13-09-2014, 04:20 PM
LASER INDUCED IGNITION OF GASOLINE DIRECT INJECTION ENGINES
LASER INDUCED.docx (Size: 908.03 KB / Downloads: 17)
INTRODUCTION
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.
LASER IGNITION SYSTEMS
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.
GAS LASERS
The Helium-neon laser (HeNe) emits 543 nm and 633 nm and is very common ineducation because of its low cost. Carbon dioxide lasers emit up to 100 kW at 9.6 μm and10.6 μm, and are used in industry for cutting and welding. Argon-Ion lasers emit 458 nm,488 nm or 514.5 nm. Carbon monoxide lasers must be cooled but can produce up to 500kW. The Transverse Electrical discharge in gas at Atmospheric pressure (TEA) laser isan inexpensive gas laser producing UV Light at 337.1 nm.
Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-Silver (HeAg) 224 nm and Neon-Copper (NeCu) 248 nm are two examples. These lasershave particularly narrow oscillation linewidths of less than 3 GHz (0.5 picometers) [6]making them candidates for use in fluorescence suppressed Raman spectroscopy.
CHEMICAL LASERS
Chemical lasers are powered by a chemical reaction, and can achieve high powers incontinuous operation. For example, in the Hydrogen fluoride laser (2700-2900 nm) andthe Deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen ordeuterium gas with combustion products of ethylene in nitrogen trifluoride
SOLID-STATE LASERS
Solid state laser materials are commonly made by doping a crystalline solid host withions that provide the required energy states. For example, the first working laser wasmade from ruby, or chromium-doped sapphire. Another common type is made from neodymium-doped yttrium aluminium garnet (YAG), known as Nd:YAG. Nd:YAG
lasers can produce high powers in the infrared spectrum at 1064 nm. They are used forcutting, welding and marking of metals and other materials, and also in spectroscopy andfor pumping dye lasers. Nd:YAG lasers are also commonly doubled their frequency toproduce 532 nm when a visible (green) coherent source is required.
Ytterbium, holmium, thulium and erbium are other common dopants in solid statelasers
SEMICONDUCTOR LASERS
Laser diodes produce wavelengths from 405 nm to 1550 nm. Low power laser diodes areused in laser pointers, laser printers, and CD/DVD players. More powerful laser diodesare frequently used to optically pump other lasers with high efficiency. The highestpower industrial laser diodes, with power up to 10 kW, are used in industry for cuttingand welding. External-cavity semiconductor lasers have a semiconductor active mediumin a larger cavity. These devices can generate high power outputs with good beamquality, wavelength-tunable narrow-linewidth radiation, or ultrashort laser pulses.
Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers whoseemission direction is perpendicular to the surface of the wafer. VCSEL devices typicallyhave a more circular output beam than conventional laser diodes, and potentially could bemuch cheaper to manufacture. As of 2005, only 850 nm VCSELs are widely available,with 1300 nm VCSELs beginning to be commercialized [7], and 1550 nm devices an areaof research. VECSELs are external-cavity VCSELs. Quantum cascade lasers aresemiconductor lasers that have an active transition between energy sub-bands of anelectron in a structure containing several quantum wells
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 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 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 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.
RESULTS OF EXPERIMENT
Results of the experiments are summarized in fig9. Fig. 9 shows that laser ignition hasadvantages compared to conventional spark plug ignition. Compared to conventionalspark plug ignition, laser ignition reduces the fuel consumption by several per cents.Exhaust emissions are reduced by nearly 20%. It is important that the benefits from laserignition can be achieved at almost the same engine smoothness level, as can be seen fromfig.9. Additionally, a frequency-doubled Nd:YAG laser has been used to examinepossible influences of the wavelength on the laser ignition process. No influences couldbe found. Best results in terms of fuel consumption as well as exhaust gases have beenachieved by laser ignition within the fuel spray. As already mentioned, it is not possibleto use conventional spark plugs within the fuel spray since they will be destroyed veryrapidly. Laser ignition doesn’t suffer from that restriction.
CONCLUSION
The applicability of a laser-induced ignition system on direct injected gasolineengine has been proven. Main advantages are the almost free choice of the ignitionlocation within the combustion chamber, even inside the fuel spray. Significantreductions in fuel consumption as well as reductions of exhaust gases show thepotential of the laser ignition process.
At present, a laser ignition plug is very expensive compared to a standard
electrical spark plug ignition system and it is nowhere near ready for deployment.But the potential and advantages certainly make the laser ignition more attractive inmany practical applications.