LASER Ignition System

LASER IGNITION SYSTEMBy,

Avinash Bhagawati Divyang Choudhary

Rishav Raj

6th SEM Mechanical Engineering

INTRODUCTIONIt's widely accepted that the internal combustion engines will continue to power our vehicles.

Hence, as the global mobilisation of people and goods increases, advances in combustion and after-treatment are needed to reduce the environmental impact of the continued use of IC engine vehicles. To meet environmental legislation requirements, automotive manufacturers continue to address two critical aspects of engine performance, fuel economy and exhaust gas emissions. New engines are becoming increasingly complex, with advanced combustion mechanisms that burn an increasing variety of fuels to meet future goals on performance, fuel economy and emissions. The spark plug has remained largely unchanged since its invention, yet its poor ability to ignite highly dilute air-fuel mixtures limits the potential for improving combustion efficiency.

With

INTRODUCTION CONTINUED...

Spark ignition (SI) also restricts engine design, particularly in new engines, since the spark position is fixed by the cylinder head location of the plug, and the protruding electrode disturbs the cylinder geometry and may quench the combustion flame kernel. So, many alternatives are being sought after to counter these limitations. One of the alternative is the laser ignition system (LIS) being described here. Compared to a conventional spark plug, a LIS should be a favourable ignition source in terms of lean burn characteristics and system flexibility.

So, in this paper we'll be discussing the implementation and impact of LIS on IC engines.

PROCESSES & MECHANISMSWhen the ignition switch is turned on current flows from the

battery to the ignition coil. Current flows through the Primary winding of the ignition coil where one end is connected to the contact breaker. A cam which is directly connected to the camshaft opens and closes the contact breaker (CB) points according to the number of the cylinders. When the cam lobe pushes CB switch, the CB point opens which causes the current from the primary circuit to break. Due to a break in the current, an EMF is induced in the second

winding having more number of turns than the primary which increases the battery 12 volts to 22,000 volts. The high voltage produced by the secondary winding is then transferred to the distributor. Higher voltage is then transferred to the spark plug terminal via a high tension cable. A voltage difference is generated between the central electrode

and ground electrode of the spark plug. The voltage is continuously transferred through the central electrode (which is sealed using an insulator). When the voltage exceeds the dielectric of strength of the gases

between the electrodes, the gases are ionized. Due to the ionization of gases, they become conductors and allows the current to flow through the gap and the spark is finally produced.

Spark 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).

For example it's possible to provide sufficient energy for ignition with no spark formation or provide insufficient energy with spark formation.

LASER Ignition System

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.

SO, NOW WHAT'S THE PROBLEM?

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 is not possible to ignite inside the fuel spray. 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 being sought after.

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 absorb more 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, and breakdown of the gas. Multi-photon absorption processes are usually essential for the initial stage of breakdown because the available photon energy 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 there is insufficient time for electron-molecule collision to occur. Thus this avalanche of electrons and resultant ions collide with each other producing immense heat hence creating plasma which is sufficiently strong to ignite the fuel.

NOW, HOW DOES IT WORK?The laser ignition system has a laser transmitter with a fibre-optic cable powered by the car’s

battery. It shoots the laser beam to a focusing lens that would consume a much smaller space than current spark plugs. The lenses focus the beams into an intense pinpoint of light by passing through an optical window, and when the fuel is injected into the engine, the laser is fired and produces enough energy (heat) to ignite the fuel.

NOW, HOW DOES IT WORK?...CONTINUED

The laser beam is passed through a convex lens, this convex lens diverge the beam and make it immensely strong and sufficient enough to start combustion at that point. Hence the fuel is ignited, at the focal point, with the mechanism shown above. The focal point is adjusted where the ignition is required to have.

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

1. Emission of high energy photons 2. Generation of shock waves The high energy photons, heat and ionise the charge

present in the path of laser beam which can be seen from the propagation of the flame which propagates longitudinally along the laser beam.

The shock waves carry energy out wards from the laser beam and thus help in propagation of flame as shown in the above figure.

ADVANTAGES OF LIS The 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 from any point it is possible to ignite inside the fuel spray as there is no physical component at ignition location.

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 as fuel ignition inside combustion chamber is also possible here 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 as multipoint fuel ignition is also possible. 6. Higher turbulence levels are not required due to above said advantages.

NOX EMISSIONS IN DIFFERENT IGNITION SYSTEMS IN MG/NM3

1. SI

2. LI

CONCLUSIONResearch to date on LI in engines has demonstrated improvements in combustion stability. With

proper control, these improvements can enable engines to be run under leaner conditions, with higher Exhaust Gas Recirculation  (EGR) concentrations, or at lower idle speeds without increasing the noise, vibration and harshness characteristics of a vehicle. LI gives significantly shorter power duration compared to SI. With the recent development of higher

average power and higher pulse frequency lasers, it is expected that a multi-strike LI system and associated combustion 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

play a significant role in optimisation of future engines by dynamic feedback control. The only limitations of the LIS is it's highly expensive setup.

REFERENCES[1] Lackner, M., Winter, F., What is ignition? Combustion File 256, IFRF Online Combustion Handbook, ISSN 1607-9116, International Flame Research Foundation, Ijmuiden, The Netherlands, (2004). [2] H. Kopecek, M. Lackner, F. Winter, E. Wintner, Laser ignition of methane air mixtures at pressures up to 4 MPa, Journal of Laser Physics 13 (11), 1365 (2003). [3] Kopecek, H., Lackner, M., Wintner, E., Winter, F., Laser-Stimulated Ignition in a Homogeneous Charge Compression Ignition Engine, SAE 2004 World Congress, paper No 2004-01-0937, Detroit, MI, USA (2004). [4] J. D. Dale, M. D. Checkel, P. R. Smy, Application of High Energy Ignition Systems to Engines, Prog. Energy Combust. Sci. 23, 379-398 (1997). [5] Ronney P.D., Laser versus conventional ignition of flames, Optical Engineering 33(2),510 (1994). [6] Phuoc T.X., White F.P., Laser-induced spark ignition of CH4/air mixtures, Combustion and Flame 119, 203-216 (1999). [7] Radziemski L.J., Cremers D.A., Laser-induced plasmas and applications, New York-Basel: Marcel Dekker Inc., (1989).

QUERIES, ANYONE?