With the advent of lasers in the 1960s, researcher and engineers discovered a new and powerful tool to investigate natural phenomena and improve technologically critical processes. Nowadays, applications of different lasers span quite broadly from diagnostics tools in science and engineering to biological and medical uses. In this report basic principles and applications of lasers for ignition of fuels are concisely reviewed from the engineering perspective. The objective is to present the current state of the relevant knowledge on fuel ignition and discuss select applications, advantages and disadvantages, in the context of combustion engines. Fundamentally, there are four different ways in which laser light can interact with a combustible mixture to initiate an ignition event.
They are referred to as thermal initiation, non-resonant breakdown, resonant breakdown, and photochemical ignition. By far the most commonly used technique is the non-resonant initiation of combustion primarily because of its freedom in selecting the laser wavelength and ease of implementation. Recent progress in the area of high power fibre optics allowed convenient shielding and transmission of the laser light to the combustion chamber. However, issues related to immediate interfacing between the light and the chamber such as selection of appropriate window material and its possible fouling during the operation, shaping of the laser focus volume, and selection of spatially optimum ignition point remain amongst the important engineering design challenges. One of the potential advantages of the lasers lies in its flexibility to change the ignition location. Also, multiple ignition points can be achieved rather comfortably as compared to conventional electric ignition systems using spark plugs. Although the cost and packaging complexities of the laser ignition systems have dramatically reduced to an affordable level for many applications, they are still prohibitive for important and high-volume applications such as automotive engines. However, their penetration in some niche markets, such as large stationary power plants and military applications, are imminent.
• It’s widely accepted that the internal combustion engines will continue to power our vehicles.
• Hence, as the global mobilization 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.
• 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 favorable 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
LASER INDUCED SPARK IGNITION
The process begins with multi-photon ionization 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 ionize them, leading to an electron avalanche, and breakdown of the gas.Multiphoton 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 ionization 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. The wavelength of laser depend upon the absorption properties of the laser and the minimum energy required depends upon the number of photons required for producing the electron avalanche.
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.
ADVANTAGES OF LASER INDUCED SPARK IGNITION
Location of spark plug 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.
It does not require maintenance to remove carbon deposits because of its self-cleaning property.
Leaner mixtures can be burned as fuel ignition inside combustion chamber is also possible here certainty of fuel presence is very high.
High pressure and temperature does not affect the performance allowing the use of high compression ratios.
Flame propagation is fast as multipoint fuel ignition is also possible.
Higher turbulence levels are not required due to above said advantages