LASER IGNITION SYSTEMBy

Rishav Raj

1RE11ME092

Department of

Mechanical Engineering

INTRODUCTION• It's widely accepted that the internal combustion engines

will continue to power our vehicles.

• Hence, as the global mobilisation of people and goodsincreases, advances in combustion and after-treatment areneeded to reduce the environmental impact of thecontinued use of IC engine vehicles.

• To meet environmental legislation requirements,automotive manufacturers continue to address two criticalaspects of engine performance, fuel economy and exhaustgas emissions.

• New engines are becoming increasingly complex, withadvanced combustion mechanisms that burn an increasingvariety of fuels to meet future goals on performance, fueleconomy and emissions.

• The spark plug has remained largely unchanged since itsinvention, yet its poor ability to ignite highly dilute air-fuelmixtures limits the potential for improving combustionefficiency.

INTRODUCTION CONTINUED...

• Spark ignition (SI) also restricts engine design,particularly in new engines, since the spark position isfixed by the cylinder head location of the plug, and theprotruding electrode disturbs the cylinder geometry andmay quench the combustion flame kernel.

• So, many alternatives are being sought after to counterthese limitations.

• One of the alternative is the laser ignition system (LIS)being described here.

• Compared to a conventional spark plug, a LIS should be afavourable ignition source in terms of lean burncharacteristics and system flexibility.

• So, in this paper we'll be discussing the implementationand impact of LIS on IC engines

PROCESSES & MECHANISMSSpark Ignition System

• When the ignition switch is turned on current flows from the battery tothe ignition coil. Current flows through the Primary winding of theignition coil where one end is connected to the contact breaker. A camwhich is directly connected to the camshaft opens and closes the contactbreaker (CB) points according to the number of the cylinders. When thecam lobe pushes CB switch, the CB point opens which causes the currentfrom the primary circuit to break.

• Due to a break in the current, an EMF is induced in the second windinghaving more number of turns than the primary which increases thebattery 12 volts to 22,000 volts. The high voltage produced by thesecondary winding is then transferred to the distributor. Higher voltageis then transferred to the spark plug terminal via a high tension cable.

• A voltage difference is generated between the central electrode andground electrode of the spark plug. The voltage is continuouslytransferred through the central electrode (which is sealed using aninsulator).

• When the voltage exceeds the dielectric of strength of the gasesbetween the electrodes, the gases are ionized. Due to the ionization ofgases, they become conductors and allows the current to flow throughthe gap and the spark is finally produced.

Disadvantages of Spark Plug Ignition

• The following are the drawbacks of SI:

1. Location of spark plug is not flexible as it require shielding of

plug from immense heat and fuel spray.

2. It results in high amount of NOx emission .

3. It require frequent maintenance to remove carbon deposits.

4. Leaner mixtures cannot be burned efficiently.

5. Degradation of electrodes at high pressure and temperature

6. Flame propagation is slow.

7. Multi point fuel ignition is not feasible.

8. Higher turbulence levels are required.

To overcome the above mentioned disadvantages LIS is beingsought after.

LASER Ignition System• LI requires certain conditions to be met for two basic steps to take place, spark formation (generally limited by

breakdown intensity) and subsequent ignition (generally limited by a minimum ignition energy or MIE).

• There are four mechanisms by virtue of which LI is able to ignite the air-fuel mixture.

• They are,

1. Thermal initiation (TI)

2. Non-resonant breakdown (NRB)

3. Resonant breakdown (RB)

4. Photo chemical ignition (PCI)

Amongst the above mentioned mechanisms NRB is used the most.

Laser Spark Plug

NON RESONANT BREAKDOWN• In NRB, the focused laser beam creates an electric field of sufficient intensity to cause dielectric breakdown of the air-

fuel mixture.

• The process begins with multi-photon ionisation of few gas molecules which releases electrons that readily absorbmore photons via the inverse bremsstrahlung process to increase their kinetic energy.

• Electrons liberated by this means collide with other molecules and ionise them, leading to an electron avalanche, andbreakdown of the gas.

• Multi-photon absorption processes are usually essential for the initial stage of breakdown because the available photonenergy at visible and near IR wavelengths is much smaller than the ionisation energy.

• For very short pulse duration (few picoseconds) the multi photon processes alone must provide breakdown, since thereis insufficient time for electron-molecule collision to occur.

• Thus this avalanche of electrons and resultant ions collide with each other producing immense heat hence creatingplasma which is sufficiently strong to ignite the fuel.

WORKING and MECHANISMThe laser ignition system has a laser transmitter with a fibre-optic cable powered by the car’sbattery. It shoots the laser beam to a focusing lens that would consume a much smaller spacethan current spark plugs. The lenses focus the beams into an intense pinpoint of light by passingthrough an optical window, and when the fuel is injected into the engine, the laser is fired andproduces enough energy (heat) to ignite the fuel.

WORKING and MECHANISM….CONTINUED

The laser beam is passed through a convex lens, this convexlens converge the beam and make it immensely strong andsufficient enough to start combustion at that point. Hencethe fuel is ignited, at the focal point, with the mechanismshown above. The focal point is adjusted where the ignitionis required to have.

• The plasma generated by the Laser beam results in two ofthe following actions :

1. Emission of high energy photons

2. Generation of shock waves

The high energy photons, heat and ionise the charge presentin the path of laser beam which can be seen from thepropagation of the flame which propagates longitudinallyalong the laser beam.

The shock waves carry energy out wards from the laser beamand thus help in propagation of flame as shown in the abovefigure.

Laser Ignition process along timeLaser ignition encompasses the nanosecond domain of the laser pulse itself to the duration of the entire combustion lasting several hundreds of milliseconds.

The laser energy is deposited in a few nanoseconds which leads to a shock wave generation. In the firstmilliseconds an ignition delay can be observed which has duration between 5 – 100 ms depending on themixture. Combustion can last between 100 ms up to several seconds again depending on the gas mixture,initial pressure, pulse energy, plasma size, position of the plasma in the combustion bomb and initialtemperature.

ADVANTAGES OF LISThe following are the advantages of using a LIS:

1. Location of laser is flexible as it does not require shielding from immense heat and fuel spray and

focal point can be made anywhere in the combustion chamber.

2. It does not require maintenance to remove carbon deposits because of the fact that whole

system is isolated.

3. Leaner mixtures can be burned and fuel ignition inside combustion chamber is also possible as

the certainty of fuel presence is very high.

4. High pressure and temperature does not affect the performance allowing the use of high

compression ratios.

5. Flame propagation is fast .

6. Higher turbulence levels are not required due to above said advantages.

Engine Experiments and Results

Experiment with laser ignition of the engine has been performed with a qswitched Nd:YAG laser.

The laser beam was focused into the chamber by means of alens with a focal length of 50 mm. Variations of pulse energies as well asfuel mixtures have been performed to judge the feasibility of theprocess. Results indicate that ignition-delay times are smaller andpressure gradients are much steeper compared to conventional sparkplug ignition.

Compared to conventional spark plug ignition, laser ignitionreduces the fuel consumption by several per cents. Exhaust emissions arereduced by nearly 20%. It is important that the benefits from laserignition can be achieved at almost the same engine smoothness level.

Research Engine Setup

NOX EMISSIONS IN DIFFERENTIGNITION SYSTEMS IN MG/NM3

Self-Cleaning PropertiesAnother important question with a laser ignition system is itsreliability. It is clear that the operation of an engine causes verystrong pollution within the combustion chamber. Depositscaused by the combustion process can contaminate the beamentrance window and the laser ignition system will probably fail.

To quantify the influence of deposits on the laserignition system, the engine has been operated with a spark plugat different load points for more than 20 hours with an installedbeam entrance window.

As can be seen in fig, the window was soiled with a dark layer ofcombustion deposits. Afterwards, a cold start of the engine wassimulated. Already the first laser pulse ignited the fuel/airmixture. Following laser pulses ignited the engine withoutmisfiring, too. After 100 cycles the engine was stopped and thewindow was disassembled. As can be seen from fig, all depositshave been removed by the laser beam.

Self–Cleaning Properties

CONCLUSION

• Research to date on LI in engines has demonstrated improvements in combustion stability. With propercontrol, these improvements can enable engines to be run under leaner conditions, with higher ExhaustGas Recirculation (EGR) concentrations, or at lower idle speeds without increasing the noise, vibration andharshness characteristics of a vehicle.

• LI gives significantly shorter power duration compared to SI. With the recent development of higheraverage power and higher pulse frequency lasers, it is expected that a multi-strike LI system and associatedcombustion control can reduce the probability of misfires under high levels of dilution.

• The prospects for LI are also particularly exciting from a control perspective, from optical sensing of the in-cylinder combustion, to the array of possible ignition activation and control mechanisms.

• It is anticipated that this, combined with the capability to control the ignition location and timing, will playa significant role in optimisation of future engines by dynamic feedback control.

• The only limitations of the LIS is it's highly expensive setup.

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