21-06-2011, 03:26 PM
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ABSTRACT
Laser ignition has become an active research topic in recent years because it has the potential to replace the conventional electric spark plugs in engines Compared to conventional spark ignition. Laser ignition allows more flexible choice of the ignition location inside the combustion chamber with the possibility to ignite even inside the fuel spray .Modern engines are required to operate under much higher compression ratios, faster compression rates, and much leaner fuel-to-air ratios than gas engines today. It is anticipated that the igniter in these engines will face with pressures as high as 50MPa and temperatures as high as 4000 K. Using the conventional ignition system, the required voltage and energy must be greatly increased (voltages in excess of 40 kV) to reliably ignite the air and fuel mixture under these conditions. Increasing the voltage and energy does not always improve ignitability but it does create greater reliability problem. Experiments with the direct injection engine have been carried out at the fundamental wavelength of the Nd:YAG laser as well as with a frequency doubled system Experiments show that above a certain threshold intensity of the laser beam at the window even highly polluted surfaces could be cleaned with the first laser pulse which is important for operation in real world engines .The objective of this paper is to review past work to identify some fundamental issues underlying the physics of the laser spark ignition process and research needs in order to bring the laser ignition concept into the realm of reality. The purpose of this paper is to prove that laser induced spark ignition can be used in gasoline direct injection engines.
CHAPTER 1
INTRODUCTION
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 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. 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.
CHAPTER 2
CONVENTIONAL SPARK IGNITION
2.1. 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 requires 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.
Economic as well as environmental considerations compel to overcome above
disadvantages and use a better system.
All the above drawbacks are overcome in laser ignition system explained as follows.
CHAPTER 3
LASER IGNITION SYSTEMS
3.1. 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 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 and 70 mm in length approximately. Laser rod is excited by xenon filled lamp, which surrounds it. Both are enclosed in highly 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 flash tube pumps it.
