1

CHAPTER-1

INTRODUCTION

1.1 General introduction

The diesel engine is the most efficient power plant among all known types of internal

combustion engines. Heavy trucks, urban buses, and industrial equipment are powered

almost exclusively by diesel engines all over the world and diesel powered passenger cars

are increasingly popular. For the foreseeable future, the worlds transportation needs will

continue to rely on the diesel engine and its gasoline counterpart. However, both engine

technologies are evolving at an ever increasing pace to meet two major challenges: lower

emissions and increased energy efficiency.

Unlike spark ignited engines where the combustible mixture is predominantly

homogeneous, diesel combustion is heterogeneous in nature. Diesel fuel is injected into a

cylinder filled with high temperature compressed air. Emissions formed as a result of

burning this heterogeneous air/fuel mixture depend on the prevailing conditions not only

during combustion, but also during the expansion and especially prior to the exhaust

valve opening. Mixture preparation during the ignition delay, fuel ignition quality,

residence time at different combustion temperatures, expansion duration, and general

engine design features play a very important role in emission formation. In essence, the

concentration of the different emission species in the exhaust is the result of their

formation, and their reduction in the exhaust system. Incomplete combustion products

formed in the early stages of combustion may be oxidized later during the expansion

stroke. Mixing of unburned hydrocarbons with oxidizing gases, high combustion chamber

temperature, and adequate residence time for the oxidation process permit more complete

combustion. In most cases, once nitric oxide (NOx) is formed it is not decomposed, but

may increase in concentration during the rest of the combustion process if the temperature

remains high. Fig 1.1 summarizes the sources of unburned hydrocarbons (HC) and NOx

in direct injected diesel engines. Species formed in both the premixed and diffusion

(mixing controlled) combustion phases [1] are shown.

During combustion, oxygen combines with hydrogen carbon to form water (H2O), carbon

monoxide (CO) and carbon dioxide (CO2). The nitrogen in the fuel combines with oxygen

and forms nitrogen oxide (NO2). Remaining fuel goes unburnt resulting in smoke and ash.

Exhaust gas constituents consist of partly burned carbon monoxide, nitrogen oxides

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pollute the air. The oxides of nitrogen together with hydrocarbons react in the presence of

sunlight and form petrochemical smog.

Fig: 1.1 Pollutant formation mechanisms in DI combustion system

Internal combustion engines are significant contributors to air pollution that can be

harmful to human health and the environment. Most of these pollutants originate from

various non ideal processes during combustion, such as incomplete combustion of fuel,

reactions between mixture components under high temperature and pressure, combustion

of engine lubricating oil and oil additives as well as combustion of hydrocarbon

components of diesel fuel, such as sulfur compounds and fuel additives. Common

pollutants include unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides

(NOx) or particulate matter (PM). Total concentration of pollutants [2] in diesel exhaust

gases typically amounts to some tenths of one percentthis is schematically illustrated in

Fig 1.2.

Fig: 1.2 Relative concentrations of pollutant emissions in diesel exhaust gas

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As a result smog is created, the atmosphere becomes dirty and breathing becomes

difficult. Its bad effect includes crop damage, eye irritation, objectionable odour, decrease

of visibility, cracking in rubber etc. Smog is a kind of fog mixed with other substances.

The smog covers the cities like a blanket for days together during winter. The heat

generated in large cities tends to circulate air within a dome-like shape.

The substitution of burnt gas oxygen rich air reduces the proportion of the cylinder

contents available for combustion. This causes a correspondingly lower heat release and

peak cylinder temperature, and reduces the formation of NOx. The presence of an inert

gas in the cylinder further limits the peak temperature. The gas to be recirculated may

also be passed through an EGR cooler, which is usually of the air/water type. This

reduces the temperature of the gas, which reduces the cylinder charge temperature when

EGR is employed. This has two benefits the reduction of charge temperature results in

lower peak temperature, and the greater density of cooled EGR gas allows a higher

proportion of EGR to be used. On a diesel engine the recirculated fraction may be as high

as 50% under some operating conditions. Advantages of EGR are reduces NOx emission

and improved engine life through reduced cylinder temperatures (particularly exhaust

valve life).

Reduced emission characteristics and improved efficiency are always the primary area of

consideration in internal combustion engine design sector. They were generally done by

conducting the experimentation. But this traditional process always had several

limitations. They were time consuming, cost consuming and have some errors. This

difficulty can be overcome by using CFD studies. With the increasing advancement in

computational power of modern computers, CFD has found its application in diesel

combustion. This is now widely used by many automobile industries not only for design

and analysis of engine but also for the whole vehicle analysis. the many types of models

for engine combustion process, multidimensional computational fluid dynamics (CFD)

models is gaining momentum due to its capability to predict the gas flow patterns,

combustion phenomenon and emission characteristics etc.

In this work a detailed study has been carried out between two conditions in order to have

a clear clarification in studying the change in pressure, temperature and emission

characteristics. Two conditions are: - engine operated with EGR and without EGR were

taken into consideration. The combustion chambers were modelled a sector geometry of

30 using ANSYS WORKBENCH and analysis part were carried out using ANSYS

Fluent 14.5 package. By using the finite volume method the design and analysis of

combustion chambers, emission characteristic study was done for both conditions. Proper

comparisons of the results were carried out between pressure, temperature and emission

characteristics.

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1.2 Organization of report

The report is organised in the following manner.

Chapter-1: Covers the general introduction which deals with the combustion in diesel

engine, mechanism of pollutants formation, concentration of pollutants and CFD

method of combustion analysis.

Chapter-2: Covers the brief introduction and working principle of compression

ignition engine, combustion in CI engine, Brief discussion on emissions, mechanism

of NOx formation, pollution hazards and human health, introduction to CFD

governing equations and stages that are used in computational fluid dynamics,

introduction to fluent steps involved in solving problem. It also reviews the previous

research works that was conducted by others people in the same area. Some of the

relevant materials including technical papers, journals and books taken from those

researches will be discussed.

Chapter-3: Covers methodology adopted for the combustion analysis of diesel engine.

Here a detailed study has been carried out between two conditions in order to have a

clear clarification in studying the change in pressure, temperature and emission

characteristics. Two conditions are: - engine operated with EGR and without EGR

were taken into consideration. The combustion chambers were modelled (a sector

geometry of 30 using ANSYS WORKBENCH and analysis part were carried out

using CFD tool (ANSYS Fluent 14.5 package). By using the finite volume method

the design and analysis of combustion chambers, emission characteristic study was

done for both conditions.

Chapter-4: This chapter is related to results and discussions. Validation of CFD

results with experimental datas and comparative study of diesel engine with EGR and

without EGR are discussed in this chapter. It represents the pressure variation,

temperature variation, velocity magnitude, emission characteristics and effect of

temperature on NOx in graphical as well as contours and plots in order to compare the

diesel engine with EGR and without EGR cases.

Chapter-5: Covers conclusions, they are drawn from the contour plots and graphical

results are summarised.

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CHAPTER-2

LITERATURE REVIEW

2.1 Introduction

This chapter covers the working principle of compression ignition engine, combustion in

CI engine and brief discussion on emissions. Mechanism of NOx formation, pollution

hazards and those effects on human health. Introduction to CFD, governing equations and

stages that are used in computational fluid dynamics. Introduction to fluent and steps

involved in solving problem. It also reviews the previous research works that was

conducted by others people in the same area.

2.2 Working principle of compression ignition engine

An engine in which the combustion process starts when the air-fuel mixture self-ignites

due to high temperature in the combustion chamber caused by high compression. CI

engines are often called diesel engines and working principle of diesel engine [3] as

shown in Fig 2.1.

Fig 2.1 Working principle of diesel engine

First stroke - suction (intake)

The piston travels from TDC to BDC with the intake valve open and exhaust

valve closed. This creates an increasing volume in the combustion chamber,

which in turn creates a vacuum. The resulting pressure differential through the

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intake system from atmospheric pressure on the outside to the vacuum on the

inside causes air to be pushed into the cylinder. No fuel is added to the incoming

air.

Second stroke - compression

When the piston reaches BDC, the intake valve closes and the piston travels back

to TDC with all valves closed. This compresses the air-fuel mixture, raising both

pressure and temperature in the cylinder. Air is compressed and compressed to

higher pressures and temperature. Late in the compression stroke fuel is injected

directly into the combustion chamber, where it mixes with the very hot air. This

causes the fuel to evaporate and self-ignite, causing combustion to start.

Third stroke - power

With all valves closed, the high pressure created by the combustion process

pushes the piston away from TDC. This is the stroke which produces the work

output of the engine cycle. As the piston travels from TDC to BDC, cylinder

volume is increased, causing pressure and temperature to drop. Combustion is

fully developed by TDC and continues at about constant pressure until fuel

injection is complete and the piston has started towards BDC as.

Fourth stroke - exhaust

By the time the piston reaches BDC, exhaust blow down is complete, but the

cylinder is still full of exhaust gases at approximately atmospheric pressure. With

the exhaust valve remaining open, the piston now travels from BDC to TDC in the

exhaust stroke. This pushes most of the remaining exhaust gases out of the

cylinder into the exhaust system at about atmospheric pressure, leaving only that

trapped in the clearance volume when the piston reaches TDC. Near the end of the

exhaust stroke BTDC, Exhaust-Blow down Late in the power stroke, the exhaust

valve is opened and exhaust blow down occurs. Pressure and temperature in the

cylinder are still high relative to the surroundings at this point, and a pressure

differential is created through the exhaust system which is open to atmospheric

pressure. This exhaust gas carries away a high amount of enthalpy, which lowers

the cycle thermal efficiency. Opening the exhaust valve before BDC reduces the

work obtained during the power stroke but is required because of the finite time

needed for exhaust blow down. Intake valve starts to open, so that it is fully open

by TDC when the new intake stroke starts the next cycle. Near TDC the exhaust

valve starts to close and finally is fully closed sometime ATDC. This period when

both the intake valve and exhaust valve are open is called valve overlap.

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2.3 Combustion in CI engines

Combustion in a compression ignition engine [3] is quite different from that in an SI

engine. Whereas combustion in an SI engine is essentially a flame front moving through a

homogeneous mixture, combustion in a CI engine is an unsteady process occurring

simultaneously at many spots in a very non-homogeneous mixture at a rate controlled by

fuel injection. Air intake into the engine is unthrottled, with engine torque and power

output controlled by the amount of fuel injected per cycle. Because the incoming air is not

throttled, pressure in the intake manifold is always at a value close to one atmosphere.

This makes the pump work loop of the engine cycle very small, with a corresponding

better thermal efficiency compared to an SI engine. For CI engines, only air is contained

in the cylinder during the compression stroke, and much higher compression ratios are

used in CI engines. Compression ratios of modern CI engines range from 12 to 24. Fuel is

injected into the cylinders late in the compression stroke by one or more injectors located

in each cylinder combustion chamber. Injection time is usually about 20 of crankshaft

rotation, starting at about 15 BTDC and ending about 5 ATDC. After injection the fuel

must go through a series of events to assure the proper combustion process.

Atomization

Fuel drops break into very small droplets. The smaller the original drop size

emitted by the injector, the quicker and more efficient will be this atomization

process.

Vaporization

The small droplets of liquid fuel evaporate to vapour. This occurs very quickly

due to the hot air temperatures created by the high compression of CI engines.

High air temperature needed for this vaporization process requires a minimum

compression ratio in CI engines of about 12:1. About 90% of the fuel injected into

the cylinder has been vaporized within 0.001 second after injection. As the first

fuel evaporates, the immediate surroundings are cooled by evaporative cooling.

This greatly affects subsequent evaporation. Near the core of the fuel jet, the

combination of high fuel concentration and evaporative cooling will cause

adiabatic saturation of fuel to occur. Evaporation will stop in this region, and only

after additional mixing and heating will this fuel be evaporated.

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Mixing

After vaporization, the fuel vapour must mix with air to form a mixture within the

A/F range which is combustible. This mixing comes about because of the high

fuel injection velocity added to the swirl and turbulence in the cylinder air the

non-homogeneous distribution of air-fuel ratio that develops around the injected

fuel jet. Combustion can occur within the equivalence ratio limits of Cp= 1.8

(rich) and Cp= 0.8 (lean).

Self-Ignition

At about 8 BTDC, 6-8 after the start of injection, the air fuel mixture starts to

self-ignite. Actual combustion is preceded by secondary reactions, including

breakdown of large hydrocarbon molecules into smaller species and some

oxidation. These reactions, caused by the high-temperature air, are exothermic and

further raise the air temperature in the immediate local vicinity. This finally leads

to an actual sustained combustion process.

Combustion phases

Fig 2.2 Typical DI engine heat release rate diagram identifying different diesel

combustion phases

Fig 2.2 shows the typical DI engine heat release rate with crank angle identifying

different diesel combustion phases [1] as follows.

Ignition delay (ab): The period between the start of fuel injection into the

combustion chamber and the start of combustion (determined from the change in

slope on the P- diagram, or from a heat release analysis of the p() data, or from

a luminosity detector).

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Premixed or rapid combustion phase (bc): In this phase combustion of the fuel

which has mixed with air to within the flammability limits during the ignition

delay period occurs rapidly in a few crank angle degrees. When this burning

mixture is added to the fuel which becomes ready for burning and burns during

this phase, the HHR (high heat release rates) characteristics of the phase result.

Mixing-controlled combustion phase (cd): Once the fuel and air which

premixed during the ignition delay have been consumed the burning rate or heat

release rate is controlled by the rate at which mixture become available for

burning. While several processes are involved-liquid fuel atomization,

vaporization, mixing of fuel vapor with air, preflame chemical reactions-the rate

of burning is controlled in this phase primarily by the fuel vapor-air mixing

process. The heat release rate may or may not reach a second (usually lower) peak

in this phase, it decreases as this phase progresses.

Late combustion phase (de): Heat release continues at lower rate well into the

expansion stroke. There are several reasons for this. A small fraction of the fuel

may not yet have burned. A fraction of the fuel energy is present in soot and fuel

rich combustion products and can still be released. The cylinder charge is non

uniform and mixing during this period promotes more complete combustion and

less dissociated product gases. The kinetics of the final burnout processes become

slower as the temperature of the cylinder gases form during expansion.

2.4 Emissions from diesel engine

2.4.1 Carbon monoxide (CO)

Colourless and odourless gases slightly denser than air. Residence time and turbulence in

the combustion chamber, flame temperature and excess O2 affect CO formation.

Conversion of CO to CO2 in the atmosphere is slow and takes 2 to 5 months. It reduces

the oxygen carrying capacity of blood. It causes health effect such as coma and death. It is

one of the green house gases so it increases the globe temperature.

2.4.2 Carbon dioxide (CO2)

Carbon dioxide is a greenhouse gas associated with global warming, resulting mainly

from increased combustion of fossil fuels including motor vehicle fuels. Motor vehicle

CO2 emissions are part of the anthropogenic contribution to the growth of CO2

concentrations in the atmosphere which is causing climate change.

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2.4.3 Hydrocarbon compounds (HC)

Compounds consist of carbon and hydrogen and include a variety of other volatile

organic compounds (VOCs). Most HCs are not directly harmful to health at

concentrations found in the ambient air. Through chemical reactions in the troposphere,

they play an important role in forming NO2 and O3 which are health and environmental

hazards. Among various HC, methane (CH4) is absent from these reactions. Remaining

HC, non methane hydrocarbons (NMHC) are reactive in the formation of secondary air

pollutants. NMHC are photo chemically reactive.

2.4.4 Particulates

Particulates are solid or liquid toxic particle emitted from vehicle due to the destruction of

exhaust pipe and engine component. Due to high temperature and formation of rust in

exhaust chamber produce dust particle to the atmosphere. It travels to long distance with

the help of wind or any other external agency. It cause severe lung problem.

2.4.5 Smoke

Smoke is produced because of insufficient mixing of fuel and air. It contains carbon

monoxide and carbon di oxide. Smog is produced by smoke. It gives harmful effects to

men such as asthma, eczema, omphysema, troubles, lung and stomach cancer.

2.4.6 Oxides of nitrogen (NOx)

Fuel containing nitrogen is burnt in combustion chamber. Lot of nitrogen gas is emitted

from exhaust of the vehicle. Nitrogen reacts with atmospheric oxygen to form nitrogen

dioxide and nitric acid. Nitric acid corrodes the metal and non-metallic material. The

formation of acid rain is due to the presence of nitric acid.

Mechanism of NOx formation

A major hurdle in understanding the mechanism of formation and controlling its

emission is that combustion is highly heterogeneous and transient in diesel

engines. While NO and NO2 are lumped together as NOx, there are some

distinctive differences between these two pollutants. NO is a colourless and

odourless gas, while NO2 is a reddish brown gas with pungent odour. Both gases

are considered toxic, but NO2 has a level of toxicity 5 times greater than that of

NO. Although NO2 is largely formed from oxidation of NO, attention has been

given on how NO can be controlled before and after combustion NO is formed

during the post flame combustion process in a high temperature region. The

principal source of NO formation is the oxidation of the nitrogen present in

atmospheric air. The nitric oxide formation chain reactions [4] are initiated by

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atomic oxygen, which forms from the dissociation of oxygen molecules at the

high temperatures reached during the combustion process.

The principal reactions governing the formation of NO from molecular nitrogen

are shown in equation (2.1),

N2 + O NO + N

N + O2 NO + O

N + OH NO + H (2.1)

Chemical equilibrium consideration indicates that for burnt gases at typical flame

temperatures, NO2/NO ratios should be negligibly small. While experimental data

show that this is true for spark ignition engines, in diesels, NO2 can be 10 to 30%

of total exhaust emissions of oxides of nitrogen. A plausible mechanism for the

persistence of NO2 is as follows. NO formed in the flame zone can be rapidly

converted to NO2 via reactions such as equation (2.2),

NO + HO2 NO2 + OH (2.2)

Subsequently, conversion of this NO2 to NO occurs via equation (2.3),

NO2 + O NO + O2 (2.3)

Unless the NO2 formed in the flame is quenched by mixing with cooler fluid. This

explanation is consistent with the highest NO2/NO ratio occurring at high load in

diesels, when cooler regions which could quench the conversion back to NO are

widespread The local atomic oxygen concentration depends on molecular oxygen

concentration as well as local temperatures. Formation of NOx is almost absent at

temperatures below 2000 K. Hence any technique, that can keep the instantaneous

local temperature in the combustion chamber below 2000 K, will be able to reduce

NOx formation.

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EGR method for NOx reduction

Fig 2.3: Schematic exhaust gas recirculation

Exhaust gas recirculation (EGR) [5] is used for controlling the NOx emissions. EGR is an

effective technique of reducing NOx emissions from the diesel engine exhaust. EGR

involves replacement of oxygen and nitrogen of fresh air entering in the combustion

chamber with the carbon dioxide and water vapour from the engine exhaust. The

recirculation of part of exhaust gases into the engine intake air increases the specific heat

capacity of the mixture and reduces the oxygen concentration of the intake mixture. These

two factors combined lead to significant reduction in NOx emissions as shown in Fig 2.3.

Introduction of EGR has combinations of some these effects

Depletion of oxygen in the intake charge

Increased intake temperature due to mixing with EGR

Increased specific heat of intake charge

Recycling of unburned hydrocarbons (opportunity for re-burn)

EGR (%) is defined as the mass percentage of the recirculated exhaust in total intake

mixture shown in below,

% EGR = Mass of air admitted wit hout EGR Mass of air admitted with EGR

Mass of air admitted without EGR

25% EGR were adopted. The engine was operated with 25% EGR to study the

combustion, emission characteristics of the diesel engine.

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2.5 Pollution hazards and human health

The major pollutants emitted by motor vehicles including CO, CO2, NOx, HC and

suspended particulate matter (SPM), have damaging effects on both human health and

ecology [6] as shown in Table 2.1. The human health effects of air pollution vary in the

degree of severity, covering a range of minor effects to serious illness, as well as

premature death in certain cases. Most of the conventional air pollutants are believed to

directly affect the respiratory and cardio-vascular systems. In particular, high levels of

SPM are associated with increased mortality, morbidity and impaired pulmonary

function. Lead prevents haemoglobin synthesis in red blood cells in bone marrow, impairs

liver and kidney function and causes neurological damage.

Table 2.1 Various pollutants and their effect on human health and on the natural environment

Pollutants

Effects on Human Health

Effects on the Natural

Environment

Carbon

monoxide

(CO)

Can affect the cardio-vascular

system, exacer-baiting

cardiovascular disease symptoms,

particularly angina; may also

particularly affect foetuses, sickle

cell anaemic and young children.

Can affect the central nervous

system, impairing physical

coordination, vision and judgement,

creating nausea and headaches.

__

Carbon

dioxide (CO2)

__ Major contributor to global

warming, climate change

Nitrogen

oxides (NOX)

Nitrogen dioxide (NO2) can affect

the respiratory system. Nitrogen

monoxide (NO) and nitrogen

dioxide (NO2), where they play a

part in photochemical some

formation, pulmonary disease,

impairment of lung function and

eye, nose and thread irritations.

NO and NO2 can contribute

significantly to acid deposition

damaging aquatic eco-systems and

other eco-systems such as forests

NOx can also have a fertilizing

effect on forests

Particulate

matter

Fine particulate matter may be

toxic in itself or may carry toxic

(including carcinogenic) trace

substance, and can alter the

immune system. Fine particulate

can penetrate deeply into the

respiratory system irritating lung

tissue and causing long-term

disorders.

Fine particulate can significantly

reduce visibility. High dust and

soot levels are associated with a

general perception of dirtiness of

the environment

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2.6 Introduction to CFD

The detail analysis of combustion in any engine is one of the most important key factors

to confirm its efficient design wherein computational fluid dynamics (CFD) proves to be

an efficient tool. It is observed from the experiment and CFD, CFD tool is good technique

to reduce all regulated emission from diesel engine.

2.6.1 Governing equations in CFD

There are mainly three equations [7] we solve in computational fluid dynamics problem.

They are continuity equation, momentum equation (Navier Stokes equation) and energy

equation. The flow of most fluids may be analyzed mathematically by the use of two

equations. The first, often referred to as the continuity equation, requires that the mass of

fluid entering a fixed control volume either leaves that volume or accumulates within it. It

is thus a "mass balance" requirement posed in mathematical form, and is a scalar

equation. The other governing equation is the momentum equation, or Navier-Stokes

equation, and may be thought of a momentum balance" The Navier-Stokes equations are

vector equations, meaning that there is a separate equation for each of the coordinate

directions (usually three).

Continuity equation

It is based on the principle of conservation of mass [7]. Net mass flow out of

control volume = Time rate of decrease of mass inside control volume.

Mass conservation equation is shown in equation (2.4)

Momentum (Navier Stokes) equation

It is based on the law of conservation of momentum [7], which states that the net

force acting in a fluid mass is equal to change in momentum of flow per unit time

in that direction.

The force acting on a fluid element mass m is given in equation (2.5)

Newtons second law of motion is

F = m x a (2.5)

Where a is the acceleration acting in the same direction as force F

Momentum equation is shown in equation (2.6)

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Energy equation

It is based on the principle that total energy is conserved [7].

Total energy entering control volume = Total Energy leaving Control volume

Energy equation is shown in equation (2.7)

Where,

2.6.2 Stages in the computational fluid dynamics

Pre-processing

Defining the geometry of the region for computational domain.

Generating the grids for subdivision of the domain into a number of smaller, non-

overlapping sub domains.

Specifying the appropriate boundary and initial conditions.

Solver:

Approximation of unknown flow variables by means of simple functions.

Discretization by substitution of the approximation into the governing flow

equations and subsequent mathematical manipulations.

Solving the algebraic equations.

Post-processing:

Domain geometry and grid display.

Animations.

2D and 3D surface plots.

X-Y plots with different properties.

Particle tracking.

View manipulation (translation, rotation, scaling etc).

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2.6.3 Introduction to fluent

The term computational fluid dynamics (CFD) has come into use to cover all aspects of

computational techniques that can be applied to the solution of problems involving fluids

and gases. CFD studies are by no means limited to the area of engineering studies and

cover fields such as weather forecasting, geological and geographical studies, medical

applications so on. In the area of engineering studies, from which we will be drawing our

examples, CFD is primarily used as a design aid for predicting the performance

characteristics of equipment involving fluid/gas flow and heat transfer. The ability to

simulate heat transfer and fluid flow problems numerically before a prototype is built cuts

the cost and time of development by orders of magnitude. Of course, CFD must be

continuously backed up by experimentation in order to ensure that the numerical

predictions are reliable. Thus a cycle is formed involving theoretical predictions, CFD

and experimentation. Validity of new mathematical models can be tested within the

context of this relationship, with resulting improvements in the accuracy of CFD analysis.

Fluent uses finite volume numerical procedures to solve the governing equations for fluid

velocities, mass flow, pressure, temperature, species concentration and turbulence

parameters and fluid properties. Numerical techniques involve the sub-division of the

domain into a finite set of neighbouring cells known as "control volumes" and applying

the discretised governing partial differential equations over each cell. This yields a large

set of simultaneous algebraic equations, which are highly non-linear. These equations are

in turn solved by iterative means until a converged solution is achieved. The criteria of

convergence can be changed by the user, and is generally applied to the changes in the

values of all the field variables from one iteration to the next. When all the equations are

satisfied on all the discretization points there will be no change from one iteration to the

next. This theoretical convergence is not normally achievable in a finite number of steps.

Hence the selection of suitable criteria to detect near convergence becomes important.

FLUENT have the following turbulence models:

Spalart-Allmaras model

k-epsilon models (k- )

Standard k- model

Renormalization-group (RNG) k- model

Realizable k- model

k-omega models (k- )

Standard k- model

Reynolds stress model (RSM)

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2.6.4 Steps involved in solving problem

First create the geometry in the Design Modeller.

Create the grid of appropriate size and with appropriate skewness to specify the

problem domain in ANSYS Meshing.

Give the Boundary Conditions for entire domain.

Save it and export it as fluent mesh file.

Read the file in FLUENT and check the quality of mesh.

Enter values for boundary conditions, operating conditions etc.

Selecting the appropriate solver to solve the problem.

Solve the problem by initializing from mass flow inlet and specifying the number

of iterations.

Solve the problem and note down the results.

Mass flow inlet

Mass flow inlet boundary conditions are used to define the mass flow of the fluid,

along with all relevant scalar properties of the flow, at flow inlets. The total

properties of the flow are not fixed, so they will be adjusted whatever value is

necessary to provide the prescribed velocity distribution. Both velocities defined

at the inlet boundary condition and scalar quantities defined on the boundary are

used by FLUENT to compute mass flow rate, momentum fluxes, etc. at the inlet.

The mass flow rate [7] is given by Equation (2.8) below.

It is worth noting that only normal component of velocity to volume face

contributes in mass flow rate

Solution controls

Solution parameters like courant number, under relaxation factors, and

discretization schemes are manipulated in FLUENT to obtain stable solution as

fast as possible. A second order discretization was used for pressure equations and

a second order upwind scheme was used to discretize momentum, energy, and

discrete phase and combustion equations.

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2.7 Literature review

In IC Engine exhaust gas recirculation (EGR) is a technique which reduces the harmful

emission such as NOx in CI/Diesel engine. EGR is simple technique to supply already

exhausted gas again back to combustion chamber by some cooling and filtered media so

as to increase or to maintain temperature of cylinder head.

Some of the work carried out in this field is given below.

Avinash Kumar Agrawal et al., [8] Effect of EGR on the exhaust gas temperature

and exhaust Opacity in compression ignition engines. In diesel engines, NOx

formation is a highly temperature-dependent phenomenon and takes place when

the temperature in the combustion chamber exceeds 2000 K. Therefore, in order to

reduce NOx emissions in the exhaust, it is necessary to keep peak combustion

temperatures under control.

Thermal efficiency and brake specific fuel consumption are not affected

significantly by EGR. However particulate matter emission in the exhaust

increases, as evident from smoke opacity observations. A diesel engine score

higher than that of other engines in most aspects like fuel consumption and low

CO emissions, but loses in NOx emissions. EGR is proved to be one of the most

efficient methods of NOx reduction in diesel engines. The increase in particulate

matter emissions due to EGR can be taken care by employing particulate traps and

adequate regeneration techniques.

K. Rajan et al., [9] Transesterified fuels (biodiesel) from vegetable oils are

alternative fuels for diesel engines. They are renewable and offer potential

reduction in CO and HC emissions due to higher O2

contents in vegetable oil.

Many research studies have reported that exhaust from biodiesel fuel has higher

NOx emissions while HC and PM emissions are significantly lower than operated

with diesel fuel. The aim of the present investigation is to reduce NOx emissions.

Exhaust gas recirculation (EGR) is one of the most effective techniques for

reducing NOx emissions in compression ignition engines. A twin cylinder four

stroke water cooled direct injection (DI) diesel engine was used for conducting

test with (Sunflower methyl ester SFME) biodiesel blends with diesel fuel

combined with EGR technique. The results showed that for a 7.5kW power

output, B20 SFME with 15% EGR rate produce 25% less NOx emissions

compared to diesel fuel for the same level smoke emissions.

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Nithin Balakrishnan et al., [10] CFD studies of combustion in direct injection

single cylinder diesel engine using non-premixed combustion model. In this

Combustion studies of both chambers, shallow depth and hemispherical

combustion chambers were carried out. Emission characteristics of both

combustion chambers had also been carried out. The obtained results are

compared. It has been found that hemispherical combustion chamber is more

efficient as it produces higher pressure and temperature compared to that of

shallow depth combustion chamber. As the temperature increases the formation of

NOx emissions and soot formation also get increased the two combustion

chamber. Models were modeled using ANSYS WORKBENCH and the

combustion phenomena were analyzed using ANSYS FLUENT 14.5. The results

show values comparable to theoretical values. Here two combustion chambers

results were compared and following observations were made. The result is

Numerical analysis result shows that hemispherical piston head provides much

better performance than the shallow depth. This is due to the high turbulent

intensity formed within the cylinder.

N.V. Mahalakshmi et al., [11] experiments were conducted to study the

performance, emission and combustion characteristics of a diesel engine using

poon oil based fuels. In the present work, poon oil and poon oil methyl ester are

tested as diesel fuels in neat and blended forms. The blends were prepared with

20% poon oil and 40% poon oil methyl ester separately with standard diesel on

volume bases the reduction in smoke, hydrocarbons and CO emissions were

observed for poon oil methyl ester. The peak pressure obtained for diesel fuel in

combustion chamber is 67.5 bars.

Rajesh Bisane1 et al., [12] CFD analysis of a single cylinder four stroke C.I.

engine exhaust system. Each after treatment system design should be done in such

a way that considering the complete system objectives. Energy efficient exhaust

system development requires minimum fuel consumption and maximum

utilization of exhaust energy for reduction of the exhaust emissions and also for

effective waste energy recovery system such as in turbocharger, heat pipe etc.

from C.I. engine. Traditional manifold optimization has been based on tests on

Exhaust system. This trial & error method can be effective but is very expensive

& time consuming. Beside this method cannot provide any information about the

actual flow structure inside the system. This vital information can be obtained

20

using 3-D CFD analysis. The design engineers can study the flow structures &

understand whether a particular system performs correctly or not

Deepak Agarwal et al., [13] investigate the effect of EGR on soot deposits, and

wear of vital engine parts, especially piston rings, apart from performance and

emissions in a two cylinder, air cooled, constant speed direct injection diesel

engine, which is typically used in agricultural farm machinery and decentralized

captive power generation. Such engines are normally not operated with EGR. The

experiments were carried out to experimentally evaluate the performance and

emissions for different EGR rates of the engine. Emissions of hydrocarbons (HC),

NOX, carbon monoxide (CO), exhaust gas temperature, and smoke capacity of the

exhaust gas etc. were measured. Performance parameters such as thermal

efficiency, brake specific fuel consumption (BSFC) were calculated. Reductions

in NOX and exhaust gas temperature were observed but emissions of particulate

matter (PM), HC and CO were found to have increased with usage of EGR.

N. Saravanan et al., [14] used hydrogen-enriched air as intake charge in a diesel

engine adopting exhaust gas recirculation (EGR) technique with hydrogen flow

rate at 20 l/min. Experiments are conducted in a single cylinder, four stroke,

water-cooled, direct-injection. Diesel engine coupled to an electrical generator.

Performance parameters such as specific energy consumption, brake thermal

efficiency are determined and emissions such as oxides of nitrogen, hydrocarbon,

carbon monoxide, particulate matter, smoke and exhaust gas temperature are

measured. Usage of hydrogen in dual fuel mode with EGR technique results in

lowered smoke level, particulate and NOX emissions.

Renganathan Manimaran et al., [15] numerical analysis of direct injection diesel

engine combustion using extended coherent flame 3-zone model. Several

applications have proven the reliability of using multi-dimensional CFD tools to

assist in diesel engine research, design and development. CFD tools are

extensively used to reveal details about invisible in-cylinder processes of diesel

combustion so that guidance can be provided to improve engine designs in terms

of emissions reduction and fuel economy. Innovative combustion concepts can be

evaluated numerically prior to experimental tests to reduce the number of

investigated parameters. In this present work, reacting flow simulations were

performed in a single cylinder direct injection diesel engine with an improved

version of the ECFM-3Z (extended coherent flame model 3 Zones) model using

ES-ICE and STAR-CD codes. Combustion and emission characteristics are

21

studied in a sector of engine cylinder, which eliminates the tedious experimental

task with conservation in resources and time. The pressure variations during

motoring and firing conditions, temperature and heat release graphs with respect

to crank angle are plotted. Mass fractions contours of CO, NO, soot and fuel

(C12H26) and mixture density contours at TDC are plotted. It is found that higher

NOx emissions occur at peak temperatures while soot and CO emissions occur at

peak pressures.

H.E.Saleh [16] studied jojoba methyl ester (JME) has been used as a renewable

fuel in numerous studies evaluating its potential use in diesel engines. These

studies showed that this fuel is good gas oil substitute but an increase in the

nitrogenous oxides emissions was observed at all operating conditions. The aim of

this study mainly was to quantify the efficiency of exhaust gas recirculation

(EGR) when using JME fuel in a fully instrumented, two-cylinder, naturally

aspirated, four-stroke direct injection diesel engine. The tests were carried out in

three sections. Firstly, the measured performance and exhaust emissions of the

diesel engine operating with diesel fuel and JME at various speeds under full load

are determined and compared. Secondly, tests were performed at constant speed

with two loads to investigate the EGR effect on engine performance and exhaust

emissions including nitrogenous oxides (NOX), carbon monoxide (CO), unburned

hydrocarbons (HC) and exhaust gas temperatures. Thirdly, the effect of cooled

EGR with high ratio at full load on engine performance and emissions was

examined. With the application of the EGR method, the CO and HC concentration

in the engine out emissions increased. For all operating conditions, a better trade-

off between HC, CO and NOX emissions can be attained within a limited EGR

rate of 515% with very little economy penalty.

Umakant V. et al. [17] worked on the CFD modeling and experimental validation

of combustion in direct ignition engine fueled with diesel. This paper describes

the development and use of sub models for combustion analysis in direct injection

(DI) diesel engine. In the present study the Computational Fluid dynamics (CFD)

code FLUENT is used to model complex combustion phenomenon in compression

ignition (CI) engine. The experiments were accomplished on single cylinder and

DI engine, with full load condition at constant speed of 1500 rpm. Combustion

parameters such as cylinder pressure, rate of pressure rise and heat release rate

were obtained from experiment. The numerical modeling is solved by unsteady

first order implicit, taking into account the effect of turbulence. For modeling

22

turbulence Renormalization Group Theory (RNG) k- model is used. The sub

models such as droplet collision model and Taylor Analogy Breakup (TAB)

model are used for spray modeling. The wall film model is used to assess spray

wall interaction. Modeling in cylinder combustion, species transport and finite rate

Chemistry model is used with simplified chemistry reactions. The results obtained

from modeling were compared with experimental investigation. Consequences in

terms of pressure, rate of Pressure rise and rate of heat release are presented. The

rate of pressure rise and heat release Rate was calculated from pressure based

statistics. The modeling outcome is discussed in detail with combustion

parameters. The results presented in this paper demonstrate that, the CFD

modeling can be the reliable tool for modeling combustion of internal combustion

Engine.

Helgi Skuli Fridriksson [18] on CFD-analysis of heat transfer of a heavy-duty

diesel engine. In this work, a heavy-duty diesel engine was studied by employing

CFD simulations on a closed volume engine segment. These simulations were

used to evaluate both the effects from certain parameters on the wall heat transfer,

and to examine how reduction of heat transfer would affect the engine

performance and emission levels. A study of engine performance, along with an

estimation of NOx and soot emission levels, was performed on a heavy duty diesel

engine for two different combustion modes. A specific load point was used as a

validation point for each combustion mode and experimental results were used for

the validation process. Both baseline cases gave acceptable agreement with the

experimental data.

M.sc. Iliev s [19] simulation on single cylinder diesel engine and effect of

compression ratio and EGR on engine performance and emission. In this research,

the one dimensional (1D) CFD modeling of four-stroke direct injection diesel

engine is developed by AVL Boost software. The performance of a diesel engine

increases with increase in compression ratio. Variable compression technologies

in IC engines are used to increase fuel efficiency under variable loads. Exhaust

gas recirculation is a common way to control in-cylinder NOx production and is

used in most modern high speed direct injection diesel engines. However the

effect of EGR on performance, combustion and emissions production at different

compression ratios are difficult to depict. In the present work an attempt was made

to study the effects of exhaust gas recirculation on performance, combustion and

emissions of a variable compression diesel engine. The test was conducted at

23

different compression ratios with different loads and for different EGR rates. This

work present the results of the engine cycle simulation of a single cylinder, direct

injection diesel engine with different compression ratios, percentages of EGR and

loads to estimate performance, combustion and emission characteristics of the

engine using AVL Boost software. It was found that with increase in compression

ratio the specific fuel consumption decreases. The results obtained indicated that

with increase in % EGR the NOx emissions was gradually decreases at different

compression ratios due to less flame temperatures and low oxygen content in the

combustion chamber. The high degree of recirculation is suitable for higher

compression ratio because at compression ratio 19 and 10% EGR the percentage

reduction of NOx was 36%.

Ming Zheng et al., [20] Diesel engine exhaust gas recirculationa review on

advanced and novel concepts. Exhaust gas recirculation (EGR) is effective to

reduce nitrogen oxides (NOx) from Diesel engines because it lowers the flame

temperature and the oxygen concentration of the working fluid in the combustion

chamber. However, as NOx reduces, particulate matter (PM) increases, resulting

from the lowered oxygen concentration. When EGR further increases, the engine

operation reaches zones with higher instabilities, increased carbonaceous

emissions and even power losses. In this research, the paths and limits to reduce

NOx emissions from Diesel engines are briefly reviewed, and the inevitable uses

of EGR are highlighted. The impact of EGR on Diesel operations is analyzed and

a variety of ways to implement EGR are outlined. Thereafter, new concepts

regarding EGR stream treatment and EGR hydrogen reforming are proposed.

EGR is still the most viable technique that can reduce NOx dramatically. Energy

efficient after treatment systems dealing with NOx and PM simultaneously are still

in the early development stages. The inability of available catalytic after treatment

technologies further encourages aggressive uses of EGR.

Jafarmadar et al., [21] Combustion modeling for modern direct injection diesel

engines. In order to comply with stringent pollutant emissions regulations, a

detailed analysis of the engine combustion and emission is required. In this field,

computational tools like CFD and engine cycle simulation play a fundamental

role. Therefore, the goal of the present work is to simulate a high speed DI diesel

engine and study the combustion and major diesel engine emissions with more

details, by using the AVL-FIRE commercial CFD code. The predicted values of

the in cylinder pressure, heat release rate, emissions, spray penetration and in-

24

cylinder isothermal contour plots by this code are compared with the

corresponding experimental data in the literature and is derived good agreement.

This agreement makes the model a reliable tool that can use for exploring new

engine concepts.

Calculated spray penetration and in-cylinder isothermal contour plots are

compared with experimental photographs at different crank angle degrees and is

derived good agreement.

Predicted value for average pressure, heat release rate, soot and NOx emissions in

cylinder are good agreement with the corresponding experimental data.

Model can be predicts exactly start of combustion.

2.8 Objectives of the project

It is seen from the literature Avinash Kumar Agrawal et al., [8] studied that effect of EGR

on the exhaust gas temperature, exhaust opacity and NOx formation in compression

ignition engines. It is observed that NOx formation is a highly temperature-dependent

phenomenon where it is increases with increase in temperature during combustion. K

Rajan et al., [9] studied that transesterified fuels (biodiesel) from vegetable oils are

alternative fuels for diesel engines. These studies reported that exhaust from biodiesel

fuel has higher NOx emissions. Exhaust gas recirculation (EGR) is used for reduction of

NOx emission in compression ignition engines. Nithin Balakrishnan et al., [10] CFD

analysis of combustion in CI engine. In this work reported that as the temperature

increases the formation of NOx emission is increased in the combustion chamber. Deepak

Agarwal et al., [13] investigate the effect of EGR on NOx reduction in direct injection

diesel engine. Emissions of NOx, carbon monoxide (CO) carried out. Reduction in NOx is

observed but emission of CO was found to have increased with usage of EGR. Hardik B.

Charola et al., [22] evaluate the performance and emission using EGR (exhaust gas

recirculation) in compression-ignition engine fuelled with blend. In this study exhaust gas

recirculation on four stroke compression ignition engine fuelled with diesel/methanol

blends were studied to evaluate the performance and emission of engine. It is observed

that NOx emission and exhaust gas temperature reduced but emission CO were found to

have increased with usage of EGR in CI engine.

There is a need for insights the simulation process inside the engine cylinder, CFD has

become modern tool to study the complicated process of combustion in engines, instead

of using highly cost experimental setup this technique can be adopted for combustion

analysis of DI diesel engines and engine specification are taken from literature [11].The

25

computational fluid dynamics (CFD) code FLUENT 14.5 is used to model the complex

combustion phenomenon for engine without EGR and with EGR, which could play a very

important role in engine design, research, development and comparative analysis of

pressure variation, temperature variation and mass fraction of species CO, CO2 and NOx.

The main combustion products that are contained in engine exhaust gases are water vapor

(H2O), carbon dioxide (CO2), nitrogen oxides (NOx), carbon monoxide (CO). All of

these, except for the water vapor, are considered environmentally harmful. This is also

reflected in the fact that governments all over the world enact limits for the emission of

these gases. A way to reduce the formation of NOx in diesel engines is the use of EGR,

recirculated exhaust gas. Part of the exhaust gas is rerouted into the combustion chamber,

where it helps to attenuate the formation of NOx by reducing the local reaction

temperature. Therefore it has been selected as the main objectives for the project.

2.9 Statement of the project

This project contains CFD analysis of combustion phenomenon, emission characteristics

and optimization of NOx emission formation by using without EGR and with EGR in

direct injection single cylinder diesel engine using non-premixed combustion model. The

computational fluid dynamics (CFD) code FLUENT 14.5 is used to model the complex

combustion phenomenon.

Validation of results is done with pressure and crank angle plot results by comparing it

with experimental data available in the literature [11]. From experimental datas

Combustion chamber modelling has been done by using ANSYS design modeller.

Grids are generated.

Generating the mesh by using ANSYS meshing.

Non-premixed combustion model of ANSYS FLUENT 14.5 can be used to

simulate the combustion process.

Pressure and crank angle plots are drawn.

Temperature and crank angle plots are drawn.

Effects of temperature on NOx graphs are plotted.

Comparative analysis of emission characteristics for diesel engine without EGR

and with EGR is carried out.

2.10 Scope of the project

CFD analysis of combustion chamber from above points are plays a very important role

in IC engine design, research, development and optimization of emissions.

26

CHAPTER-3

METHODOLOGY

3.1 Introduction

This chapter covers the brief introduction about geometry creation in ANSYS design

modeller and geometry details are available in the literature [11], checking the mesh

quality details by grid independence and solution convergence. Fluent set-up and steps

considered for the analysis combustion phenomenon, which provides the boundary

conditions, dynamic meshing, computational models in ANSYS fluent and set-up of

exhaust gas recirculation system.

3.2 Geometry creation

Model of the I.C engine has been created using Ansys design modeller from the geometry

details are available in the literature [11] as shown in Fig 3.1.

Fig 3.1: IC engine model

Geometry details

Fig 3.2: Piston bowl design

27

Fig 3.2 shows the piston bowl design [23], geometry details are

Cylinder bore = 87.5 mm

In center piston bowl depth, hc = 5 mm

Depth of a combustion chamber in periphery, hp = 25 mm.

Fig 3.3: Geometry created in ANSYS design modeller

Fig 3.1 shows the IC engine model, it is computationally very expensive to analyze the

whole model of the I.C Engine in CFD; hence 30 sector model is taken for analysis as

shown in Fig 3.3.

3.3 Meshing details

Meshing is performed in Ansys meshing tool, the element size has been defined to be 1-

mm. The meshing details are shown in Fig 3.4 and 3.5 for complete section and cut

section respectively; convergence and grid independency are carried.

Fig 3.4: Meshing performed in ANSYS meshed tool

28

Fig 3.5: Meshed part in cut view

Fig 3.6: Meshing quality details

The quality of mesh from Fig 3.6 has been found to be 0.8638 mm maximum, 6.57710-4

mm is minimum and average values 0.22777 mm with standard deviation 0.124333 hence

skewness is less than 1mm hence we can go with this mesh. The main parameter to get

conform that our results are satisfactory are to check the convergence and grid

independency. This solution is checked for grid independency and convergence.

Grid independence

Grid independence is the criteria in which we conform that the results are

independent of the size of mesh i.e. as the mesh size is decreased the results

changes but there exists one mesh size after which the results doesnt change that

size we name as grid independent size. In our case we found that the grid

independent size is 1 mm element size, hence this size will be kept constant for all

the models.

29

Solution convergence

The numerical solution is an iterative process. A steady-state solution requires the

solution converge to an accurate approximation of the exact solution. In order to

monitor how much the solution changes with each iteration, a residual is

introduced, which is a quantity that measures the unknown error.

3.4 Fluent set-up

The Steps considered for the Analysis are-

Importing the Mesh in Fluent.

Running a transient Analysis

Fig 3.7: Transient analysis

The present solution is based on pressure based hence transient state is taken for

analysis as shown in Fig 3.7.

Selecting species transport model

The fuel is injected directly into the combustion chamber. The fuel mixes with the

high pressure air in the combustion chamber and combustion occurs. Due to the

non-premixed nature of the combustion occurring in such engines, non-premixed

combustion model of ANSYS FLUENT 14.5 can be used to simulate the

combustion process as shown in fig 3.8.

Fig 3.8: Species transport model

30

Selection of the materials.

Fig 3.9: Materials selection

Fig 3.9 shows the selection of the material let us choose the material as air and

C12H23 (chemical formula of the diesel).

3.5 Boundary conditions

Fig 3.10: Geometry with boundary conditions

Specifying the injection timings and mass flow rate.

Flow rate: 7.4 10-6

kg/s.

Start crank angle: 300 degree, 370 degree CA (TDC).

Fuel injected at 10 degree before TDC, Stop crank angle: 540 degree

Temperature: 300 K.

31

3.6 Dynamic mesh

Due to the transient nature of an internal combustion engine, it is essential to model the

motion of the components inside the chamber. This is accomplished by dynamic meshing

techniques in ANSYS Fluent. With these tools, it is possible to model the piston motion

over the course of a combustion cycle, while maintaining mesh integrity and simulation

accuracy. In order to maintain mesh integrity with such motion, Fluent offers smoothing

and remeshing options that allow the elements in a mesh to stretch, break up, and remesh

as the cylinder volume increases and decreases. The user inputs parameters such as

maximum/minimum cell size and maximum skewness that are evaluated at each time

step, then smoothing or remeshing occurs when the cell size and skewness limits are

exceeded. This enables a consistently accurate mesh throughout the range of motion

encountered in an engine cycle. Fluent offers In-Cylinder options shown in Fig 3.11

and for the simulation of internal combustion engines which greatly aid ease-of-use of

specification of engine from Table- 3.1. These options allow the use to specify operating

parameters such as engine speed, bore x stroke information, and crank information. This

effectively defines the entire simulation in terms of crank angle rotation, which lends

itself to easy visualization. It is also preferential to define events such as spark, injection,

and valve events in terms of crank degrees rather than flow time. This also reduces

likelihood of the user inputting inaccurate parameters.

Table 3.1: Engine specifications

1 Bore 87.5 mm

2 Stroke 110

3 Fuel in both cases Diesel

4 Crankshaft speed 1500 rpm

5 Crank angle step size 0.25 deg

6 Crank radius 55 mm

7 Connecting rod length 220mm

Fig 3.11: Dynamic mesh model

32

3.7 Computational models in ANSYS fluent

3.7.1 Injection model

In this model we have been using solid cone injection method, in which one injector will

inject diesel as fuel [24]. The injector will inject the fuel at the interval of crank angle

with defined mass flow rate and cetane number. The velocity magnitude, position and

axis of the model for injection have to be entered correctly in order to achieve proper

injection of the fuel.

3.7.2 Flow model

In order to accurately model the in-cylinder flow as the density varies throughout a cycle,

a three-dimensional compressible Navier-Stokes solver is utilized in ANSYS Fluent [24].

This enables realistic simulation of the effects of compressibility on the engine cycle,

such as changing fuel injection trajectories as regions of various densities are

encountered. This is coupled with a realizable K- turbulence model, which resolves

turbulent flow based on turbulent kinetic energy as well as turbulent dissipation rate,

therefore solving two transport equations. The K- model, proposed by Launder and

Spaulding, offers good accuracy for many turbulent flow scenarios. The K- turbulence

model is separated into three different subcategories: standard, RNG, and realizable. The

K- has been used for various internal combustion engine simulations with good reported

validations to experimental results.

3.7.3 Fuel evaporation

Once the fuel droplets enter the chamber they interact with the flow and dynamics of the

engine until they reach evaporation criteria [24]. When the temperature, pressure,

turbulence, or combination of above has met sufficient conditions, Fluent allows that

particle to evaporate into a gas. For diesel, the primary evaporation species is C12H23

(diesel).

3.7.4 Auto ignition model

In order to initiate combustion in Fluent, auto ignition model is offered [24]. Enable the

Ignition delay model and let the option be in Hardenberg. Set the required activation

energy and cetane number values for the model. Now enable the discrete phase model,

and choose the droplet breakup model.

3.7.5 Combustion model

To simulate the combustion in the engine, the species transport model was chosen [24].

This is appropriate for the simulation of direct injection engines, so that fuel injector

parameters can be modelled and the effects analysed systematically. This is sufficient for

modeling a direct fuel injection.

33

3.7.6 NOx formation

Three different mechanisms have been identified for the formation of nitric oxide during

the combustion of hydrocarbons, namely:

Thermal NOx

As its name suggests, it is strongly temperature dependent. It is produced by the

reaction of atmospheric nitrogen with oxygen at elevated temperatures.

Prompt NOx

The exact details of prompt NOx formation are still uncertain but are generally

believed to involve the reactions between hydrocarbon radicals and atmospheric

nitrogen. In certain combustion environments (such as low temperature, fuel-rich

conditions and short residence time), prompt NOx can be produced in significant

quantities.

Fuel NOx

This is produced by the reaction of the nitrogenous components present in liquid

or solid fossil fuel with oxygen. The fuel nitrogen is a particularly important

source of nitrogen oxide emissions for residual fuel oil and coal, which typically

contain 0.3 - 2.0% nitrogen by weight.

3.8 Exhaust gas recirculation system

Simulation process of non-premixed combustion in a direct injection single cylinder

diesel engine has been described with initial species model without EGR (Case-1) is

shown in Fig 3.12 and Table 3.2.

Fig 3.12: Initial species model for diesel without EGR

34

Table 3.2: Initial mass fraction of species

Species Fuel Oxide

C12H23 1 0

N2 0 0.7670

O2 0 0.2330

After completing the case-1 that is engine operated without EGR, Let us taken the output

average mass fraction of species at 540 deg crank angle N2, O2, C12H23, H2O, H2, CO, CO2

are tabulated in Table 3.3.

Table 3.3: The output average mass fractions

N2 0.7426

O2 0.1402

C12H23 0.00254

H2O 0.0308

H2 0.0005911

CO 0.01571

CO2 0.06738

Adopting 25% of EGR hence take the 25% of each species from output average mass

fraction of species from diesel analysis at 540 deg crank angle (without EGR) available in

the Table 3.3, calculations are carried out using equation 3.1[25].

Case-2 for EGR,

Mass fraction of species = Output of diesel analysis 25% + Initial value (1 - 25%) (3.1)

N2 = output of diesel analysis 25% + Initial value (1 - 25%)

= 0.7426 0.25 + 0.7670 (1 - 0.25)

= 0.7609

O2 = output of diesel analysis 25% + Initial value (1 - 25%)

= 0.1402 0.25 + 0.2330 (1 - 0.25)

= 0.2098

C12H23 = 0.00254 0.25

= 6.35 10-4

H2O = 0.0308 0.25

= 7.7 10-3

H2 = 0.0005911 0.25

= 1.477 10-4

35

CO = 0.01571 0.25

= 3.927 10-3

CO2 = 0.06738 0.25

= 0.01684.

Stoichiometric ratio for diesel = 14.6 (default for diesel)

Equivalence ratio = 1.2

Fuel injected = fresh fuel 75%

= 7.4 10-6

0.75

= 5.55 10-6

.

Table 3.4: Average mass fractions admitted to species model

N2 0.7609

O2 0.2098

C12H23 6.35 10-4

H2O 7.7 10-3

H2 1.477 10-4

CO 3.927 10-3

CO2 0.01684.

Fig 3.13: Species model for EGR

Mass fractions of species are added to species model in Ansys fluent from Table 3.4 as

shown in Fig 3.13 hence analysis become a with EGR.

36

CHAPTER - 4

RESULTS AND DISCUSSION

4.1 Introduction

This chapter covers the validation of pressure-crank angle results with experimental

datas available in the literature [11]. Comparative analysis of pressure variation,

temperature variation, velocity magnitude, emission characteristics and effect of

temperature on NOx for engine operated without and with EGR are carried. Graphical

representation of pressure v/s crank angle, temperature v/s crank angle, mass fraction of

species v/s crank angle and NOx v/s temperature are plotted for engine operated without

EGR and with EGR.

4.2 Validation of pressure-crank angle results

Fig 4.1 Validation of CFD results with experimental results

Mahalakshmi et al [11] conducted an experiment on a four stroke single cylinder DI diesel

engine Kirloskar TAFI make of 4.4 kW at rated speed of 1500 rpm using poon oil and its

diesel blends. In their experiment they have evaluated performance, emissions and

combustion characteristics by blending poon oil in diesel. For the CFD analysis I have

taken specification of the engine used by the Mahalakshmi et al. for validation of my

CFD works taken experimental results presented in there paper and presented as shown in

Fig 4.1. It can be observed that P- diagram of experimentation and my CFD values are

well comparable and acceptable. This validates the CFD results evaluated in this project.

0

10

20

30

40

50

60

70

320 340 360 380 400 420

Pre

ssure

, bar

Crank angle, degree

Experimental

CFD

37

4.3 Combustion chamber without EGR

Mass fraction of C12H23

Fig 4.2: Contours of mass fraction of diesel at 370 degree CA without EGR

Fig 4.3: Contours of mass fraction of diesel at 540 degree CA without EGR

38

Temperature variation

Fig 4.4: Contours of static temperature at 370 degree CA without EGR

Fig 4.5: Contours of static temperature at 540 degree CA without EGR

39

Pressure variation

Fig 4.6: Contours of static pressure at 370 degree CA without EGR

Fig 4.7: Contours of static pressure at 540 degree CA without EGR

40

Pressure-crank angle diagram

Fig 4.8: Variation of cylinder pressure with crank angle without EGR

Velocity magnitude

Fig 4.9: Contours of velocity magnitude at 360 degree CA without EGR

Fig 4.10: Contours of velocity magnitude at 370 degree CA without EGR

0

10

20

30

40

50

60

70

310 330 350 370 390 410 430

Pre

ssure

, bar

Crank angle, degree

Without EGR

41

Fig 4.11: Contours of velocity magnitude at 540 degree CA without EGR

Fig 4.2 to 4.11 shows the various contours at different crank angles without EGR. Mass

burnt fraction of diesel at 370 and 540 crank angle. Fuel is injected into the cylinder at

360 CA. The movement injected fuel is starts to burn, gradually the pressure and

temperature very close to the spray starts increasing. After 10 rotation that is 370 CA

40% of the mass of diesel get burns raising the pressure and temperature to 69.4 bars and

1923K respectively. The velocity of the flame front is the order of 90 m/sec. At the end of

power stroke that is 540 CA mass fraction of diesel burnt is almost nil, except a small

amount at nozzle tip. The pressure in the cylinder is of the order 2.44 bars and maximum

temperature is of the order of 1430K and minimum at wall is 372K. By looking at the

velocity contours we can say that there is anticlockwise current starting from centre plane

to the surface.

42

4.4 Combustion chamber with EGR

Mass fraction of C12H23 (diesel)

Fig 4.12: Contours of mass fraction of diesel at 370 degree CA with EGR

Fig 4.13: Contours of mass fraction of diesel at 540 degree CA with EGR

43

Temperature variation

Fig 4.14: Contours of static temperature at 370 degree CA with EGR

Fig 4.15: Contours of static temperature at 540 degree CA with EGR

44

Pressure variation

Fig 4.16: Contours of static pressure at 370 degree CA with EGR

Fig 4.17: Contours of static pressure at 540 degree CA with EGR

45

Pressure-crank angle diagram

Fig 4.18: Variation of cylinder pressure with crank angle with EGR

Velocity magnitude

Fig 4.19: Contours of velocity magnitude at 360 degree CA with EGR

Fig 4.20: Contours of velocity magnitude at 370 degree CA with EGR

0

10

20

30

40

50

60

70

310 330 350 370 390 410 430

Pre

ssure

, bar

Crank angle, degree

With EGR

46

Fig 4.21: Contours of velocity magnitude at 540 degree CA with EGR

Fig 4.12 to 4.21 shows the various contours at different crank angle with EGR. At 370

crank angle average mass burnt fraction is of the order of 55% against 40% of without

EGR at the end of combustion it varies from 20% to 0.028%. The maximum pressure

attained at 370 CA is 63.2 bars against 69.4 bars without EGR, the maximum

temperature attained is 2259 K.

4.5 Comparison of pressure and temperature with crank angle

Pressure v/s crank angle diagram

Fig 4.22: Comparison of cylinder pressure with crank angle for without and with EGR

0

10

20

30

40

50

60

70

310 330 350 370 390 410 430

Pre

ssure

, bar

Crank angle, degree

Without EGR

With EGR

47

Temperature v/s crank angle diagram

Fig 4.23: Comparison of temperature with crank angle for without and with EGR

Fig 4.22 and 4.23 shows the variation of pressure and temperature with crank angle for

without and with EGR. Trend of the pressure raise and temperature for without EGR and

with EGR is same. However the maximum pressure without EGR is 69.9 bars and against

63.2 bars with EGR as shown in Table 4.1. The maximum temperature without EGR

occurs at 390 CA i.e. 2500 K against 380 CA of that with EGR i.e. 2259 K as shown in

Table 4.2. Throughout the combustion temperature of the gas inside the cylinder is lower

with EGR.

Table: 4.1 Comparison of pressure

Engine Pressure at 370 CA Pressure at 540 CA

Without EGR 69.4 bars 2.40 bars

With EGR 63.2 bars 1.69 bars

Form observations the pressure in case of with EGR is reduced by 9%.

Table: 4.2 Comparison of temperature

Engine Maximum temperature Temperature at 540 CA

Without EGR 2500K 1690 K

With EGR 2259K 1600 K

Form observations the temperature in case of with EGR is reduced by 10%.

0

500

1000

1500

2000

2500

3000

340 360 380 400 420 440

Tem

par

eture

, K

Crank angle, degree

Without EGR

With EGR

48

4.6 Comparison of mass fraction of emissions

Mass fraction of CO

Fig 4.24: Contours of mass fraction of CO at 370 degree CA without EGR and with EGR

Fig 4.25: Contours of mass fraction of CO at 540 degree CA without EGR and with EGR

49

Fig 4.26: Variation of mass fraction of CO with crank angle without EGR

Fig 4.27: Variation of mass fraction of CO with crank angle with EGR

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

300 350 400 450 500 550

Mas

s fr

acti

on o

f C

O, g/k

g o

f fu

el

Crank angle, degree

Without EGR

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

300 350 400 450 500 550

Mas

s fr

acti

on o

f C

O, g/k

g o

f fu

el

Crank angle, degree

With EGR

50

Fig 4.28: Comparison of mass fraction of CO with CA for without and with EGR

Fig 4.24 and 4.25 shows the contours of mass fraction of CO at 370 and 540 CA

respectively for without and with EGR. The pattern of the CO emission for EGR is higher

than that of non EGR for the both cases. The maximum CO emission occurs at 370 CA

with EGR against 0.176 g/kg of fuel at 370 CA without EGR. Further for complete

power stroke mass fraction of CO burns is higher with EGR than that of without EGR as

shown in Fig 4.28 and Table 4.3. This attributes to incomplete combustion, due to non

availability of fresh air, delay the ignition timing and thus lower the combustion

temperature it leads to greater formation of CO in case of with EGR.

Table: 4.3 Comparison of CO emissions

Engine CO emission at 370 CA

g/kg of fuel

CO emission at 540 CA

g/kg of fuel

Without EGR 0.143 0.141

With EGR 0.176 0.157

From observations the percentage of mass fraction of CO increased by 10% in case of

with EGR as shown in Fig 4.28.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

300 350 400 450 500 550

Mas

s fr

acti

on o

f C

O, g/k

g o

f fu

el

Crank angle, degree

Without EGR

With EGR

51

Mass fraction of CO2

Fig 4.29: Contours of mass fraction of CO2 at 370 degree CA without EGR and with EGR

Fig 4.30: Contours of mass fraction of CO2 at 540 degree CA without EGR and with EGR

52

Fig 4.31: Variation of mass fraction of CO2 with crank angle without EGR

Fig 4.32: Variation of mass fraction of CO2 with crank angle with EGR

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

300 350 400 450 500 550

Mas

s fr

acti

on o

f C

O2, g/k

g o

f fu

el

Crank angle, degree

Without EGR

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

300 350 400 450 500 550

Mas

s fr

acti

on o

f C

O2, g/k

g f

uel

Crank angle, degree

With EGR

53

Fig 4.33: Comparison of mass fraction of CO2 with CA for without and with EGR

Fig 4.29 and 4.30 shows the contours of mass fraction of CO2 at 370 and 540 CA

respectively for without and with EGR. This is reverse case of mass fraction of CO

produced. Due to availability of fresh air carbon monoxide converts carbon di oxide in

case of without EGR, however with EGR due to the dilution of charge inside the

combustion chamber CO2 emission lower for full power stroke as shown in Fig 4.33 and

Table 4.4.

Table: 4.4 Comparison of CO2 emissions

Engine CO2 emission at 370 CA

g/kg of fuel

CO2 emission at 540 CA

g/kg of fuel

Without EGR 0.146 0.197

With EGR 0.157 0.140

From observations the percentage of mass fraction of CO2 was decreased by 28% in case

of with EGR compared to without EGR as shown in Fig 4.33.

The CO2 emission from diesel engines can be absorbed by plants for photosynthesis

purposes, so that the carbon dioxide level in the atmosphere is kept in balance.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

300 350 400 450 500 550

Mas

s fr

acti

on

of

CO

2, g/k

g f

uel

Crank angle, degree

Without EGR

With EGR

54

Mass fraction of NOx

Fig 4.34: Contours of mass fraction of NOx at 370 degree CA without EGR and with EGR

Fig 4.35: Contours of mass fraction of NOx at 540 degree CA without EGR and with EGR

55

Fig 4.36: Variation of mass fraction of NOx with crank angle without EGR

Fig 4.37: Variation of mass fraction of NOx with crank angle with EGR

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

300 350 400 450 500 550

Mas

s fr

acti

on o

f N

Ox

, g/k

g o

f fu

el

Crank angle, degree

Without EGR

0

0.000000

0.000001

0.000001

0.000002

0.000002

0.000003

0.000003

300 350 400 450 500 550

Mas

s fr

acti

on o

f N

Ox

, g/k

g o

f fu

el

Crank angle, degree

With EGR

56

Fig 4.38: Comparison of mass fraction of NOx with CA for without and with EGR

Fig 4.34 and 4.35 shows the contours of mass fraction of NOx at 370 and 540 CA

respectively for without and with EGR. It is observed that there is considerable reduction

of mass fraction of NOx generated in case of with EGR.

NOx emission is decreased in case of with EGR. Its formation is depends on the following

factors those are air-fuel mixture, ignition timing and amount of oxygen in the cylinder.

In presence of EGR involves replacement of oxygen and nitrogen of fresh air entering in

the combustion chamber with the carbon dioxide and water vapour from the engine

exhaust. It dilutes the intake mixture, dilution of mixture leads to enriching the air fuel

mixture and reducing the amount of oxygen in the cylinder. This chemically slows and

cools the combustion process by several hundred degrees Fig 4.41, thus reduction in NOx

formation as shown in Fig 4.38 and Table 4.5. NOx emission is the most harmful gaseous

from diesel engines; its reduction is always the aim of engine researchers and

manufacturers.

Table: 4.5 Comparison of NOx emissions

Engine NOx emission at 370 CA

g/kg of fuel

NOx emission at 540 CA

g/kg of fuel

Without EGR 8.33 10-6

1.02 10-4

With EGR 1.00 10-5

3.99 10-7

From observations, NOx is reduced in case of with EGR compared to without EGR as

shown in Fig 4.38.

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

300 350 400 450 500 550

Mas

s fr

acti

on o

f N

Ox

, g/k

g o

f fu

el

Crank angle, degree

Without EGR

With EGR

57

4.7 Effect of temperature on NOx

Fig 4.39: Variation of NOx with temperature for without EGR

Fig 4.40: Variation of NOx with temperature for with EGR

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

1660 1680 1700 1720 1740 1760 1780 1800 1820

Mas

s fr

acti

on o

f N

Ox

, g/k

g o

f fu

el

Tempareture, K

Without EGR

0.00E+00

2.00E-07

4.00E-07

6.00E-07

8.00E-07

1.00E-06

1.20E-06

1.40E-06

1.60E-06

1.80E-06

1600 1650 1700 1750 1800 1850

Mas

s fr

acti

on o

f N

Ox

, g/k

g o

f fu

el

Tempareture, K

With EGR

58

Fig 4.41: Comparison of mass fraction of NOx with temperature for without and with EGR

Fig 4.39 and 4.40 shows the variation of NOx with temperature for without and with

EGR, it shows that the mass fraction of NOx increased with increase in temperature. Due

to the principal source of NOx formation is the oxidation of the nitrogen present in

atmospheric air. The nitric oxide formation chain reactions are initiated by atomic

oxygen, which forms from the dissociation of oxygen molecules at the high temperatures

during the combustion process. Let us reduce the oxygen concentration (by using EGR) it

will reduce the dissociation of oxygen molecules and decreased the flame temperatures in

the combustion chamber it leads to the significant reduction NOx in case of with EGR as

shown in Fig 4.41.

From observations

Table: 4.6 Comparison of pressure and temperature results

Engine Operated Static pressure (bar) Static temperature (K)

Without EGR 69.40 2330

With EGR 63.20 2190

% decreased 9 6

Table: 4.7 Comparison of emissions at 540 degree CA

0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00