Definition
Economic as well as environmental constraints demand a
further reduction in the fuelconsumption and the exhaust emissions of motor
vehicles. At the moment, directInjected 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
know 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. ThisPlasma 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.
Drawbacks
Of Conventional Spark Ignition
Location
of spark plug is not flexible as it require shielding of plug from immense heat
and fuel spray.
It
is not possible to ignite inside the fuel spray.
It
require frequent maintenance to remove carbon deposits..
Leaner
mixtures cannot be burned.
Degradation
of electrodes at high pressure and temperature.
Flame
propagation is slow.
Multi
point fuel ignition is not feasible.
Higher
turbulence levels are required.
What
Is Laser?
Lasers
provide intense and unidirectional beam of light. Laser light is monochromatic
(onespecific wavelength). Wavelength of light is determined by amount of energy
releasedwhen electron drops to lower orbit. Light is coherent; all the photons
have same wavefronts that launch to unison. Laser light has tight beam and is
strong and concentrated. Tomake these three properties occur takes something
called “Stimulated Emission”, inwhich 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 crystalis manmade ruby consisting of aluminum 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 and70 mm in length approximately. Laser
rod is excited by xenon filled lamp, whichsurrounds it. Both are enclosed in
highly reflective cylinder, which directs light fromflash lamp in to the rod.
Chromium atoms are excited to higher energy levels. The excitedions meet
photons when they return to normal state. Thus very high energy is obtained
inshort pulses. Ruby rod becomes less efficient at higher temperatures, so it
is continuouslycooled with water, air or liquid nitrogen. The Ruby rod is the
lasing medium and flashtube pumps it.
Laser Induced Spark Ignition
The process begins with multi-photon ionization of few gas
molecules which releaseselectrons that readily absorb more photons via the
inverse bremsstrahlung process toincrease their kinetic energy. Electrons
liberated by this means collide with othermolecules and ionize them, leading to
an electron avalanche, and breakdown of the gas.Multiphoton absorption processes
are usually essential for the initial stage of breakdownbecause the available
photon energy at visible and near IR wavelengths is much smaller than the
ionization energy. For very short pulse duration (few picoseconds)
themultiphoton processes alone must provide breakdown, since there is
insufficient time forelectron-molecule collision to occur. Thus this avalanche
of electrons and resultant ionscollide with each other producing immense heat
hence creating plasma which issufficiently strong to ignite the fuel. The
wavelength of laser depend upon the absorptionproperties of the laser and the
minimum energy required depends upon the number ofphotons required for
producing the electron avalanche.
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 10^-3 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 isusually designed to last <100 ns.
Breakdown and heating of laser sparks depend only onthe gas,
optical, and laser parameters, while the energy balance of spark
dischargesdepends on the circuit, gas, and electrode characteristics. The
efficiency of energytransfer to near-threshold laser sparks is substantially
lower than to electrical sparks, somore 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 laserbeam. 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 forhigher minimum energy required for laser ignition than that for
electric spark.
Advantages
- Location of spark plug is flexible as it does not require shielding from immense heat and fuel spray and focal point can be made any where in thecombustion chamber from any point It is possible to ignite inside the fuel spray asthere is no physical component at ignition location.
- It does not require maintenance to remove carbon deposits because of itsself cleaning property.
- Leaner mixtures can be burned as fuel ignition inside combustion chamberis also possible here certainty of fuel presence is very high.
- High pressure and temperature does not affect the performance allowing the use of high compression ratios.
- Flame propagation is fast as multipoint fuel ignition is also possible.
- Higher turbulence levels are not required due to above said advantages
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