Seminar Report on Laser Ignition System

1. A SEMINAR REPORT
ON
“LASER IGNITION SYSTEM”
NATIONAL INSTITUTE OF TECHNOLOGY, HAMIRPUR (Hp)-177005

2. INDEX
No Topic Page No.
1. Introduction 1
2. Need for alternative ignition 1
3. Alternative ignition system 2
4 Definition 2
5 What is Laser?
5.1. Ruby Laser
3
5
6 Laser Ignition 5
7
8
Laser Induced spark system
7.1. Ignition in combustion chamber
7.2. Minimum energy required for combustion
7.3. Flame propagation in combustion chamber
7.4. Advantages of Laser induced Spark Ignition
Overview of the process
6
6
7
8
8
9

3. 1.INTRODUCTION
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.
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.
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.
2.NEED FOR ALTERNATIVE IGNITION
The need for alternative ignition in the combustion chamber of an engine can be
easily explained by the drawbacks which are observed in the conventional battery
ignition system. These are:
 Location of spark plug is not flexible as it requires shielding of plug from
immense heat and fuel spray.
 It is not possible to ignite inside the fuel spray.
 It requires frequent maintenance to remove carbon deposits.
 Leaner mixtures cannot be burned.

4.  Degradation of electrodes at high pressure and temperature.
 Flame propagation is slow.
 Multi point fuel ignition is not feasible.
 Higher turbulence levels are required.
3.ALTERNATIVE IGNITION SYSTEMS
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 powertrain system.
The most important contribution for lower fuel consumption lies in the spark ignition
(SI) engine sector, due to the outstanding thermodynamic potential which the direct fuel
injection provides.
Wall- and air-guided combustion processes already found their way into standard-
production application and serial development, whereas quite some fundamental
engineering work is still needed for combustion processes of the second generation.
Problems occur primarily due to the fact that with conventional spark ignition the place
of ignition cannot be specifically chosen, due to several reasons. By the means of laser
induced ignition these difficulties can be reduced significantly.
The combination of technologies (spray-guided combustion process and laser induced
ignition) seems to become of particular interest, since the ignition in the fuel spray is
direct and thus the combustion initiation is secure and non-wearing.
Another approach is laser ignition of a homogeneous mixture. Laser ignition,
microwave ignition, high frequency ignition are among the concepts widely
investigated.
4.DEFINITION
Laser ignition is an alternative method for igniting compressed gaseous mixture of fuel
and air. The method is based on laser devices that produce short but powerful flashes
regardless of the pressure in the combustion chamber. Usually, high voltage spark plugs
are good enough for automotive use, as the typical compression ratio of an Otto cycle

5. internal combustion engine is around 10:1 and in some rare cases reach 14:1. However,
fuels such as natural gas or methanol can withstand high compression without self-
ignition. This allows higher compression ratios, because it is economically reasonable,
as the fuel efficiency of such engines is high. Using high compression ratio and high
pressure requires special spark plugs that are expensive and their electrodes still wear
out. Thus, even expensive laser ignition systems could be economical, because they
would last longer. Economic as well as environmental constraints demand a further
reduction in the fuel consumption and the exhaust emissions of motor vehicles. At the
moment, direct Injected fuel engines show the highest potential in reducing fuel
consumption and exhaust emissions. Unfortunately, conventional spark plug ignition
shows a major disadvantage with modern spray-guided combustion processes since the
ignition location cannot be chosen optimally. It is important that the spark plug
electrodes are not hit by the injected fuel because otherwise severe damage will occur.
Additionally, the spark plug electrodes can influence the gas flow inside the combustion
chamber. It is well known that short and intensive laser pulses are able to produce an
“optical breakdown” in air. Necessary intensities are in the range between 1010-
1011W/cm2.1, 2 at such intensities, gas molecules are dissociated and ionized Within
the vicinity of the focal spot of a laser beam and a 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.
5.WHAT IS LASER?
Lasers provide intense and unidirectional beam of light. Laser light is
monochromatic (one specific wavelength). Wavelength of light is determined by
amount of energy released when electron drops to lower orbit. Light is coherent; all
the photons have same wave fronts that launch to unison. Laser light has tight beam
and is strong and concentrated. To make these three properties occur takes
something called “Stimulated Emission”, in which photon emission is organized.
Main parts of laser are power supply, lasing medium and a pair of precisely aligned
mirrors. One has totally reflective surface and other is partially reflective (96 %).
The most important part of laser apparatus is laser crystal. Most commonly used
laser crystal is manmade ruby consisting of aluminium oxide and 0.05% chromium.
Crystal rods are round and end surfaces are made reflective.
A laser rod for 3 J is 6 mm in diameter and 70 mm in length approximately. Laser
rod is excited by xenon filled lamp, which surrounds it. Both are enclosed in highly

6. reflective cylinder, which directs light from flash lamp in to the rod. Chromium
atoms are excited to higher energy levels. The excited ions meet photons when they
return to normal state. Thus very high energy is obtained in short pulses. Ruby rod
becomes less efficient at higher temperatures, so it is continuously cooled with
water, air or liquid nitrogen. The Ruby rod is the lasing medium and flashtube pumps
it.
Fig.5.1. Laser in its non lasing state.
Fig.5.2. The flash tube fires and injects light into the ruby rod. The light excites atoms
in the ruby.
Fig.5.3. Some of these atoms emit photons.
Fig.5.4. Photons run in a directional ruby axis, so they bounce back and forth off the
mirrors. As they pass through the crystal, they stimulate emission in other atoms.

7. Fig.5.5. Monochromatic, single phase columnated light leaves the ruby through the half
silvered mirror laser light.
5.1. RUBY LASER
Monochromatic, single-phase, columnated light leaves the ruby
through the half-silvered mirror - laser light.
Fig.5.6. working of ruby Laser
6.Laser ignition
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 in:
• 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

8. ignition, it requires highly energetic photons (UV to deep UV region). Therefore, these
two types of interaction do not appear to be relevant for the 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
localised plasma with temperatures in the order of 106
K and pressures in the order of
102
MPa as mentioned in.
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 ionise 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 ionisation (MPI), no wavelength dependence is expected for
this initiation path. It is very unlikely that the first free electrons are produced by
multiphoton ionisation because the intensities in the focus (1010
W/mm2
) are too low to
ionise gas molecules via this process, which requires intensities of more than 1012
W/mm2
.
7.LASER INDUCED SPARK IGNITION
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.
7.1.IGNITION IN COMBUSTION CHAMBER

9. Fig.7.1. Ignition in 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, with the mechanism shown above. The focal point is adjusted where
the ignition is required to have.
7.2. MINIMUM ENERGY REQUIRED FOR IGNITION
The minimum ignition energy required for laser ignition is more than that for electric
spark ignition because of following reasons:
An initial comparison is useful for establishing the model requirements, and for
identifying causes of the higher laser MIE. First, the volume of a typical electrical
ignition spark is 103
cm3
. The focal volume for a typical laser spark is 10-5
cm3
.
Since atmospheric air contains _1000 charged particles/cm3
, the probability of finding
a charged particle in the discharge volume is very low for a laser spark.
Second, an electrical discharge is part of an external circuit that controls the power
input, which may last milliseconds, although high power input to ignition sparks is
usually designed to last < 100 ns. Breakdown and heating of laser sparks depend only
on the gas, optical, and laser parameters, while the energy balance of spark discharges
depends on the circuit, gas, and electrode characteristics. The efficiency of energy
transfer to near-threshold laser sparks is substantially lower than to electrical sparks, so
more power is required to heat laser sparks.
Another reason is that, energy in the form of photons is wasted before the beam reach
the focal point. Hence heating and ionizing the charge present in the path of laser beam.
This can also be seen from the propagation of flame which propagates longitudinally
along the laser beam. Hence this loss of photons is another reason for higher minimum
energy required for laser ignition than that for electric spark.

10. 7.3. FLAME PROPAGATION IN COMBUSTION
CHAMBER
Fig.7.2. Flame propagation.
7.4. ADVANTAGES OF LASER INDUCED SPARK
IGNITION
Location of spark plug is flexible as it does not require shielding from immense heat
and fuel spray and focal point can be made anywhere in the combustion chamber from
any point It is possible to ignite inside the fuel spray as there is no physical component
at ignition location.
 It does not require maintenance to remove carbon deposits because of its self-
cleansing property.
 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 => 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 of a spark plug.
 high load/ignition pressures possible => increase in 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.
 fuel-lean ignition possible.

11. 8.OVERVIEW OF THE PROCESS
An overview of the processes involved in laser-induced ignition covering several
orders of magnitude in time is shown:
Fig.7.3
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 leads to a shock wave generation. In
the first milliseconds an ignition delay can be observed which has a 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.
Despite the advantages of the system there are a couple of disadvantages of laser
ignition:
• high system costs
• concept proven, but no commercial system available yet.
REFERENCES
 J.D. Dale, P.R. Smy, R.M. Clements, "Laser Ignited Internal Combustion Engine - An
Experimental Study", Society of Automotive Engineers
 J. Ma, D. Alexander, and D. Poulain, “Laser spark ignition and combustion characteristics of
methane-air mixtures,” Combustion and Flame, pp. 492–506, 1998