LASER INDUCED IGNITION FOR I C ENGINES

LASER INDUCED IGNITION FOR IC ENGINES

A Seminar Report submitted in partial fulfillment of the requirements for the award of

BACHELOR OF TECHNOLOGY

IN

MECHANICAL ENGINEERING

Under University of Calicut

by

SUDHEESH SANKAR. S. R

(APAMEME026)

DEPARTMENT OF MECHANICAL ENGINEERING

Aryanet Institute of Technology

Velikkad, Mundur, Palakkad- 678592

MARCH - 2016

DEPARTMENT OF MECHANICAL ENGINEERINGARYANET INSTITUTE OF TECHNOLOGY

VELIKKAD, PALAKKAD, PIN 678 592

CERTIFICATE

Certified that the seminar titled LASER INDUCED IGNITION FOR I C

ENGINES is a bonafide record of the work done by SUDHEESH SANKAR S R

(APAMEME026) under my supervision and guidance, and is submitted in March

2016 in partial fulfillment of the requirements for award of the Degree of Bachelor of

Technology in Mechanical Engineering under University of Calicut.

Project Guide Prof. V GopinathanLathesh.K Head of the DepartmentAssistant Professor Department of Mechanical Engg.Department of Mechanical Engg

Place : PALAKKAD

Date :

Seminar report Laser Induced Ignition for I C engines

ACKNOWLEDGEMENT

While bringing out this seminar to its final form, I came across a number of

people whose contributions in various ways helped me alot and they deserve special

thanks. It is a pleasure to convey my gratitude to all of them.

I would like to express my deepest gratitude to Dr. M.R. VIKRAMAN,

Principal, Aryanet Institute of Technology, Palakkad for fostering an excellent

academic climate in the college and for his support and encouragement throughout the

course period.

I wish to thank Prof. V GOPINATHAN, Head of the Department,

Mechanical Engineering, Aryanet Institute of Technology, Palakkad, for providing all

the facilities and support.

I am thankful to Mr.SREEJITH M (Seminar Co-ordinator of Mechanical

Engineering), Assistant Professor, Department of Mechanical Engineering, Aryanet

Institute of Technology, Palakkad for his advice and invaluable supervision of the

seminar.

I would like to express my indebtedness to my seminar guide Mr.LATHESH K,

Assistant Professor, Department of Mechanical Engineering, Aryanet Institute of

Technology, Palakkad for his encouragement, suggestions and support throughout this

work.

I would like to take this opportunity to thank my friends who spent their

valuable time and shared their knowledge for helping me to complete the seminar

with the best possible result.

SUDHEESH SANKAR S R

_____________________________________________________________________Department of Mechanical Engineering, AIT (2012-2016)

iii


ABSTRACT

With the advent of lasers in the 1960s, researcher and engineers discovered a new and

powerful tool to investigate natural phenomena and improve technologically critical

processes. Nowadays, applications of different lasers span quite broadly from

diagnostics tools in science and engineering to biological and medical uses. In this

report basic principles and applications of lasers for ignition of fuels are concisely

reviewed from the engineering perspective. The objective is to present the current

state of the relevant knowledge on fuel ignition and discuss select applications,

advantages and disadvantages, in the context of combustion engines. Fundamentally,

there are four different ways in which laser light can interact with a combustible

mixture to initiate an ignition event. They are referred to as thermal initiation, non-

resonant breakdown, resonant breakdown, and photochemical ignition. By far the

most commonly used technique is the non-resonant initiation of combustion primarily

because of its freedom in selecting the laser wavelength and ease of implementation.

Recent progress in the area of high power fiber optics allowed convenient shielding

and transmission of the laser light to the combustion chamber. However, issues related

to immediate interfacing between the light and the chamber such as selection of

appropriate window material and its possible fouling during the operation, shaping of

the laser focus volume, and selection of spatially optimum ignition point remain

amongst the important engineering design challenges. One of the potential advantages

of the lasers lies in its flexibility to change the ignition location. Also, multiple

ignition points can be achieved rather comfortably as compared to conventional

electric ignition systems using spark plugs. Although the cost and packaging

complexities of the laser ignition systems have dramatically reduced to an affordable

level for many applications, they are still prohibitive for important and high-volume

applications such as automotive engines. However, their penetration in some niche

markets, such as large stationary power plants and military applications, are imminent.


iv


CONTENTS

SL No. TITLE Page No.

LIST OF TABLES viii

LIST OF FIGURES viii

LIST OF SYMBOLS x

LIST OF ABBREVIATIONS xi

1 INTRODUCTION 1

2 LITERATURE REVIEW 2

3BACKGROUND STUDY OF IGNITION IN ICENGINE 5

3.1 What is ignition 5

3.2 Ignition types 5

3.2.1 Compression Ignition (CI) or Auto Ignition 5

3.2.2 Induced Ignition 5

3.3 Conventional Spark Plug 6

____________________________________________________________________Department of Mechanical Engineering, AIT (2012-2016)

v


3.4 Sparking Plug Ignition 6

3.4.1 Drawbacks of Conventional Ignition System 7

4 LASER 9

4.1 Types of laser 12

5 LASER IGNITION 14

5.1 Types of laser ignition 15

5.2 Laser ignition along time 17

5.3 Ignition in combustion chamber 18

5.4 Mechanism of laser ignition 18

5.5 Principle of laser ignition 20

5.6 Arrangement of laser ignition system 20

5.6.1 Laser spark plug 21

5.7 Working of laser ignition system 22


vi


6 EXPERIMENTS AND RESULTS 24

6.1 Combustion chamber experiments 24

6.2 Engine experiments 24

6.3 Results of the experiment 26

6.4Additional possibilities for the application

of laser Ignition 29

6.5 Advantages of laser ignition 30

6.6 Future Research Needs and Shortcomings 31

7 CONCLUSION 32

8 REFERENCE 33


vii


LIST OF TABLES

Table No. TITLE Page No.

6.1

Technical data of the research engine and

the ND: YAG laser used for the

experiments

LIST OF FIGURES

SL No. TITLE Page No.

3.1 Conventional spark plug 9

3.2 Four stroke engine cycle 10

4.1 Principal components of a laser 13

4.2 Lasing action diagram 14

5.1Optical breakdown in air generated by aND: YAG laser 17

5.2 Non resonant breakdown 20

_________________________________________________________________Department of Mechanical Engineering, AIT (2012-2016)

viii

25


5.3 Stages of ignition with respect to time 17

5.4 Ignition inside combustion chamber 18

5.5 Principle of laser ignition 20

5.6 Laser arrangement with respect to engine 21

5.7 Laser plug 21

5.8Laser ignition system for multi cylinder

engine 22

6.1Research engine with the q-switched Nd: YAG

laser system 25

6.2Comparison of performance parameters of S Iwith respect to L I engines 26

6.3 Self cleaning property 27

6.4 Flame front propogation 28

6.5Comparison of NOX emissions of different

ignition 29

6.6 Flame front 29 0 after ignition 29

6.7Variation of ignition energy with respect to

combustion chamber temperatures 30


ix


LIST OF SYMBOLS

I Intensity of an electromagnetic wave

E Electric field strength

D Diameter of the laser beam

M Beam quality

λ Wave length of laser beam

f Focal length of the optical element

T Temperature

P Pressure

k Boltzmann’s constant


x


LIST OF ABBREVIATIONS

IMEP Indicated Mean Effective Pressures

COV Coefficient Of Variation

SIS Spark Ignition System

LIS Laser Ignition System

IC Engine Internal Combustion Engine

µs Nano Second

Mj Milli-Joule

Mpa Mega Pascal

MPI Multi Photon Ionization

DOHC Double-Overhead-Camshaft


Xi


PPM Particles Per Million

CH4 Methane

CO2 Carbon Dioxide

NO X Oxides Of Nitrogen

MEP Mean Effective Pressure

Is Build-Up Intensity

Es Build-Up Energy

MPE Minimum Pulse Energy For Ignition

_____________________________________________________________Department of Mechanical Engineering, AIT (2012-2016)

Xii


Department of Mechanical Engineering, AIT (2012-2016)1

CHAPTER - 1

INTRODUCTION

It's widely accepted that the internal combustion engines will continue to power our vehicles.

Hence, as the global mobilization of people and goods increases, advances in combustion and

after-treatment are needed to reduce the environmental impact of the continued use of IC

engine vehicles. To meet environmental legislation requirements, automotive manufacturers

continue to address two critical aspects of engine performance, fuel economy and exhaust gas

emissions. New engines are becoming increasingly complex, with advanced combustion

mechanisms that burn an increasing variety of fuels to meet future goals on performance, fuel

economy and emissions. The spark plug has remained largely unchanged since its invention,

yet its poor ability to ignite highly dilute air- fuel mixtures limits the potential for improving

combustion efficiency. Spark ignition (SI) also restricts engine design, particularly in new

engines, since the spark position is fixed by the cylinder head location of the plug, and the

protruding electrode disturbs the cylinder geometry and may quench the combustion flame

kernel.

So, many alternatives are being sought after to counter these limitations. One of the

alternative is the laser ignition system (LIS) being described here. Compared to a

conventional spark plug, a LIS should be a favorable ignition source in terms of lean burn

characteristics and system flexibility . So, in this paper we'll be discussing the implementation

and impact of LIS on IC engines.



CHAPTER 2

LITREATURE REVIEW

The history of laser induced ignition has progressed in three distinct directions. The first is the

theoretical analysis of the breakdown phenomenon in which the physical ignition processes at

the molecular level of the ignition event are investigated. The second is comprised of

laboratory experiments conducted to gain insight and understanding of the ignition process

and to help test and tune previously developed theories. The third consists of experiments and

analysis performed on slider-crank piston engines to gauge the effectiveness of laser energy

ignition on the engine operating parameters.

Past theoretical studies have lead to the statistical development and comparison of ignition

delay and ignition probability models to experimental observations allowing the direct

correlation of gas (usually methane) concentration to ignitability and ignition delay.

Theoretical analysis has also led to the development of shock wave heating models, which aid

in the explanation of the propagation of hot expanding gas, produced by the laser spark, that

perpetuates the combustion process. Further theoretical examinations and the availability of

experimental data have allowed researchers to develop more precise estimations of the

minimum required laser induced breakdown energy required for ignition of combustible gases

as well as focal length effects.

The method by which the laser induces breakdown in a combustible gaseous mixture has

been divided into four basic processes: thermal heating, non resonant breakdown,

resonant breakdown and photochemical excitation. Thermal heating takes place where the

laser beam is incident on a solid target and induces excitation by heating the target or by

exciting a rotational or vibrational modes of oscillation in the surrounding gas.

Resonant breakdown occurs when the incident radiation ionizes the gas molecules and frees

up electrons to absorb the radiation energy and in turn ionize other gas molecules leading to

avalanche breakdown. Photochemical ignition occurs when a single photon dissociates a

molecule thus allowing the ionized constituents to react with the surrounding gases. Non-

resonant breakdown occurs when laser light is focused into a gas and the electrical field

component of the light is strong enough to initiate the electrical breakdown of the gas. The

non-resonant breakdown mechanism is the predominant factor governing the results presented

in this work.



Lasers promise less pollution and greater fuel efficiency, but making small, powerful lasers

has, until now, proven hard. To ignite combustion, a laser must focus light to approximately

100 giga-watts per square centimeter with short pulses of more than 10 millijoules each.In the

past, lasers that could meet those requirements were limited to basic research because they

were big, inefficient, and unstable. Nor could they be located away from the engine, because

their powerful beams would destroy any optical fibers that delivered light to the cylinders.

This problem overcame by making composite lasers from ceramic powders. In this the

powders is heated and fuse into optically transparent solids and embeds metal ions in them to

tune their properties. Ceramics are easier to tune optically than conventional crystals. They

are also much stronger, more durable, and thermally conductive, so they can dissipate the heat

from an engine without breaking down. The use of laser ignition to improve gas engine

performance was initially demonstrated by J. D. Dale in 1978. However, with very few exceptions,

work in this area has for the last 20 years been limited to laboratory experimentation employing large,

expensive and relatively complicated lasers and laser beam delivery systems.

Experimental studies have been vital to extending the value of the theoretical

examinations and in gaining a further understanding of the combustion process. Combustion

vessel and open flame jet experimentation with methane (CH4) and other combustible gases

have proven invaluable in the search for better fuel economy and emissions and provide a

better understanding of the general ignition and combustion processes. Results of the laser

spark combustion vessel studies has indicated a shortened ignition delay and higher peak

pressures than an electrical spark ignited combustion event. Some studies investigated the

ignition energy effect on the combustion process and found that for stoichiometric conditions

the amount of energy had only a slight pressure dependence, however more energy was

required for breakdown as the equivalence ratio approached either lean or rich conditions

Other studies have examined multipoint laser ignition as a means of gaining quicker

combustion which allows for higher thermal efficiency due to reduced time for thermal losses

during the combustion event and overall shorter travel distances for the flames .The most

promising result of the combustion vessel examination of laser ignition is the ability of the

optical energy to ignite and more readily burn lean mixtures. This offers the potential for

extending the lean limit in spark ignited engines which .Laser ignition studies performed on

internal combustion engines have allowed researchers to directly study the effect that laser



induced ignition has on the operating and emissions characteristics of an operating engine.

Past and recent studies have indicated a higher and quicker combustion pressure rise with

laser ignition.

The experimentation performed by Dale et al., used a gasoline-fueled stoichiometric

operating internal combustion engine for testing. The research performed by Ma et al.,

involved a motored slider crank mechanism that was not self sustaining.

Researchers from Japan's National Institutes of Natural Sciences (NINS) are creating

laser igniters that could one day replace spark plugs in automobile engines. The team from

Japan built its laser from two yttrium aluminum- gallium (YAG) segments, one doped with

neodymium, the other with chromium. They bonded the two sections together to form a

powerful laser only 9 millimeters in diameter and 11 millimeters long (a bit less than half an

inch). The composite generates two laser beams that can ignite fuel in two separate locations

at the same time. This would produce a flame wall that grows faster and more uniformly than

one lit by a single laser. The laser is not strong enough to light the leanest fuel mixtures with a

single pulse. By using several 800- picosecond-long pulses, however, they can inject enough

energy to ignite the mixture completely.A commercial automotive engine will require 60 Hz

(or pulse trains per second), The team has already tested the new dual-beam laser at 100 Hz.

The team is also at work on a three-beam laser that will enable even faster and more uniform

combustion. The laser-ignition system, although highly promising, is not yet being installed

into actual automobiles made in a factory. Scientist team from Japan is, however, working

with a large spark-plug company and with DENSO Corporation, a member of the Toyota

Group.



CHAPTER 3

BACKGROUND STUDY OF IGNITION IN IC ENGINE

3.1 What is ignition

Ignition is the process of starting radical reactions until a self-sustaining flame has developed.

One can distinguish between auto ignition, induced ignition and photo-ignition, the latter

being caused by photolytic generation of radicals

3.2 Ignition types

3.2.1 Compression Ignition (CI) or Auto Ignition

At certain values of temperature and pressure a mixture will ignite spontaneously, this is

known as the auto ignition or compression ignition

3.2.2 Induced Ignition

A process where a mixture, which would not ignite by it, is ignited locally by an ignition

source (i.e. Electric spark plug, pulsed laser, microwave ignition source) is called induced

ignition. In induced ignition, energy is deposited, leading to a temperature rise in a small

volume of the mixture, where auto ignition takes place or the energy is used for the generation

of radicals. In both cases subsequent flame propagation occurs and sets the mixture on fire.

3.3 Conventional Spark Plug

Fig.3.1

Conventional spark plug



A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a

device for delivering electric current from an ignition system to the combustion chamber of

a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while

containing combustion pressure within the engine. A spark plug has a metal threaded shell,

electrically isolated from a central electrode by a porcelain insulator. The central electrode,

which may contain a resistor, is connected by a heavily insulated wire to the output terminal

of an ignition coil or magneto. The spark plug's metal shell is screwed into the

engine's cylinder head and thus electrically grounded. The central electrode protrudes through

the porcelain insulator into the combustion chamber, forming one or more spark gaps between

the inner end of the central electrode and usually one or more protuberances or structures

attached to the inner end of the threaded shell and designated the side, earth,

or ground electrode(s).

Spark plugs may also be used for other purposes; in Saab Direct Ignition when they

are not firing, spark plugs are used to measure ionization in the cylinders – this ionic current

measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire

measurement function. Spark plugs may also be used in other applications such

as furnaces wherein a combustible fuel/air mixture must be ignited. In this case, they are

sometimes referred to as flame igniters

3.4 Sparking Plug Ignition

Conventional spark plug ignition has been used for many years. For ignition of a fuel-air

mixture the fuel-air mixture is compressed and at the right moment a high voltage is applied

to the electrodes of the spark plug.

When the ignition switch is turned on current flows from the battery to the

ignition coil. Current flows through the Primary winding of the ignition coil where one end is

connected to the contact breaker. A cam which is directly connected to the camshaft opens

and closes the contact breaker (CB) points according to the number of the cylinders. When the

cam lobe Pushes CB switch, the CB point opens which causes the current from the primary

circuit to break. Due to a break in the current, an EMF is induced in the second winding

having more number of turns than the primary which increases the battery 12 volts to 22,000

volts.



The high voltage produced by the secondary winding is then transferred to the

distributor. Higher voltage is then transferred to the spark plug terminal via a high tension

cable. A voltage difference is generated between the central electrode and ground electrode of

the spark plug. The voltage is continuously transferred through the central electrode (which is

sealed using an insulator). When the voltage exceeds the dielectric of strength of the gases

between the electrodes, the gases are ionized. Due to the ionization of gases, they become

conductors and allow the current to flow through the gap and the spark is finally produced.

Fig 3.2

Four stroke engine cycle

3.4.1 Drawbacks of Conventional Ignition System

• Location of spark plug is not flexible as it requires shielding of plug from immense heat and

fuel spray

• Ignition location cannot be chosen optimally.

• Spark plug electrodes can disturb the gas flow within the combustion chamber.

• It is not possible to ignite inside the fuel spray.

• It requires frequent maintenance to remove carbon deposits.



• Leaner mixtures cannot be burned, ratio between fuel and air has to be within the correct

range

• Degradation of electrodes at high pressure and temperature.

• Flame propagation is slow.

• Multi point fuel ignition is not feasible.

• Higher turbulence levels are required.

• Erosion of spark plug electrodes.



CHAPTER - 4

LASER

The word LASER is an acronym. It stands for Light Amplification by the Stimulated

Emission of Radiation. By "radiation", however, the acronym refers to a radiant vibration, not

an emission of radioactive particles. In other words, the emissions of lasers are in the form of

light, and the frequencies can range anywhere from infra-red to ultraviolet. Those lasers of

interest to the laser display industry, however, are mostly those whose output is visible (from

red to deep blue).

As the acronym suggests, lasers work through a process called stimulated emission.

The lasers we typically employ are called ion gas lasers, due to the fact that they utilize a gas

or a mixture of gases as the lasing medium. These work because certain gases are "easily"

coerced to produce visible light through this process.

Fig 4.1

Principal components of a laser

The stimulation comes in the form of electricity, which excites the atoms of the gas: as

the electrons in these atoms are given more energy, they tend to jump to a higher orbit. These

unnaturally high orbits, however, don't last long, and the electrons fall back to their proper

orbital shells, to be once again excited by the influx of electricity. It is this process of the



electrons returning to their original orbits that creates the laser light we see (actually, it's more

appropriate to call it a jump, for an electron falls from orbit in a span of time infinitesimally

small): During this jump back down, the extra energy is released from each atom as a packet

called a photon (light). Moreover, if this photon collides with another already excited atom,

that atom is also stimulated to emit a photon...but this new photon will be vibrating perfectly

in step (in-phase) with the colliding photon, and will be traveling on the exact same course.

Photons are released, however, in haphazard directions. In order to get them aligned into the

tight beam of light with which we're familiar, the tubes in which the atoms of gas are excited

must be mirrored on both ends. Any photon that now happens to randomly travel exactly

perpendicular with the mirrors on both ends (which inevitably happens) will cause a

remarkable chain of events: The drama begins with the photon's 'cloning' when it bounces off

the mirror and collides with another excited atom. Those two in-phase photons then collide

with two more excited atoms, making four photons traveling in-phase, and exactly down the

length of the laser tube. This process is then repeated in a geometric progression of photons

parading exactly down the laser tube, colliding with more excited atoms, creating more

photons, reflecting off the mirrors, and repeating and amplifying the process over and over

again. The laser light we see is finally released through the front mirror, whose reflective

coating was designed to be partially transparent. In this way, a small percentage of those

perfectly aligned photons is allowed to escape, forming the thin, straight, coherent, and

beautiful beams we call LASER light.



Fig 4.2

Lasing action diagram

Different colors of light, as specific frequencies, are produced by different gases.

Argon gas, for instance, produces colors ranging from emerald green to beautiful deep blues.

Krypton gas produces a palette from deep reds to light blues. A laser incorporating a mixture

of these two gases can produce all the colors unique to those individual gases...

simultaneously. It is these Krypton/Argon mixed-gas ion lasers that are typically utilized in

laser projection hardware.

To sum up, laser light provides a quality of light unmatched by any other light source

in the world: It's coherent, meaning again that the waves of light are vibrating perfectly in step

with each other (in-phase); It's monochromatic, meaning that only very pure, specific

frequencies (colors) of light are created (though several frequencies can be created

simultaneously with the same laser); and it's low-divergent, meaning it keeps its power

contained within a narrow, barely widening beam.



4.1 Types of laser

∑ Gas lasers

The helium-neon laser (hene) emits 543 nm and 633 nm and is very common in education

because of its low cost. Carbon dioxide lasers emit up to 100 kw at 9.6 μm and10.6 μm, and

are used in industry for cutting and welding. Argon-ion lasers emit 458 nm, 488 nm or 514.5

nm. Carbon monoxide lasers must be cooled but can produce up to 500kw. The transverse

electrical discharge in gas at atmospheric pressure (tea) laser is an inexpensive gas laser

producing uv light at 337.1 nm. Metal ion lasers are gas lasers that generate deep ultraviolet

wavelengths. Helium-silver (heag) 224 nm and neon-copper (necu) 248 nm are two examples.

These lasers have particularly narrow oscillation line widths of less than 3 ghz (0.5 pico

meters) making them candidates for use in fluorescence suppressed raman spectroscopy.

∑ Chemical laser

Chemical lasers are powered by a chemical reaction, and can achieve high powers in

continuous operation. For example, in the hydrogen fluoride laser (2700-2900 nm) and the

deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium

gas with combustion products of ethylene in nitrogen trifluoride.

∑ Excimer lasers

Excimer lasers produce ultraviolet light, and are used in semiconductor manufacturing and in

lasik eye surgery. Commonly used excimer molecules include f2 (emitting at 157 nm), arf

(193 nm), krcl (222 nm), krf (248 nm), xecl (308 nm), and xef (351nm).

Solid-state lasers

Solid state laser materials are commonly made by doping a crystalline solid host with ions

that provide the required energy states. For example, the first working laser was made from

ruby, or chromium-doped sapphire. Another common type is made from neodymium-doped

yttrium aluminium garnet (yag), known as nd:yag. Nd:yag lasers can produce high powers in

the infrared spectrum at 1064 nm. They are used for cutting, welding and marking of metals

and other materials, and also in spectroscopy and for pumping dye lasers. Nd:yag lasers are

also commonly doubled their frequency to produce 532 nm when a visible (green) coherent

source is required. ytterbium, holmium, thulium and erbium are other common dopants in

solid state lasers. The ho-yag is usually operated in a pulsed mode, and passed through optical



fibre surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and

pulverize kidney and gall stones. Titanium-doped sapphire (ti: sapphire) produces a highly

tunable infrared laser, used for spectroscopy. Solid state lasers also include glass or optical

fibre hosted lasers, for example, with erbium or ytterbium ions as the active species. These

allow extremely long gain regions, and can support very high output powers because the

fibre’s high surface area to volume ratio allows efficient cooling and its wave guiding

properties reduce thermal distortion of the beam.

∑ Semiconductor lasers

Laser diodes produce wavelengths from 405 nm to 1550 nm. Low power laser diodes are used

in laser pointers, laser printers, and cd/dvd players. More powerful laser diodes are frequently

used to optically pump other lasers with high efficiency. The highest power industrial laser

diodes, with power up to 10 kw, are used in industry for cutting and welding. External-cavity

semiconductor lasers have a semiconductor active medium in a larger cavity. These devices

can generate high power outputs with good beam quality, wavelength-tunable narrow-line

width radiation, or ultra-short laser pulses.

Vertical cavity surface-emitting lasers (vessels) are semiconductor lasers whose

emission direction is perpendicular to the surface of the wafer. Vessel devices typically have a

more circular output beam than conventional laser diodes, and potentially could be much

cheaper to manufacture. As of 2005, only 850 nm vessels are widely available, with 1300 nm

vessels beginning to be commercialized [7], and 1550 nm devices an area of research. Vessels

are external-cavity vessels. Quantum cascade lasers are semiconductor lasers that have an

active transition between energy sub-bands of an electron in a structure containing several

quantum wells

∑ Dye lasers

Dye lasers use an organic dye as the gain medium. The wide gain spectrum of available dyes

allows these lasers to be highly tunable, or to produce very short-duration pulses (on the order

of a few femto seconds).



CHAPTER 5

LASER IGNITION

Laser ignition, or laser-induced ignition, is the process of starting combustion by the stimulus

of a laser light source.Laser ignition uses an optical breakdown of gas molecule caused by an

intense laser pulse to ignite gas mixtures. The beam of a powerful short pulse laser is focused

by a lens into a combustion chamber and near the focal spot and hot and bright plasma is

generated.

Fig 5.1Optical breakdown in air generated by a ND: YAG laser. At a wavelength of 1064 nm,

at 532nm

The process begins with multi-photon ionization of few gas molecules which releases

electrons that readily absorb more photons via the inverse bremsstrahlung process to increase

their kinetic energy. Electrons liberated by this means collide with other molecules and ionize

them, leading to an electron avalanche, and breakdown of the gas. Multiphoton absorption

processes are usually essential for the initial stage of breakdown because the available photon

energy at visible and near IR wavelengths is much smaller than the ionization energy. For

very short pulse duration (few picoseconds) the multiphoton processes alone must provide

breakdown, since there is insufficient time for electron-molecule collision to occur. Thus this

avalanche of electrons and resultant ions collide with each other producing immense heat

hence creating plasma which is sufficiently strong to ignite the fuel. The wavelength of laser



depend upon the absorption properties of the laser and the minimum energy required depends

upon the number of photons required for producing the electron avalanche.

5.1 Types of laser ignition

Basically, energetic interactions of a laser with a gas may be classified into one of the

following four schemes as described in.

∑ Thermal initiation

In thermal initiation of ignition, there is no electrical breakdown of the gas and a laser beam is

used to raise the kinetic energy of target molecules in translational, rotational, or vibrational

forms. Consequently, molecular bonds are broken and chemical reaction occur leading to

ignition with typically long ignition delay times. This method is suitable for fuel/oxidizer

mixtures with strong absorption at the laser wavelength. However, if in a gaseous or liquid

mixtures is an objective, thermal ignition is unlikely a preferred choice due to energy

absorption along the laser propagation direction. Conversely, this is an ideal method for

homogeneous or distributed ignition of combustible gases or liquids. Thermal ignition method

has been used successfully for solid fuels due to their absorption ability at infrared

wavelengths.

• Non-resonant breakdown

In nonresonant breakdown ignition method, because typically the light photon energy is

invisible or UV range of spectrum, multiphoton processes are required for molecular

ionization. This is due to the lower photon energy in this range of wavelengths in comparison

to the molecular ionization energy. The electrons thus freed will absorb more energy to boost

their kinetic energy (KE), facilitating further molecular ionization through collision with other

molecules. This process shortly leads to an electron avalanche and ends with gas breakdown

and ignition. By far, the most commonly used technique is the nonresonant initiation of

ignition primarily because of the freedom in selection of the laser wavelength and ease of

implementation.



Fig 5.2

Non resonant breakdown

• Resonant breakdown

The resonant breakdown laser ignition process involves, first, a nonresonant multiphoton

dissociation of molecules resulting to freed atoms, followed by a resonant photo ionization of

these atoms. This process generates sufficient electrons needed for gas breakdown.

Theoretically, less input energy is required due to the resonant nature of this method.

• Photochemical mechanisms

In photochemical ignition approach, very little direct heating takes place and the laser beam

brings about molecular dissociation leading to formation of radicals (i.e., highly reactive

chemical species), if the production rate of the radicals produced by this approach is higher

than the recombination rate (i.e., neutralizing the radicals), then the number of these highly

active species will reach a threshold value, leading to an ignition event. This (radical) number

augmentation scenario is named as chain-branching in chemical terms.



5.2 Laser ignition along time

Laser ignition encompasses the nanosecond domain of the laser pulse itself to the duration of

the entire combustion lasting several hundreds of milliseconds.

Fig 5.3

Stages of ignition with respect to time

The laser energy is deposited in a few nanoseconds which lead to a shock wave generation. In

the first milliseconds an ignition delay can be observed which has duration between 5 – 100

ms depending on the mixture. Combustion can last between 100 ms up to several seconds

again depending on the gas mixture, initial pressure, pulse energy, plasma size, position of the

plasma in the combustion bomb and initial temperature



5.3 Ignition in combustion chamber

Fig 5.4

Ignition inside combustion chamber

The laser beam is passed through a convex lens, this convex lens diverge the beam and make

it immensely strong and sufficient enough to start combustion at that point. Hence the fuel is

ignited, at the focal point. The focal point is adjusted where the ignition is required to have.

5.4 Mechanism of laser ignition

It is well know that short and intensive laser pulses are able to produce an “optical

breakdown” in air. Necessary intensities are in the range between 1010 to 1011W/cm2. At such

intensities, gas molecules are dissociated and ionized within the vicinity of the focal spot of a

laser beam and hot plasma is generated. This plasma is heated by the incoming laser beam and

a strong shock wave occurs. The expanding hot plasma can be used for the ignition of fuel-gas

mixtures. By comparing the field strength of the field between the electrodes of a spark plug

and the field of a laser pulse it should be possible to estimate the required laser intensity for

generation of an optical breakdown. The field strength reaches values in the range of

approximately 3×104V/cm between the electrodes of a conventional spark plug. Since the

intensity of an electromagnetic wave is proportional to the square of the electric field strength

I ∝E2, one can estimate that the intensity should be in the order of 2 × 106 W/cm2, which is

several orders of magnitude lower as indicated by experiments on laser ignition. The reason is

that usually no free electrons are available within the irradiated volume. At the electrodes of a

spark plug electrons can be liberated by field emission processes. In contrary, ionization due

to irradiation requires a “multiphoton” process where several photons hit the atom at nearly


Department of Mechanical Engineering, AIT ( 2012-2016 )19

the same time. Such multiphoton ionization processes can only happen at very high irradiation

levels (in the order of 1010to 1011W/cm2)

where the number of photons is extremely high. For example, nitrogen has an ionization

energy of approximately 14.5 eV, whereas one photon emitted by a Nd:YAG laser has

an energy of 1.1 eV, thus more than 13 photons are required for ionization of nitrogen.

The pulse energy of a laser system for ignition can be estimated by the following

calculation. The diameter d of a focused laser beam is

D = 2 × wf = 2 × M2 × ………. (1)

where M2 is the beam quality, F is the focal length of the optical element and D is the

diameter of the laser beam with the wavelength λ. Now it is assumed that the laser beam

irradiates a spherical volume.

V =^ …………………….(2)

From the thermodynamical gas equation the number of particles N in a volume V is

N = ……………………..(3)

With the pressure p, temperature T and Boltzmann’s constant k = 1.38 × 10 -23J/K. Inside the

irradiated volume, N molecules have to be dissociated where first the dissociation energy Wd

is required and finally 2N atoms are ionized (ionization energy Wi). Using known values for

Wd= 9.79 eV and Wi= 14.53 eV for nitrogen, the energy for dissociating and ionizing all

particles inside the volume can be calculated as

W = ( ) × (Wd + 2Wi) ……(4)

For a spot radius of about 100 μm the equation gives a maximum energy of approximately 1

mJ.Since not all particles inside the irradiated volume have to be ionized, even

smallerenergies should be sufficient for generation of an optical breakdown.

It is assumed that the intensity which is necessary for the generation of an optical breakdown

processes is related to the pressure of the gas

I α 1/Pn

With n =1…5 depending on the mechanism of multiphoton process. Higher pressures, like in

a combustion chamber should ease the ignition process what favors the laser induced ignition.



5.5 Principle of laser ignition

Fig 5.5Principle of laser ignition

The laser beam is passed through a convex lens, this convex lens diverge the beam and make

it immensely strong and sufficient enough to start combustion at that point. Hence the fuel is

ignited, at the focal point, with the mechanism shown above. The focal point is adjusted

where the ignition is required to have.

5.6 Arrangement of laser ignition system

A laser ignition device for irradiating and condensing laser beams in a combustion chamber of

an internal combustion engine so as to ignite fuel particles within the combustion chamber,

includes: a laser beam generating unit for emitting the laser beams; and a condensing optical

member for guiding the laser beams into the combustion chamber such that the laser beams

are condensed in the combustion chamber.



Fig 5.6

Laser arrangement with respect to engine

5.6.1 Laser spark plug

Fig 5.7

Laser plug



5.7 Working of laser ignition system

Fig 5.8

Laser ignition system for multi cylinder engine

The laser ignition system has a laser transmitter with a fibre-optic cable powered by the car’s

battery. The average power requirements for a laser spark plug are relatively modest. A four

stroke engine operating at maximum of 1200 rpm requires an ignition spark 10 times per

second or 10Hz (1200rpm/2x60). For example 1-Joule/pulse electrical diode pumping levels

we are readily able to generate high mill joule levels of Q-switched energy. This provides us

with an average power requirement for the laser spark plug of say approximately 1-Joule

times 10Hz equal to approximately 10 Watts .It shoots the laser beam to a focusing lens that

would consume a much smaller space than current spark plugs. The lenses focus the beams

into an intense pinpoint of light by passing through an optical window. The laser beam is

passed through a convex lens, this convex lens diverge the beam and make it immensely

strong and sufficient enough to start combustion at that point. Hence the fuel is ignited, at the

focal point, with the mechanism shown above. The focal point is adjusted where the ignition

is required to have. when the fuel is injected into the engine, the laser is fired and produces

enough energy (heat) to ignite the fuel

Hence the fuel is ignited, at the focal point, with the mechanism shown above. The

focal point is adjusted where the ignition is required to have. The plasma generated by the



Laser beam results in two of the following actions :

1. Emission of high energy photons

2. Generation of shock waves

The high energy photons, heat and ionize the charge present in the path of laser beam which

can be seen from the propagation of the flame which propagates longitudinally along the laser

beam.

The shock waves carry energy out wards from the laser beam and thus help in propagation of

flame



CHAPTER 6

EXPERIMENTS AND RESULTS

6.1 Combustion chamber experiments

As a feasibility test, an excimer laser has been used for ignition of inflammable gases inside a

combustion bomb. The laser used for the first experiments was a Lambda Physik LPX205,

equipped with an unstable resonator system and operated with KrF, delivering pulses with a

wavelength of 248 nm and a duration of approximately 34 ns with maximum pulse energy of

400 mJ.10 The combustion chamber has had a diameter of 65 mm and a height of 86mm, with

a resulting volume of 290cm3 and was made of steel.

The laser beam was focused into the chamber by means of a lens with a focal length of

50 mm. Variations of pulse energies as well as gas mixtures have been performed to judge the

feasibility of the process. Results indicate that ignition-delay times are smaller and pressure

gradients are much steeper compared to conventional spark plug ignition

6.2 Engine experiments

Since the first feasibility experiments could be concluded successfully, an engine was

modified for laser ignition. The engine has been modified by a replacement of the

conventional spark plug by a window installed into a cylindrical mount. The position of the

focusing lens inside the mount can be changed to allow variations of the location of the initial

optical breakdown. First experiment with laser ignition of the engine have been performed

with an excimer laser, later a q switched ND: YAG has been used

The replacement of the excimer laser was mainly caused by the

fact that especially at very low pulse energies the excimer laser shows strong energy

fluctuations. Pulse energies, ignition location and fuel/air ratios have been varied during the

experiments. The engine has been operated at each setting for several hours, repeatedly. All

laser ignition experiments have been accompanied by conventional spark plug ignition as

reference measurements.



Fig 6.1

Research engine with the q-switched Nd: YAG laser system

Table 6.1

Technical data of the research engine and the ND: YAG laser used for the experiments

Research engine switched Nd:YAG

No. of cylinders1

Pump source Flash lamp

No. of valves 1 Wavelength 1064 or 532 nmInjector Multihole Max. pulse

energy1064 or 532 nm

Stroke 85 mm Pulse duration 6 ns

Bore 88 mm Power consumption

1 kW

Displacement vol.

517 cm3 Beam diameter 6 mm

Comp. ratio 11.6 Type Quantel Brilliant



6.3 Results of the experiment

Results of the experiments are summarized in fig shows that laser ignition has advantages

compared to conventional spark plug ignition. Compared to conventional spark plug ignition,

laser ignition reduces the fuel consumption by several per cents. Exhaust emissions are

reduced by nearly 20%. It is important that the benefits from laser ignition can be achieved at

almost the same engine smoothness level, as can be seen from .

Fig 6.2

Comparison of performance parameters of S I with respect to L I engines

Additionally, a frequency-doubled Nd: YAG laser has been used to examine possible

influences of the wavelength on the laser ignition process. No influences could be found. Best

results in terms of fuel consumption as well as exhaust gases have been achieved by laser

ignition within the fuel spray. As already mentioned, it is not possible to use conventional

spark plugs within the fuel spray since they will be destroyed very rapidly. Laser ignition

doesn’t suffer from that restriction.



Another important question with a laser ignition system is its reliability. It is clear that the

operation of an engine causes very strong pollution within the combustion Chamber. Deposits

caused by the combustion process can contaminate the beam entrance window and the laser

ignition system will probably fail. To quantify the influence of deposits on the laser ignition

system, the engine has been operated with a spark plug at different load points for more than

20 hours with an installed beam entrance window. As can be seen in fig the window was

soiled with a dark layer of combustion deposits. Afterwards, a cold start of the engine was

simulated. Already the first laser pulse ignited the fuel/air mixture. Following laser pulses

ignited the engine without misfiring, too. After 100 cycles the engine was stopped and the

window was disassembled.

Fig 6.3

Self cleaning property

As can be seen from fig 6.3 all deposits have been removed by the laser beam. Additional

experiments showed that for smooth operation of the engine the minimum pulse energy of the

laser is determined by the necessary intensity for cleaning of the beam entrance window.

Estimated minimum pulse energies are too low since such “self-cleaning” mechanisms are not

taken into account. Engine operation without misfiring was always possible above certain

threshold intensity at the beam entrance window. For safe operation of an engine even at cold

start conditions increased pulse energy of the first few laser pulses would be beneficial for

cleaning of the beam entrance window.



Fig 6.4

Flame front propogation

The above figure explains the flamefront propagation inside the combustion chamber during

combustion .Plasma had the maximum emission peak 14 ns after the laser was fired and laser

plasma UV-emission persisted for about 80 ns .Minimum laser pulse energy (MPE) for

ignition is decreases with increasing initial pressure.The time of pressure rise in case of laser

ignition is shorter than the spark ignition.

Engines would produce less NOx if they burnt more air and less fuel,

but they would require the plugs to produce higher energy sparks in order to do so. Less NOx

emission

Fig 6.5

Comparison of NOX emissions of different ignition



6.4 Additional possibilities for the application of laser Ignition

To fully utilize the potentialities of laser ignition, the developer must understand and master

the interrelationships in the engine perfectly. There is no sense in utilizing only the NOX

advantages with a costly system and not paying attention to the specific fuel consumption.

Consequently, additional measures must be taken to maintain the fuel consumption level

under conditions of extremely lean operation and even to improve it. In this regard,

researchers place great emphasis on its experience with high turbulence to accelerate

combustion (HEC concept). However, there are also other innovative approaches possible

with laser ignition. One tested approach is so-called multi-point ignition, which has been

investigated not only in terms of the theoretical approach, but also through studies dealing

with combustion vessels.

Fig 6.6

Flame front 29 0 after ignition

As an example, Figure presents the result of the calculated flame front of a 4-point Laser

ignition after 29°CA in operation at Lambda 2.05. In this manner, the spark duration (90 %)

can be reduced approximately to less than half (NOX level 30 ppm)

Another approach is to improve ignition conditions and flame propagation by

increasing combustion chamber temperatures. As well, this allows the required ignition

energies to be reduced considerably. An example regarding this approach is shown in Figure

below.



Fig 6.7

Variation of ignition energy with respect to combustion chamber temperatures

With a very lean mixture it is possible to reduce the required ignition energy by about 30 %

by increasing the temperature by 50°C (from150 to 200°C). At full load the temperatures at

the firing point are a good deal higher. To be able to better understand the interrelationships,

the tests with the combustion vessel were extended to a temperature level of 400°C. The

results in the case of methane are presented in Figure 11. Using this approach, the required

ignition energy can be kept at less than 2 mj up to Lambda 2.2. Knowledge of the global

interrelationships is therefore very important for the design of the laser.

6.5 Advantages of laser ignition

The main advantages of laser ignitions are given below:

∑ A choice of arbitrary positioning of the ignition plasma in the combustion cylinder

∑ Absence of quenching effects by the spark plug electrodes

∑ Ignition of leaner mixtures than with the spark plug; lower combustion temperatures and

less Nox emissions

∑ No erosion effects as in the case of the spark plugs, lifetime of a laser ignition System

expected to be significantly longer than that ofa spark plug

∑ High load/ignition pressures possible, increasein efficiency\

∑ Precise ignition timing possible



∑ Exact regulation of the ignition energy deposited in the ignition plasma

∑ Easier possibility of multipoint ignition

∑ Shorter ignition delay time and shorter combustion time

∑ The thermodynamic requirements of a high compression ratio and a high power density are

fulfilled well by laser ignition

6.6 Future Research Needs and Shortcomings

Delivering the beam through free space and channeling it into the combustion chamber

through the optical plug achieved the best results – reducing the Coefficient of Variation,

making combustion smoother and more fuelefficient. The team was particularly keen to

deliver the beam via optical fiber, since this was likely to be less susceptible to engine

vibration and could facilitate improved engine layout. They tried out a range of optical fibers,

including silica and sapphire, and experimented with different internal fiber structures, core

sizes and beam coupling optics. Delivering the beam via optical fiber proved to be more

difficult than the research team had hoped. The fiber didn’t respond well to engine vibration,

which increased the divergence of the output beam and reduced the beam mode quality.

Bending the fiber was also problematical and up to 20 per cent of the beam energy was lost

with small bend diameters, while tight bends caused the fiber to fail altogether after a period.

What’s more, the high density of laser energy can cause immediate or long term degradation,

leading to loss of beam transmission – and therefore loss of ignition. Careful design of laser

parameters, fiber coupling and choice of optical media is crucial to avoid this. These problems

can be solved with further research.



CHAPTER - 7

CONCLUSION

Laser induced ignition of I C engines has been examined. The feasibility of a laser-induced

ignition system on a direct injected gasoline engine has been proven in long-term

experiments. Main advantages are are the almost free choice of the ignition location within

the combustion chamber, even inside the fuel spray. Significant reductions in fuel

consumption as well as reductions of exhaust gases show the potential of the laser ignition

process. Results indicate that pollution of the beam entrance window is not critical as

expected, even heavily polluted windows have had no influence on the ignition characteristics

of the engine. Measurements show that the required pulse energy for successful ignition

decreases with increasing pressure. Laser ignition is nonintrusive in nature; high energy can

be rapidly deposited, has limited heat losses, and is capable of multipoint ignition of

combustible charges. More importantly, it shows better minimum ignition energy requirement

than electric spark systems with lean and rich fuel/air mixtures. It also possesses potentials

for combustion enhancement and better immunity to spurious signals that may accidentally

trigger electric igniters. Although the laser will need to fire more than 50 times per second to

produce 3000 RPM, it will require less power than current spark plugs. The lasers can also

reflect back from inside the cylinders to relay information based on fuel type used and the

level of ignition, enabling cars to readjust the quantities of air and fuel for optimum

performance. At present, a laser ignition plug is very expensive compared to a standard

electrical spark plug ignition system and it is nowhere near ready for deployment. But the

potential and advantages certainly make the laser ignition more attractive in many practical

applications. It was found for the laser ignition tests with hydrogen that with higher initial

pressures the minimum pulse energy for ignition (MPE) decreases. That behaviour was also

found for methane. Fuel-lean biogas/air mixtures exhibit a slower combustion process

resulting in lower peak pressure and flame emission compared to methane-air mixtures of

similar air to fuel equivalence ratio. The applicability of the laser induced ignition as a future

ignition system for combustion engines with spray-guided combustion process could be

proved with the basic research.



CHAPTER - 8

REFERENCE

[1] Swapnil S. Harel, Mohnish Khairnar, Vipul Sonawane, “Laser Ignition System for IC

Engines”, International Journal of Science and Research (IJSR), Volume 3 Issue 7,

July2014.

[2] J. Ma, D. Alexander, and D. Poulain, “Laser spark ignition and combustion

characteristics of methane-air mixtures,” Combustion and Flame, pp. 492–506, 1998

[3] J. Syage, E. Fournier, R. Rianda, and R. Cohn, “Dynamics of flame propagation using

laser-induced spark initiation: Ignition energy measurements,” Journal of Applied

Physics, pp. 1499–1507, 1988.

[4] Lambda Physik, Manual for the LPX205 Excimer Laser,1991

[5] R. Hill, “Ignition-delay times in laser initiated combustion,” Applied Optics, pp.

2239–2242, 1981

[6] J. Ma, D. Alexander, and D. Poulain, “Laser spark ignition and combustion

Characteristics of methane-air mixtures,” Combustion and Flame 112 (4), pp.492–506,

1998

[7] J. Syage, E. Fournier, R. Rianda, and R. Cohn, “Dynamics of flame propagation

Using laser-induced spark initiation: Ignition energy measurements,” Journal of

Applied Physics 64 (3), pp. 1499–1507, 1988.

[8] Lambda Physik, Manual for the LPX205 Excimer Laser, 1991

[9] P. Ronney, “Laser versus conventional ignition of flames,” Opt. Eng. 33 (2), pp.

510–521, 1994.

[10] R. Hill, “Ignition-delay times in laser initiated combustion,” Applied Optics. 20

(13), pp. 2239–2242, 1981