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A review on Laser
Ignition System

December 2017

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Abdul Rahim
Nasir, Hafiz Muhammad Imran, Hassan Mansoor*

*Department of
Mechanical Engineering

University of
Engineering and Technology, Lahore.

LASER IGITION
SYSTEM

Abstract

Ignition systems are a vital part
of our world and they play very key roles in matters pertaining to our food,
comfort and travelling. Since their invention they have been subject to
numerous improvements. Main types of ignition engines are the Spark Ignition
and Compression Ignition engines. In the case of a SI Engine the time of
ignition and the position of propagation of combustion are important factors in
determining the integrity of the structure and the power generation process.
Spark plugs suffer due to damage to the electrodes plus the position of the
flame generation are not flexible. Laser Ignition does not have these problems
and provides advantages over the usual spark plug by reducing the pollutants
and improving the combustion process. 
The most commonly used technique is non-resonant initiation of combustion because
it is comparatively easy to apply and we can select a wavelength of our choice.
Hot plasma is formed in the gas which falls in the focal point of the high
power laser. This plasma initiates the self-propagating combustion.

Introduction

Reduced pollutants emission and
lesser consumption of fuel are the major concerns regarding ignition systems.
Induced ignition such as Laser ignition systems can provide high power
densities and improved compression ratios. Moreover the location of flame
generation can be chosen easily in this system.

Disadvantages of
Conventional Ignition Systems

·        
Location of spark plug is not flexible as it requires
shielding of plug from immense heat and fuel spray

• 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.

• Multi point fuel ignition is not feasible.

• Higher turbulence levels are required.

• Erosion of spark plug electrodes.

 

LASER Ignition

Laser ignition systems direct a highly intense LASER beam onto gas
molecules which causes breakdown followed by ignition. Hot plasma is generated
in the focal spot of the laser focused by a lens. Multi-photon ionization
initiates the phenomena by affecting initially very few gas molecules to release
electrons that are capable of absorbing more photons due to the inverse
bremsstrahlung process and an increase in their kinetic energy takes place. An
electron avalanche occurs which causes the breakdown of gas molecules occurs
due to the collisions of electrons with other molecules. Multi-photon absorption
processes are a prerequisite for the initial stages of breakdown since the
available photon energy at visible and near IR wavelengths is minute as
compared to the ionization energy. The multi-photon processes alone must
provide breakdown for very short pulse duration (few picoseconds), since there
is insufficient time for electron-molecule collision to occur. Plasma, sufficiently
strong to ignite the fuel, as a
result of this avalanche of electrons and collision of resultant ions with each
other. 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.iii  iii

Resonant Breakdown

Free atoms are generated by anon-resonant multi-photon breakdown of
molecules. These atoms are then ionized. The gas is broken down and
theoretically lesser energy is required due to resonant nature of the method.iv

Properties of the
Optical Window

The materials used for the
window should be transparent for laser emission. The second prerequisite is that the
window should be able to endure the high energy density of the laser. The
shorter the focal length of the lens, the higher generally the laser light intensity
of the passing laser beam becomes at the window surface. The self-cleaning
ability of the window should be effective.v

Results of Experimentation

Laser ignition reduces exhaust emissions by 20%vi.
Varying the wavelength of the laser beam has no influence on the ignition
process. The beam entrance window is cleaned by the ‘self-cleaning’ property of
the laser provided increased pulse energy of the first few lasers is provided. Even
with a heavily polluted beam entrance windows the direct injection engine could
be operated successfully. The required pulse energy for successful ignition
decreases with increasing pressure.vii  viii
ix

Effect of flow velocity and temperaturex

Laser induced spark ignition offers the potential for greater reliability
and consistency in ignition of lean air/fuel mixtures. This increased
reliability is essential for the application of gas turbines as primary or
secondary reserve energy sources in smart grid systems, enabling the
integration of renewable energy sources whose output is prone to fluctuation
overtime. By varying flow velocity Beduneau investigated that the minimum
energy for laser ignition is increased by increasing flow velocity for both
lean and rich mixtures. The combustion energy also increased as wavelength of
light is increased because focus is disturbed. The temperature effects the
ignition system in a way that at higher temperature the minimum energy required
for combustion is high in both lean and rich mixtures. At low temperature less
energy is required for lean mixture. But in both cases of temperature the
central span for combustion remains same. xi               

Effect of laser pulse energy

In laser ignition system, the laser beam of a specific focal length is focused
with the help of a lens  on a  point. The plasma is produced when of focus
the energy produced at the spot of focus will exceed the energy required for
the plasma formation .As for as the concern of laser energy on the ignition
system, it effects the combustion system. If we used the laser bean of low
energy the peak pressure obtained in combustion chamber is reduced with the
reduction in flame speed.                         If we will use a laser
beam of high energy, the minimum pulse energy for the start of combustion is
reduced as the laser light of short wavelength is used. A high energy wave will
reduce the time for the formation of the flame .The high energy will provide
the required temperature earlier than vice versa. Exhaust gas temperature for
higher energy laser pulse was slightly higher at all loads, leading to
relatively higher NOx formation. Other regulated emissions such as total
unburned hydrocarbons, carbon monoxide and carbon dioxide were insignificant in
the exhaust.

 We can enhance the ignition
phenomenon by the use of incident laser light. In this technique when laser
light is focused inside the cylinder, combustion starts at different locations
inside the combustion chamber of an engine. This proposed method of laser
ignition is known as “multipoint ignition system”. Two methods have been adopt
for laser positioning. In first method it is placed on cylinder sand in second
method it is not fixed but it is mobile. Both these methods adopt flexible
optical fibers for the transportation of laser light. It was found that the
position of the output window in the optical path was paramount to the
suitability of a given pulse energy for the optical system For the single pulse
experimental set-up, 100% removal of the carbon layer was possible at distances
from the focal position of up to 25 mm, beyond which only partial removal was
possible. A dual pulse technique is applied in order to increase the combustion
locations, in which two pulses of equal pulse energy were delivered
sequentially with a separation of 2 ns. In this technique along with the
combustion of gas inside the cylinder the full removal of carbon deposits is
also achieved which was not possible for single pulse laser techniques. This is
significant as it increases the range over which the ignition location can be
varied, a key advantage of laser based ignition systems. xii

Application of additional approaches with laser ignition

There are multiple innovative approaches that can be employed with laser
ignition systems:

·        
Using multi-point ignition

·        
Increasing the combustion chamber temperatures

·        
Using diffractive lenses to generate multiple plasma
sparks at different points in the combustion chamber

 

 

Comparative Advantages of Laser Ignitionxiii

SI System

LI System

Less intense spark

More intense spark

Location of
combustion initiation is restricted

Free choice of
ignition location

Leaner mixtures
cannot be burned

Leaner fuels burn
effectively

Flame propagation
is slow

Relatively fast
flame propagation

Multi-point fuel
ignition is not feasible

Easier possibility
of multi-point ignition

More NOx emission

Less NOx emission

 

Conclusion

Laser ignition system uses laser technology for producing stable
combustion with lean mixtures also which can prevent the formation of NOx, thus
increasing the fuel efficiency. Spark plugs ignite the area near top dead
center where rich mixtures are present and lot of heat is absorbed by the metal
walls and conducted to the down area, the fuel inside the combustion chamber
cannot be burnt properly. However, with LI technology we can have optimum spark
locations by focusing the laser on multiple areas with no of lenses, which will
shorten the flame travel distance. Flame kernel shifting may take place, and
heat transfer loses can be reduced from walls of cylinder. Multiple spark plugs
with over loaded engine cylinder heads can be reduced. Spark plug
isn’t efficient in case of dilute air fuel mixtures and electrode protrusion
may take place and thus quenching the flame kernel. Experiments conducted at
university of Liverpool found the mechanism of self-cleaning of optical window
by consecutive laser pulses previously which was a hurdle in this technologyxiv.
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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References

 

i Laser ignited engines: progress, challenges
and prospects Geoff Dearden and Tom Shenton School of Engineering,
University of Liverpool, Liverpool L69 3GH, UK

ii
Laser Ignition in Internal Combustion Engines – A Contribution to a Sustainable
Environment M. Lackner*, F. Winter Institute of Chemical Engineering,
Vienna University of Technology,Austria. J. Graf, B. Geringer Institut für Verbrennungskraftmaschinen,
Vienna University of Technology, Austria. 
M. Weinrotter, H. Kopecek, E.
Wintner.

 

iii
Laser Ignition System in IC Engine
Hrushikesh Mantri and Prof. Manoday Ramteke, 1,2Department of Mechanical
Engineering, Smt. Kashibai Navale College of Engineering, Pune, India

 

iv
Laser Ignition in Internal Combustion Engines – A Contribution to a Sustainable
Environment M. Lackner*, F. Winter Institute of Chemical Engineering,
Vienna University of Technology,Austria. J. Graf, B. Geringer Institut für Verbrennungskraftmaschinen,
Vienna University of Technology, Austria. 
M. Weinrotter, H. Kopecek, E.
Wintner.

 

v
Laser Ignition in Internal Combustion Engines  Pankaj Hatwar, Durgesh Verma
*(Lecturer, Department of Mechanical Engineering, Nagpur Institute of
Technology/Nagpur University, India

 

vi
LASER IGNITION SYSTEM IN I C ENGINES FOR
CLEANER ENVIRONMENT: A REVIEW  ABHISHEK
SAXENA Assistant Professor, Department of Mechanical Engineering, IMS
Engineering College, Ghaziabad, UP, India

 

vii
Effect of laser pulse energy on laser ignition of port fuel injected
hydrogen engine Anuj Pal, Avinash
Kumar Agarwal 

 

viii
Laser Ignition in Internal Combustion Engines  Pankaj Hatwar, Durgesh Verma
*(Lecturer, Department of Mechanical Engineering, Nagpur Institute of
Technology/Nagpur University, India

 

ix Laser Ignition System for IC Engines Swapnil S. Harel1, Mohnish Khairnar2, Vipul
Sonawane3 1, 2Dr. Babasaheb Ambedkar Technical University, Lonere,
Raigad, Maharashtra, India 3Saraswati College of Engineering, Kharghar, New
Mumbai, Maharashtra, India

x
Effect of flow velocity and temperature on ignition characteristics in laser
ignition J.Griffiths , M.J.W.Riley
b, A.Borman a, C.Dowding

 

xi
Effect of laser induced plasma ignition timing and location on Diesel spray
combustion  José V. Pastor ?, José M. García-Oliver,
Antonio García, Mattia Pinotti

 

xii Laser-induced
ignition modeling of HMX Karl V.
Meredith a, Matthew L. Gross b,?, Merrill W. Beckstead

 

xiv
Review and recent developments of laser ignition for internal combustion
engine application Mohamed H. morsy

 

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