AN OVERVIEW OF LASER IGNITION SYSTEM
A
Seminar Report
On
AN OVERVIEW OF LASER IGNITION SYSTEM
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
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AN OVERVIEW OF LASER IGNITION SYSTEM
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)
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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.
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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.
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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
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LIST OF FIGURES AND TABLES
TABLE
SR no Name Page no
Table 1- Overview of the various
interdependencies
24
1. INTRODUCTION
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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
AN OVERVIEW OF LASER IGNITION SYSTEM
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
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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
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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
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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
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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.
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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)
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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.
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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
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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.
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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.
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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
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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.
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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
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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
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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
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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
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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
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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.
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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
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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
DEPARTMENT OF MECHANICAL ENGINEERING Page 26
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