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Flame kernel development

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different relative airfuel ratios (l1.21.7) and the images were interrogated for temporal propaga-tion of ame front. Pressure-time history inside the combustion chamber was recorded and analyzed.

This data is useful in characterizing the laser ignition of natural gasair mixture and can be used in

developing an appropriate laser ignition system for commercial use in SI engines.

withdegradblems haveimprothe n(CNG)

engine operation, higher pressure of combustible charge can be

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exceeds certain threshold intensity level, breakdown of medium

Contents lists available at ScienceDirect

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Optics and Lasers

Optics and Lasers in Engineering ] (]]]]) ]]]]]]one is the spark creation due to the local deposition of energy.E-mail address: [email protected] (A.K. Agarwal).used. This further increases in-cylinder pressure towards the end occurs leading to the formation of a plasma spark, whose sizedepends on the numerical aperture (NA) of the focused laser beam.If the energy content of the spark is high enough, the mixtureignites. Laser ignition can be divided into two main parts. The rst

0143-8166/$ - see front matter & 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.optlaseng.2011.04.015

n Corresponding author. Tel.: 91 512 259 7982; fax: 91 512 259 7408.Pleasvoluthis leads to considerable reduction in power density of theengine. To compensate for lower power density due to leaner

laser are focused by a lens system inside the chamber containingcombustible fuel-air mixture. If the peak intensity in the focal regionengines can be potentially operated at relatively higher compres-sion ratios, thus leading to higher thermal efciencies. The mostimportant pollutant of concern from CNG fueled engine is oxidesof nitrogen (NOx). Emissions from a CNG fueled engine can befurther improved by igniting leaner fuelair mixtures, however

system is a desirable option, which will overcome these litions to achieve higher engine efciency by igniting an ultramixture in reciprocating engines.

Laser is an alternative ignition source for engines. Shortpulses of few nanoseconds pulse duration delivered by a Q-swifor some of these problems. CNG is regarded as one of the mostpromising alternative fuels and is probably the cleanest commer-cial fuel. This fact has resulted in an increased interest in usingCNG as a fuel for transport engines all over the world. CNG hashigher octane number compared to gasoline, thus CNG fueled

these electrodes. An increase in chamber pressure with the sameelectrode distance means an increase in the required secondarycoil voltage applied to the spark plug. Therefore in order to realizecleaner combustion of leaner CNGair mixture at higher chamberlling pressures, a durable high-energy electrode-less ignition1. Introduction

The world is presently confrontedfuel depletion and environmentalconcern towards environmental prosions, stringent emission regulationthe world. Thus, an alternative andwill be helpful in coping up withregulations. Compressed natural gase cite this article as: Srivastava DK,me combustion chamber. Opt Laser& 2011 Elsevier Ltd. All rights reserved.

the twin crises of fossilation. With increasingdue to vehicular emis-been imposed all overved engine technologyew requirements andis one of the solutions

of compression stroke, i.e. at the time of combustion. Howeverleaner airfuel mixtures combined with higher pressures at thetime of ignition require relatively much higher voltages, whenconventional spark plug technology is used. Providing the neces-sary spark energy at these relatively higher voltages to operatethese engines signicantly reduces the lifetime of spark plugs [1].The amount of the energy released at the spark electrodesdepends mainly on the pressure of the combustion chambertowards the end of compression stroke and the distances betweenFlame kernel characterization of laser iin a constant volume combustion cham

Dhananjay Kumar Srivastava, Kewal Dharamshi, A

Engine Research Laboratory, Department of Mechanical Engineering, Indian Institute o

a r t i c l e i n f o

Article history:

Received 13 January 2011

Received in revised form

27 April 2011

Accepted 27 April 2011

Keywords:

Laser ignition

Constant volume combustion chamber

a b s t r a c t

In this paper, laser-indu

Experiments were perform

compression stroke condit

conditions except turbulen

locations, which are used

were conducted at 10 b

combustion phenomena w

CMOS camera. Flame kern

journal homepage: www.eet al. Flame kernel characteEng (2011), doi:10.1016/j.oition of natural gasair mixtureer

ash Kumar Agarwal n

hnology Kanpur, Kanpur 208016, India

ignition was investigated for compressed natural gasair mixtures.

in a constant volume combustion chamber, which simulate end of the

s of a SI engine. This chamber simulates the engine combustion chamber

of airfuel mixture. It has four optical windows at diametrically opposite

laser ignition and optical diagnostics simultaneously. All experiments

chamber pressure and 373 K chamber temperature. Initial stage of

visualized by employing Shadowgraphy technique using a high speed

evelopment of the combustible fuelair mixture was investigated under

vier.com/locate/optlaseng

in Engineeringrization of laser ignition of natural gasair mixture in a constantptlaseng.2011.04.015

kernel growth in methaneair mixtures at 10 bar and different

D.K. Srivastava et al. / Optics and Lasers in Engineering ] (]]]]) ]]]]]]2This can be achieved in any gas. Breakdown is associated withplasma formation and shock wave generation. The second part ofthe laser ignition is the ignition itself based on a positive balancebetween the deposited energy and the losses. In this case, a amekernel can develop.

There are four mechanisms by which laser radiations interactwith medium/combustible fuelair mixtures: thermal ignition[24], photochemical ignition [57], resonant ignition [89] andnon-resonant breakdown [10]. Non-resonant breakdown of gas ismore favorable because it does not require a close match betweenthe laser wavelength and the target molecules [11]. This processgenerally begins with multi-photon ionization of a few gasmolecules, which release electrons that can then readily absorbmore photons, increasing their kinetic energy. The electronsliberated by this means collide with other molecules and ionizethem, leading to an electron avalanche and breakdown of the gas.It is important to note that this process requires initial seedelectrons. These electrons are produced from impurities presentin the combustible gas mixtures [12], which absorb the laserradiations and lead to very high local temperatures and as aconsequence, free electrons start the avalanche process. Multi-photon processes are usually essential for the initial stages ofbreakdown because the available laser photon energy similar towhat is employed in this work is approximately 1 eV, whereas theionization potential for most molecules is more than 10 eV [13].Initial ame growth from laser ignition resembles in some waysthe process of electric spark ignition; however, the initial stages ofenergy deposition differ considerably. In laser ignition, most ofthe energy transfers to plasma within the pulse duration of laser[14] is of the order of nano-seconds whereas in electric sparkignition, energy transfer lasts in microseconds range.

There are several potential benets of laser ignition over theconventional spark plug. Detailed advantages of laser ignitionwere reviewed by Paul [15]. The choice of location of plasmainside the combustion chamber is one of the several importantadvantages of laser ignition. Location of ignition initiator sparkcould be placed at any optimum location inside the combustionchamber using a suitable focal length of lens, which is notpractically feasible in any conventional spark plug engine. Thisway, ame propagation distance could be reduced and combus-tion duration could also be decreased. This may also potentiallyhelp in ignition of relatively leaner airfuel mixtures, where theslower combustion is the main issue. Since the laser ignition doesnot employ any spark electrode, there is no erosion effect asobserved in case of spark plug engines therefore the life span oflaser ignited engine system is expected to be signicantly longerthan that of spark plug [16]. A diode-pumped laser ignitionsystem has potential lifetime up to 10,000 h compared to sparkplug lifetimes of the order of 20004000 h. McMillian et al. [17],McIntyre et al. [18], Tauer et al. [19] have developed a miniaturelaser that can be mounted directly on the cylinder head. Multi-point ignition in the combustion chamber is also possible withlaser ignition. Phuoc [20] found that multi-point ignition ofcombustible gas mixture increases the combustion chamberpressure and shortens the combustion duration. This furtherenhances the possibility of using laser ignition system for ignitinglean airfuel mixture and enhances the combustion speed.

In the present investigation, early stages of laser-inducedignition of a CNGair mixture at 10 bar lling pressure wereexperimentally investigated for potential application in anengines simulated environment. To improve the understandingof in-cylinder combustion, it is important to understand the amepropagation. The burning speed or rate of ame propagation is afundamental parameter, which inuences the engine perfor-mance and emissions. In the early stages of ignition, the relative

importance of the shape and development of ame kernel is

Please cite this article as: Srivastava DK, et al. Flame kernel charactevolume combustion chamber. Opt Laser Eng (2011), doi:10.1016/j.oair/fuel equivalence ratios. The data points of this study exhibiteda signicant spread because only one picture was taken duringeach test and temporal growth of ame kernel was composed of asequence of ignition tests. However, it was concluded that amegrew faster at stoichiometric air/fuel ratio than the lean air/fuelmixtures. Tewari and Wilson [24] investigated the effect of highfrequency electric eld on the ame propagation generated bylaser-induced spark. They conducted experiments at one atmo-spheric pressure in mixtures of methaneair, methaneoxygenargon and hydrogenair. They found that the ame propagationrate increases to almost double in the methaneair mixture inpresence of high frequency electric eld however the identicaleld condition has an insignicant effect on the ame propaga-tion rate in the hydrogenair mixture. Beduneau and Ikeda [25]investigated the laser-induced spark kernel in a premixed laminarmethaneair burner. The ame kernel is in asymmetric toroidalshape, which is caused by the expansion mode of the shock wave.The asymmetric behavior was attributed in part to the plasmacharacteristics. In the initial stages of ame kernel growth, ameexpansion velocity strongly correlated to the spark energy. In thelater stage of expansion, the velocity was found to depend mainlyon the relative airfuel ratios. There are several others researchers[2629] who performed optical investigations to visualize theame evolution. The objective of this study is to investigate thelaser ignition behavior of CNGair mixture in a constant volumecombustion chamber. CNG is regarded as one the most promisingalternative fuel for the engines; thus performing laser ignition inCNGair mixture would be one step closer towards the develop-ment of laser red natural gas engine. To understand thecombustion, ame kernel shape and propagation in the earlystages of combustion was investigated for different relative airfuel ratios. In addition to providing time resolved images of amekernel growth and its speed, pressure-time history inside thecombustion chamber was also investigated.

2. Experimental setup

2.1. Experimental apparatus

The schematic of experimental setup is depicted in Fig. 1. Laserignition of CNGair mixture was performed in a speciallydesigned constant volume combustion chamber to gain thefundamental information like minimum laser ignition energyrequired for plasma generation, ame kernel development andgrowth, ame speed and pressure-time history inside thecombustion chamber, whose conditions typically represent the endof the compression stroke conditions of a typical internal combus-tion engine. The internal diameter and length of combustionchamber is 72 and 220 mm, respectively. Constant volume com-particularly high. At this time, the expansion speeds for theplasma growth are low. In the earlier work, Srivastava et al. [21]measured the size and propagation behavior of laser plasma inatmospheric air. It was found that the plasma propagates towardsthe incoming laser. This backward moving plasma (towards thefocusing lens) grows much faster than the forward movingplasma (along the direction of laser). Phuoc and White [22]reported the laser spark size in methaneair mixture at 1 atm. Itwas observed that laser plasma elongated in the direction of laserbeam. The shape of laser spark was oval and lean for richmethaneair mixture whereas it becomes cylindrical in shapefor stoichiometric and near stoichiometric methaneair mixture.The spark length and radius were about 0.8 and 0.3 mm, respec-tively. Lackner et al. [23] also investigated the laser-induced amebustion chamber was able to simulate the real engine combustion

rization of laser ignition of natural gasair mixture in a constantptlaseng.2011.04.015

ratios and this was kept xed for all the experiments in thisinvestigation.

For all the experiments, moisture-free compressed air andcommercially available CNG were used for performing combus-tible fuelair mixtures. Commercially available CNG contains95.6% methane, 1.2% ethane, 1.4% carbon dioxide and 1.7%nitrogen [30]. Properties of the CNG are given in Table 1.

For achieving the required relative airfuel ratio (l) ofthe gaseous mixtures (1.21.7 in this study), it was necessaryto measure the partial pressure of air and CNG using a high-resolution digital manometer. Gas lling arrangement is shown inFig. 2. Cylinders lled with air and CNG are tted with pressureregulators. Outlet pressure of the cylinder is kept slightly higherthan what is required for a particular airfuel ratio. Gases owfrom the cylinder via the high pressure regulator through the highpressure pipe to the combustion chamber. To achieve therequired airfuel ratio inside the combustion chamber, partialpressure of gas was measured by digital manometer. Requiredairfuel ratio was controlled according to Daltons law of partialpressures. CNG was lled rst because required partial pressureof CNG was low for all intended airfuel ratios. Then, air was lledwith high partial pressures. High turbulence generated by airlling inside the combustion chamber helps in the formation of ahomogeneous mixture. Additionally, the mixture of air and fuelare left for 1 min to doubly ensure the thermal stabilization andhomogeneity of the combustible mixture. After combustion using

D.K. Srivastava et al. / Optics and Lasers in Engineering ] (]]]]) ]]]]]] 3chamber conditions except turbulence. It was provided with fouroptical windows at diametrically opposite locations so that theycould be used for laser ignition and optical diagnostics/Shadow-graphy simultaneously. Combustion chamber was designed to beable to withstand 300 bars static pressure and could be heated upto 300 1C.

A Q-switched Nd: YAG laser (NanoL 200-30, Litron UK) wasused for the ignition of CNGair mixture, which is capable ofdelivering maximum pulse energy up to 200 mJ and a pulseduration of 69 ns at full width half maximum (FWHM) atfundamental wavelength. The beam diameter was 5 mm (1/e2).An aperture of 2.5 mm was inserted between the safety shutterand an output coupler of the laser to enhance the beam qualityand improve the M2 value. This resulted in maximum pulseenergy being limited to 38 mJ/ pulse. The beam prole and M2

values were measured using a laser beam prolometer (Win-CamD, DataRay Inc. USA). The laser energy could be attenuatedcontinuously using an external wave plate/ polarizer setup with-out affecting any laser parameters such as pulse duration orspatial beam prole. The energy of each pulse was measuredusing a pyro-electric detector and laser energy meter (FieldMax,Coherent UK). A piezoelectric pressure sensor (6052C, KistlerSwitzerland) was installed in the chamber for measuring pres-sure-time history inside the combustion chamber. A high speedcamera (SA 1.1, Photron UK) was used to visualize the amekernel growth. Minimum frame rate for this camera is 5400 fps atmaximum resolution and 6,75,000 fps at minimum resolution. Awhite light source (OSL1-EC, Thorlabs USA) was used to illumi-nate the ame kernel.

2.2. Experimental procedure

Fig. 1. Schematic diagram of laser ignition shadowgraphy of CNGair mixture.Before conducting experiments, it was necessary to character-ize the laser beam. Laser beam prole and beam quality wasmeasured by a laser beam proler. Laser beam prole and beamquality are important parameters in laser ignition. These affectthe minimum energy required for formation of plasma andinitiation of combustion. Laser beam was expanded from 2.5 to15 mm using a set of lenses. A 100 mm focal length of plano-convex lens was used to focus the laser beam. The location offocal point was kept inside the combustion chamber in such away that the plasma is formed in front of the orthogonal windowsso that laser plasma and ame kernel growth can be visualized asshown in Fig. 1. It was necessary to measure minimum laser pulseenergy required for the formation of plasma in atmosphericconditions as well as at 10 bar pressure. Plasma formation prob-ability was calculated at different pulse energies and chamberpressures. Laser pulse energy for combustion experiments waschosen such that the laser ignition was successful for all airfuel

Please cite this article as: Srivastava DK, et al. Flame kernel charactevolume combustion chamber. Opt Laser Eng (2011), doi:10.1016/j.olaser ignition, exhaust valve was opened and the contents of thecombustion chamber are evacuated by a vacuum pump so that noresidual gases are left out in the CVCC. It was also ushed outusing fresh air to remove the traces of exhaust. The CVCC thusbecomes ready for next combustible mixture preparation for thenext combustion event.

The pressure-time history inside the combustion chamber wasrecorded using a piezoelectric pressure transducer. The signalsfrom the pressure transducer were amplied using a chargeamplier and were recorded in a digital storage oscilloscope.Then pressure-time history signals were sent to computer forfurther data analysis.

Table 1Properties of CNG.

Sl. no. Properties Values

1. Relative density 0.64

2. Auto ignition temperature (1C) 5403. Flammability range (% v/v) 515

4. Octane no. 127

Fig. 2. Gas lling arrangement for the constant volume combustion chamber withprecise control of relative airfuel ratio. 1High pressure pipe, 2Pressureregulating valve, 3Digital manometers and 4Vacuum Pump.


The early development of ame kernel was observed byShadowgraphy technique [31]. Collimated white light beam wasdirected into the combustion chamber and ame kernel wascaptured on the other side with the help of the high speed cameraoperating at 54,000 fps. The camera was also used to trigger thelaser system. Computer, camera, laser and oscilloscope are syn-chronized for combustion and visualization of ame kernel.Camera is triggered from the computer software. A 5 V outputsignal is generated by camera. This signal is divided into two sothat laser and oscilloscope are triggered simultaneously. Laser istriggered in the external mode so that as soon as laser receivessignal from the camera it will re the laser pulse. If the energydeposited in the plasma is above the critical value for combustion,then combustion event takes place and is recorded by the camera.After receiving the trigger signal from camera, oscilloscope isactivated in the measurement mode. During combustion, varia-tion of pressure inside the combustion chamber is recorded bypiezoelectric transducer and stored in the computer connectedto oscilloscope.

investigated rst. Shape of plasma is shown qualitatively inFig. 4a. Laser enters from right to left. A prior study by Srivastavaet al. [21] measured the shape and propagation behavior ofplasma generated in atmospheric air. The maximum diameterand length of the laser plasma 30 ns after the ring of the laserwere 0.01 and 0.27 mm, respectively, as depicted in Fig. 4b. It wasobserved that plasma growth took place towards the incominglaser beam much faster compared to along the laser beam. Thereason for this reverse propagation of plasma is that the layers ofgas outside the plasma, although transparent to the laser beam,get heated by the plasma radiation. This outside gas close to theplasma will in turn get ionized to such an extent that it willstrongly absorb the laser beam [32]. As a result, these gas layerswill then get further heated rapidly and their temperaturesincrease. By this time, another layer of plasma near to the laserwould become strongly absorbing, and hence the boundary of theplasma will move toward the focusing lens.

Once the plasma is generated in the atmospheric air, itbecomes pertinent to study the probability of plasma formationupon ring a laser pulse of certain energy. Fig. 5 shows the plasmaformation probability in atmospheric air and 10 bar of air pres-sure (in the chamber lled with only air) with different laserpulse energies. Plasma formation probability is dened as numberof successful breakdown events divided by number of attempts.For each pulse energies investigated, laser was red 500 times.The breakdown threshold was dened as the laser energy atwhich the air would breakdown for more than 50% of theattempts.

It can be noticed very clearly from Fig. 5 that the air break-down pulse energy thresholds at atmospheric condition and

Laser Pulse Energy (mJ/Pulse)Fig. 5. Plasma formation probability in air at atmospheric pressure and 10 barpressure.

D.K. Srivastava et al. / Optics and Lasers in Engineering ] (]]]]) ]]]]]]4Fig. 3. Laser beam prole at 1 m distance from laser head (a) 2-D prole, (b) 3-Dprole, (c) Line prole of beam along X-axis and (d) Line prole of beam along3. Results and discussion

Laser beam prole is a critical parameter for laser ignitionexperiments. It is usually necessary to measure it in order toensure the adequate beam quality prole. Before starting theexperiments in combustion chamber, ofine test was carried outto characterize the laser beam. Laser beam prolometer was usedto measure the beam quality with a cavity aperture of 2.5 mm at1 m distance from the laser head. Fig. 3a and b show 2-D and 3-Dbeam proles, respectively. Fig. 3c and d show the line prolealong the X and Y axis, respectively.

It can be seen from Fig. 3 that the laser beam prole deviatesslightly from TEM00 mode. Beam quality factor (M

2 value) wasmeasured for the cavity aperture of 2.5 mm. M2 denes thefocussability of laser beam. It is directly related to the diameterof beam at focal point. M2 value of perfectly Gaussian laser beamis 1.0. In the present case, M2 value of laser beam was observed tobe 4.6. For laser ignition, the M2 value closer to the Gaussian laserbeam is desired however it also leads to severe reduction inmaximum laser pulse energy therefore a balance between the twois required to be found out.

For systematic investigation of laser ignition of fuelair mix-tures, plasma generated in atmospheric condition needs to beY-axis.

Please cite this article as: Srivastava DK, et al. Flame kernel charactevolume combustion chamber. Opt Laser Eng (2011), doi:10.1016/j.oFig. 4. (a) Plasma formation in air (b) emission photography of the laser plasma30 ns after the laser pulse [21].

0102030405060708090

100

0 3 6 9 12 15 18 21

Plas

ma

Form

atio

n Pr

obab

ility

(%) 1 bar

10 bar10 bar pressure were 17 and 7 mJ, respectively. In summary, the


breakdown threshold energy decreases with the increasing cham-ber pressure. The reason is that at higher pressure conditions, thenumber of gas molecules in the focal region increases and laserenergy can be absorbed more efciently. In an engine operatingwith lean airfuel mixture, in-cylinder pressure at the time ofignition should increase in order to compensate for the resultingpower density loss. From Fig. 5, it could be seen that this willcreate a favorable condition in case of laser ignition, where laserpulse energy required to breakdown decreases with increasingchamber pressure. This trend is completely opposite to the oneobserved in conventional spark plug system. It is reported thatthe spark energy required for igniting the mixture increases withincreasing in-cylinder pressures [1]. Since at 9 mJ laser pulseenergy, the plasma formation probability is 100% at 10 barchamber pressure, a notch higher level of pulse energy i.e.12 mJ pulse energy is chosen for carrying out the combustionexperiments in the present study in order to be able to cover thewhole spectrum of experimental conditions.

Once the minimum laser pulse energy for this set of experi-ments was determined, ignition experiments and ame kernelvisualization of different airfuel ratios were carried out in theconstant volume combustion chamber. The images showing thedevelopment of the early ame kernel stages and its growth withtime were recorded by employing Shadowgraphy technique. Highspeed camera was used to capture the image at 54,000 fps. Theconsecutive images were captured at an interval of 18.5 ms for anygiven single combustion event. These images provided useful

At the early stages of ame development (to92.5 ms, notshown in these pictures), a toroidal shape of the kernel wasobserved. Toroidal shape of ame kernel shape is similar to theone observed in conventional spark electrode ignition system.Maly and Vogel [33], conducted experiments in methaneairmixture using conventional spark plug. They changed the spark

T = 1.87ms T =2.70ms T = 3.53ms T = 4.37ms

Fig. 8. Shadowgraph image of ame kernel development (l1.4).

T = 37s T= 370s T = 703s T = 1036s

T = 2.37ms T = 3.70ms T = 5.03ms T = 6.36ms


T = 2.70ms T = 4.37ms T = 6.03ms T = 7.70ms

T = 37s T = 370s T = 703s T = 1036s


T = 6.03ms T = 11.03ms T = 16.02ms T = 21.02ms

T = 37s T= 370s T = 703s T = 1036s


D.K. Srivastava et al. / Optics and Lasers in Engineering ] (]]]]) ]]]]]] 5information about the ame kernel development, and the shapeof the ame kernel as a function of time for different airfuelratios (l1.21.7). Figs. 611 show the images of ame kerneldevelopment process at different time scales. Time scale of rstfour images was kept constant in all gures however inFigs. 911, the time scale for the last four images was observedto be different because of longer combustion duration for theleaner mixtures. In all these gures, laser beam is entering fromright to the left. Vertical and horizontal dimensions of all imageswere kept constant at 1.15 and 1.45 cm, respectively.

T = 37s T= 370s T = 703s T = 1036s

T = 1.87ms T = 2.70ms T = 3.53ms T = 4.37ms


T = 37s T= 370s T = 703s T = 1036s

T = 1.87ms T = 2.70ms T = 3.53ms T = 4.37msFig. 7. Shadowgraph image of ame kernel development (l1.3).

Please cite this article as: Srivastava DK, et al. Flame kernel charactevolume combustion chamber. Opt Laser Eng (2011), doi:10.1016/j.oT = 37s T= 370s T = 703s T = 1036senergy to see its effect on ame kernel size and expansion


velocity. A smooth ame front appeared for high-energy arc.Initially, the shape of kernel was cylindrical for very short timethen spherical changing to toroidal for intermediate time andspherical again for longer time durations. Other researchers[3436] also visualized the ame kernel formation in combustionchamber for different airfuel mixture using conventional sparkplug. The evaluation behavior of ame kernel was found bespherical. In the present experiment with laser ignition, the toroidshape continues to develop radially away from the spark centertill 92.5 ms. After approximately t92.5 ms, a front lobe is formedand propagates towards the incoming laser beam. This is apeculiar feature of laser-induced ignition. Similar expansionbehavior of laser ignited ame kernel is observed by otherresearchers as well [14,23,37]. The shape of the ame kernel isobserved to be structurally identical for all airfuel ratios. It isobserved from Figs. 10 and 11 that the front lobe of the amekernel disappeared after approximately 1 ms for relative airfuelratios of 1.6 and 1.7 (image 5 onwards). Based on these observa-

for every airfuel ratio, the images were analyzed and averagevalues are presented in these graphs to reduce effect of experi-mental errors. Centroid of the rst kernel is taken as origin forcalculating distance in the three directions.

It can be observed from Fig. 12 that during the early stages(t666 ms), ame propagation distance increases rapidly, sug-gesting higher ame velocity initially. For leaner mixtures (l1.6and 1.7), ame propagation distance in X-direction decreases withtime in the later stages of combustion (t1.16 ms) because theleaner mixture is unable to sustain the combustion in front lobebeyond a limit, resulting in reduction in ame velocity. However,for relatively richer mixtures (l1.2, 1.3 and 1.4), ame propaga-tion distance increases up to 2.8 ms and then it becomes almostconstant. The maximum distance of ame propagation observedfor lambda 1.2 is 0.53 cm.

Flame kernel propagation in the direction of laser beam, i.e.X direction is shown in Fig. 13. It can be observed from thisgure that the propagation of ame kernels were varying almostlinearly for relative airfuel ratio (l1.2, 1.3 and 1.4) with timeand this suggest almost constant ame velocity. The maximumpropagation distance for lambda 1.2 is 0.3 cm. It can be concludedfrom Figs. 12 and 13 that forward propagation distance of amekernel i.e. in the direction of laser beam, is less than thepropagation distance of ame kernel in backward direction, i.e.opposite to laser beam direction, for all the relative airfuel ratio.

Propagation of ame kernel in Y and Y direction wasalmost identical and therefore only one direction is reported.Fig. 14 shows the temporal variation of ame kernel in Ydirection. It can be seen from the gure that the maximum ame

Fig. 14. Temporal variation of ame kernel development in the orthogonal to thelaser beam propagation (Y-direction).

D.K. Srivastava et al. / Optics and Lasers in Engineering ] (]]]]) ]]]]]]6tions, shape of the laser-induced ame kernel can be thought tobe having two stages of development. In early stage (to92.5 ms)of ame kernel development, the kernel develops radially to forma toroidal shape. In the latter stage (t492.5 ms) of development, afront lobe is formed, which propagates backwards towards theincoming laser beam. This expansion phenomenon is alsoreported earlier [14] however the reasons for this have not beenexplained fully and convincingly. Spiglanin et al. [37] proposedpossible explanation for the formation of front lobe and suggestedthat this phenomenon may be related to the initial ow eldcreated by the propagation of a radiation transport wave up tothe laser beam, arising from the high rate of energy transferred atthe leading edge of the plasma. An additional factor may be thepreheating of the gases by the focused laser beam that ignites themixture. This preheating gas readily ignites in a ame front thatpropagates much faster than it would through cold combustiblemixture.

After understanding the ame kernel development, it is logicalto analyze the temporal variation of the ame kernel develop-ment in various directions with time inside the combustionchamber for varying relative airfuel ratios. This informationcan be attained from the analysis of different photographicimages captured by the high speed camera. Temporal develop-ment of ame kernel was analyzed using MATLAB. Lasers direc-tion of propagation is taken as X and direction opposite to thelaser propagation is taken as X , which is also the direction ofpropagation of front lobe. The temporal variation of ame kerneldevelopment in X , X and Y direction are given in Figs. 1214.Five combustion events are carried out under identical conditions

Fig. 12. Temporal variation of ame kernel development in the direction of

opposite to laser beam propagation (X-direction).

Please cite this article as: Srivastava DK, et al. Flame kernel charactevolume combustion chamber. Opt Laser Eng (2011), doi:10.1016/j.oFig. 13. Temporal variation of ame kernel development in the direction of laserbeam propagation (X direction).kernel propagation distance was 0.38 cm for relative fuelair


earlier observations, which indicate that the rich fuelairmixtures give higher ame velocities and faster propagation ofame kernel. It can also be noticed that the peak cylinder pressuredecreases with leaner mixtures, as expected. The experimentswere also carried out with very lean mixture (l1.7) however thepressure-time history showed unacceptable variations reectinguncertain combustion behavior. Further leaner mixtures couldnot be ignited by lasers and this suggested that for a practicalapplication of laser ignition system applied to the engine will notbe able to deal with CNGair mixtures leaner than l1.6. Thisstatement is however subject to the given type of optics. Ifimproved optics and better quality laser beam is used, possiblylean combustion limit in an engine can be pushed further.

To study the event-to-event variation in combustion pressure,peak pressure variations were analyzed for different airfuelratios. One of the important advantages of laser ignition is the

dt

D.K. Srivastava et al. / Optics and Lasers in Engineering ] (]]]]) ]]]]]] 7Fig. 15. Flame propagation speed in the X-direction.

05

101520253035404550

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Pre

ssur

e (b

ar)

Time (ms)

Lambda = 1.2Lambda = 1.3

Lambda = 1.4 Lambda = 1.5Lambda = 1.6ratios (l1.2 and 1.3). The ame kernel propagation distance wasfound to consistently decrease with the leaner mixtures and it isfound to be lowest for l1.7. These ndings are almost similar tothe ones for X directions suggesting that the volumetric growth ofthe ame kernel reduces substantially with reduction of airfuelmixture strength.

Flame speed was derived from propagation distance. Fig. 15shows the propagation speed of ame in direction opposite tolaser beam direction, i.e. X-direction for different relative airfuelratio. It is observed from gure that ame propagation speeddecreases with increasing relative airfuel ratio and alsodecreases with time. Initially propagation speed for higher rela-tive airfuel ratios, i.e. leaner mixture strength is observed to behigher than that of relatively richer mixtures however thispropagation speed decreases very fast. At 0.33 ms, ame propaga-tion speed for lambda 1.2 and 1.6 are 5.54 and 4.07 m/s,respectively, while at 6.67 ms, it becomes 1.62 and 3.21 m/s,respectively. Propagation speed becomes negative for lambda 1.5,1.6 and 1.7 at some time because of reduction of ame propaga-tion distance and suggests shrinking of ame kernel, particularlyfor leaner mixture (l1.6 and 1.7).

It is also important to experimentally evaluate the pressure-time history in the combustion chamber for varying fuelairmixture as this will provide vital information about the kind ofpressure rise, which can be expected in an engine system ignitedusing laser.

Fig. 16 shows a pressure-time history of the combustionchamber for different relative airfuel (l1.21.6) at initialchamber lling pressure and temperature of 10 bar and 373 K,respectively, ignited by laser. It can be clearly seen from thisgure that there is a clear trend towards longer combustionduration with leaner CNGair mixtures. This is also supported by

Fig. 16. Pressuretime history of the combustion chamber for different l.

Please cite this article as: Srivastava DK, et al. Flame kernel charactevolume combustion chamber. Opt Laser Eng (2011), doi:10.1016/j.oI. The cylinder charge was considered to behave as an ideal gas.II. Distribution of thermodynamic properties inside the combus-

tion chamber was considered to be uniform.III. Dissociation of combustion products was neglected.IV. Heat transfer from the combustion wall is neglected in

this model.

Since experiments were done in constant volume combustionchamber, the rate of volume change parameter in heat releaseequation will be zero. Rate of heat release for constant volumecombustion chamber will therefore be:

dQ

dt 1g1

VdP

dt

0

5

10

15

20

25

30

35

40

45

50

1.2 1.3 1.4 1.5 1.6

Pea

k P

ress

ure

(bar

)

Relative Air-Fuel Ratio (-)TFi

rizaptlahe following assumptions were made in this calculation.g1 V dt g1 P dtdQ 1

dP g

dVRexact regulation of deposited energy in the focal volume. So it isexpected that the variation in combustion is lower in case of laserignition. Fig. 17 shows the variation of peak combustion pressureinside the combustion chamber for different airfuel ratios. Errorbars in Fig. 17 show that there is very little variation in peakchamber pressure. There is a good repeatability of experimentsand the observed data.

Rate of heat release (ROHR) was calculated from the acquiredpressure-time history data of the CVCC using zero dimensionalheat release analysis model [38].

ate of heat release was calculated asg. 17. Peak pressure variations for laser ignition of different airfuel ratios.

tion of laser ignition of natural gasair mixture in a constantseng.2011.04.015

for providing funding for carrying out this project. The Engine

ignition in 02/03 mixtures. Applied Physics B 1985;37:18995.[3] Hill RA. Ignition-delay times in laser initiated combustion. Applied Optics

D.K. Srivastava et al. / Optics and Lasers in Engineering ] (]]]]) ]]]]]]84. Summary

In this study, laser-induced ignition of CNGair mixtures wasexperimentally investigated. Experiments were conducted in aconstant volume combustion chamber at 10 bar initial llingpressure and 373 K temperature. A Q-switched Nd: YAG laserwas used for the ignition of CNGair mixture at the fundamentalwavelength (1064 nm). The beam quality was measured using anoptical prolometer. Plasma was generated in the atmosphericcondition as well as 10 bar chamber pressure, and the minimumenergy required for plasma generation with 50% probability werefound to be 17 and 7 mJ, respectively. It was observed thatbreakdown threshold energy decreases with increase in chamberpressure. This trend is completely opposite to the one observed inconventional spark plug system. Since at 9 mJ laser pulse energy,the plasma formation probability is 100% at 10 bar chamberand the net heat release is

Q Z t0

dQ

dt

dt

where Q is net heat release, t is the time, V is the volume ofcombustion chamber, P is combustion pressure and g is the ratioof specic heat.

Fig. 18 shows the net heat release for different relative airfuelratio. Net heat release decreases with increase in relative airfuelmixture. Signicant differences in slope of heat release indicatesthe variations in combustion duration. Combustion durationincreases with increasing relative airfuel i.e. for leaner mixtures.This observation is also supported by the decrease in ame speedwith increasing relative airfuel. Heat release for lambda 1.2 is12.4 kJ whereas for leaner mixture, (lambda1.6) it is 8.8 kJ.

Fig. 18. Net heat release versus time for different relative airfuel ratios.pressure, 12 mJ pulse energy is chosen for carrying out thecombustion experiments in the present study in order to be ableto cover the whole spectrum of experimental conditions.

Ignition experiments and ame kernel visualization of differ-ent airfuel ratios were carried out in the constant volumecombustion chamber. The images showing the development ofthe early ame kernel stages and its growth with time wererecorded by employing Shadowgraphy technique. At the earlystages of ame development (to92.5 ms), a toroidal shape of thekernel was observed. The toroid continues to develop radiallyaway from the spark center till 92.5 ms. After t92.5 ms, a frontlobe is formed and propagates towards the incoming laser beam.This is a peculiar feature of laser-induced ignition. The shape ofthe ame kernel is observed to be structurally identical for allairfuel ratios of the combustible airfuel mixtures. Based onthese observations, shape of the laser-induced ame kernel can bethought to be having two stages of development. In early stage

Please cite this article as: Srivastava DK, et al. Flame kernel charactevolume combustion chamber. Opt Laser Eng (2011), doi:10.1016/j.o1981;20(13):223956.[4] Hill RA, Laguna GA. Laser initiated combustion of CH4O2 mixtures. Optics

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[2] Raffel B, Warnatz J, Wolfrum J. Experimental study of laser-induced thermal(to92.5 ms) of ame kernel development, the kernel developsradially to form toroidal shape. In the latter stage (t492.5 ms) ofdevelopment, a front lobe is formed, which propagates backwardstowards the incoming laser beam. Front lobe of ame kerneldisappeared after approximately 1 ms for leaner mixtures (l1.6and 1.7). The temporal variation of the ame kernel developmentin various directions with time inside the combustion chamberfor different airfuel ratios is analyzed by interrogation ofdifferent images. The ame kernel propagation distance wasfound to be consistently decreasing with leaner mixtures and itfound to be the lowest for (l1.7). These ndings are almostsimilar to the ones for X directions suggesting that the volumetricgrowth of the ame kernel reduces substantially with reductionof airfuel mixture strength. The pressure-time history of thecombustion chamber for different airfuel mixture was investi-gated. There was a clear trend towards longer combustionduration with leaner CNGair mixtures. This is also supportedby earlier observations, which indicate that the richer fuelairmixtures give higher ame velocities and faster propagation ofame kernel. It can also be noticed that the peak cylinder pressuredecreases for leaner mixtures, as expected. A practical applicationof laser ignition system applied to the engine will not be able todeal with CNGair mixtures leaner than (l1.6).

Acknowledgments

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D.K. Srivastava et al. / Optics and Lasers in Engineering ] (]]]]) ]]]]]] 9Please cite this article as: Srivastava DK, et al. Flame kernel charactevolume combustion chamber. Opt Laser Eng (2011), doi:10.1016/j.orization of laser ignition of natural gasair mixture in a constantptlaseng.2011.04.015

Flame kernel characterization of laser ignition of natural gas-air mixture in a constant volume combustion chamberIntroductionExperimental setupExperimental apparatusExperimental procedure

Results and discussionSummaryAcknowledgmentsReferences

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