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AN OVERVIEW OF LASER IGNITION SYSTEM

A

Seminar Report

On


By

NILESH H. BANKAR

Under the guidance of

Prof (Dr) D. N. KAMBLE

Submitted in partial fulfillment of the requirement for

T. E. (Mechanical Engineering)

2013-2014

University of Pune

Department of Mechanical Engineering

Sinhgad Technical Education Society’s

Sinhgad Academy of Engineering, Kondhwa, Pune-411048

DEPARTMENT OF MECHANICAL ENGINEERING


Sinhgad Technical Education Society’s

Sinhgad Academy of Engineering, Kondhwa, Pune

Certificate

This is to certify that NILESH H. BANKAR, Examination No _________ of TE (Mech Engg) has

submitted his Seminar Report on AN OVERVIEW OF LASER IGNITION

SYSTEM under the guidance of Prof (Dr) D. N. KAMBLE towards the partial fulfillment

of the requirement for T.E.(Mechanical Engineering), University of Pune for the Academic

year 2013-14.

Prof.(Dr) D N KAMBLE Prof.P.M.Sonawane Prof.S.C.Shilwant (Guide) (Seminar Coordinator) (H O D)

(Examiner 1) (Examiner 2)



ACKNOWLEDGEMENT

It is with a feeling of great pleasure that I would like to articulate my most sincere

heartfelt gratitude to Prof.(Dr) D N KAMBLE, Department of Mechanical Engineering,

Sinhgad Academy of Engineering for guiding me throughout the course of preparing

the Seminar. I am greatly indebted to him for his constructive suggestions and

criticism from time to time during the course of progress of our work. I express my

sincere thanks to Prof P.M.Sonawane, my Project Co-coordinator, Sinhgad

Academy of Engineering for providing us the necessary facilities in the department

and for his valuable guidance, constant encouragement and kind help at different

stages for the execution of this seminar work. I also express my sincere gratitude to,

to Prof.S.C.Shilwant, Head of the Department of Mechanical Engineering for his

timely help during the course of work. I would also like to thank my parents without

whose inspiration and blessings I would not have been able to reach this far. Last but

not the least I will be grateful to all my friends who have stood by me, especially,

when I needed them to during the course of the Seminar.



ABSTRACT

Sustainability with regard to internal combustion engines is strongly linked to the

fuels burnt and the overall efficiency. Laser ignition can enhance the combustion

process and minimize pollutant formation. This paper is on laser ignition of

sustainable fuels for future internal combustion engines. Ignition is the process of

starting radical reactions until a self-sustaining flame has developed.

In technical appliances such as internal combustion engines, reliable ignition is

necessary for adequate system performance. Ignition strongly affects the formation of

pollutants and the extent of fuel conversion. This paper presents experimental results

on laser-induced ignition for technical applications.

Laser ignition tests were performed with the fuels hydrogen and biogas in a static

combustion cell and with gasoline in a spray-guided internal combustion engine. A

Nd: YAG laser with 6 ns pulse duration, 1064 nm wavelength and 1-50 mJ pulse

energy was used to ignite the fuel/air mixtures at initial pressures of 1-3 MPa.

Schlieren photography was used for optical diagnostics of flame kernel development

and shock wave propagation. Compared to a conventional spark plug, a laser ignition

system should be a favorable ignition source in terms of lean burn characteristics and

system flexibility. Yet several problems remain unsolved, e.g. cost issues and the

stability of the optical window. Different window configurations in engine test runs

are compared and discussed.



TABLE OF CONTENTS

Sr. no Contents Page no.

i Acknowledgement 3

ii Abstract 4

iii Table of contents 5

iv List of Figures, Tables 6

1 Introduction 7

1.1 Previous ignition systems 7

1.2 Limitations 7

2 Laser ignition system need? 8

2.1 Advantages of laser ignition 8

3 Laser ignition system 9

3.1 Principle of working 10

4 Optical spark plug 13

5 Experimental setup and analysis of experiment 17

5.1 Experiment 17

5.2 Results 21

6 Conclusion 25

7 References 26



LIST OF FIGURES AND TABLES

TABLE

SR no Name Page no

Table 1- Overview of the various

interdependencies

24

1. INTRODUCTION


Sr. no Name Page no

Figure 1 Semi-Section of the dual electrical/optical plug 13

Figure 2 Research engine with the q-switched Nd:YAG

laser system (top)

14

Figure 3 The optical plug compared to a conventional

electrical spark plug

14

Figure 4 Experimental setup 17

Figure 5 Pressure history in the combustion chamber after ignition applying MPE for ignition

20

Figure 6 MPE to ignite the mixture versus mixture ratio 21

Figure 7 Pressure history in the combustion chamber after ignition applying MPE

22

Figure 8 Pressure history in the combustion chamber after ignition applying MPE for ignition; λ = 3, initial temperature = 393 K , initial pressure = 1–2:8 MPa

23


Internal combustion engines play a dominant role in transportation and energy

production. Even a slight improvement will translate into considerable reductions in

pollutant emissions and impact on the environment

1.1 Previous ignition systems-

The two major types of internal combustion engines are the Otto and the Diesel

engine. The Former relies on an ignition source to start combustion, the latter works

in auto-ignition mode. Ignition is a complex phenomenon known to strongly affect the

subsequent combustion. It is especially the early stages that have strong implications

on pollutant formation, flame propagation and quenching. The spark ignited Otto

engine has a widespread use and has been subject to continuous, sophisticated

improvements. The ignition source, however, changed little in the last 100 years. An

electrical spark plug essentially consists of two electrodes with a gap in between

where, upon application of a high voltage, an electrical breakthrough occurs.

1.2 Limitations-

The protection of the resources and the reduction of the CO2 emissions with the aim

to limit the greenhouse effect require a lowering of the fuel consumption of motor

vehicles. Great importance for the reduction lies upon the driving source. Equally

important are the optimization of the vehicle by the means of a reduction of the

running resistance as well as a low-consumption arrangement of the entire power train

system.

Electrode erosion

Restricted positioning

CO2 emission

Flame Quenching

2. NEED OF LASER IGNITION SYSTEM



A laser based ignition source, i.e. replacing the spark plug by the focused beam of a

pulsed laser, has been envisaged for some time. Also, it was tried to control auto

ignition by a laser light source. The time scale of a laser-induced spark is by several

orders of magnitude smaller than the time scales of turbulence and chemical kinetics.

In the importance of the spark time scale on the flame kernel size and NOx production

is identified. A laser ignition source has the potential of improving engine combustion

with respect to conventional spark plugs. Conventional ignition systems, like spark

plugs or heating wires are well suited but suffer from disadvantages. Electrode

erosion, influences the gas flow as well as restricted positioning possibilities are the

main motives in search of alternatives to conventional ignition systems.

2.1 Following the main advantages of laser ignition:

• A choice of arbitrary positioning of the ignition plasma in the combustion cylinder,

• Absence of quenching e6ects by the spark plug electrodes,

• Ignition of leaner mixtures than with the spark plug ⇒lower combustion

temperatures less NOx emissions

• No erosion effects as in the case of the spark plugs and Lifetime of a laser ignition

system is expected to be longer than that of a spark plug,

• High load/ignition pressures are possible ⇒ increase in efficiency,

• Precise ignition timing possible

• Reducing the overall ignition package costs, weight and energy requirements

3. LASER IGNITION SYSTEM



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

stimulus of a laser light source. Basically, energetic interactions of a laser with a gas

may be classified into one of the following four schemes as described:

• Thermal breakdown

• Non-resonant breakdown

• Resonant breakdown

• Photochemical mechanisms

In the case of thermal interaction, ignition occurs without the generation of an

electrical breakdown in the combustible medium. The ignition energy is absorbed by

the gas mixture through vibrational or rotational modes of the molecules; therefore no

well-localized ignition source exists. Instead, energy deposition occurs along the

whole beam path in the gas.

According to the characteristic transport times therein, it is not necessary to deposit

the needed ignition energy in a very short time (pulse). So, this ignition process can

also be achieved using quasi continuous wave (cw) lasers.

Another type, resonant breakdown, involves non-resonant multi-photon dissociation

of a molecule followed by resonant photo ionization of an atom. As well as

photochemical ignition, it requires highly energetic photons (UV to deep UV region).

Therefore, these two types of interaction do not appear to be relevant for this study

and practical applications. In these experiments, the laser spark was created by a non-

resonant breakdown. By focusing a pulsed laser to a sufficiently small spot size, the

laser beam creates a high intensity and high electric fields in the focal region. This

results in a well localized plasma with temperatures in the order of 106 K and

pressures in the order of 102 MPa .The most dominant plasma producing process is

the electron cascade process: Initial electrons absorb photons out of the laser beam via

the inverse bremsstrahlung process. If the electrons gain sufficient energy, they can

ionize other gas molecules on impact, leading to an electron cascade and breakdown

of the gas in the focal region. It is important to note that this process requires initial

seed electrons. These electrons are produced from



impurities in the gas mixture (dust, aerosols and soot particles) which are always

present. These impurities absorb the laser radiation and lead to high local temperature

and in consequence to free electrons starting the avalanche process. In contrast to

multiphoton ionization (MPI), no wavelength dependence is expected for this

initiation path. It is very unlikely that the first free electrons are produced by

multiphoton ionization because the intensities in the focus (1010 W/mm2) are too low

to ionize gas molecules via this process, which requires intensities of more than 1012

W/mm2

An overview of the processes involved in laser-induced ignition covering several

orders of magnitude in time is shown in Fig. 1. Laser ignition encompasses the

nanosecond domain of the laser pulse itself to the duration of the entire combustion

lasting several hundreds of milliseconds. 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

3.1 Principle of working

If electron diffusion out of the irradiated volume is neglected, the number of electrons

increases exponentially during the laser pulse with the duration t

N = N0et/τ = N02k, …………..(1)

Where,

τ is the characteristic time constant of the cascade process and

k is the number of generations of electrons at the end of the laser pulse.

Finally, the number of electrons exceeds the breakdown threshold and a bright and hot plasma is generated. Multiplication time constant is usually quite short (approx. 1 ns). Reaction velocities of combustion processes are several orders of magnitude slower. As a result, laser ignition fulfils the requirements on a well defined ignition time since there is almost no time



delay between the laser pulse and the development of hot plasma. The required pulse

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

roughly. It is well known that the diameter d of a focused laser beam depends on the

wavelength, the diameter of the unfocused beam and the focusing optics.

d = 2 · wf = 2 ·M2 2/π λF/D …………..(2)

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 _. A reasonable radius wf is in the

range of approximately 100 μm and within a spherical volume

V = 4πw3 f /3 ………………………….(3)

the number of molecules depend on the pressure and temperature according tothe

ideal gas law:

N = pV/kT………………………………(4)

With the pressure p,

Temperature T and

Boltzmann’s constant k = 1.38 · 10−23J/K.

Since not all molecules within the irradiated volume will be dissociated and ionized,

one can assume that approximately 1013 electrons will be present at the end of the

laser pulse. Dissociation and ionization requires a certain amount of energy which has

to be delivered by the laser beam. First the dissociation energy Wd is required and

finally 2N atoms are ionized (ionization energy Wi). Using known values12 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 = N · (Wd + 2Wi)………………………(5)

For a spot radius of about 100 μm the estimation gives required pulse energy for

ionization in the order of approximately 0.1 mJ.



After a successful ignition event the flame propagates through the combustible.

Usually, one can distinguish between different types of combustion processes. Slow

combustion processes (deflagrations): Reaction velocity is mainly determined by heat

conductivity. Propagation velocity is less than the speed of sound. Fast combustion

processes (detonations): Reaction velocity is determined by a strong shock front

moving at supersonic velocity. Propagation velocity is greater than the speed of

sound. Slow combustion processes are easier to control and are not as violent as fast

combustion processes. Pressure and temperature gradients inside deflagrations are

always smaller and stress on components is lower, too. In the case of very high heat of

reaction the relation between the temperatures which can be achieved during a

deflagration and a detonation approach a threshold value greater one:

T detonation/T deflagration=2γ2/γ + 1 ………………..(6)



4.OPTICAL SPARK PLUG

To verify the viability of the laser ignition concept, an experiment using a specially

designed combined electrical spark / optical plug, as devised. The combined plug is a

modified Kistler Type 6117, combined pressure sensor and spark plug, with the

pressure sensor removed. The plug has the focusing lens at the top and a window at

the bottom to protect the lens and to inhibit pressure waves in the optical cavity. Tests

were conducted using retarded spark timing, of 5◦ BTDC and advanced LI to

determine whether combustion had been initiated by the laser or spark through

observation of the phase of the pressure trace. For measurement and analysis of laser

beam parameters at various locations along the optical train (including beam energy,

average power, beam diameter, transverse intensity profiles and divergence), a series

of off-line tests were first performed by using a test path arrangement which

mimicked the optical path to the engine. In the analysis of beam diameter and

transverse intensity profiles, both near field and far field profiles were obtained. The

laser used in engine testing was a Q-switched Nd: YAG laser (Neodymium doped

Yttrium Aluminum Garnet) operating at the fundamental wavelength of 1064 nm and

in single mode (i.e. with an almost Gaussian beam profile) with low M2 < 2. Energy

up to 20 mJ per pulse was available. The maximum repetition rate is 50 Hz. Initially

tests were conducted using a pulse length of 6–8 ns, which is the standard pulse length

for this laser when operated in Q-switched mode, but this was then increased to values

in the range 12–16 ns by extending the optical cavity length. The camera system used

for this analysis was initially a Spiricon 100A Laser Beam Analyzer (LBA), and

finally an infrared Photon 7290A camera system, both placed at different locations

along the optical path to obtain intensity profiles and beam diameter. The laser pulse

energy, pulse-to-pulse variation and average power were measured using a GENTEC

ED-200+ meter with a GENTEC Solo PE monitor laser energy meter and an Ophir

30A-P-RP-NIR head with Ophir Nova display laser power meter respectively. By

combining the near and far field profiles with the energy per pulse, the energy density

was obtained. During engine laser ignition tests, a small percentage of the laser beam

was diverted with a beam splitter and used for online monitoring of energy levels.



AnAgilent 54641Aoscilloscope and an Alphalas UPD-300IR1 photodiode (operated

between wavelengths of 800 and 1800 nm and having a rise time of <300 ps) were

used to measure the pulse length by the Full Width Half Maximum value (FWHM).

The fast photodiode shows the temporal shape of the pulse of the laser. The extended

laser cavity also reduced the peak power density of the output laser beam, which was

found to reduce the likelihood of laser-induced damage to transparent optical

elements in the optical train (which had been an issue with the 6ns duration pulses).

The 1.4mm diameter output beam from the laser was expanded and collimated using a

Galilean telescope, before being further propagated via a turning mirror and a

focusing lens to complete the optical train.

Figure1 -Semi-Section of the dual electrical/optical plug



Figure 2 - Research engine with the q-switched Nd:YAG laser system

(top)

Figure 3- The optical plug compared to a conventional

electrical spark plug.



The possibility of choosing the location of the focal point in the cylinder is a

significant advantage for the combustion process. It is possible to position the plasma

exactly in the middle of the cylinder. High load/ignition pressure of the gas engine for

optimum efficiency performance demand increasing spark plug voltage leading to

enhanced erosion of the electrodes. Therefore, it is a main aim to increase the lifetime

of an ignition system and minimize the service efforts. A diode pumped laser ignition

system has potential lifetimes up to 10 000 h compared to spark plug lifetimes in the

order of 1000 h. The e6ectiveness of a diode pumped laser is about 10%.

Furthermore, with the possibility of multipoint ignition the combustion can be started

with two or more plasmas at di6erent points but at the same time in the cylinder which

again shortens the total combustion time. The experimental literature devoted to laser

ignition of gas mixtures so far has stressed the low-pressure and low-temperature

regimes. This publication extends the existing.Endings for hydrogen–air mixtures at

initial pressures up to 4 MPa and initial temperatures of 473 K. Gas engines usually

operate at these parameters.



5. EXPERIMENTAL SETUP AND ANALYSIS OF EXPERIMENT

5.1 EXPERIMENT

The experimental setup for the laser ignition experiments with emphasis on the optical

scheme of the igniting beam is depicted is depicted in fig.

Figure - 4 Experimental setup

The beam of a Q-switched Nd: YAG (Quantel Brilliant) laser was focused into a high

pressure, constant volume combustion chamber through a sapphire window with a

thickness of 5 mm. The laser pulse duration was about 5 ns (FWHM ... full-width at

half-maximum, measured by a Newport InGa As Photodiode and a Lecroy

9450 Oscilloscope 350 MHz 10 Gs=s), the beam had a diameter of 5:7 mm (1=e2),

and the beam quality described bythe M2 factor was less than two. Pulse energies

needed forignition varied between 1–50 mJ. The laser could be attenuated

continuously by using an external wave plate/polarizer setup without a6ecting any

other laser parameters such aspulse duration or spatial pro3le of the beam. A part of

the laser beam (about 4%) was used to measure the energy of each pulse by using a

pyro electric detector (SpectroLasPEM21) and an energy measuring unit (LEM2020).

After the beam sampler the laser beam was guided by three mirrors

(reNectivity 99:5%) to a spherically corrected convex lens with a focal length of 60∼

mm (120 mm). In the combustion chamber the laser beam was focused to about30 m



focal waist (for 60 mm focal length) thereby producing plasma and starting the

combustion. The infrared emissions of the combustion were measured with an

InGaAs photo detector (Thorlabs PDA400).

1—Nd: YAG-Laser (Quantel Brilliant);

2—beam attenuator (wave plate/polarizer);

3—wave plate;

4— beam sampler (4%);

5—pyro electric detector (SpectroLasPEM21);

6—laser energy measuring unit (LEM2020);

7—mirrors (reNection 99:5%);∼

8—spherically corrected convex lens f =60 mm;

9— sapphire window (thickness = 5 mm);

10—combustion chamber;

11—InGaAs photo detector (800–1800 nm) (Thorlabs PDA400);

12—personal computer;

13—piezoelectric pressure transducer(Kistler 7061B);

14—charge amplifier;

15--digital storage oscilloscope.

The combustion process was characterized by its pressure history measured with a

piezoelectric pressure transducer (Kistler 7061B; response time 8:8 _s). The signal

from the sensor had to be ampli3ed with a charge ampli3er and was recorded in a

digital storage oscilloscope. Both the pressure signal and the laser pulse were

recorded by a personal computer.



For all experiments, compressed air (water free) and hydrogen with a purity of¡99:9%

were used in order to yield data relevant for practical applications. For achieving the

intended ratio of the gaseous mixture components according to the method of partial

pressures (Dalton), it was necessary to measure the partial pressures of hydrogen and

air byusing a high resolution (resolution = 100 Pa) pressure meter.

The compressibility of the gases was neglected. In order to avoid explosions during

the hydrogen 3lling process, the chamber was 3rst evacuated to about 2:5 kPa and

only then hydrogen was 3lled in up to the calculated pressure for the di6erent

mixtures. After this operation was complete, air was introduced into the combustion

chamber. To reliably ensure a homogenous mixture; hydrogen being the species of

lower partial pressure was added in 3rst.In this way, homogeneity could be easily

achieved by the turbulences of the following incoming stream of air. Also because of

the very high diffusion coefficient of hydrogen

(D=61 mm2=s, at atmospheric pressure and 298 K) homogeneity was assured. For

comparison, methane has a diffusion coefficient of 16 mm2=s (at atmospheric

pressure and298 K). Further on, a one minute pause before each ignition attempt has

been carried out to guarantee a homogeneous mixture.The interior diameter and

lengths of the combustion chamber were 70 and 220 mm, respectively. The maximum

3llingpressure of one load was 4:2 MPa and the chamber was



5.2 RESULTS

1. Pressure history in the combustion chamber after ignition applying MPE for

ignition:

If the air/fuel equivalence ratio is increasing (leaner mixtures), the peak pressure is

decreasing but the total combustion time is increasing

Figure 5 Pressure history in the combustion chamber after ignition applying MPE for ignition



2. MPE to ignite the mixture versus mixture ratio:

It can be observed that with leaner mixtures and decreasing initial pressures the MPE

needed to ignite the mixture increased. With higher pressures the number of

molecules in the focal region is increased and the laser beam can be absorbed more

efficiently in the gas

Figure 6 MPE to ignite the mixture versus mixture ratio



3. Pressure history in the combustion chamber after ignition applying MPE:For higher

initial pressures the peak pressure, ignition delay and total combustion time is

increasing but MPE is decreasing.

Figure – 7 Pressure history in the combustion chamber after ignition applying

MPE



4. Pressure history in the combustion chamber after ignition applying MPE for

ignition:

Especially at this boundary knocking occurred only at lower filling pressures. With

higher initial 3lling pressures no knocking could be observed. Richer gas mixtures

only have a knocking combustion with no dependency on the

filling pressure

Figure8 Pressure history in the combustion chamber after ignition applying MPE for

ignition; λ = 3, initial temperature = 393 K , initial pressure = 1–2:8 MPa



Table 1- overview of the various interdependencies

Peak

Pressure

MPE Time until

peak

pressure

Ignition

delay

λ↑ ↓ ↑ ↑ ↑

p↑ ↑ ↓ ↑ ↑

T↑ ↓ ↓ = =

PE↑ = = = =

Focal

Length↑

↑ ↑ ↓ ↓

Plasma size↑ ↑ ↑ ↓ ↓

↑, increasing; ↓, decreasing; =; no signi3cant change in the combustion process; _, air

equivalence ratio; p, initial combustion chamber pressure; T, initial combustion

chamber temperature; PE, pulse energy; MPE, minimum pulse energy needed for

ignition; ignition delay, time when 5% of peak pressure is reached.



6. Conclusion

The following conclusions are drawn from the details for an overview of laser ignition

system

Plasma propagates towards the incoming laser beam

Plasma had the maximum emission peak 30 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



7. References

1. “Application of laser ignition to hydrogen–air mixtures at high pressures”; Martin Weinrottera, Herbert Kopeceka, Ernst Wintnera, Maximilian Lacknerb,a-Photonics Institute, Gusshausstrasse 27, A-1040, Wien, AustriabInstitute of Chemical Engineering, Getreidemarkt 9/166, A-1000 Wien, AustriaAccepted 31 March 2004

2. “Laser Ignition in Internal Combustion Engines - A Contribution to aSustainable Environment”; M. Lackner*Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, A-1060 Wien,Austria, F. Winter, J. Graf, B. GeringerInstitut für Verbrennungskraftmaschinen, Vienna University of Technology, Getreidemarkt 9/315, A-1060 Wien,Austria

3. “Laser Ignition of an IC Test Engine using an Nd: YAG Laser and the Effect of Key Laser Parameters on Engine Combustion Performance”; R. Dodd(1), J. Mullett, S. Carroll(2), G. Dearden(1),A.T. Shenton(1), K. G. Watkins(1), G. Triantos(1),S. Keen1) The University of Liverpool (Dept. of Engineering, PO Box 147,Brownlow Hill, Liverpool, L69 3GH, UK) email: g. [email protected]) Ford Motor Co. Ltd. Dunton, UK.2) GSI Lumonics Ltd. (Cosford Lane, Swift Valley Rugby, Warwickshire,CV21 1QN, UK) ,June 12, 2007

4. “Laser Ignition in Internal Combustion Engines”; Pankaj Hatwar(1), Durgesh Verma(2), *(Lecturer, Department of Mechanical Engineering, Nagpur Institute of Technology/Nagpur University, India ** (Lecturer, Department of Mechanical Engineering, Nagpur Institute of Technology/Nagpur University, India Mar-Apr 2012


final

Documents

laserinduced ignition

laser ignition tests

reliable ignition

yag laser

guidance of prof dr

sonawane prof

prof p

dr d n kamble prof