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INTRODUCTION An ignition system is a system for igniting a fuel-air mixture. It is best known in the field of internal combustion engines but also has other applications, e.g. in oil-fired and gas- fired boilers. The earliest internal combustion engines used a flame, or a heated tube, for ignition but these were quickly replaced by systems using an electric spark. The power required for the motion of a vehicle is obtained by controlled combustion of fuel/air mixture in the cylinder of the engine. The energy produced due to this combustion is converted and transmitted from the engine to the wheels by means of mechanical linkages. The process of combustion of fuel is initiated by the ignition system. The ignition system has two tasks to perform. First, it must create a voltage high enough (20,000+) to arc across the gap of a spark plug, thus creating a spark strong enough to ignite the air/fuel mixture for combustion. Second, it must control the timing of that the spark so it occurs at the exact right time and send it to the correct cylinder. In IC engines there are 2 types of ignition based on the fuel being used: 1. spark ignition (petrol engine) 2. compression ignition (diesel engine) Spark ignition In a petrol engine, the fuel and air are usually pre-mixed before compression. The pre- mixing was formerly done in a carburetor, but now (except in the smallest engines) it is done by electronically controlled fuel injection. In this system fuel entering the engine cylinder is ignited by means of a spark. The required amount of fuel is induced into the cylinder during suction stroke. This fuel is ignited during the compression stroke by a spark produced by a spark plug. Due to the combustion of fuel large amount of heat and high pressure gases are produced which expand causing linear motion of the piston. 1
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Page 1: 54022569 Laser Ignition System

INTRODUCTION

An ignition system is a system for igniting a fuel-air mixture. It is best known in the field of internal combustion engines but also has other applications, e.g. in oil-fired and gas-fired boilers. The earliest internal combustion engines used a flame, or a heated tube, for ignition but these were quickly replaced by systems using an electric spark.

The power required for the motion of a vehicle is obtained by controlled combustion of fuel/air mixture in the cylinder of the engine. The energy produced due to this combustion is converted and transmitted from the engine to the wheels by means of mechanical linkages. The process of combustion of fuel is initiated by the ignition system.

The ignition system has two tasks to perform. First, it must create a voltage high enough (20,000+) to arc across the gap of a spark plug, thus creating a spark strong enough to ignite the air/fuel mixture for combustion. Second, it must control the timing of that the spark so it occurs at the exact right time and send it to the correct cylinder.

In IC engines there are 2 types of ignition based on the fuel being used:

1. spark ignition (petrol engine)2. compression ignition (diesel engine)

Spark ignition

In a petrol engine, the fuel and air are usually pre-mixed before compression. The pre-mixing was formerly done in a carburetor, but now (except in the smallest engines) it is done by electronically controlled fuel injection.In this system fuel entering the engine cylinder is ignited by means of a spark. The required amount of fuel is induced into the cylinder during suction stroke. This fuel is ignited during the compression stroke by a spark produced by a spark plug. Due to the combustion of fuel large amount of heat and high pressure gases are produced which expand causing linear motion of the piston.

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Based on the method by which spark is created and distributed, ignition systems are classified as:

1. Mechanical ignition System2. Electronic ignition system3. Distributor less ignition system

Mechanical ignition system

The Mechanical Ignition System was used prior to 1975. It was mechanical and electrical and used no electronics.

The ignition system consists of a battery, ignition coil, distributor, distributor cap, rotor, plug wires and spark plugs.

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Typical mechanical ignition system:

Battery

The system is powered by a lead-acid battery, which is charged by the car's electrical system using a dynamo or alternator. It provides power to the system.

Distributor

The heart of the system is the distributor. The distributor contains a rotating cam driven by the engine's drive, a set of breaker points, a condenser, a rotor and a distributor cap. First, it is responsible for triggering the ignition coil to generate a spark at the precise instant that it is. Second, the distributor is responsible for directing that spark to the proper cylinder. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder.

Distributor cap, rotor, plug wires

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The ignition coil's secondary windings are connected to the distributor cap. The extremely high voltage from the coil's secondary -– often higher than 1000 volts—causes a spark to form across the gap of the spark plug. This, in turn, ignites the compressed air-fuel mixture within the engine.

Contact breaker

The contact breaker is operated by an engine-driven cam, and the position of the contact breaker is set so that they open (and hence generate a spark) at the exactly correct moment needed to ignite the fuel at the top of the piston's compression stroke.

Ignition coil

The ignition coil consists of two transformer windings sharing a common magnetic core—the primary and secondary windings.

a. Primary windingThe coil primary winding contains 100 to 150 turns of heavy copper wire. This wire must be insulated so that the voltage does not jump from loop to loop, shorting it out. If this happened, it could not create the primary magnetic field that is required.

b. secondary windingThe coil secondary winding circuit contains 15,000 to 30,000 turns of fine copper wire, which also must be insulated from each other. The secondary windings sit inside the loops of the primary windings. To further increase the coils magnetic field the windings are wrapped around a soft iron core. To withstand the heat of the current flow, the coil is filled with oil which helps keep it cool.

An alternating current in the primary induces alternating magnetic field in the coil's core. Because the ignition coil's secondary has far more windings than the primary, the coil is a step-up transformer which induces a much higher voltage across the secondary windings. For an ignition coil, one end of windings of both the primary and secondary are connected together. This common point is connected to the battery. The other end of the primary is connected to the points within the distributor. The other end of the secondary is connected, via the distributor cap and rotor, to the spark plugs.

Spark plug

The ignition system's sole reason for being is to service the spark plug. It must provide sufficient voltage to jump the gap at the tip of the spark plug and do it at the exact right time, reliably on the order of thousands of times per minute for each spark plug in the engine.The modern spark plug is designed to last many thousands of miles before it requires replacement. The heat range of a spark plug dictates whether it will be hot enough to burn

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off any residue that collects on the tip, but not so hot that it will cause pre-ignition in the engine. Pre-ignition is caused when a spark plug is so hot, that it begins to glow and ignite the fuel-air mixture prematurely, before the spark. The gap on a spark plug is also important and must be set before the spark plug is installed in the engine. If the gap is too wide, there may not be enough voltage to jump the gap, causing a misfire. If the gap is too small, the spark may be inadequate to ignite a lean fuel-air mixture, also causing a misfire.

Ignition coil showing primary and secondary windings:

Cross-section of a spark plug:

Electronic ignition system

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The disadvantage of the mechanical system is the use of breaker points to interrupt the low-voltage high-current through the primary winding of the coil; the points are subject to mechanical wear where they ride the cam to open and shut, as well as oxidation and burning at the contact surfaces from the constant sparking. They require regular adjustment to compensate for wear, and the opening of the contact breakers, which is responsible for spark timing, is subject to mechanical variations. In addition, the spark voltage is also dependent on contact effectiveness, and poor sparking can lead to lower engine efficiency. A mechanical contact breaker system cannot control an average ignition current of more than about 3 A while still giving a reasonable service life and this may limit the power of the spark and ultimate engine speed. In the electronic ignition system, the points and condenser were replaced by electronics. On these systems, there were several methods used to replace the points and condenser in order to trigger the coil to fire. One method used a metal wheel with teeth, usually one for each cylinder. This is called an armature. The advantage of this system, aside from the fact that it is maintenance free, is that the control module can handle much higher primary voltage than the mechanical points. Voltage can even be stepped up before sending it to the coil, so the coil can create a much hotter spark, on the order of 50,000 volts instead of 20,000 volts that is common with the mechanical systems. These systems only have a single wire from the ignition switch to the coil since a primary resistor is no longer needed.

Distributor less ignition system

The coil in this type of system works the same way as the larger, centrally-located coils. The engine control unit controls the transistors that break the ground side of the circuit, which generates the spark. This gives the ECU total control over spark timing. Systems like these have some substantial advantages. First, there is no distributor, which is an item that eventually wears out. Also, there are no high-voltage spark-plug wires, which also wear out. And finally, they allow for more precise control of the spark timing, which can improve efficiency, emissions and increase the overall power of a car.

DRAWBACKS OF CONVENTIONAL SPARK IGNITION:

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1. Location of spark plug is not flexible as it requires shielding of plug from immense heat and fuel spray.2. It is not possible to ignite inside the fuel spray.

3. It requires frequent maintenance to remove carbon deposits. 4. Leaner mixtures cannot be burned. 5. Degradation of electrodes at high pressure and temperature. 6. Flame propagation is slow. 7. Higher turbulence levels are required. 8. Economic as well as environmental considerations compel to overcome above disadvantages and use a better system.

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LASERS

A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons. The term "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation.

Typically, very intense flashes of light or electrical discharges pump the lasing medium and create a large collection of excited-state atoms (atoms with higher-energy electrons). It is necessary to have a large collection of atoms in the excited state for the laser to work efficiently. In general, the atoms are excited to a level that is two or three levels above the ground state. This increases the degree of population inversion. The population inversion is when the number of atoms in the excited state is more than the number in ground state. The excited electrons have energies greater than the more relaxed electrons. Just as the electron absorbed some amount of energy to reach this excited level, it can also release this energy. As the figure below illustrates, the electron can simply relax, and in turn rid itself of some energy.

This emitted energy comes in the form of photons (light energy). The photon emitted has a very specific wavelength (color) that depends on the state of the electron's energy when the photon is released. Two identical atoms with electrons in identical states will release photons with identical wavelengths. The photon that any atom releases has a certain wavelength that is dependent on the energy difference between the excited state and the ground state. If this photon should encounter another atom that has an electron in the same excited state, stimulated emission can occur. The first photon can stimulate or induce atomic emission such that the subsequent emitted photon vibrates with the same frequency and direction as the incoming photon. The other key to a laser is a pair of mirrors, one at each end of the lasing medium. Photons, with a very specific wavelength and phase, reflect off the mirrors to travel back and forth through the lasing medium. In the process, they stimulate other electrons to make the downward energy jump and can cause the emission of more photons of the same wavelength and phase. A cascade effect occurs, and soon we have propagated many, many photons of the same wavelength and

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phase. The mirror at one end of the laser is "half-silvered," meaning it reflects some light and lets some light through. The light that makes it through is the laser light.

The figure below illustrates the stages in laser light formation by stimulated emission:

Types of lasers

There are many different types of lasers. The laser medium can be a solid, gas, liquid or semiconductor. Lasers are commonly designated by the type of lasing material employed:

1. Solid-state lasers: have lasing material distributed in a solid matrix2. Gas lasers3. Excimer lasers: use reactive gases, such as chlorine and fluorine, mixed with inert

gases such as argon, krypton or xenon.4. Dye lasers: use complex organic dyes, such as rhodamine 6G, in liquid solution or

suspension as lasing media.5. Semiconductor lasers

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Properties of laser light

1. Monochromatic - photons of one wavelength. In contrast, ordinary white Light is a combination of different wavelengths.2. Directional- laser light is emitted as a narrow beam and in a specific direction. This property is referred to as directionality. 3. Coherent - The light from a laser is said to be coherent. This means that the wavelengths of the laser light are in phase.

Here are some typical lasers and their emission wavelengths:

Laser Type Wavelength (nm)

Argon fluoride (UV) 193

Krypton fluoride (UV) 248

Xenon chloride (UV) 308

Nitrogen (UV) 337

Argon (blue) 488

Argon (green) 514

Helium neon (green) 543

Helium neon (red) 633

Rhodamine 6G dye (tunable) 570-650

Ruby (CrAlO3) (red) 694

Nd:Yag (NIR) 1064

Carbon dioxide (FIR) 10600

LASER IGNITION SYSTEM IN SI ENGINES

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The use of laser ignition to improve gas engine performance was initially demonstrated by J. D. Dale in 1978.However, with very few exceptions, work in this area has for the last 20 years been limited to laboratory experimentation employing large, expensive and relatively complicated lasers and laser beam delivery systems. More recently, researchers at GE-Jenbacher, Mitsubishi Heavy Industries, Toyota, National Energy Technology Lab and Argonne National Lab have obtained and/or built smaller high peak power laser spark plugs.

Unlike many earlier laboratory laser systems, these smaller lasers are now mounted directly onto the engine cylinder head so as to fire the laser beam directly into the chamber. This arrangement allows the laser to become a direct replacement for the traditional high voltage electrical spark-gap plug. Further reductions in laser size, price and complexity will help the laser spark plug become a commercial reality and a viable competitor to the traditional high voltage spark-gap plug.

The Otto or SI engine is today characterized by low pollutant emissions. The very efficient exhaust gas treatment makes power drives for nearly equal zero emission operation possible. There is however need for improvement of fuel consumption and the higher carbon dioxide emissions compared to the Diesel equivalent.

Advancing the state of art of ignition systems for lean burn, stationary, natural gas fuelled engines is crucial to meet increased performance requirements. As the demand for higher engine efficiencies and lower emissions drive stationary, spark-ignited reciprocating engine combustion to leaner air/fuel operating conditions and higher in-cylinder pressures, increased spark energy is required to maintain stable combustion and low emissions.

To compensate power density losses due to leaner operation, high pressure of initial charge is used to increase in-cylinder pressure at the time of combustion. However, an important parameter is the ignition under extreme conditions, lean combustible mixture and high initial pressure, requiring high voltage when using conventional spark plug technology. Providing the necessary spark energy to operate these engines significantly reduces the lifetime of spark plug and its effectiveness in transmitting adequate energy as an ignition source. Laser ignition offers the potential to improve ignition system durability, reduce maintenance, as well as to improve engine combustion performance.

Breakdown voltage of the spark plugs of a large gas engine depending on the test duration at two different BMEP levels:

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It can seen from the graph of spark voltage vs. test duration at different brake mean effective pressures, the spark voltage required for ignition increases with pressure in spark ignition system.

It has been known for some time already that when a short laser impulse is focused in air a strong spark is generated which is connected with an audible bang - "ignition plasma". The plasma formed in the process bears a certain resemblance to the one generated during the electrical discharge between two electrodes; it was therefore obvious at the start of the studies to ignite the mixture in a combustion engine also by a focused laser impulse instead of a conventional ignition spark.

The method by which the laser induces breakdown in a combustible gaseous mixture has been divided into four basic processes: thermal heating, non resonant breakdown, resonant breakdown and photochemical excitation. Thermal heating takes place where the laser beam is incident on a solid target and induces excitation by heating the target or by exciting rotational or vibrational modes of oscillation in the surrounding gas. Resonant breakdown occurs when the incident radiation ionizes the gas molecules and frees up electrons to absorb the radiation energy and in turn ionize other gas molecules leading to an avalanche breakdown. Photochemical ignition occurs when a single photon dissociates a molecule thus allowing the ionized constituents to react with the surrounding gases. Non-resonant breakdown occurs when laser light is focused into a gas and the electrical field component of the light is strong enough to initiate the electrical breakdown of the gas. The non-resonant breakdown mechanism is the predominant factor governing the results presented in this work.Tests conducted on laser ignition:

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Laser ignition tests were performed with gasoline in a spray-guided internal combustion engine. An Nd: YAG laser with 6 ns pulse duration, 1064 nm wavelength and 1-50 mJ pulse energy was used to ignite the fuel/air mixtures at initial pressures of 1-3 MPa. Schlieren photography was used for optical diagnostics of flame kernel development and shock wave propagation.

The engine, a one-cylinder research engine, was deployed for the investigation of spray guided combustion initiated by a laser. The focus of sustainability is on laser ignition for enhanced combustion and efficiency.

Pressure within the combustion chamber has been recorded as well as fuel consumption and exhaust gases. The laser was triggered at well defined positions of the crankshaft, just as with conventional ignition systems. Pulse energies, ignition location and fuel/air ratios have been varied during the experiments. The engine has been operated at each setting for several hours, repeatedly. All laser ignition experiments have been accompanied by conventional spark plug ignition as reference measurements.

Technical data of the research engine and the Nd:YAG laser used for the experiments:

Research engine Q-switched Nd:YAG laserNumber of cylinders: 1 Pump source: Flash lampNumber of valves: 1Wavelength 1064 or 532 nmInjector: Multi-hole Maximum pulse energy: 160 mJStroke: 85 mm Pulse duration: 6 nsBore: 88 mm Power Consumption: 1 kWDisplacement volume: 517 cm3Beam diameter: 6 mmCompression ratio: 11.6 Type: Quantel Brillian

Results of the test:

Results of the experiments show that laser ignition has advantages compared to conventional spark plug ignition. Compared to conventional spark 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 laser ignition can be achieved at almost the same engine smoothness level. Additionally, a frequency-doubledNd:YAG laser has been used to examine possible influences of the wavelength on the laser ignition process. No influences could be found. Best results in terms of fuel consumption as well as exhaust gases have been achieved by laser ignition within the fuel

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spray. As already mentioned, it is not possible to use conventional spark plugs within the fuel spray since they will be destroyed very rapidly. Laser ignition doesn’t suffer from that restriction.

Graphical comparison of laser and spark ignition with respect to fuel consumption, smoothness of running and emissions from test results:

It can be inferred from the graph that laser ignition system has better properties than spark ignition system, with respect to fuel consumption, smoothness of operation and amount of emissions expelled.

Reliability:

Another important question with a laser ignition system is its reliability. It is clear that the operation of an engine causes very strong pollution within the combustion chamber. Deposits caused by the combustion process can contaminate the beam entrance window and the laser ignition system will probably fail. To quantify the influence of deposits on the laser ignition system, the engine has been operated with a spark plug at different load points for more than 20 hours with an installed beam entrance window. The window was soiled with a dark layer of combustion deposits. Afterwards, a cold start of the engine was simulated. The first laser pulse ignited the fuel/air mixture. Following laser pulses ignited the engine without misfiring, too. After 100 cycles the engine was stopped and the window was disassembled. All deposits have been removed by the laser beam. Engine operation without misfiring was always possible above certain threshold intensity at the beam entrance window.Comparison of laser ignition – spark plug ignition with respect to reliability:

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From the graph of ignition pressure vs. air-fuel ratio it can be seen that 100% reliability can be achieved in laser ignition at a higher air-fuel ratio of 2.05, than in spark ignition. Hence leaner mixture can be used in laser ignition system to achieve 100% reliability.

Comparison of NOx emission potentials of various ignition methods:

The graph below shows NOx emission levels in different ignition systems. It can be seen that NOx emissions are considerably lesser in case of laser ignition system. This is due to the fact that lean air-fuel mixtures can be burnt easily in laser ignition systems, leading to lower combustion chamber temperatures and hence lower NOx emissions.

NOX in [mg/Nm³]

Direct Pre chamber Direct Pre chamber

Spark ignition Laser ignition Diesel pilot ignitionPressure History in Combustion Chamber:

15

330

240

70

250

190

0

50

100

150

200

250

300

350

ign ition re lia b ility o f la se r ign itio n

A /F re l ( )

1 ,40 1 ,5 0 1 ,60 1 ,7 0 1 ,8 0 1 ,90 2 ,0 0 2 ,10 2 ,2 0 2 ,30

p init

(bar

)

1 0

1 5

2 0

2 5

3 0

3 5

4 0

0 ,0 0 0 ,2 5 0 ,5 0 0 ,7 5 1 ,0 0

ign ition re lia b ility o f spa rk p lu g ign itio n

A /F re l ( )

1 ,40 1 ,50 1 ,60 1 ,7 0 1 ,8 0 1 ,90 2 ,00 2 ,10 2 ,20 2 ,30

p init

(bar

)

1 0

1 5

2 0

2 5

3 0

3 5

4 0

0 ,0 0 0 ,2 5 0 ,5 0 0 ,7 5 1 ,0 0

For 100% ignition reliability at 30 barA/Frel = 2.05 (laser ignition)A/Frel = 1.74 (spark ignition)

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Below is a graph of excess pressure vs time taken for ignition at different air fuel ratio. Consider the air/fuel ratio of 3.0 for both spark plug and laser ignition. It can be seen that the excessive pressure in case of laser ignition is higher. Since minimum energy required for ignition in case of laser is inversely proportional to excessive pressure, ignition takes place faster in laser ignition system. Therefore ignition delay is less incase of laser ignition system. This also reduces knocking tendency.

Here λ=air-fuel ratio

KNOCK AND MISFIRE LIMITS:

As can be seen in the figure below the brake mean effective pressure range for misfire is reduced in the case on laser ignition. Also knocking in case of laser ignition starts at a higher pressure.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

0 100 200 300 400 500 600 700 800 900 1000

Time (ms)

Ex

ce

ss

Pre

ss

ure

(M

Pa

)

λ = 2.5, laser

λ = 3.0, laser

λ = 3.5, spark plug

λ = 3.5, laserλ = 3.0, spark plug

λ = 2.5, spark plug

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Principle of Laser Ignition:

Convex lens

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Laser beam

Focused laser

Plasma I>Ithreshold

flame kernel E>Eignition

Mixture burning

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Laser Arrangement with Respect to Engine:

Parts of laser ignition system

A laser ignition device for irradiating and condensing laser beams in a combustion chamber of an internal combustion engine so as to ignite fuel particles within the combustion chamber, includes: a laser beam generating unit for emitting the laser beams; and a condensing optical member for guiding the laser beams into the combustion chamber such that the laser beams are condensed in the combustion chamber

Power source

The average power requirements for a laser spark plug are relatively modest. A four stroke engine operating at maximum of 1200 rpm requires an ignition spark 10 times per second or 10Hz (1200rpm/2x60). For example 1-Joule/pulse electrical diode pumping levels we are readily able to generate high millijoule levels of Q-switched energy. This provides us with an average power requirement for the laser spark plug of say approximately 1-Joule times 10Hz equal to approximately 10 Watts.

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Nd: YAG laser

It is the most suitable laser beam generating unit in laser ignition system. Nd: YAG (neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12) is a crystal that is used as a lasing medium for solid-state lasers. The dopant, triply ionized neodymium, typically replaces yttrium in the crystal structure of the yttrium aluminium garnet (YAG), since they are of similar size. Generally the crystalline host is doped with around 1% neodymium by atomic percent. They typically emit light with a wavelength of 1064 nm, in the infrared. However, there are also transitions near 940, 1120, 1320, and 1440 nm. Nd:YAG lasers operate in both pulsed and continuous mode. Pulsed Nd:YAG lasers are typically operated in the so called Q-switching mode: An optical switch is inserted in the laser cavity waiting for a maximum population inversion in the neodymium ions before it opens. Then the light wave can run through the cavity, depopulating the excited laser medium at maximum population inversion.

Physical and chemical properties of Nd:YAG:Properties of YAG crystal:Formula: Y3Al5O12Molecular weight: 596.7Crystal structure: CubicHardness: 8–8.5 (Moh)Melting point: 1950 °C (3540 °F)Density: 4.55 g/cm3Refractive index of Nd:YAGWavelength (μm) Index n (25°C) 0.8 1.8245 0.9 1.8222 1.0 1.8197 1.2 1.8152 1.4 1.8121

Properties of Nd: YAG at 25°C (with 1% Nd doping):Formula: Y2.97Nd0.03Al5O12Weight of Nd: 0.725%Atoms of Nd per unit volume: 1.38×1020 /cm3Emission wavelength: 1064 nmTransition: 4F3/2 → 4I11/2Duration of fluorescence: 230 μsThermal conductivity: 0.14 W·cm−1·K−1Specific heat capacity: 0.59 J·g−1·K−1Thermal expansion: 6.9×10−6 K−1dn/dT: 7.3×10−6 K−1Young's modulus: 3.17×104 K·g/mm−2Poisson's ratio: 0.25Resistance to thermal shock: 790 W·m−1

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Combustion chamber window

Since the laser ignition system is located outside the combustion chamber a window is required to optically couple the laser beam. The window must:

• Withstand the thermal and mechanical stresses from the engine.• Withstand the high laser power.• Exhibit low propensity to fouling.

During combustion deposits may form on the window which may be organic or inorganic. These deposits may block the laser beam. Usually these are carbon deposits. When laser beam passes through these deposits, they het heated up and evaporate. This phenomenon is called ABLATION and process is called SELF CLEANING.

The photo below shows self cleaning of the window due to ‘ablation’.

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Optic fiber wire

It is used to transport the laser beam from generating unit to the focusing unit.

Focusing unit

A set of optical lenses are used to focus the laser beam into the combustion chamber. The focal length of the lenses can be varied according to where ignition is required. The lenses used may be either combined or separated.

FLAME PROPAGATION IN COMBUSTION CHAMBER:

Schlieren photography is a visual process used to view the flow of fluids of varying density. It is used to view the various stages of flame propagation after laser ignition as shown below:

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• Plasma propagates towards the incoming laser beam.

• Plasma had the maximum emission peak 30 ns after the laser was fired and laser plasma UV-emission persisted for about 80 ns.

Approach for multipoint ignition

Laser ignition system can also be used for multipoint ignition in engines. The laser beam generated will be divided 2 or more beams by means of diffraction grating. Each beam is directed by optic fiber and focused into their respective laser spark plugs.

Practical Laser ignition Requirements:

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Mechanical Requirements:

Laser spark plug designs must perform under engine mount shock and vibration conditions. Testing to shock and vibration specifications for engine mounted products will help to validate the durability and design life of the laser spark plug. It appears that large stationary Advanced Reciprocating Engines Systems (ARES) will most likely subject the laser spark plug to substantial long term vibration and limited shock.Automotive requirements are limited to shock and vibration compliance of random vibration frequency testing at less than 15 g’s.Environmental Requirements: Lasers and optical instrumentation designed for outdoor use are typically hermitically sealed backfilled with dry inert gas. Diode Pumped Solid State Lasers are most sensitive to environmental temperature fluctuations as the diode pump wavelength changes with temperature. This can be especially troublesome in Nd: YAG and other crystal host lasers as their pump band width tends to be narrow. Glass host DPSS lasers provide broad pump band widths allowing them to traverse through -30 to +50 degrees C temperature operating range without the need for diode thermal conditioning.The ideal laser spark plug requires maximum performance over large temperature ranges with minimum thermal conditioning. Decreasing the laser’s thermal conditioning requirements makes the laser design less complicated and less expensive to build and maintain.

Peak Power Requirements:

The peak power requirements for the laser spark are relatively high. Formation of a plasma or “laser spark” in free space air is not difficult if you start with Megawatt class (nanosecond pulse width - milli joule energy level) laser pulses. As the engine cylinder head pressure increases, the required laser pulse peak power level for air breakdown decreases. With a multiple lens focusing system it is plausible that one could reliably project a laser spark into a high pressure cylinder head utilizing lower Kilowatt class pulse power densities. Passive Q-switched lasers also allows for generation of a multiple laser pulse output or “pulse train.” The first pulse of a pulse train initiates the plasma and successive pulses feed more energy into the plasma causing the plasma to expand. For neodymium lasers the pulses are typically separated by a few 10’s of microseconds. The net result of pulse train operation is longer sustained plasma containing higher energy.

The main advantages of laser ignition are given:

•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.

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• 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.

The disadvantages of laser ignition are:

• High system costs. • Concept is proven, but no commercial system available yet.

Challenges:

•Propagation of laser pulse through fiber optics.•Development of a compact, robust and economic laser source.•Durability of windows.

Conclusion

Tests were performed to gauge the performance of laser ignition system as compared to spark igntition system, in a gasoline engine. Nd:YAG laser was used. Energy required for ignitio with rising pressure, knocking, misfire , Nox emissions, fuel consumption, smoothness of operation and reliabilty were studied.

The physical differences of the two ignition sources are summarized:

Apart from the differences in the plasma temperature, the different efficiency factor of the energy output to the gas to be ignited is another and quite essential characteristic of the laser ignition.The following fact is quite decisive: The amount of the spark energy at the electrodes of the classical spark ignition depends mainly on the pressure of the combustion chamber and on the distances of the electrodes. An increase in pressure with the same electrode distance means an increase of the required secondary voltage. The energy requirement is exactly the opposite for generating plasma in air by means of a laser. This is another quite essential advantage of the laser ignition concept especially for engines with high cylinder pressure.

The physical differences of the ignition systems, especially the higher temperature of the ignition plasma in the case of laser ignition, clearly reduce the ignition delay and thus improve the quiet running of the engine.

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The position of the place of ignition for the mixture in the combustion chamber is an equally important influence factor for the initiation of the ignition. For the laser ignition the starting point of the ignition can be located at a defined optimal place in the combustion chamber and in a defined flow field. In contrast to conventional spark plug ignition, the point of ignition (spark) from a laser can be positioned at a considerable distance from potential heat sinks thus eliminating problems involving flame kernel heat transfer quenching common in spark plugs. High peak power laser pulses can be focused to a point to create strong sparks with high surface area. With a homogenous distribution of the mixture in the engine one can hereby realize efficiency factor advantages because of shorter flame paths. In the near full load range this will result in increased knock resistance and thus ultimately in performance advantages.

Principle, parts, flame propagation, self cleaning, approach for multipoint ignition, Practical Laser ignition Requirements, advantages and disadvantages of laser ignition system were studied. Laser ignition system was found to be more advantageous than spark ignition in some regards, but also has its disadvantages which if overcome, it can be used for commercial applications.

Bibliography

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The content of this document has been derived from the following sources:

http://www.netl.doe.gov/publications/proceedings/02/naturalgas/3-1.pdfhttp://www.seminarprojects.com/Thread-laser-ignition-systemhttp://www.processeng.biz/docs/laser_Konferenzbeitrag_IFRF_proceedings.pdfhttp://www.iitk.ac.in/erl/laserignition.htmlhttp://www.faqs.org/patents/app/20080264371http://publik.tuwien.ac.at/files/pub-mb_2880.pdfhttp://www-pub.iaea.org/MTCD/publications/PDF/P_1357_CD_web/presentations/s6-3s.pdfhttp://www.kigre.com/files/nd41.pdfhttp://www.lasers.org.uk/paperstore/Ignition2.pdfhttp://autorepair.about.com/od/glossary/ss/igntn-sys_descr.htmhttp://www.familycar.com/Classroom/ignition.htmhttp://en.wikipedia.org/wiki/Ignition_systemhttp://auto.howstuffworks.com/ignition-system4.htm

Thank you

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