NATURE OF PLASTIC DEFORMATION

NATURE OF PLASTIC DEFORMATION

The change of any dimension or shape of an object under the action of external forces is generally considered as a deformation.

When external forces are applied on an object, then the deformation along the direction of the applied force is called longitudinal deformation where as any deformation along its transverse directions are called as lateral deformations.

When external forces are applied in an object, the object will be deformed first. Due to this deformation crystal structure is also deformed and thus creating an unbalanced internal resisting force, which neutralizes the external force and a condition of equilibrium is achieved as deformation stopped.

When the deformation per unit length is small, the material shows a remarkable ability to recover its original shape and size as the external forces are removed.

Hence, as the external force is withdrawn, the deformation will be vanished.

This type of deformation is called elastic deformation and this property of the material is known as Elasticity.

During the elastic phase of deformation, no permanent change in crystal structure happens, but as the magnitude of the applied force increases, resistance due to the change or distortion of the crystal structure becomes insufficient and as a result crystal dislocation occurs.

Plastic deformation is the deformation which is permanent and beyond the elastic range of the material. Very often, metals are worked by plastic deformation because of the beneficial effect that is imparted to the mechanical properties by it.

The necessary deformation in a metal can be achieved by application of large amount of mechanical force only or by heating the metal and then applying a small force.

The deformation of metals which is caused by the displacement of the atoms is achieved by one or both of the processes called slip and twinning. These two are the prominent  mechanisms  of  plastic  deformation,  namely  slip  and  twinning.

SLIP AND TWINNING

Slip is the prominent mechanism of plastic  deformation in metals. It involves sliding of blocks of crystal over one other along definite  crystallographic planes, called slip planes.

It is analogous to a deck of cards when it is pushed from one end. Slip occurs when shear stress  applied exceeds a critical value. 

During  slip  each  atom  usually  moves  same  integral  number  of  atomic  distances  along  the  slip  plane  producing a  step  but  the  orientation  of  the  crystal remains the same.

 Generally  slip  plane  is  the  plane  of  greatest  atomic  density, and  the  slip  direction  is  the   close  packed  direction  within  the  slip  plane.

Twining : Portion  of  crystal  takes  up  an  orientation  that  is  related  to  the  orientation  of  the  rest  of  the  untwined  lattice  in  a  definite, symmetrical  way.

The  twinned  portion  of  the  crystal  is  a  mirror  image  of  the  parent  crystal.

The plane of symmetry is called twinning plane.

The  important role of twinning in plastic deformation is that it causes changes in plane orientation so that further slip can occur.

On the macroscopic scale when plastic deformation occurs, the metal appears to flow in the solid state along specific directions which are dependent on the type of processing and the direction of applied force.

The crystals or grains of the metal get elongated in the direction of metal flow. This flow of metal can be seen under microscope after polishing and suitable etching of the metal surface. These visible lines are called as “fibre flow lines".

Since the grains are elongated in the direction of flow, they would be able to offer more resistance to stresses acting across them. As a result, the mechanically worked metals called wrought products would be able to achieve better mechanical strength in specific orientation, that of the flow direction.

Since it is possible to control these flow lines in any specific direction by careful manipulation of the applied fibres. It is possible to achieve optimum mechanical properties.

The metal of course, would be weak along the flow lines. The wastage of material in metal working processes is either negligible or very small and the production rate is in general very high. These two factors give rise to the economy in production.

HOT WORKING AND COLD WORKING

The metal working processes are traditionally divided into hot working and cold working processes.

The division is on the basis of the amount of heating applied to the metal before applying the mechanical force. Those processes, working above the recrystallisation temperature, are termed as hot working processes whereas those below are termed as cold working processes.

Under the action of heat and the force, when the atoms reach a certain higher energy level, the new crystals start forming which is termed as recrystallisation.

Recrystallisation destroys the old grain structure deformed by the mechanical working, and entirely new crystals which are strain free are formed.

The grains in fact start nucleating at the points of severest deformation.

Recrystallisation temperature as defined by American Society of Metals is "the approximate minimum temperature at which complete recrystallisation of a cold worked metal occurs within a specified time".

The recrystallisation temperature is generally between one-third to half the melting point of most of the metals. The recrystallisation temperature also depends on the amount of cold work a material has already received. Higher the cold work, lower would be the recrystallisation temperature as shown in Fig. given below.

Though cold work affects the recrystallisation temperature to a great extent, there are other variables which also affect the recrystallisation temperature

In hot working, the process may be carried above the recrystallisation temperature with or without actual heating.

For example, for lead and tin the recrystallisation temperature is below the room temperature and hence working of these metals at room temperature is always hot working. Similarly for steels, the recrystallisation temperature is of the order of 1000^oC, and therefore working below that temperature is still cold working only.

In hot working, the temperature at which the working is completed is important since any extra heat left after working will aid in the grain growth, thus giving poor mechanical properties.

The effect of temperature of completion of hot working is profound. A simple heating where the grain start growing after the metal crosses the recrystallisation temperature. But, if it is cooled without any hot working, the final grain size would be larger than the grain size in the initial stage of heating.

Again, after heating, if the metal is worked before cooling the result is the reduction in size. It is due to the process of recrystallisation, that new grain will be started to form and the final grain size is reduced. This phenomena rises due to working of metal at recrystallisation, that gives rise to a large number of nucleation sites for the new crystals to form.

But if the hot working is completed much above the recrystallisation temperature the grain size start increasing and finally may end up with coarse grain size.

This increase the size of the grains occurs by a process of coalescence of adjoining grains and is a function of time and temperature.

This is not generally desirable. If the hot working is completed just above the recrystallisation temperature, then the resultant grain size would be fine. The same is schematically shown for hot rolling operation.

THERMODYNAMICS: BASIC CONCEPTS AND DEFINITIONS

UNIT- I:

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Fundamental Concepts and Definitions; Terminology, Definition and Scope; Microscopic and Macroscopic Approaches; Engineering Thermodynamics; it's definition and practical applications; Systems and Control volumes; Characteristics of System boundary and Control Surfaces; Surroundings and fixed, moving and imaginary boundaries; Thermodynamic States, state point; identification of a state through properties; definitions and units; extensive, intensive and specific properties, Thermodynamic planes and coordinate systems using properties; Change of state, path and processes; Quasi-static processes; Reversible processes, Restrained and unrestrained processes; Thermodynamic Equilibrium; diathermic wall, Zeroth Law of thermodynamics, Temperature as an important properties.

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Q.1) What is the meaning of Thermodynamics?

Ans:) The branch of science that deals with energy and its movements in the space is generally known as Thermodynamics. The study of this science is based upon experimental values and common experiences and the laws are empirical in thermodynamics.

¤ Introduction:

The most of general sense of thermodynamics is the study of energy and its relationship to the properties of matter. All activities in nature involve some interaction between energy and matter. Thermodynamics is a science that governs the following:

• (i) Energy and its transformation
• (ii) Feasibility of a process involving transformation of energy
• (iii) Feasibility of a process involving transfer of energy
• (iv) Equilibrium processes

More specifically, thermodynamics deals with energy conversion, energy exchange and the direction of exchange.

¤ Areas of Application of Thermodynamics:

All natural processes are governed by the principles of thermodynamics. However, the following engineering devices are typically designed based on the principles of thermodynamics.

Automotive engines, Turbines, Compressors, Pumps, Fossil and Nuclear Power Plants, Propulsion systems for the Aircrafts, Separation and Liquefaction Plant, Refrigeration, Air-conditioning and Heating Devices.

The principles of thermodynamics are summarized in the form of a set of axioms. These axioms are known as four thermodynamic laws:

• Zeroth law of thermodynamics,
• First law of thermodynamics,
• Second law of thermodynamics, and
• Third law of thermodynamics.

The Zeroth Law deals with thermal equilibrium and provides a means for measuring temperatures.

The First Law deals with the conservation of energy and introduces the concept of internal energy.

The Second Law of thermodynamics provides with the guidelines on the conversion of internal energy of matter into work. It also introduces the concept of entropy.

The Third Law of thermodynamics defines the absolute zero of entropy. The entropy of a pure crystalline substance at absolute zero temperature is zero.


¤ Different Approaches in the Study of Thermodynamics:

There are two ways through which the subject of thermodynamics can be studied

• Macroscopic Approach
• Microscopic Approach

¤ Macroscopic Approach:

Consider a certain amount of gas in a cylindrical container. The volume (V) can be measured by measuring the diameter and the height of the cylinder. The pressure (P) of the gas can be measured by a pressure gauge. The temperature (T) of the gas can be measured using a thermometer. The state of the gas can be specified by the measured P, V and T . The values of these variables are space averaged characteristics of the properties of the gas under consideration. In classical thermodynamics, we often use this macroscopic approach. The macroscopic approach has the following features.

• The structure of the matter is not considered.
• A few variables are used to describe the state of the matter under consideration. The values of these variables are measurable following the available techniques of experimental physics.

¤ Microscopic Approach:

On the other hand, the gas can be considered as assemblage of a large number of particles each of which moves randomly with independent velocity. The state of each particle can be specified in terms of position coordinates ( x_i , y_i , z_i ) and the momentum components ( p_xi , p_yi , p_zi ). If we consider a gas occupying a volume of 1 cm³ at ambient temperature and pressure, the number of particles present in it is of the order of 10²⁰. The same number of position coordinates and momentum components are needed to specify the state of the gas. The microscopic approach can be summarized as:

• A knowledge of the molecular structure of matter under consideration is essential.
• A large number of variables are needed for a complete specification of the state of the matter.

¤ Zeroth Law of Thermodynamics: 

This is one of the most fundamental laws of thermodynamics. It is the basis of temperature and heat transfer between two systems. Suppose we take three thermodynamic system named System A, System B and System C. Now let that system A is in thermal equilibrium with system B. By thermal equilibrium we mean that there is no heat transfer between system A and system B when they are brought in contact with each other. Now, suppose system A is in thermal equilibrium with system C too and there is no contact between system B and system C. It implies that although system B and C are isolated from each other, they will remain at thermal equilibrium to each other. It means that there will be no heat transfer between system B and C, when they are brought in contact with each other. This is called the Zeroth Law of thermodynamics.


¤ Basis of Temperature: 

When two bodies are kept at contact with each other and if there is no heat transfer between them we say that their body temperatures are same. It means that temperature is the property of a system which decides whether there will be any heat transfer between two different bodies. Heat transfer always occur from a higher temperature body to a lower temperature body. Further whenever there is any heat inflow to a body, it raises its temperature and conversely, if heat outflow occurs from a system it lowers its temperature.

Suppose we take two bodies one of which is at higher temperature than the other. Now when we bring the bodies at contact, heat will be transformed from a higher temperature body to that of lower temperature. Then what will be its effect, we may ask as a result of this heat transfer? Is this heat transfer a perpetual process? Our common life experiences tell us that it will not be the case. Although, at first heat transfer will take place, but its amount will be gradually decreased and after some time, a situation will come when there will be no heat transfer between the bodies or the bodies will come to a state of thermal equilibrium with each other. So, what is the reason for that? Can we justify the situation?

Yes, we can justify it as the hotter body releases heat to the colder body, the temperature of the hotter body decreases where as the temperature of the colder body increases and after sufficient time both the bodies will have equal temperature and a state of thermal equilibrium will be achieved.


¤ Temperature Measurement: 

We know the temperature of a body can be measured with a thermometer. How can we actually calculate the temperature of a body with the help of thermodynamics?


¤ Thermometer:

A thermometer is a temperature measuring instrument. It is made of a thin capillary glass tube, one end is closed and the other end is fitted with metallic bulb full of mercury. The mercury is in thermal equilibrium with the metallic bulb. Therefore, the temperature of the mercury is equal to the temperature of the metallic bulb. 
Mercury has a good coefficient of volume expansion and it means that as the temperature of the mercury increases, its volume increases too and as a result mercury column inside the capillary rises up. 

The capillary tube has been graduated with the help of calibrating with standard temperature sources. Therefore, the temperature of the mercury can be measured from the height of mercury column as the tube is finely graduated. 

Whenever we want to measure the temperature of a body, we kept the body in contact with the metallic bulb of the thermometer. When thermal equilibrium is established between the body and the metallic bulb of the thermometer, the temperature of both the body will be equal again the metallic bulb is in thermal equilibrium with mercury then the temperature of the mercury will be equal to the temperature of the metallic bulb and the temperature of the object.


As we can measure the temperature of the mercury from the column height, hence we can also determine the temperature of the object as they are equal to each other.

DISCUSSION:
Microscopic basis of temperature and pressure:
Here we shall try to discuss the basis of temperature and pressure only qualitatively, without any mathematical expression. 






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Thermodynamic Systems: 


If we want to analyze movement of energy over space, then we must define the space that would be used for the observation, we would call it as a System, separated from the adjoining space that is known as "Surroundings", by a boundary that may be real or may be virtual depending upon the nature of the observation. The boundary is called as System Boundary. So, we shall now define a system properly.


A thermodynamics system refers to a three dimensional space occupied by a certain amount of matter known as ''Working Substance'', and it is the space under consideration. It must be bounded by an arbitrary surface which may be real or imaginary, may be at rest or in motion as well as it may change its size and shape. All thermodynamic systems contain three basic elements:


• System boundary: The imaginary surface that bounds the system.
• System volume: The volume within the imaginary surface.
• The surroundings: The surroundings are everything external to the system.


So we get a space of certain volume where Energy Transfer (movement of energy) is going on, what may or may not be real, and distinct, it may be virtual (in case of flow system ), again if real boundary exists, then it may be fixed (rigid boundary like constant volume system) or may be flexible (like cylinder-piston assembly). For a certain experiment the system and surroundings together is called Universe.

The interface between the system and surroundings is called as "System boundary", which may be real and distinct in some cases where as some of them are virtual, but it may be real, solid and distinct. If the air in this room is the system, the floor, ceiling and walls constitutes real boundaries. The plane at the open doorway constitutes an imaginary boundary.



Classification of Thermodynamic Systems:

Systems can be classified as being (i) closed, (ii) open, or (iii) isolated.

(i) Closed System:

A thermodynamic system may exchange mass and energy with its surroundings. There are systems which allow only energy transfer with surroundings in the form of either heat transfer or work transfer or both heat and work transfer between a system and its surroundings. In these types of system, any sorts of mass transfer between the system and its surroundings are prohibited. These types of systems are classified as closed system. Examples of closed thermodynamic systems include a fluid being compressed by a piston inside a cylinder, a bomb calorimeter. In a closed system although energy content may vary over a period of time, but the system will always contain the same amount of matter.

(ii) Open System or Control Volume: 

An open system is a region in space defined by a boundary across which matter may flow in addition to work and heat exchange between the system and the surroundings. So, in an open system, the boundaries must have one or more opening through which mass transfer may take place in addition to work and heat transfer. Most of the engineering devices are examples of open system. Some examples are (a) a gas expanding from a container through a nozzle, (b) steam flowing through a turbine, and (c) water entering a boiler and leaving as steam. The boundary of an open system may be real or imaginary and it is called as control surface. The space inside an open system is called as control volume.

(iii) Isolated System:  

In an isolated system, there is no interaction between a system and its surroundings. Hence, the quantities of mass and energy in these types of system doesn’t change with time or we can say mass and energy remain constant in an isolated system. If there is no change in energy of a system, it indicates that there is neither any kind of heat transfer nor any kind of work transfer.  Our universe as a whole can be regarded as an isolated system.

Property, Equilibrium and State: 

A property is any measurable characteristic of a system. The common properties include: 

• pressure (P)

• temperature (T)

• volume (V)

• velocity (v)

• mass (m)

• enthalpy (H)

• entropy (S)

Properties can be intensive or extensive. Intensive properties are those whose values are independent of the mass possessed by the system, such as pressure, temperature, and velocity. Extensive properties are those whose values are dependent of the mass possessed by the system, such as volume, enthalpy, and entropy. 

Extensive properties are denoted by uppercase letters, such as volume (V), enthalpy (H) and entropy (S). Per unit mass of extensive properties are called specific properties and denoted by lowercase letters. For example, specific volume v = V/m, specific enthalpy h = H/m and specific entropy s = S/m 

*Note that work and heat are not properties. They are dependent of the process from one state to another state.

When the properties of a system are assumed constant from point to point and there is no change over time, the system is in a thermodynamic equilibrium.

The state of a system is its condition as described by giving values to its properties at a particular instant. For example, gas is in a tank. At state 1, its mass is 2 kg, temperature is 160°C, and volume is 0.1 m3. At state 2, its mass is 1 kg, temperature is 80°C, and volume is 0.2  m³..

A system is said to be at steady state if none of its properties changes with time.

State:

It is the condition of a system as defined by the values of all its properties. It gives a complete description of the system. Any operation in which one or more properties of a system change is called a change of state.

Phase:

It is a quantity of mass that is homogeneous throughout in chemical composition and physical structure. Examples of phase are solid, liquid, vapour, gas. Phase consisting of more than one phase is known as heterogenous system, where as if it consists of only one phase, it is called as homogenous system.

Process, Path and Cycle: 

The changes that a system undergoes from one equilibrium state to another are called a process. The series of states through which a system passes during a process is called path.

In thermodynamics the concept of quasi-equilibrium processes is used. It is a sufficiently slow process that allows the system to adjust itself internally so that its properties in one part of the system do not change any faster than those at other parts.

When a system in a given initial state experiences a series of quasi-equilibrium processes and returns to the initial state, the system undergoes a cycle. For example, the piston of car engine undergoes Intake stroke, Compression stroke, Combustion stroke, Exhaust stroke and goes back to Intake again. It is a cycle.

Quasi-static Processes:

Although the processes can be restrained or unrestrained, in practical purpose we need restrained processes.

A quasi-static process is one in which,

• The deviation from thermodynamic equilibrium is infinitesimal.

• All states of the system passes through are equilibrium states.

In a cylinder-piston assembly, several small weights are placed on the piston as shown in the figure. If we remove a weight, the pressure on the enclosed gas will be reduced by an infinitesimal amount. If we remove these weights one by one very slowly, then the pressure on the gas will be reduced by very small amount very slowly. Every time we remove a weight, the equilibrium state will be changed to a new equilibrium state at a very slow rate, such that the system will be appeared at a static condition as the change is infinitesimally small and the rate of change is also very small. The path of the change will be a series of quasi-equilibrium states. These types of processes are known as quasi-static processes.  

Equilibrium States:

A system is said to be in an equilibrium state if its properties will not be changed without some perceivable effect in the surroundings.

• Equilibrium generally requires all properties to be uniform throughout the system.

• There are mechanical, thermal, phase, and chemical equilibrium.

Nature has a preferred way of directing changes. As examples, we can say,

• Water flows from a higher to a lower level

• Electricity flows from a higher potential to a lower one

• Heat flows from a body at higher temperature to the one at a lower temperature

• Momentum transfer occurs from a point of higher pressure to a lower one.

• Mass transfer occurs from higher concentration to a lower one


Equilibrium state will be achieved when there will not be any change of the values of the properties of a system. Neither the system will exchange 
Heat Energy nor any Work exchange nor any kind of mass exchange with its surroundings. There are mainly three kind of Equilibrium and they are as follows.

* Thermal Equilibrium
* Mechanical Equilibrium
* Chemical Equilibrium


Thermal Equilibrium: 

When two bodies are in contact, there will be heat exchange between the bodies if and only there exists a temperature difference (ΔT) between the bodies.

Due to the temperature difference between the bodies, heat will flow from the high temperature body to the low temperature body. 

As a result of this heat transfer, the temperature of the hot body will be decreased and the temperature of the cold body will be increased.

When the temperature of both the bodies becomes equal to each others, the flow of heat stops. This equilibrium condition is known as the Thermal Equilibrium. 


Mechanical Eqiilibrium : 

If there exists a pressure gradient (ΔP) inside a system, between two systems or between a system and its surroundings, then the interface surface will experience a net force not equal to zero and due to which work transfer will happen where the system having higher pressure will do work against the lower pressure system. 

Due to this work transfer, pressure of the high pressure system will be decreased as energy has flown out of the system. On the other hand, the pressure in the low pressure system will be increased. When the pressure becomes equal in both sides, the work energy flow will be stopped and this state is known as the state of Mechanical Equilibrium.;

Chemical Equilibrium:

If there exists a chemical potential (Δμ) within the components of the system or between the system and surroundings, then there will be a spontaneous chemical reaction which will try to neutralize the chemical potential, after sometimes when the chemical potential becomes zero, the reaction stops and then there will not be any more changes in chemical properties of the system. This condition is called Chemical Equilibrium.

When a system attains thermal, mechanical and chemical equilibrium simultaneously, the state of the system is called in a "THERMODYNAMIC EQUILIBRIUM".

THE IMPORTANCE OF MANUFACTURING ENGINEERING

If we carefully think about human civilization, one shall notice an wonderful fact about human beings. The thing that made us different from other hominids is the skill to manufacture tools. We just triumphed due to our ability to make primitive tools out of stone and metals during the dawn of the civilizations. Since then much time has passed and we have entered into a Machine Era and man has been still continuously engaged in converting the natural resources into useful products by adding value to them through machining and other engineering activities applying on the raw materials. Manufacturing is the sub branch of Engineering which involves the conversion of raw materials into finished products.

The conversion of natural resources into raw materials is normally taken care of by two sub branches of engineering viz. Mining and Metallurgy Engineering. The value addition to the raw materials by shaping and transforming it to final products generally involves several distinct processes like castings, forming, forging, machining, joining, assembling and finishing to obtain a completely finished product.

Understanding Manufacturing Engineering largely based upon three engineering activities and they are Designing,  Production and Development of new more efficient techniques.

At the Design stage, engineering design mainly concentrates on the optimization of engineering activities to achieve most economical way to manufacture a goods from raw materials. It also chooses the raw materials and impart the requisite engineering properties of materials like hardness, strength, elasticity, toughness by applying various heat treatment to them.

During the production stages, the selection of the important process parameters to minimize the idle time and cost, and maximizing the production and its quality is very important.

The New Technologies must be implemented to adapt to the changing scenarios of the markets and demands to make the sales competitive and sustainable.

ASSUMPTIONS CONSIDERED IN ANALYZING AIR STANDARD CYCLE:

AIR STANDARD CYCLE:

• In true sense, internal combustion engines in which combustion of fuels occurs inside the engine cylinder can not be defined as cyclic heat engines. The temperature generated during combustion is very high so that engines must be water cooled to prevent the damage of the engine due to thermal shock. The working fluid here is a mixture of air and fuel that undergoes permanent chemical changes due to combustion and the products of combustions must be exhausted and driven out of the cylinder so that fresh charges can be admitted. Therefore, it does not complete a full thermodynamic cycle.
• The engine cycle analysis is an important tool in the design and study of
• Internal Combustion Engines. 
•  A thermodynamic cycle is defined as a series of processes through which the working fluid progresses and ultimately return to the original state. 
•  Although the thermodynamic cycles are closed cycles and actual engine 
• A real thermodynamic analysis of such an engine quite complex. Hence, we simplified the operation of an I.C. Engine by introducing somewhat idealized version of a real thermodynamic processes occur inside an IC Engine, and this idealized thermodynamic cycles are called "Air standard cycle." In an air standard cycle, a certain mass of a perfect gas like air operates in a complete thermodynamic cycle, where heat is added and rejected reversibly with external heat reservoirs, and all the processes in the cycle are reversible. Air is assumed to behave like a perfect gas, and like a perfect gas, its specific heats are assumed to be constant (although they are certain functions of temperature). These air standard cycles are conceived in such a manner that they may correspond to the operations of internal combustion engines.
•  Although, there are numerous such air standard cycles, the important of them are

a) Otto Cycle (used for petrol engine)

b) Diesel Cycle (used for diesel engine)

c) Mixed, limited pressure or Dual Cycle (used for hot spot engine)

d) Stirling Cycle

e) Ericsson Cycle

To make the analysis simpler, certain assumptions are made during the analysis of air standard cycle. They are as following,

• i) The working substance is a perfect gas obeying the gas equation pV = mRT.
• ii) The working fluid is a fixed mass of air either contained in a closed system or flowing at a constant rate round a closed cycle.
• iii) The physical constants of the working fluid will be those of air.
• iv) The working medium has constant specific heats.
• v) The working media doesn't undergo any chemical change throughout the cycle.

OTTO CYCLE:

The Otto cycle is a thermodynamic cycle used in gasoline (petrol) engines to convert the chemical energy stored in fuel into useful work. It is a four-stroke cycle, consisting of four processes: intake, compression, combustion, and exhaust.

During the intake stroke, the fuel-air mixture is drawn into the engine cylinder as the piston moves downward. During the compression stroke, the mixture is compressed by the upward motion of the piston, which raises the temperature and pressure of the mixture. Near the end of the compression stroke, the spark plug ignites the mixture, causing a rapid combustion that generates a high-pressure wave that drives the piston downward, producing power. This is the power stroke. Finally, during the exhaust stroke, the spent gases are expelled from the cylinder as the piston moves upward.

The Otto cycle is an idealized model of the engine, assuming that the combustion occurs instantaneously and that there are no losses due to friction, heat transfer, or other factors. In practice, real engines operate less efficiently than the idealized model, due to these losses.

The Otto cycle is named after its inventor, Nikolaus Otto, a German engineer who patented the four-stroke engine in 1876. The cycle is widely used in modern gasoline engines, which have been refined and optimized over more than a century of development to achieve high levels of performance, efficiency, and reliability.

DIESEL CYCLE:

The Diesel cycle is a thermodynamic cycle used in diesel engines to convert the chemical energy stored in fuel into useful work. It is a four-stroke cycle, consisting of four processes: intake, compression, combustion, and exhaust.

During the intake stroke, air is drawn into the engine cylinder as the piston moves downward. During the compression stroke, the air is compressed by the upward motion of the piston, which raises the temperature and pressure of the air. Near the end of the compression stroke, fuel is injected into the cylinder, which ignites due to the high temperature and pressure of the air. The fuel-air mixture combusts, generating a high-pressure wave that drives the piston downward, producing power. This is the power stroke. Finally, during the exhaust stroke, the spent gases are expelled from the cylinder as the piston moves upward.

The Diesel cycle is similar to the Otto cycle but differs in that it does not rely on a spark plug to ignite the fuel. Instead, the fuel is injected directly into the cylinder and ignites due to the heat of the compressed air. This allows diesel engines to operate at a higher compression ratio than gasoline engines, which leads to higher efficiency and better fuel economy.

The Diesel cycle is named after Rudolf Diesel, a German inventor who patented the diesel engine in 1892. Diesel engines are widely used in a variety of applications, including cars, trucks, buses, ships, and generators. They are known for their efficiency, durability, and reliability.

Mixed, limited pressure or Dual Cycle (used for hot spot engine):

The Mixed or Dual Cycle is a thermodynamic cycle used in hot-spot engines, which are a type of internal combustion engine that combines elements of diesel and gasoline engines. The cycle is also sometimes referred to as the Limited Pressure cycle.

The Dual Cycle is a combination of the Otto and Diesel cycles. It uses the diesel combustion process, where fuel is injected directly into the cylinder and ignited by the heat of compressed air, but also includes a spark plug like in the Otto cycle. During the intake stroke, air is drawn into the cylinder, and during the compression stroke, the air is compressed to a higher pressure and temperature than in the Otto cycle. Fuel is injected into the cylinder, and the spark plug ignites the fuel-air mixture, creating a flame that spreads through the cylinder. The combustion of the fuel-air mixture produces high pressure and temperature, which drives the piston downward, producing power. Finally, during the exhaust stroke, the spent gases are expelled from the cylinder as the piston moves upward.

The Dual Cycle is designed to provide the advantages of both the diesel and gasoline engines, namely high efficiency and low emissions. It allows for a higher compression ratio than the Otto cycle, which leads to better fuel economy, while also reducing the emission of pollutants like nitrogen oxides (NOx) and particulate matter. The Dual Cycle is used in some specialized applications, such as large marine engines and certain military vehicles. However, it is not as widely used as the Otto and Diesel cycles in most everyday applications.

STERLING CYCLE:

The Stirling cycle is a thermodynamic cycle used in Stirling engines, which are a type of heat engine that converts heat energy into mechanical work. Unlike traditional internal combustion engines, Stirling engines operate on an external heat source, which can be supplied by any fuel source that can produce heat, such as wood, coal, or natural gas.

The Stirling cycle consists of four processes: heating, expansion, cooling, and compression. During the heating process, the working fluid (typically a gas such as helium or hydrogen) is heated by an external heat source, causing it to expand and drive a piston outward. During the expansion process, the expanding gas continues to drive the piston outward, producing mechanical work. During the cooling process, the working fluid is cooled by a heat sink (usually air or water), causing it to contract and pull the piston inward. Finally, during the compression process, the compressed gas is pushed back to the starting point, ready to begin the cycle again.

The Stirling cycle is designed to maximize efficiency by minimizing the losses associated with traditional internal combustion engines, such as friction and heat transfer. However, Stirling engines have a relatively low power-to-weight ratio and are less suitable for high-speed applications. They are typically used in specialized applications, such as in submarines, where quiet operation and long running times are important.

The Stirling engine was invented in the early 19th century by Robert Stirling, a Scottish clergyman, and engineer. Despite its potential benefits, the Stirling engine has not been widely adopted in mainstream applications due to its complexity and high cost compared to other types of engines. However, research and development continue to explore ways to improve the efficiency and practicality of Stirling engines.

FUEL USED IN IC ENGINES AND REFINERY PROCCESSES; EME-505

FUEL USED IN IC ENGINES

An article on fossil fuels

Internal Combustion Engines are the generators of the energy mainly used for transportation. Almost more than 90% of the total IC Engines run on fossil fuels or different derivatives of petroleum.

IC Engines are a kind of open cycle heat engine where heat is supplied to the engine by the combustion of working fluids thus releasing huge amount of energy due to the combustion processes of the working fluids. Combustible working fluids are called fuels.

The natural petroleum oil is the largest single source of internal combustion engine fuels. Petrol and Diesel are the most used among them. The boiling point of petrol is 30°C to 200°C and that of diesel oil is from 200°C to 375°C.

Fuels of most of the IC Engines are the derivatives of Petroleum like gasoline, diesel oil, kerosene, jet fuel etc. All of these fuels are produced during the fractional distillation of Petroleum Oil obtained from crude from oil wells.

The fuels used in the IC Engines are designed to satisfy the performance requirements of the engine system in which they are used. As a result the fuels must have certain

• (i) physical,
• (ii) chemical and
• (iii) combustion properties.

Following are the some characteristics a fuel must have in order to produce the desirable output to the engine performance.

1. A fuel must have a large energy density to be capable to release huge amount of energy during its combustion in side the combustion chamber.
2. A fuel must posses a good combustion quality to produce large amount of energy in smooth way.
3. A fuel must have high thermal stability or pre-ignition may occur.
4. A fuel must show a low deposit forming tendency else gum formation and other deposit forming processes will hamper the combustion process.
5. A fuel must be non-toxic, easy to handle and storage.

CRUDE PETROLEUM OIL:

Petroleum or often referred as "Crude Oil" is a naturally occurring inflammable mixtures of liquid and mud and it contains a complex mixture of different hydrocarbons of various molecular weights. It is mainly recovered through a process called "Oil Drilling".

Oil Wells and Gas Wells:

An oil well produces mainly crude oil with some natural gas dissolved in it. In contrast a gas well produces natural gases although it may contain heavier hydrocarbons like pentane, hexane or hepthane in gaseous state due to the extreme pressure and temperature inside the well, but at surface conditions condensation starts and forms "Natural Gas Condensate" or simply known as Condensate.



COMPOSITIONS OF CRUDE WELL:

Basically, crude well is the muddy mixtures of different hydrocarbons of different molecular weights. Alkanes, Cyclo-alkanes or napthenes, aromatics. It contains nitrogen, oxygen, sulfur and phosphorous. It may also contains metallic compounds too.

Four different types of hydrocarbon molecules appear in crude oil. The relative percentages are widely varied from oil to oil. They are:

• i) Paraffins (alkanes,  C_nH_{2n + 2} )
• ii) Olefins (alkenes, C_nH_2n),
• iii) Napthenes (cyclo-alkanes, C_nH_2n ),
• iv) Aromatics (having benzene ring, C_nH_{2n - 6}).

It is then refined by fractional distillation in oil refinery to obtain a large number of consumer products, from petrol or gasoline, diesel to kerosene, heavy oil, fuel oil, asphalt, chemical reagents, plastics etc.

Most of the derivatives of the petroleum have been used as fuel or heating purpose. The major products of a petroleum refinery are:

• (i) Gasoline,
• (ii) Kerosene,
• (iii) Diesel Oil,
• (iv) Fuel oil,
• (v) Heavy Oil,
• (vi) Lubricating Oil,
• (vii) Asphalts

INTRODUCTION: 

As the demands for gasoline, kerosene/ jet fuel and diesel oil are maximum, refineries around the world have started to convert heavy fuels and other higher hydrocarbons into gasoline, kerosene and diesel oil. To perform this, refineries have adopted several thermo-chemical processes those can convert high molecular weight hydrocarbons into lighter ones by breaking them.

GENERAL REFINERY PROCESSES:

Petroleum refining has evolved continuously in response to changing consumer demand for better and different products. The original requirement was to produce kerosene as a cheaper and better source of light than whale oil. The development of the internal combustion engine led to the production of gasoline and diesel fuels. The evolution of the airplane created an initial need for high-octane aviation gasoline and then for jet fuel, a sophisticated form of the original product, kerosene. Present-day refineries produce a variety of products including many required as feedstock for the petrochemical industry.

a) Distillation Processes:

The first refinery, opened in 1861, produced kerosene by simple atmospheric distillation. Its by-products included tar and naphtha. It was soon discovered that distilling petroleum under vacuum could produce high-quality lubricating oils. However, for the next 30 years kerosene was the product consumer wanted. Two significant events changed this situation. The invention of the electric light decreased the demand for kerosene and the invention of the internal combustion engine created a demand for diesel fuel and gasoline (naphtha). 

b) Thermal Cracking Processes:

With the advent of mass production and World War I, the number of gasoline-powered vehicles increased dramatically and the demand for gasoline grew accordingly. However, distillation processes produced only a certain amount of gasoline from crude oil. In 1913, the thermal cracking process was developed, which subjected heavy fuels to both pressure and intense heat, physically breaking the large molecules into smaller ones to produce additional gasoline and distillate fuels. Visbreaking, another form of thermal cracking, was developed in the late 1930's to produce more desirable and valuable products. 

c) Catalytic Processes:

Higher-compression gasoline engines required higher-octane gasoline with better antiknock characteristics. The introduction of catalytic cracking and polymerization processes in the mid- to late 1930's met the demand by providing improved gasoline yields and higher octane numbers.   Alkylation, another catalytic process developed in the early 1940's, produced more high-octane aviation gasoline and petrochemical feedstock for explosives and synthetic rubber. Subsequently, catalytic isomerization was developed to convert hydrocarbons to produce increased quantities of alkylation feedstock. Improved catalysts and process methods such as hydrocracking and reforming were developed throughout the 1960's to increase gasoline yields and improve antiknock characteristics. These catalytic processes also produced hydrocarbon molecules with a double bond (alkenes) and formed the basis of the modern petrochemical industry. 

d) Treatment Processes:

Throughout the history of refining, various treatment methods have been used to remove non-hydrocarbons, impurities, and other constituents that adversely affect the properties of finished products or reduce the efficiency of the conversion processes. Treating can involve chemical reaction and/or physical separation. Typical examples of treating are chemical sweetening, acid treating, clay contacting, caustic washing, hydrotreating, drying, solvent extraction, and solvent dewaxing. Sweetening compounds and acids desulfurize crude oil before processing and treat products during and after processing. 

Following the Second World War, various reforming processes improved gasoline quality and yield and produced higher-quality products. Some of these involved the use of catalysts and/or hydrogen to change molecules and remove sulfur. 

 Basics of Hydrocarbon Chemistry:

Crude oil is a mixture of hydrocarbon molecules, which are organic compounds of carbon and hydrogen atoms that may include from one to 60 carbon atoms. The properties of hydrocarbons depend on the number and arrangement of the carbon and hydrogen atoms in the molecules. The simplest hydrocarbon molecule is one carbon atom linked with four hydrogen atoms: methane. All other variations of petroleum hydrocarbons evolve from this molecule. 

 

Hydrocarbons containing up to four carbon atoms are usually gases, those with 5 to 19 carbon atoms are usually liquids and those with 20 or more are solids. The refining process uses chemicals, catalysts, heat, and pressure to separate and combine the basic types of hydrocarbon molecules naturally found in crude oil into groups of similar molecules. The refining process also rearranges their structures and bonding patterns into different hydrocarbon molecules and compounds. Therefore it is the type of hydrocarbon (paraffinic, naphthenic, or aromatic) rather than its specific chemical compounds that is significant in the refining process. 

Principal Groups of Hydrocarbon

• Paraffins - The paraffinic series of hydrocarbon compounds found in crude oil have the general formula C_nH_2n+2 and can be either straight chains (normal) or branched chains (isomers) of carbon atoms. The lighter, straight chain paraffin molecules are found in gases and paraffin waxes. Examples of straight-chain molecules are methane, ethane, propane, and butane (gases containing from one to four carbon atoms), and pentane and hexane (liquids with five to six carbon atoms). The branched-chain (isomer) paraffins are usually found in heavier fractions of crude oil and have higher octane numbers than normal paraffins. These compounds are saturated hydrocarbons, with all carbon bonds satisfied, that is, the hydrocarbon chain carries the full complement of hydrogen atoms.
  • Example of simplest hydrocarbon molecule: Methane (CH₄), Examples of straight chain paraffin molecule (Butane) and branched paraffin molecule (Isobutane) with same chemical formula (C₄H₁₀)
    
    
    
    
• Aromatics - Aromatics are unsaturated ring-type (cyclic) compounds which react readily because they have carbon atoms that are deficient in hydrogen. All aromatics have at least one benzene ring (a single-ring compound characterized by three double bonds alternating with three single bonds between six carbon atoms) as part of their molecular structure. Naphthalenes are fused double-ring aromatic compounds. The most complex aromatics, polynuclears (three or more fused aromatic rings), are found in heavier fractions of crude oil.
  • Example of simple aromatic compound: Benzene (C₆H₆), Examples of simple double-ring aromatic compound: Naphthalene (C₁₀H₈)
    
    
    
    
• Naphthenes - Naphthenes are saturated hydrocarbon groupings with the general formula C_nH_2n, arranged in the form of closed rings (cyclic) and found in all fractions of crude oil except the very lightest. Single-ring naphthenes (monocycloparaffins) with five and six carbon atoms predominate, with two-ring naphthenes (dicycloparaffins) found in the heavier ends of naphtha.
  • Example of typical single-ring naphthene: Cyclohexane (C₆H₁₂), Examples of naphthene with same chemical formula (C₆H₁₂) but different molecular structure: Methyl cyclopentane (C₆H₁₂)

Other Hydrocarbons

• Alkenes - Alkenes are mono-olefins with the general formula C_nH_2n and contain only one carbon-carbon double bond in the chain. The simplest alkene is ethylene, with two carbon atoms joined by a double bond and four hydrogen atoms. Olefins are usually formed by thermal and catalytic cracking and rarely occur naturally in unprocessed crude oil.
  • Example of simples Alkene: Ethylene (C₂H₄), Typical Alkenes with the same chemical formula (C₄H₈) but different molecular structures: 1-Butene and Isobutene
    
    
    
    
• Dienes and Alkynes - Dienes, also known as diolefins, have two carbon-carbon double bonds. The alkynes, another class of unsaturated hydrocarbons, have a carbon-carbon triple bond within the molecule. Both these series of hydrocarbons have the general formula CnH2n-2. Diolefins such as 1,2-butadiene and 1,3-butadiene, and alkynes such as acetylene,occur in C5 and lighter fractions from cracking. The olefins, diolefins, and alkynes are said to be unsaturated because they contain less than the amount of hydrogen necessary to saturate all the valences of the carbon atoms. These compounds are more reactive than paraffins or naphthenes and readily combine with other elements such as hydrogen, chlorine, and bromine.
  • Example of simplest Alkyne: Acetylene (C₂H₂), Typical Diolefins with the same chemical formula (C₄H₆) but different molecular structures: 1,2-Butadiene and 1,3-Butadiene

Non-hydrocarbons

• Sulfur Compounds -  Sulfur may be present in crude oil as hydrogen sulfide (H₂S), as sulfur compounds such as mercaptans, sulfides, disulfides, thiophenes, etc. or as elemental sulfur. Each crude oil has different amounts and types of sulfur compounds, but as a rule the proportion, stability, and complexity of the compounds are greater in heavier crude-oil fractions. Hydrogen sulfide is a primary contributor to corrosion in refinery processing units. Other corrosive substances are elemental sulfur and mercaptans. Moreover, the corrosive sulfur compounds have an obnoxious odor.  Pyrophoric iron sulfide results from the corrosive action of sulfur compounds on the iron and steel used in refinery process equipment, piping, and tanks. The combustion of petroleum products containing sulfur compounds produces undesirables such as sulfuric acid and sulfur dioxide. Catalytic hydrotreating processes such as hydrodesulfurization remove sulfur compounds from refinery product streams. Sweetening processes either remove the obnoxious sulfur compounds or convert them to odorless disulfides, as in the case of mercaptans.
  
  
• Oxygen Compounds -  Oxygen compounds such as phenols, ketones, and carboxylic acids occur in crude oils in varying amounts. 
  
  
• Nitrogen Compounds -  Nitrogen is found in lighter fractions of crude oil as basic compounds, and more often in heavier fractions of crude oil as nonbasic compounds that may also include trace metals such as copper, vanadium, and/or nickel. Nitrogen oxides can form in process furnaces. The decomposition of nitrogen compounds in catalytic cracking and hydrocracking processes forms ammonia and cyanides that can cause corrosion. 
  
  
• Trace Metals -  Metals, including nickel, iron, and vanadium are often found in crude oils in small quantities and are removed during the refining process. Burning heavy fuel oils in refinery furnaces and boilers can leave deposits of vanadium oxide and nickel oxide in furnace boxes, ducts, and tubes. It is also desirable to remove trace amounts of arsenic, vanadium, and nickel prior to processing as they can poison certain catalysts. 
  
  
• Salts -  Crude oils often contain inorganic salts such as sodium chloride, magnesium chloride, and calcium chloride in suspension or dissolved in entrained water (brine). These salts must be removed or neutralized before processing to prevent catalyst poisoning, equipment corrosion, and fouling. Salt corrosion is caused by the hydrolysis of some metal chlorides to hydrogen chloride (HCl) and the subsequent formation of hydrochloric acid when crude is heated. Hydrogen chloride may also combine with ammonia to form ammonium chloride (NH₄Cl), which causes fouling and corrosion. 
  
  
• Carbon Dioxide -  Carbon dioxide may result from the decomposition of bicarbonates present in or added to crude, or from steam used in the distillation process. 
• Naphthenic Acids -  Some crude oils contain naphthenic (organic) acids, which may become corrosive at temperatures above 450° F when the acid value of the crude is above a certain level.

 Major Refinery Products

• Gasoline. The most important refinery product is motor gasoline, a blend of hydrocarbons with boiling ranges from ambient temperatures to about 400 °F. The important qualities for gasoline are octane number (antiknock), volatility (starting and vapor lock), and vapor pressure (environmental control). Additives are often used to enhance performance and provide protection against oxidation and rust formation.
• Kerosene. Kerosene is a refined middle-distillate petroleum product that finds considerable use as a jet fuel and around the world in cooking and space heating. When used as a jet fuel, some of the critical qualities are freeze point, flash point, and smoke point. Commercial jet fuel has a boiling range of about 375°-525° F, and military jet fuel 130°-550° F. Kerosene, with less-critical specifications, is used for lighting, heating, solvents, and blending into diesel fuel.
• Liquified Petroleum Gas (LPG). LPG, which consists principally of propane and butane, is produced for use as fuel and is an intermediate material in the manufacture of petrochemicals. The important specifications for proper performance include vapor pressure and control of contaminants.
• Distillate Fuels. Diesel fuels and domestic heating oils have boiling ranges of about 400°-700° F. The desirable qualities required for distillate fuels include controlled flash and pour points, clean burning, no deposit formation in storage tanks, and a proper diesel fuel cetane rating for good starting and combustion.
• Residual Fuels. Many marine vessels, power plants, commercial buildings and industrial facilities use residual fuels or combinations of residual and distillate fuels for heating and processing. The two most critical specifications of residual fuels are viscosity and low sulfur content for environmental control.
• Coke and Asphalt. Coke is almost pure carbon with a variety of uses from electrodes to charcoal briquets. Asphalt, used for roads and roofing materials, must be inert to most chemicals and weather conditions.
• Solvents. A variety of products, whose boiling points and hydrocarbon composition are closely controlled, are produced for use as solvents. These include benzene, toluene, and xylene.
• Petrochemicals. Many products derived from crude oil refining, such as ethylene, propylene, butylene, and isobutylene, are primarily intended for use as petrochemical feedstock in the production of plastics, synthetic fibers, synthetic rubbers, and other products.
• Lubricants. Special refining processes produce lubricating oil base stocks. Additives such as demulsifiers, antioxidants, and viscosity improvers are blended into the base stocks to provide the characteristics required for motor oils, industrial greases, lubricants, and cutting oils. The most critical quality for lubricating-oil base stock is a high viscosity index, which provides for greater consistency under varying temperatures.

Common Refinery Chemicals

• Leaded Gasoline Additives: Tetraethyl lead (TEL) and tetramethyl lead (TML) are additives formerly used to improve gasoline octane ratings but are no longer in common use except in aviation gasoline.
• Oxygenates: Ethyl tertiary butyl ether (ETBE), methyl tertiary butyl ether (MTBE), tertiary amyl methyl ether (TAME), and other oxygenates improve gasoline octane ratings and reduce carbon monoxide emissions.
• Caustics: Caustics are added to desalting water to neutralize acids and reduce corrosion. They are also added to desalted crude in order to reduce the amount of corrosive chlorides in the tower overheads. They are used in some refinery treating processes to remove contaminants from hydrocarbon streams.
• Sulfuric Acid and Hydrofluoric Acid: Sulfuric acid and hydrofluoric acid are used primarily as catalysts in alkylation processes. Sulfuric acid is also used in some treatment processes.
  
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MOCK TEST OF I.C. ENGINE; EME-505

1^st SESSIONAL EXAMINATION 2012-13

BRANCH – ME

SEMESTER- V

Course: B.Tech.                                                                                                                Year: 3^rd

Sub: IC Engines & Compressors                                                           Subject Code: EME- 505

Maximum Marks: 30                                                                                               Time: 1.30 hrs

Section A:                                                                                                       2x6 = 12

Q.1) Answer the following question in brief                                                                                  

(a)    Explain the difference between inlet valve and inlet port.

(b)   What is cracking?

(c)    Define volumetric efficiency of an IC engine with diagram.  

(d)   What are the assumptions of a air standard cycle?

(e)    Explain the concept of indicator thermal efficiency.

(f)    What are the general refinery processes?

Section B:

Q.2) Answer any three of the following questions                                         6 x 3 = 18

(a) Compare between four stroke and two stroke IC engines.

(b)  What is compression ratio? What is its range for (i) the SI engine (ii) CI engine? What factors limit the compression ratio in each type of engine?

(c) An ideal Otto cycle has compression ratio of 8 and initial conditions are 1 bar and 15°C. Heat added during constant volume process is 1045 kJ/kg. Find:

(i)                 Maximum cycle temperature

(ii)               Air standard efficiency

(iii)             Work done per kg of air

(iv)             Heat rejected

Take: C_v = 0.7175 kJ/kg-K and γ = 1.4

(d)  A diesel engine develops 5 kW. It’s indicated thermal efficiency is 30% and mechanical efficiency 57%. Estimate the fuel consumption in (i) kg/hr, (ii) litres/hr,

(iii) Indicated specific fuel consumption, and (iv) brake specific fuel consumption.

L.C.V. of diesel oil = 42000 kJ/kg.

(e) Compare the actual cycle and its deviation from theoretical cycles in IC engines.

OCTANE AND CETANE NUMBERS

Self ignition temperature (SIT) of a fuel is the temperature at which the fuel ignites on its own without spark. If large amount of mixture in an engine cylinder auto ignites, there will be a rapid rise in pressure causing direct blow on engine structure accompanied by thudding sound. This causes vibrations in the engine. The phenomenon is called knocking.

If however, a small pocket of fuel-air mixture auto ignites, pressure waves are generated which travel with the speed of sound across the cylinder. These pressure waves are of such small duration that indicator diagram mechanism fails to record them. These waves interact within themselves and with the cylinder walls, creating characteristics ping sound. The phenomenon is called pinking.

The engine runs rough, overheats and loses efficiency due to knocking and pinking.

The processes of knocking and pinking are related to the nature of the fuel and relative merits of the fuel are decided on the basis of their anti-pinking and anti-knock property. The merit is measured by octane number such that a fuel of high octane number will be liable to less pink or knock as compared to a fuel of low octane number in the same engine. It is important to note that the same fuel will show same tendency to pink or knock in all engines.

Commonly used fuel in SI engines is a mixture of iso-octane and n-heptane. Iso-octane has minimum tendency to knock and this fuel is arbitrarily assigned an octane number of 100 (ON = 100) where as n-heptane has maximum knocking tendency with ON = 0. The octane number of a given fuel is percentage of iso-octane in the mixture of iso-octane and n-heptane. Thus a fuel other than mixture of iso-octane and n-heptane if assigned an ON of 80, it means, it will knock under standard operating condition similar to the mixture of 80% iso-octane and 20% n-heptane.

The tendency to knock in an engine increases with the increase in compression ratio. The highest compression upto which no knocking occurs in a given engine is called highest useful compression ratio (HUCR).

Certain chemical compounds when added to the fuel successfully suppress the knocking tendency. Tetra-ethyl lead [Pb(C₂ H₅)₄] also commonly called TEL and tetra-methyl lead [Pb(CH₃)₄] also referred to as TML are effective dopes in the automobile fuel to check knocking. They are called as anti-knocking agents. However, because of lead poisoning effects TEL and TML are not being used now-a-days. In stead, some organic auto knocking agents have been developed to check the undesirable effects like knocking.

In CI engine air alone is compressed to a compression ratio of 15 to 20 (commonly). The fuel is injected under a pressure of 120 to 210 bars about 20° to 35° before TDC. As the fuel in the engine starts to evaporate the pressure in the cylinder drops and it delays the ignition process by a small amount. The time between beginning of injection and the beginning of combustion is known as the delay period which consists of time for atomization, vapourization and mixing along with time of chemical reaction prior to auto-ignition. The combustion of fuel continues in the expansion and is called after burning. Increased delay period causes accumulation of atomized fuel in the combustion chamber and as the pressure and temperature continue to rise at one instant, the bulk of fuel auto-ignites. This would result in high forces on the structure of the engine causing vibration and rough running.

The CI engine fuel rating is based on ignition delay and is measured in terms of cetane number. Cetane fuel [C₁₆ H₃₄] has very low delay period and is arbitrarily assigned a cetane number of 100. Another fuel a α-methyl-napthalene [C₁₁ H₁₀] has poor ignition quality and is assigned zero cetane number. The volume percentage of cetane in a mixture of cetane and a-methyl naphthalene is the cetane number of the fuel that produces same delay period as the mixture under specified test conditions. Additives such as methyl nitrate, ethyl thio-nitrate and amyl nitrate increase cetane number of a fuel respectively by 13.5%, 10% and 9% if added to the extent of 0.5%.