Combustion in the Spark Ignition Engine

7/31/2019 Combustion in the Spark Ignition Engine

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COMBUSTION IN THE SPARK IGNITION ENGINE

Combustion in a spark ignition engine begins with a spark initiated in the air-fuel mixture.After a delay, a flame forms which propagates across the combustion chamber burning the fuel-airmixture as it goes. The pressure and temperature increase up to a certain maximum and then

decrease. The flame reaches the other end of the combustion chamber and gets quenched signifyingthe end of the combustion process.

INTRODUCTION

An enormous amount of theoretical and experimental research has been carried out on thesubject of combustion of homogenous, premixed, fuel-air mixtures. Basic studies have been carriedout in constant volume bombs, shock tubes, and other apparatus that simulate the actual workingconditions. Studies have been carried out in piston-cylinder assemblies, in rapid compressionmachines that simulate the combustion processes only, and in actual engines.

In those engines where the process of combustion follows the scenario presented above, thatis, a process involving the propagation of a flame and the burning of the mixture as it goes along, thecombustion process is termed as normal. Abnormalcombustion is said to occur when the process offlame propagation is not normal, and there is a premature burning of the charge, and otherphenomena take place. Some examples of abnormal combustion are detonation or knock, pre-ignition, and after- or post-ignition.

IGNITION

Ignition, by implication, is merely a prerequisite of combustion so the study of combustionmust begin with the consideration of the phenomenon of combustion to establish a criterion on

which to decide whether, in a particular case, ignition has occurred or not. In terms of its simplestdefinition, ignition has no degree, intensively or extensively. Either combustion of a medium isinitiated or it is not initiated. It is, therefore, sensible to consider ignition from the standpoint of thebeginning of the combustion process that it initiates.

Normal combustion can be regarded, in effect, as a zone of burning, propagated through amedium by means of heat transfer and diffusion, in a manner that constitutes a wave in the broadsense of the term. The zone of burning is the reaction zone and the propagation of the reaction orburning is the combustion wave. The flame can also be stationary; that is, it remains in one positionwith respect to the datum while the combustible mixture (of the fuel and air) flows into it, like in agas burner of a boiler or gas turbine. As seen earlier, however, the general concepts of a reaction

zone and burning velocity can be retained.

In the presence of a heat sink, like a solid surface, the dynamics of combustion are modified.From a phenomenological point of view, the reaction zone will not approach the heat sink closerthan a certain distance, which produces a dead space. If two heat sinks, each with its dead space, aremoved towards one another, the dead spaces will eventually combine to form a zone of quenching,into which a combustion wave cannot propagate. For example, in a flame trap of the wire mesh type,the gap between the wires is less than the quenching distance, which is a critical dimensiondepending upon the nature of the combustible mixture, among other factors.


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The ignition process though virtually connected with the initiation of combustion; in general,it is not associated with the gross behavior of the combustion waves. Instead, it is a local, small-scalemanifestation that takes place inside a combustion wave. Therefore, in the search for theoreticalsupport for the phenomena of ignition and combustion, it is more rewarding to look at processesoccurring inside the wave. One such treatment of the problem requires, a hypothesis, concepts ofexcess enthalpy and minimal flame; among other things, it demonstrates the existence of the quenchdistance. It is assumed that a spherical shell of burning gas enclosing and emanating from a sphere ofburnt gas can approximately represent a spherical combustion wave of minimum diameter. Thepropagation of this or any wave is simply the transport of energy across the wave front in thedirection of propagation. Two contributions to the problem exist, namely,

(a) the diffusion of the chemical reactants and the products of the reaction and(b) the conduction of heat from the burned to the adjacent unburned gas.

Furthermore, if it is now assumed, with some justification, that the diffusion transport can beneglected, in comparison with the conduction of heat, it can be shown, from combustion wave

theory that the burned gas inside the wave has enthalpy, that is, energy in a particular state, in excessof the ambient level given by the following formula

h = (k/Vb)(Tb - Tu) .. (1)

where h = enthalpy per unit area of the wave,k = coefficient of thermal conductivity,Vb= burning velocityTb= temperature of the burned gasTu= temperature of the unburned gas

For a spherical flame of surface area d2

, where d is the diameter of the sphere, the totalexcess enthalpy is given by

Htheo = d2h .. (2)

Now, if the hypothetical flame has a diameter less than the minimum, the volume of theenclosed sphere of burned gas will be too small to provide the required amount of excess enthalpy toallow the propagation to continue. Thus, the reaction will cease and the flame will become extinct.This minimum diameter is clearly related to, and must be less than, the quenching distance.

Energy Basic Requirements for Ignition

Self-ignition is the limiting case in which the systems have been brought to the state - byexpenditure of energy - such that no further internal energy, intended solely for the ignition process,is required to initiate combustion. The distinction between energy supplied to a system that is notself-igniting, to bring about ignition, and the work done on the system to make it self-igniting, issubtle and essentially practical since it is convenient to distinguish between the two methods ofignition in practical applications. There is no fundamental difference between the two processes.

An ignition process obeys the law of conservation of energy. It is treated as the balance of


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energy between that provided by an external source, that is released by chemical reaction, and thatdissipated to the surroundings by thermal conduction, convection, and radiation and mass transferwhich, in turn, are affected by experimental conditions. It has already been stated that Htheo given inEq. 2 gives the total excess enthalpy required to cause the flame to be self-sustaining and promoteignition.

It is, therefore, reasonable to suppose that the basic requirement of the source of ignition isthat it supplies this energy within a small volume compatible with the dimensions of the minimalflame in a time short enough to ensure that only a negligible amount of energy is lost over and abovethat required to establish a flame. Thus, the rate of supply of energy, or power, is as important as thetotal energy supplied.

Spark Ignition

A small electric spark of short duration is the ideal requirement for ignition. However, theenergy content need not be high. The actual energy required for the spark to cause ignition of acombustible mixture is not yet truly concepted.

A spark is caused by applying a sufficiently high voltage between two electrodes separatedby explosive gas in the gap. It is possible to pass small electric sparks through the gas (in the gap)without producing ignition. When the spark energy is increased, that is, when the voltage across theelectrodes is raised above a certain critical value (below which a spark may not even occur) athreshold energy is eventually obtained at which the spark becomes incendiary in the sense that acombustion wave propagates from the spark through the volume of gas. This minimum ignitionenergy is a function of experimental variables such as the parameters of the explosive gas and theconfiguration of the spark gap.

Paschen's Law states that the critical voltage (at which spark would occur) is a function of

the product of the dimensions of the gap and the gas pressure. Also, the manner in which voltage israised to the critical value, configuration and condition of the electrodes and the nature of thecombustible mixture are all-important in relation to the energy required.

Capacitance Spark

In Fig. 1, which shows a part of the circuit used to study spark ignition effects (given byBlaue et al1), when the condenser is charged to a voltage V equal to the critical or break downvoltage of the gap, the condenser will discharge as a spark across the gap. In the absence of anyresistance or inductance in the circuit a total energy of CV2 in the condenser will be dissipated atthe gap and, except for losses, will appear as energy in the simple, brief spark available for ignition.One important loss is due to quenching at the electrodes.

From a series of experiments in which the composition, pressure, and temperature of theexplosive gas were held constant and the length of the spark gap was systematically changed, curvesof the maximum ignition energy versus distance between the electrodes were obtained. Two typicalcurves obtained are shown in Fig. 2.

13rd Symposium. On Combustion, p 363.


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One curve corresponds to a series of experiments in which the electrode terminals weretipped with stainless steel spheres of 1.5 mm diameter. In the other series, the electrodes weresimilarly tipped and in addition were flanged by glass plates.

The curves for the glass plate electrodes take a rather sharp vertical turn around 0.08-inch (2-mm). For the other type of electrode too, the curve moves up at this point but more gradually. Theenergy requirement increases and ignition becomes impossible eventually. This is due to thequenching effect. The critical distance, wherein ignition is suppressed completely, is the quenchingdistance. Experiments have shown that its value is substantially independent of the mode of ignition.Thus, the same, or nearly the same, value of the quenching distance is obtained from experiments inwhich the explosive gas is enclosed in a rectangular channel bounded by two parallel plates andignited at one end by a pilot flame. The material of the wall has not been found to affect thequenching distance; evidently glass metal are equally effective as heat sinks since thermalconductivities of solids exceed those of gases by orders of magnitude. In the present experiments theuse of flanges made from an electric non-conductor such as glass was indicated because in this waythe sparks remained centered between the electrodes. Metal plates were also used occasionally butthe sparks had a tendency to cross at random anywhere between the plates and frequently crossed

from one plate edge to the other, thus causing ignition even though the plates were within thequenching distance. When the metal plates behaved, the results were not different from thoseobtained with glass-flanged electrodes. Since a glass surface is usually conductive, irregulardischarges and particularly corona discharges were also observed with glass flanges. Coating theglass with a trace of paraffin wax effectively eliminated this source of error. In contrast to the sharpvertical turn of the curve corresponding to the glass-flanged electrodes, the other curve in Fig. 2 risesgradually as the electrode distance is decreased below the quenching distance. The quenching effectis not so marked and it can be compensated by an increased supply of energy so that ignition can beobtained even with very small electrode distance. It is noteworthy, however, that the beginning of therising part coincides with the quenching distance of the glass-flanged electrodes; that is, thequenching effect, although much weaker, extends over the same gap length.

Similar experiments performed on 8.5% methane-air mixtures with three different electrodeconfigurations, namely, both electrodes flanged by glass plates, negative electrode flanged by glassplate and positive electrode tipped with 1.5 mm sphere, and vice versa, indicate that at electrodedistances larger than the quenching distance, the size and shape of the electrodes do not significantlyaffect the value of the minimum ignition energy, whereas within the quenching distance theinfluence of these factors is pronounced; that is, the actual minimum ignition energy for gaps smallerthan the quenching distance may vary appreciably for different electrodes. See Fig. 3.

The minimum ignition energy and the quenching distance increase with decreasing pressure.Beyond the quenching distance, the minimum ignition energy is seen to be virtually independent ofelectrode distance over a considerable range of distances and pressures. Outside this range theminimum ignition energy is seen to increase with increasing electrode distance. From Fig. 4 it is seenthat when pressure was decreased from 1 to 0.2 atm, the minimum energy required rose by a factorof 10.

Figure 5 shows the minimum ignition energy versus distance between plate electrodes whenextended to very large spark energies. It is seen that the quenching distance does not diminish whenlarge sparks are used, but on the contrary, it increases. The fact that the electrodes are to be placedapart for reducing energy requirement above the quenching distance is that large sparks produced


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turbulence, which caused an increase in heat loss to the plates and thus outweighed the additionalenergy. The plates must be separated further in order to obtain ignition. Also, minimum energyrequirements may not be substantially constant at higher spark gaps as shown above.

When the minimum ignition energy for air and various hydrocarbons is plotted it is seen thatthe minima of the energy curves for the various compounds occur at nearly identical energy values.The minima shifts to richer than stoichiometric mixtures as the number of carbon atoms in the fuelincreases. For methane, the minima occurs at an equivalence ratio of 0.9 whereas for heptane itoccurs at 1.8 and hexene and benzene at 1.75.

It is observed that the value of H theo agrees well with actual enthalpy values for hightemperature, fast burning mixtures of fuels with oxygen or oxygen-enriched air whereas the lowtemperature, slow-burning fuel-air mixtures show considerably lower experimental values ofenthalpy. This may be due to the fact that the model considers the transport of thermal energy in theflame and does not allow for transport of chemical energy by the inter-diffusion of reactants andreaction products; this effect appears to be more significant in slow burning mixtures than in fastburning ones.

For high temperature, fast burning mixtures, the H theo varies as d3 whereas for slow burning

mixtures the slope decreases to somewhat lower than d2. The curves of minimum ignition energyversus quenching distance, when both quantities are taken on the log scale are independent of thetype of fuel and proportion and pressure of the mixture. See Fig. 6.

Minimum spark energy and quenching distance decreases with increase in initialtemperature (or temperature of the unburned mixture).

Effect of Electrode Configuration

The minimum energy continues to decrease with decrease in electrode gap if the electrode issmaller. Large electrodes produce greater quenching at or below the quenching distance and thisrequired higher minimum energy.

Effect of Electrode Material

The material of the electrode has an effect, the effect being that ignition energy for electrodegaps larger than the quenching distance was found to vary with different materials and increasedafter any change to a material with higher melting point. Thus, energy required increased each timethe material was changed from cadmium to aluminum, gold and platinum. It is believed that not allthe energy released at the gap participates in the ignition process. An amount that varies with thematerial is lost to the electrodes, possibly causing slight evaporation.

Effect of Series Resistance

By imposing a resistance in series with the spark circuit, some energy will be dissipated thereduring discharge, and total energy required, that is, circuit energy stored in the capacitor, is likely tobe greater. But the energy required at the gap, calculated from the gap current and voltage, decreased


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appreciably with increase in the resistance of the circuit. Rose and Priede2 were able to reduce theminimum ignition energies of hydrogen-air mixtures by this method. By this, the discharge period isincreased and this reduces shock wave formation. Gap geometry and material have also beenstudied.

Effect of Stray Inductance

An inductance L will cause an inductive energy given by

Li2

where i is the current and will affect the discharge current. If we put

i = dQ/dt

where Q is the charge, then

L d2Q/dt2 + R dQ/dt + Q/C = 0

where R is the resistance and C is the capacitance.

If R2 < 4L/C discharge is oscillatory

If R2 4L/C discharge is non-oscillatory

For very small values of R, a small, stray inductance will cause an oscillatory discharge.Introducing a series resistance will ensure a non-oscillatory discharge when inductance isdeliberately held to a minimum. As seen above, the larger the resistance the longer the discharge

time. However, discharge continues only while the spark gap remains conductive; a situation that ismaintained while the voltage is above the breakdown potential of the gap but depends on theresidual ionization below this value. A simple capacitance spark is bright but of short duration; itoccurs mainly but not entirely, in the gas. The spectrum of the material of electrode is also present tosome extent.

Induction Spark

If a current i flowing through an inductance L (Fig. 7) is interrupted, the collapse of thecurrent will cause, in the circuit, an induced voltage given by

L di/dt

where di/dt is the rate of collapse of current.

Arcing will occur at the interrupter contacts. The energy given by

Li2

2 7th Symposium on Combustion, p 436-445 and 454.


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originally stored in the magnetic field, will be divided between the arc-heat at the contacts and otherlosses in the system. Thus, the interrupter contacts can be used as spark electrodes and the systemcould ignite the surrounding combustible mixture if the minimum ignition energy requirements aremet. This, however, is not a sure method of ignition. An obvious method of improvement is by usingan induction coil in place of a simple coil, a circuit breaker in the primary circuit and a spark gap inthe high voltage secondary circuit. This usually introduces a capacitance in the circuit and thedischarge no longer consists of an induction spark alone.

An induction spark is less bright than a capacitance spark but of a longer duration.

Induction Coil Spark

The induction coil spark has a discharge that is generally described as having twocomponents, namely, capacitance and inductance. In the circuit, as shown in Fig. 8, the dischargeoccurs thus:

On completion/breaking of the primary circuit of the transformer, an electromagnetic force(emf) is generated in the secondary circuit proportional to the rate of change of current in theprimary. This charges the condenser in parallel with the spark gap or the self-capacitance of thespark circuit. If and when the breakdown potential of the spark gap is reached, discharge occurs,initially in the form of a brief capacitance spark of energy, which is given by

CV2 - losses

However, if a current, i, has been established in the primary circuit, total energy to bedissipated is equal to the work done, that is, if L is the inductance, then

Total energy = Li2

Therefore, ignoring resistance, there will be a residual electromagnetic energy given by

Li2 - CV2

after the capacity discharge has occurred. It is also likely that the spark gap will remain ionized for abrief period after completion of the capacity component favoring continuation of the discharge of theresidual energy, at lower current and voltage, until the voltage falls to a value too low to maintaindischarge. Thus, in the induction coil discharge, the capacitance component is always present. If thecondenser or self-capacitance of the secondary circuit is not charged up to the breakdown potentialof the electrode gap, no discharge will occur. But once the capacity spark is obtained, conditions atthe gap may be sufficiently favorable to allow a proportion of the residual electromagnetic energy,arising from the work done in establishing a current in the primary circuit, to be discharged as aninduction component, sometimes referred to as an arc orflame. Two distinct phases of breakdownhave been given, namely,

(1) a phase of high current density and high electric field and

(2) a phase of comparatively low current, a low electric field and high gas temperature; the socalled induction component which, due to the high temperature and longer spark duration,


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can be responsible for appreciable erosion of the electrodes.

This causes pronounced lines in a spectrum corresponding to the materials of the electrodes;the fainter gas lines are due to the first, or capacitance phase.

The division of energy between the two phases can be arranged at will; the capacitancecomponent, CV2, can be changed by varying C and/or V, the breakdown voltage. Altering the gapchanges V. If C and V are large enough the inductance component may be negligible. If either C orV is further increased, without increasing Li2, spark will fail owing to insufficient energy. Achange in L or i will change the inductance component.

Thus, the minimum energy required for discharge is CV2. However, this is not a practicalminimum since the inductance component will not normally be negligible, that is, the primarycircuit energy Li2, the inductance, will be greater than CV2, the capacitance.

In general, the total circuit energy required for ignition will be greater for an induction coilspark than for a purely capacitance spark. This is also true in respect of a spark obtained from a

magneto, which consists of an electric generator and an induction coil, which are combined forconvenience and compactness.

The discharge of current in the secondary gap of a basically induction coil system can have ahigh degree of asymmetry: this apparent rectification varies in magnitude and direction with thelength of the gap: the reversal of direction indicates a polarity effect that depends upon thedimensions of the gap. Over 50% of the original energy in the primary circuit may appear as directcurrent (DC) in the spark discharge; this is part of the so-called "inductance" component. Its othercontribution is that induced by residual oscillations or decay of current in the primary circuit.However, for specific values of the spark gap and constants for the circuit, including the degree ofinductive coupling between the primary and secondary circuits, the unidirectional component may

contain all the energy still in the primary circuit, after the initial, very brief, capacitive discharge hasoccurred. That is, there will be no residual oscillations in the primary circuit and the inductancecomponent will be wholly unidirectional. Spark discharge has sometimes known to fail at aparticular gap setting; changes in the gap setting or polarity reversal are required to restore the spark.Hence, the importance of polarity in certain circumstances.

Effect of Gas Movement on Spark Ignition

In a combustible mixture of gas and air, laminar and turbulent flows have opposing effectson the requirements of ignition though experimental results do not entirely support this. One reportshowed that for laminar flow, the likelihood of ignition increased with increase in gas velocity. Thisis attributed to the tendency of the initial flame to escape with the aid of gas flow from the quenchingeffect of electrodes. On the other hand, when the gas flowed turbulently, ignition tended to besuppressed, probably due to increase of diffusivity of gases in the eddies.

Sometimes, turbulence is deliberately introduced to promote better mixing; the degree ofwhich can affect requirements of ignition and subsequent combustion. For example, the combustionchamber of a spark ignition engine usually has a squish area under the cylinder head in order tointroduce turbulence. See Fig. 9. Usually, this has a beneficial effect on the subsequent combustioninitiated by the spark. In these conditions, provision of an adequate spark is usually considered to be


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of secondary importance. Whether such turbulence, deliberately introduced to promote better mixingor a change in flow characteristics, facilitates ignition or not, depends on whether it makes themixture ratio in the vicinity of the spark gap more favorable for ignition than that in the absence ofturbulence. Thus, turbulence can have an effect, favorable or not, in ignition, owing to a change inthe local mixture ratio. However, the subsequent total combustion of the entire mixture is anothermatter. That is, a proportioning of the mixture in relation to the geometry of the combustionchamber could favor ignition. Thus, by definition, ignition is the initiation of combustion but not thecompletion of the combustion of the whole charge.

Drawbacks of Electric Sparks

Electric sparks are very hot and fast-acting ignition sources. Because the discharge time of anelectric spark is very short (of the order of 10 -8 to 10-7 s), the energy that is imparted to the gas at theend of the discharge period is highly concentrated, so that a very steep temperature profile with avery high temperature at the center is established. In this initial stage of flame development, thechemical liberation is insufficient to maintain such steep temperature profiles, so that the profilebroadens and the temperature at the center decreases within a period of time which depends on the

physical and chemical properties of the gas, and provided that the discharge energy is sufficient, theprofile develops to that of a minimal flame and thence continues to propagate as a combustion waveand the temperature in the center becomes approximately the flame temperature.

The process of spark ignition depends on many parameters such as energy, peak voltage,duration of discharge, geometry of the spark gap, and its location relative to the particular geometryof the compressed charge.

However, the conventional spark plug system is quite ineffective when igniting a leanmixture below an equivalence ratio of about 0.8 for gasoline and air.

Some studies have been carried out on high-energy ignition systems. Such systems give anincreased peak power to the spark and also increase the duration of the spark.

The development is based on the premise that by proper control of the manner in which theplasma is generated by the spark discharge, leading in particular to the prolongation of its duration,more unburned mixtures can be exposed to it, an effect that can be greatly enhanced by the use ofsquish whereby a significant amount of mixture can be caused to pass by the spark gap and be mixedwith the plasma, generating thereby in effect, a system of distributed ignition sources.

Tests have shown on single as well as multi-cylinder engines that increasing the gap width,its projection and the duration of the spark (with duration enhanced from 1 to 2 ms to 5 to 10 ms),the mixtures 10-15% leaner than those ignited by the conventional spark plug) can be ignited.According to Aiman3, based on the ignition system of Johnston and Neuman4, the amount of exhaustgas recirculation, which a single cylinder engine tolerated without misfire, increased when theduration of the spark was increased at a given arc current. This may be due to multiple opportunitiesfor ignition or due to increased ignition energy.

3 GMR-2230, 1976

4 SAE Paper No. 750348


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The use of multiple ignition points (usually limited to two spark plugs) as compared to asingle spark plug has also improved the combustion characteristics of the spark ignition engine.

APPLICATION TO PETROL ENGINE IGNITIONIGNITION SYSTEMS COMMONLY USED IN SPARK IGNITION ENGINES

1. Battery ignition system where the high voltage is obtained with an ignition coil (coil ignitionsystem).

2. Battery ignition system where spark energy is stored in a capacitor and transferred as a highvoltage pulse to the spark plug by means of a special transformer (capacitive dischargeignition or CDI system).

3. Magneto ignition system where the magneto - a rotating magnet or armature - generates thecurrent used to produce a high voltage pulse.

The combustion process is initiated by a spark, which is provided by a magneto or batteryand coil. The spark occurs between the simple electrodes of a spark plug; the spark is triggered off bythe timed operation of a contact breaker in series with the primary winding of a coil or magneto. Thecircuit constants are so chosen so that a collapse of the primary current is sufficient to ensure a sparkdischarge between the spark plug electrodes. They are fitted on all production cars and requiremaintenance but is adequate for the purpose. An improved system would be one with reducedmaintenance, increased reliability, and extended range of satisfactory operation of the engine.

Within limits normally encountered in engine operation, provided the energy of the spark is

sufficient, it has little or no influence on establishment of combustion although excess energy can beharmful for the spark plug electrodes because it causes erosion of the electrodes and can lead topreignition. The energy for spark in the circuit provided by conventional systems variesbetween about 10 to 100 millijoules (mJ) with energy of approximately 1 mJ stored in theself-capacity of the plug and cable, and thus available for immediate release at the spark plugelectrodes. In a single cylinder engine test, 0.2 mJ was found to be sufficient.

Other researchers have estimated that the normal total 20 mJ is divided into 2.5 mJ releasedduring the capacitance phase of the spark discharge for 1 microsecond (s) and 17.5 mJ for theinduction phase for 1 millisecond (ms). Normally, ignition can be attributed to the capacitancecomponent, although, in cases of more difficult sparks caused, for example, by wet or fouled spark

plugs, the induction components assists. The conventional automobile engine ignition systemdelivers sufficient energy based on a minimum average spark gap of about 0.8 mm, over the wholeengine operating range.

About 0.2 mJ of energy is required to ignite a quiescent stoichiometric mixture at normalengine conditions by means of a spark.

For substantially richer and leaner mixtures, and where the mixture flows past the electrodes,


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the energy required may be an order of magnitude greater (about 3 mJ).

Conventional ignition systems deliver between 30 to 50 mJ of electrical energy to the spark.Due to the physical characteristics of the discharge modes discussed above, only a fraction of theenergy supplied to the spark gap is transmitted to the gas mixture.

Radiation losses are small throughout.

The end of the breakdown occurs when a hot cathode spot develops, turning the dischargeinto an arc; heat losses to the electrodes then become substantial.

The breakdown phase reaches the highest power level (about 1 MW), but the energy suppliedis small (0.3 to 1 mJ). The glow discharge has the lowest power level (about 10 W) but the highestenergy (30 to 100 mJ), due to its long discharge time. The arc phase lies in between.

Energy supplied by a conventional system is not invariable. Now, the total circuit energy isgiven by

Li2

where i is the current in the primary coil at the time it is interrupted by the contact breaker. Duringthe period when the contacts are closed, current increases from 0 to i. If this period is less than thetime constant for the growth of the current (which is constant for the circuit) the current will be lessthan 63% of its maximum. For a period equal to three times the time constant, the current will havegrown to 95% of its maximum value. Thus, a requirement for the development of almost themaximum current and hence almost the maximum energy is a sufficiently long period betweenclosing and opening of the contacts, a condition met only at low engine speeds. This sets an energylimit for low engine speed, because of the maximum current the contact breaker points can safelyinterrupt. At high engine speeds, the period during which the contacts are closed can be too short for

growth of the current to be sufficient to provide adequate energy or voltage, so misfiring may occur.Although it is desirable that the duration of closure of the contacts be as long as possible, it is limitedby the need to have the contacts open for the period comparable with the time constant for sparkdischarge. If a magneto instead of a battery provides the electrical energy, the problem at high speedis alleviated automatically since the electrical output of the magneto is greatest at high speeds.However, at low speeds, a much reduced and possibly an insufficient amount of electrical energy isavailable. This is a well-known limitation of the magneto

The introduction of solid-state circuitry for spark ignition does not violate the basic conceptsof energy requirements. The efficiency is increased while total energy dissipated may be reduced.The reliability of the system and repeatability of the operation may be improved. The simplestdesign merely assists the conventional contact breaker to interrupt the primary current while otherfeatures remain unchanged. In more advanced designs, the contact breaker is eliminated. The fullestapplication of solid-state circuitry affords, additionally, a form of spark discharge that is probablymore favorable for ignition in adverse conditions. However, a relatively simple, transistor-assistedcontact breaker system can offer a two-fold advantage. Because the current interrupted by the CBpoint is much reduced, an appreciable increase in primary current is possible. Since current appearsto the power of 2 in the relation Li2, it would permit a considerable reduction in primaryinductance, L, for a given amount of energy which, in turn, would reduce the time constant for thegrowth of the current in the circuit. Thus the limitations of energy at high engine speeds can be


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alleviated. Another modern device, the surface discharge plug, is based on the principle whichpermits the plug to fire even when contaminated with deposits to a degree that would virtually causethe short circuiting of a normal type of spark plug

The voltage required to breakdown the resistance of a plug gap decreases with increase intemperature of the electrode. In some engines, incorrect polarity (positive polarity of the hotter,central electrode of the spark plug) can cause a 35-40% increase in the breakdown voltage. Theenergy content of the capacitance component of the spark, li2, is raised. If either the availableenergy is insufficient to provide this amount plus losses or the voltage, V, is, for some reason,attainable, there will be no spark

The voltage requirement for sparking shows the important difference between controlledlaboratory experiments and the actual working ignition system of an engine. In the laboratory,difficulties of obtaining breakdown voltage of the spark gap are first overcome, in readiness of theexperiment, and a selection, by trail and error, of the correct capacitance, C, the minimum energy interms of CV2, is readily provided. In a practical, engine ignition system, C is fixed, and two aspectsof V must be considered. First, the required breakdown voltage will vary over a period of time with,

for example, the electrode gap, temperature and cylinder pressure. Secondly, the attainable voltagewill depend upon conditions of the spark plug and associated circuits. Therefore it is feasible that afailure to spark could arise not necessarily as a result of insufficient Li2 energy in the primarycircuit to meet minimum energy requirements (Li2 + losses) but merely because, perhaps, owing toinsulation leakage, the required breakdown voltage could not be attained. Thus, energy would play asubordinate role to voltage.

A spark can arc from one plug electrode to the other only is a sufficiently high voltage isapplied. When breakdown of the resistance of the gap occurs, ionizing streamers then propagatefrom one electrode to the other. The impedance of the gap decreases drastically when the streamerreaches the opposite electrode, and the current through the gap increases rapidly. This stage of the

discharge is called the breakdown phase.

It is followed by the arc phase; where the thin, cylindrical plasma expands largely due toheat conduction and diffusion and, with inflammable mixtures, the exothermic reactions, which leadto a propagating flame, develop.

This may be followed by a glow discharge phase, where, depending on the details of theignition system, the energy storage device, e.g., the ignition coil, will dump its energy into thedischarge circuit.

TheBreakdown phase is characterized by a high voltage (about 10 kV), high current (about200 A) and an extremely short duration (about 10 ns). A narrow (about 40 m diameter) cylindricalionized gas channel is established very early. The energy supplied is transferred almost without lossto this column. The temperature and pressure in the column rise very rapidly to values up to about60,000 K and a few hundred atmospheres respectively. A strong shock or blast wave propagatesoutward, the channel expands, and, as a result, the plasma temperature and pressure fall. Some 30%of the plasma energy is carried away by the shock wave; however, most of this is regained sincespherical blast waves transfer most of their energy to the gas within a small (about 2 mm diameter)sphere into which the breakdown plasma soon expands.


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A breakdown phase always precedes arc and glow discharges; it creates the electricallyconductive path between the electrodes. TheArc phase voltage is low (


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diversity, yet not enough to cause unacceptable penalties in other directions, is a matter ofexpediency or opinion6.

Namazian et al7 studied the process of flame initiation by using spark photographs atintervals of 0.01ms (about 0.08 crank angle degree) after spark initiation. They observed a kernel(high temperature zone around the discharge) immediately following the spark discharge, which fillsup the entire electrode gap within 2 degrees after the discharge while the spark is no longer visible.Subsequent emerge of a flame structure similar in appearance to the fully developed flame isobserved a few degrees after the spark.

A simple criterion for the successful ignition is that the rate of heat release in the smallflame zone surrounding the spark should exceed the total rate of heat loss.

This implies that the characteristic dimension of the flame kernel should equal or exceedthe quenching distance.

Assuming the kernel to be a sphere of diameter d, minimum energy deposition to achieveignition is

3'(min) / 6pE C T d=

Minimum ignition energy in air at 1 atm, 20C

Fuel E (10-5J)Methane 33Ethane 42Propane 40n-Hexane 95Iso-Octane 29Acetylene 3Hydrogen 2Methanol 21

The quench distance and distance to obtain the optimum gap are taken equal. Ballal andLefebvre found d (gap) = 5d (quench)

Low pressure experiments are not easily applied to an engine. In an engine, if the velocitypast the plug is high, the kernel may detach from the plug and move down stream before itdevelops into a full-fledged flame front. In many cases the effective point of ignition is not thespark plug location. Such high flow velocities are not typical, but can be produced by high swirl.Under some conditions, the plug may act as a flame holder and the flame may rotate, thusproducing an apostrophe shaped burned gas volume, the narrow end being at the spark plug.

For more typical cases, the spark is located on axis and the flame kernel is only slightly distortedby the local turbulent velocity. This distortion changes the surface to volume ratio of the kerneland the wall surface area in contact with the kernel. Both these effects change the heat transfer.Such fluctuations in heat transfer affect the kernel growth rate, especially for lean mixtures. This

6 Hurtley, D., "Ignition Part I", Automobile Engineer, 59(1969): 96 and "Ignition Part II",

Automobile Engineer, 59(1969): 148.

7 SAE Paper No. 800044.


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leads to CBCV of the start of rapid combustion.

In a piston engine, the cylinder gas turbulent intensity is closely related to large-scalevelocity patterns in the cylinder. These patterns are initiated by the flow as it enters the cylinderfrom the intake port. Swirlingflows around the cylinder axis and vertical vortex motions calledtumble are two common structures. These large-scale patterns are modified by piston speed andcombustion chamber shape as they undergo compression. It is decay of these large flows whichprovides the major source of turbulence at the time of ignition. In the case of low swirl, tumblemotions dominate. Tumble motions break down under compression more readily than do swirlmotions, and thus give high turbulence intensity at the time of ignition. Creation of a definiteswirl pattern, however, reduces cyclic variations. Low swirl rates allow the less repeatable tumblemotions to dominate, giving large cyclic variations. Thus the desire to create high burningvelocities may lead to flows which cause cyclic variability because of the flow interaction withthe flame kernel.

From the basic experiments it can be concluded that the conditions which are mostconducive to spark ignition and which lower the minimum spark energy are:

1. Low burning velocity2. High initial temperature3. High mean reaction rate4. Low volumetric heat capacity, Cp5. Low thermal conductivity6. High total pressure7. Nearly stoichiometric mixture8. Low turbulence intensity9. Electrode separation distance close to quench distance

In spark ignition, homogeneous-charge engines, the lean limit of flame propagation can

be extended by:1. Increasing mixture homogeneity2. Decreasing charge dilution3. Increasing compression ratio4. Decreasing engine speed5. More central spark plug location6. Use of multiple spark plugs

In a quiescent mixture, minimum ignition energy is less than 1mJ. An order of magnitudemore energy is required for flowing mixtures. Conventional S.I engine ignition systems deliver 30to 50 mJ of energy.

A larger initial kernel size is important because it reduces the surface to volume (area tovolume) ratio. Thus the larger the radius the lower th e area to volume ratio. The larger kernelsurface area may also be affected by a larger range of turbulent eddy scales, thus increasing theinitial flame speed.

IGNITION BY AN ELECTRICALLY HEATED WIRE


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If the same amount of source energy were delivered by an electric current, over a time largerthan the time of development of a minimal flame, the temperature at the core would drop below theflame temperature, the heat liberation in the reaction zone would not attain a balance with theoutflow of heat into the preheat zone, and the flame would become extinct. On the other hand, if thecurrent flow were continued for a longer period the temperature profile ultimately would becomesufficiently broad, and the temperature in the core sufficiently high, so that the heat liberation in thereaction zone overbalances the outflow of heat and ignition occurs.

Energy in a wire of resistance R carrying current i for a time t is given by iRt.

If the length of the wire does not exceed half the quenching distance for a particular mixture,it will remain within the sphere of the minimal flame and the significant energy will be that of thewhole wire and not the energy per unit length. According to one investigator, the energy required inthe wire to achieve an ignition probability of 50% increases with the period of heating and the wirediameter. It was noted that sometimes ignition was obtained with energies greater than that requiredto melt the wire, hence it was concluded that the flow of current, sometimes continued after fusion ofthe wire until ignition occurred. This is feasible, as the circuit is complete until the wire actually

collapses into droplets after which electric cores form and are extinguished as the droplets separate.Thus under certain conditions, attainment of ignition does not indicate the threshold of ignitionenergy, but merely shows that the wire has melted.

The system is based on the concept that the combustible mixture extracts initiallyinstantaneously (up to 1 ms) from the source the minimum energy required for ignition and that theremainder of the energy supplied is wasted.

Ignition by heated metal strips has also been investigated. The temperature of the metal stripswas raised electrically until the surrounding mixture was ignited. When ignition temperatures wererecorded, the catalytic effect of certain metals became apparent. Convection current in the gas

tended to increase ignition temperature, indicating that when the gas flowed over the heated strip, itreceived less heat than it would have received under quiescent conditions.

Three independent quantities characterize the ignition threshold of a slow source:

(1) the total heating time or the time during which the current flows called the critical heatingperiod.

(2) the total energy delivered during this time called the critical source energy which defines thecurrent strength and

(3) the temperature Tc in the core at the end of the heating period called the critical sourcetemperature.

Figure 9 shows the general form of the relationship between the three quantities.The minimum ignition energy, denoted by hc, corresponds to a very short (zero) value of the

critical heating period. The corresponding critical source temperature has a value lower than thesource temperature, Tb. As the critical heating period is increased (and the current strength iscorrespondingly decreased) the critical source temperature decreases and the critical source energyincreases. Initially these changes are large and become gradually smaller. The temperature curve


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becomes asymptotic which is inherent in Arrhenius law that at low temperature changes in a fewdegrees of temperature produce large changes in the rate of chemical reaction.

IGNITION BY FLAME OR HOT JET

Research into the effects of size and temperature of flame on their ignition characteristics has

been carried out but there appears to be no experimental evidence of threshold ignition energy suchas exists in spark ignition system. This may be due to the presence of too many influential factors.For example though it is possible to control a flame to a given energy dissipation per unit time, as fora heated wire the time required to stabilize the flame is likely to exceed the critical ignition time. Fora turbulent flame introduction of kinetic energy would complicate the assessment of energy. It isknown, however that a turbulent flame or jet entering a combustion chamber, an appreciably leanerfuel air mixture than can be performed by a spark.

If the threshold ignition energy defined by spark ignition were distributed in space and time,as in the energy of a flame it is very unlikely that ignition would be achieved. Therefore if thresholdenergy exists for ignition by a flame, it is likely to be higher than that for a spark. However since it isknown that under certain conditions, a flame can ignite a mixture when a spark will not, somefeatures of the flame must be particularly favorable for the initiation of combustion. It is likely thatthe volume is relatively large, the energy presumably sufficiently dense, and the area of contactbetween the source of heat and the reactants large. A degree of turbulence in the flame wouldpromote a favorable increase in the mixing of the flame with the combustible medium although toomuch turbulence might hinder combustion. Also a flame is not only a source of heat but alsocontains a mixture of reactants and products of combustion, it has a high concentration of freechemical radicals, which may mean that energy is available in a particularly advantageous, form.

Turbulence in a hot igniting jet which raises the ignition temperature and small eddies due tobreak up of the jet, is likely to be less efficient agents than large volumes of hot gas in a laminarflow. A hot air jet is better than a hot nitrogen jet for a stoichiometric fuel air mixture. Also the

richer the mixture, up to the fuel rich limit, the more readily it ignites with hot air. This suggests thatan igniting jet containing oxygen favor the ignition of a fuel rich mixture. On the other hand, thepresence of hydrogen in a relatively inert gas jet drastically reduces the ignition temperature of amethane air mixture and the most ignitable mixture approaches the lean limit. The reason suggestedis that first hydrogen ignites with the surrounding air and then the resulting flame triggers theexplosion of the methane air mixture. Methane appears to inhibit the first stage so that a rich mixturewith methane becomes less ignitable. Similar effects have been noted with carbon monoxide presentin the jet.

The ignition system using flame jet ignition consists of a divided chamber with its volume 2to 3 percent of the clearance volume of the engine8,9 with one or more sharp edged orifices of cross

section area 3 to 5 mm per cm of pre-chamber volume to produce the flame jets. The overallpressure built up in the pre-chamber is carefully maintained at a sub-critical level so that the jet isessentially subsonic in order to prolong as much as possible the process of partial oxidation of therich mixture where the combustion was started (by means of a spark plug in the pre-chamber) as it isejected into the main charge containing a large amount of excess air. The equivalence ratio in the

8 Gussak SAE 750890

9 Gussaket alSAE 790692


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pre-chamber varies between 1.4 to 2.5 while that in the main chamber it is of the order of 0.5.

The jet comes out at high velocity and the ensuing turbulence it creates shears the flameapart so that in effect it is temporarily extinguished. As a consequence, a large number of small sizeturbulent kernels of flamelets, or active particles are seeded throughout the charge. After a shortinduction period, rapid combustion of the lean mixture in the main chamber is thus initiated at alarge number of distributed ignition sites.

The lean misfire limit has been claimed to be raised to an air-fuel ratio of 33:1 with a definiteimprovement in the fuel economy and a significant decrease in the fuel octane requirement. Ignitionis believed to be due to the action of the methyl radicals in addition to the hydrogen atomsmentioned earlier, which enhances the chain branching mechanism. Explanations to the phenomenaare based on the thermal theory10.

Figure 13 & 14 show some designs of combustion chamber arrangements to produce a hotjet which would ignite the charge in the main chamber.

A flame or hot jet, whether laminar or turbulent, is probably a less efficient source of ignition

than one in which dissipation of energy is confined to a small volume and a short duration. But thereare several features of the flame/hot jet and both appear to be capable of igniting combustiblemixture under circumstances in which other ignition systems fail. A threshold energy conceptprobably applies but a carefully devised experiment would be required to prove the point.

The foregoing is neither authoritative nor exhaustive but defines quantitatively the problemof ignition by a flame or hot jet, placing the role of ignition in its proper perspective. Evidently, thistype of ignition system warrants further study even at steady flow conditions, let alone at pulseoperation conditions required in the internal combustion engines.

The ignition of a relatively lean air-fuel mixture by a flame - in particular, a burning jet - is

not simply a matter of energy. At least, it is partly attributable to other features of such a flame thatare more favorable to combustion than is an electric spark.

PLASMA JET IGNITION

In the plasma jet igniter, the spark discharge is confined to a recessed cavity provided with adischarge orifice, while the electrical power supply is augmented by the addition of a condenser thathelps discharge at a relatively low voltage and high current through the spark generated in aconventional manner by a high voltage, low current ignition system10,11. The circuit consists of aconventional high voltage ignition coil, which is used to produce an electric spark that closes thecircuit by the ionized passage it creates. This causes a condenser, charged from 900 to 1200 volts, to

shorten, forming high temperature plasma. Stored energy of up to 10 joules can easily be employed,but typically only 1 or 2 joules are required. The high temperature plasma is created so rapidly thatthe cavity is pressurized, causing a supersonic jet of plasma to be issued through the orifice andpenetrate into the charge10.

Experiments conducted in a constant volume bomb where the combustible mixture was

10 Dale and Oppenheim SAE 810146.

11 SAE Paper No. 830479


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initially at rest, and at atmospheric pressure and room temperature, have revealed the following:

1. The plasma jet entered the combustion chamber in the form of a turbulent flame, which wasembedded in a blast wave by a hemispherical shock front.

2. The gas dynamic effects of the blast wave were dissipated by the time combustion started,after a delay of about 1 ms so that ignition took place in the turbulent zone of the plume.

3. The depth of penetration of the jet was solely a function of its initial velocity; it could thus becontrolled by the amount of energy deposited in the cavity, as well as its size and that of theexit orifice.

4. In direct contrast to spark ignition, which produced a laminar flame, which later becameturbulent, here combustion was initiated in the form of a turbulent flame, which upon leavingthe plume tended to acquire a laminar character. As a consequence, the normal burningspeed, which was initially quite high, decreased monotonically as the flame kernel expanded.

5. It was found that the most effective feedstock for ignition was hydrocarbons initially in theliquid state. This was believed to be due to the action of the hydrocarbon atom, which was inabundance in the plasma created from such feedstocks.

6. Plasma jets were shown to be capable of igniting gaseous mixtures below the normalflammability limit.

Studies on single- and multi-cylinder engines have shown that with plasma jets, leanmixtures of the order of 18:1 could be ignited. According to Ref. 11, equivalence ratios as low as0.54 can be ignited.

The drawback is that plasma jet igniters require more electrical energy (1 J) as compared toconventional igniters (50 mJ) per pulse. As a result they also suffer from high electrode erosion rates.More work still needs to be done to obtain data on the performance, fuel economy and emissioncharacteristics. Its main advantage is its extremely short discharge time: about 20 s, which makes itcapable of igniting lean mixtures.

Figure 15 shows the schematic of a plasma jet ignition. Figure 16 (a) to (d) showcomparisons of plasma jet (PJ) system with standard spark (SS). PJ system have an extended leanlimit (25:1) compared to SS (22:1), produce higher power (Fig 16 (b) & (c)) and lower sfc (16 (b) &(c)) and combustion duration is shorter (16 (a) & (d)).


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PHOTOCHEMICAL IGNITION

Norrish12 studied the combustion of air-fuel mixtures by photochemical methods. Thesemethods are based on the phenomenon of photolysis, a method of established significance tochemical kinetic studies of oxidation reactions. Figure 1013 shows a photochemical igniter. Since the

igniter is effective only with windows transmitting in vacuum, ultra-violet radiation, it wasconcluded that ignition is, in effect, caused by the action of oxygen atoms. This is caused by thedissociation of oxygen atoms, which in turn is caused by radiation below 245 nm (nanometers),absorption of 180 nm being most efficient in this respect. It was found specifically that for ignition ofa hydrocarbon-air mixture the critical concentration of oxygen atoms was of the order of 1014 atomsper cubic centimeter. It was established, moreover, that the energy requirement to initiatecombustion is essentially independent of the air-fuel ratio of the mixture. The most interestingfeature of this ignition system is that it is evidently capable of initiating combustion under conditionssimilar to the plasma jet system with about the same expenditure of energy. However, the plasma is,in this case, physically separated from the mixture by the window, guaranteeing that, unlike in thecase of the plasma jet, its effect is solely chemically kinetic in nature.

MICROWAVE IGNITION

Ward14 studied the possibility of applying microwave energy to a burning hydrocarbon fuel-air mixture in a conventional internal combustion engine in order to stimulate the burning of a leanmixture. He showed that typical flame front electron plasma properties and combustion chambergeometries are such that significant conversions of microwave energy to electron energy occur. Inturn, the electrons will give up a substantial portion of their excess energy to excitation of internalenergy levels of molecules, which are known to accelerate chemical reaction rates. In this wasmicrowave energy could be used to excite the relatively cool flame front molecules to more stablestates.

Ward and Tu15 carried out a theoretical study on the effects of microwave in treating themixture at the flame front with particular application to engines. Effective transfer of microwaveenergy to the flame front electron energy occurs. The feasibility, therefore, of efficient microwaveheating of the flame front electrons in engines was established. These conclusions corroboratedflame experiments carried out later13.

LASER IGNITION

1210th Symposium on Combustion, p 1-18, 1963.13SAE Paper No. 81014614Journal of Microwave Power, 12(3), 187-199, Sept. 1977.15 Combustion and Flame, vol. 32, p 57-71, 1978.


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The ignition of lean mixtures and diluted mixtures (diluted with exhaust gases) becomes aproblem. By raising the spark energy and the spark duration the spark is still tied to the relativelycold chamber wall resulting in slow initial combustion wave development. Using the energy burstfrom a focused laser beam permits the spark location to be moved away from the chamber wall andthe use of higher spark energies without heating the protruding ground electrodes to temperatureshigh enough to cause pre-ignition.

According to Hickling and Smith16 who carried out studies using several hydrocarbon fuel-air mixtures in a bomb at elevated temperatures and pressures, the laser spark will ignite very leanmixtures. Ai-fuel ratios of up to 31:1 ( = 0.43) were successfully ignited.

Dale et al17 operated a single cylinder engine using energy bursts from a focused carbondioxide laser as the ignition source operating at 16 m wavelength. Carbon dioxide laser has theadvantage of being far more efficient than those operating in the visible and near infraredwavelengths and also breakdown can be achieved with lower pulse energies. The minimum energyrequired in the spark burst of the laser to ignite the mixture seems to be dictated by the energyrequired to obtain breakdown in air at the same pressure. Raising the spark energy above this

minimum level is desirable to produce a steady running engine. They were also able to ignite amixture containing up to 16% recirculated exhaust gas, provided to reduce nitric oxide, whichincreased if a laser system was used as igniter.

The laser beam was focused at a small point within the mixture to achieve breakdown,generating thereby a plasma kernel that acts as the ignition source. Dale et alfocused the laser beamnear the center of the combustion chamber.

Figure 17 compares cylinder pressure traces between laser, plasma jet and standard spark.The laser and PJ systems show stable running at 22.5:1 A/F, The SS shows no firing.

PUFF-JET IGNITION

The nature of the plume of hot gases, which expand from the cavity of the plasma jet igniter,

16 SAE 740114, Transactions17 SAE 780239


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has been shown to be similar to atmospheric thermals. In addition to this, the mixing of a plasma jetplume with the ambient gas has been shown to be very rapid due to the turbulent nature of theplume.

The puff-jet ignition concept described here involves the rapid mixing of a turbulent plumecreated by injecting a small volume of combustible gas such that a thoroughly well mixed, nearstoichiometric, turbulent puff is formed in the region of the spark plug electrodes. The rapidmixing of the plume results in a near stoichiometric mixture in the vicinity of the spark gap, thusproducing charge stratification in the combustion chamber.

Pitt et al18carried out experiments in a combustion bomb. The fuel injected was methanegas. The charge to be ignited was a mixture of methane and air. They were able to ignite mixtures ofequivalence ratio 0.67 with a low energy spark. Other advantages in using this technique includereduced delay and improvement in burn rate. Further improvement in delay period was possible ifhydrogen was injected instead of methane without any improvement in burn rate. The systemcompares well with plasma jet igniter devices with lower energy consumption and reduced electrodeerosion.

The system was later tried in a methane-fueled single cylinder engine19. The small fuelquantity (about 1% of the total fuel charge) was admitted under pressure by a fast acting valve. Aturbulent puff of gas that rapidly mixed with the surrounding lean mixture was directed towards thespark gap to create a local region of stoichiometric mixture, which could ignite and produce arapidly growing ignition kernel. This ignition kernel was similar to that produced during the earlyphases of plasma jet ignition.

The technique required less than 100 mJ of electrical energy compared to 1 J for the plasmajet ignition system. The electrodes were found to be less eroded. While it had features similar to theignition and combustion characteristics of the direct injection stratified charge system, it did not

require bulk cylinder swirl flows to augment the mixing and combustion rate of the pilot fuel. Thisshould allow for simpler combustion chamber design.

Fisheret al20 have found that by using a puff-jet igniter, output power increased when usedunder lean mixture conditions. They found it suitable for natural gas because it burns more slowlyand is more difficult to ignite than conventional fuels. Hence it is useful for lean methane-airmixtures. It produces less cycle-to-cycle variations in power output.

18 Combustion Science and Technology, vol. 35, p 277-285, 1984.19 Pitt et al, Combustion Science and Technology, vol. 38, p 217-225, 198420 SAE Paper No. 860538

Combustion in the Spark Ignition Engine

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