transfers would be far too slow for any real benefits to be realised. Internal combustion engines
work on non cyclic processes because the fuel-air mix enters the system and products of combustion exit
the system. . However theoretical cycles based on the hypothesis that air is the working fluid in a closed
system receiving an rejecting energy to external sinks allows provide very crude estimations on the
theoretical efficiencies possible internal combustion engines.

For the purpose of the air standard cycles the suction and exhaust strokes are not considered.T

The Otto Cycle or constant volume cycle has been proposed to provide an approximation of the 4 stroke
Internal combustion cycle designed by Otto. The diesel cycle is used to approximate a cycle with heat
being added at constant pressure..
Otto Cycle

The Otto cycle is comprised of four reversible processes of air in a closed system:
? a -> c adiabatic compression,

Page 4

Disclaimer: The information on this page has not been checked by an independent person. Use this
information at your own risk.
ROYMECH

Home
Thermos Index
Thermodynamics Cycles

"What is the greatest fraction of the heat transfer from a energy source is it possible to convert into work ?
i.e. what is the limiting efficiency of conversion ?"

Carnot introduced a theoretical gas cycle based on ideal reversible process which provides this information

The work done through during a complete cycle is determined using the relationships identified on
webpage Polytropic processes....

From the general relationship for adiabatic polytropic processes the following relationship is
identified. Relationships

From the table it can clearly be seen that the total work done by the carnot cycle is Rm(T1 - T1 )loge r = Q1 -
Q2.

The energy supplied = RmT1 loge r = Q1. Therefore

This is the maximum efficiency achievable by an reversible thermodynamic cycle working with a ideal perfect gas.

The following relationship results from the above....

For the purpose of the air standard cycles the suction and exhaust strokes are not considered.T

The Otto cycle is comprised of four reversible processes of air in a closed system:
? a -> c adiabatic compression,

THERMODYNAMICS - THEORY

Reversible and Irreversible Process

Examples of Reversible and Irreversible
Processes
Click to View Movie (52 kB)

A process is reversible if, after it has been carried out,
it is possible to restore both the system and its entire
surroundings to exactly the same states they were in
before the process. If the system and its surroundings
cannot return to their initial states at the end of the
reversed process, this process is an irreversible
process.
A system can be restored to its initial state following a
process, regardless if the process is reversible or not.
If the surroundings can also be restored to its initial
state, the process is reversible. Otherwise, the
process is irreversible.
Reversible process does not occur in nature. It is the
idealization of actual process and serves as an
idealized model to which actual process can be
compared.
The factors that cause a process to be irreversible are
called irreversibilities. They include:
? heat transfers through a finite temperature
difference
? unrestrained expansion of a gas
? mixing of two gases
? friction
? electric current flow through a resistance
? inelastic deformation
? chemical reactions
The process is irreversible if any of these effects
present.

Internally and Externally Reversible
Processes

When a process is carried out, irreversibilities can be
found within the system as well as in the system's
surroundings. A process is called internally reversible
if the system can be restored through exactly the
same equilibrium states which the system goes
through. No irreversibilities occur within the
boundaries of the system as it goes through the
process.
If no irreversibilities occur outside the system
boundaries during the process, the process is called
Page 5

Disclaimer: The information on this page has not been checked by an independent person. Use this
information at your own risk.
ROYMECH

Home
Thermos Index
Thermodynamics Cycles

"What is the greatest fraction of the heat transfer from a energy source is it possible to convert into work ?
i.e. what is the limiting efficiency of conversion ?"

Carnot introduced a theoretical gas cycle based on ideal reversible process which provides this information

The work done through during a complete cycle is determined using the relationships identified on
webpage Polytropic processes....

From the general relationship for adiabatic polytropic processes the following relationship is
identified. Relationships

From the table it can clearly be seen that the total work done by the carnot cycle is Rm(T1 - T1 )loge r = Q1 -
Q2.

The energy supplied = RmT1 loge r = Q1. Therefore

This is the maximum efficiency achievable by an reversible thermodynamic cycle working with a ideal perfect gas.

The following relationship results from the above....

For the purpose of the air standard cycles the suction and exhaust strokes are not considered.T

The Otto cycle is comprised of four reversible processes of air in a closed system:
? a -> c adiabatic compression,

THERMODYNAMICS - THEORY

Reversible and Irreversible Process

Examples of Reversible and Irreversible
Processes
Click to View Movie (52 kB)

Internally and Externally Reversible
Processes

The Carnot Cycle

The Carnot Cycle (1-2): Reversible
Isothermal Expansion
Click to View Movie (36 kB)

The Carnot Cycle (2-3): Reversible Adiabatic
Expansion
Click to View Movie (40 kB)

The Carnot Cycle (3-4): Reversible
Isothermal Compression
Click to View Movie (40 kB)

Heat engine operates on a cycle. The efficiency of
heat engine depends on how the individual processes
are executed. The most efficient cycles are reversible
cycles, that is, the processes that make up the cycle
are all reversible processes.
Reversible cycles cannot be achieved in practice.
However, they provide the upper limits on the
performance of real cycles.
Carnot cycle is one of the best-known reversible
cycles. The Carnot cycle is composed of four
reversible processes. Consider an adiabaticpiston-
cylinder device that contains gas. The four reversible
processes that make up the Carnot cycle are as
follows:
? Reversible Isothermal Expansion (process 1-
2):
Heat transfer between the heat source and
the cylinder occurs with an infinitesimal
temperature difference. Hence, it is a
reversible heat transfer process. Gas in the
cylinder expands slowly, does work to its
surroundings, and remains at a constant
temperature TH. The total amount of heat
transferred to the gas during this process is
QH.
? Reversible adiabatic expansion (process 2-3):
The heat source is removed, and the gas
expands in an adiabatic manner. Gas in the
cylinder continues to expand slowly, do work
to its surroundings till the temperature of the
gas drops from TH to TL. Assuming the piston
moves frictionless and the process to be
quasi-equilibrium, the process is reversible as
well as adiabatic.
? Reversible isothermal compression (process
3-4):
The cylinder is brought into contact with a
heat sink at temperature TL. The piston is
pushed by an external force and which does
work on the gas. During the compression, the
gas temperature maintains at TL and the
process is a reversible heat transfer process.
The total amount of heat rejected to the heat

FAQs on Thermodynamics Cycles - Notes, Mechanical Engineering, Semester

1. What is a thermodynamic cycle?

A thermodynamic cycle is a series of processes that a system undergoes to transfer heat and perform work. It is a closed loop path that returns the system to its initial state. These cycles are used to analyze and design various heat engines, refrigeration systems, and power plants.

2. What are the main types of thermodynamic cycles?

The main types of thermodynamic cycles are the Carnot cycle, Rankine cycle, Brayton cycle, and Otto cycle. The Carnot cycle is an idealized cycle that consists of four reversible processes and is used to establish the maximum efficiency that any heat engine operating between two temperature reservoirs can achieve. The Rankine cycle is commonly used in steam power plants and consists of four processes: compression, heat addition, expansion, and heat rejection. It is used to convert heat into mechanical work. The Brayton cycle, also known as the gas turbine cycle, is used in gas turbine engines and consists of four processes: compression, heat addition, expansion, and heat rejection. It is used to convert chemical energy into mechanical work. The Otto cycle is used in spark-ignition engines (e.g., gasoline engines) and consists of four processes: compression, heat addition, expansion, and heat rejection. It is used to convert chemical energy into mechanical work.

3. How does a thermodynamic cycle work?

A thermodynamic cycle works by taking a working fluid through a series of processes that allow it to transfer heat and perform work. The cycle begins with the fluid at a certain initial state, and then it goes through a sequence of changes, such as compression, heating, expansion, and cooling. These processes are carefully designed to maximize efficiency and achieve the desired output, such as mechanical work or cooling.

4. What is the purpose of a thermodynamic cycle?

The purpose of a thermodynamic cycle is to analyze and design systems that involve the transfer of heat and conversion of energy. It allows engineers to determine the efficiency and performance of heat engines, refrigeration systems, and power plants. By studying thermodynamic cycles, engineers can optimize the design and operation of these systems to achieve the desired output and minimize energy losses.

5. What are some real-life applications of thermodynamic cycles?

Thermodynamic cycles have numerous real-life applications. Some examples include: - Steam power plants, which use the Rankine cycle to generate electricity by converting heat from burning fossil fuels into mechanical work. - Gas turbine engines, which use the Brayton cycle to produce thrust in aircraft or generate electricity in gas turbine power plants. - Refrigeration systems, which use the vapor compression cycle to remove heat from a space and provide cooling. - Internal combustion engines, such as gasoline engines, which use the Otto cycle to convert chemical energy into mechanical work in cars and motorcycles.

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