2. INTRODUCTION
• It'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.
3. 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
4. PROCESSES & MECHANISMS
Spark Ignition System
• When 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.
5. 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 being
sought after.
6. 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
7. 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.
8. WORKING and MECHANISM
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.
9. WORKING and MECHANISM….CONTINUED
The laser beam is passed through a convex lens, this convex
lens converge 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.
10. Laser Ignition process along time
Laser 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 first
milliseconds an ignition delay can be observed which has duration between 5 – 100 ms depending on the
mixture. 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 initial
temperature.
11. 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.
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.
12. Engine Experiments and Results
Experiment with laser ignition of the engine has been performed with a q
switched Nd:YAG laser.
The laser beam was focused into the chamber by means of a
lens with a focal length of 50 mm. Variations of pulse energies as well as
fuel mixtures have been performed to judge the feasibility of the
process. Results indicate that ignition-delay times are smaller and
pressure gradients are much steeper 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.
Research Engine Setup
15. Self-Cleaning Properties
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.
As can be seen in fig, the window was soiled with a dark layer of
combustion deposits. Afterwards, a cold start of the engine was
simulated. Already 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. As can be seen from fig, all deposits
have been removed by the laser beam.
Self–Cleaning Properties
16. CONCLUSION
• Research 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.
17. REFERENCES
[1] Bergmann and Schaefer, Lehrbuch der Experimentalphysik: Elektrizit¨at und Magnetism us, vol. 2, Walter de Gruyter Berlin, 1981.
[2] D. R. Lidde, ed., CRC Handbook of Chemistry and Physics, CRC Press, 2000
[3] J. Ma, D. Alexander, and D. Poulain, “Laser spark ignition and combustion Characteristics of methane-air mixtures,” Combustion and Flame
112 (4), pp. 492–506, 1998
[4] J. Syage, E. Fournier, R. Rianda, and R. Cohn, “Dynamics of flame propagation
Using laser-induced spark initiation: Ignition energy measurements,” Journal of Applied Physics 64 (3), pp. 1499–1507, 1988.
[5] Lambda Physik, Manual for the LPX205 Excimer Laser, 1991
[6] M. Gower, “Krf laser-induced breakdown of gases,” Opt. Commun. 36, No. 1, pp. 43–45, 1981.
[7] M. Lavid, A. Poulos, and S. Gulati, “Infrared multiphoton ignition and Combustion enhancement of natural gas,” in SPIE Proc.: Laser
Applications in Combustion and Combustion Diagnostics, 1862, pp. 33–44, 1993.
[8] P. Ronney, “Laser versus conventional ignition of flames,” Opt. Eng. 33 (2), pp. 510–521, 1994.
[9] R. Hill, “Ignition-delay times in laser initiated combustion,” Applied Optics. 20 (13), pp. 2239–2242, 1981.
[10] T. Huges, Plasma and laser light, Adam Hilg
