8 Spark Ignition

Spark Ignition 1 Dr M Lawes, Univ. Leeds

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Spark IgnitionDr M Lawes, School of Mechanical Engineering, University of Leeds

IntroductionIgnition is the process of supplying sufficient energy to the reactants to cause them to react anddevelop into a self-sustaining reaction zone in which exothermic chemical reactions occur. Thesemust provide at least as much heat as that which is lost to the surroundings by heat transfer. In dieselengines, auto-ignition occurs due to the high temperature and pressure caused by compression. Inspark ignition and gas turbine engines, an electric ignition system provides, at the spark gap, hightemperature plasma which, in turn, provides the energy and active species required to initiate flamepropagation. In gas turbines, a fuel spray often is used and, therefore, the ignition system mustprovide sufficient energy to evaporate the fuel droplets prior to ignition. Gas turbines may requirestarting at a wide range of operating conditions which range from normal start to altitude relight atwhich the pressure and temperature are very low. Conversely, the conditions at which reciprocatingspark ignition engine combustion is initiated are more favourable, but many ignitions per minute arerequired and these must be repeatable if cycle to cycle variations in the combustion event are to beminimised.

Electric sparksShown in Fig. 1 is a schematic of a representative ignition system. An inductor and/or capacitorsupply a high voltage. This is delivered through a high voltage cable, having known impedance andcontaining a radio interference damping resistor, to a spark plug which has its own inductance,capacitance and gap resistance.

Figure 1. Schematic of a representative ignition circuit Figure 2. Variations of voltage and currentduring a typical spark discharge.

Discharge modesShown in Fig 2 are the variations of current and voltage during a typical spark discharge. Four phases,or modes, are observed and these are: pre-breakdown phase; breakdown; arc and glow.


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1 Pre-breakdown

Initially, the gas between the electrodes represents a perfect insulator. When a large electricalpotential (K Volts) is applied across the spark gap, the electric field causes any randomly existingelectrons in the gap to move towards the anode (positive electrode). Any electrons that collide withmolecules may ionise them (strip off electrons) to produce more electrons such that the number ofelectrons increases rapidly. This process is augmented by photoelectric processes in which UVradiation from excited ions liberate, with high efficiency, electrons at the cathode surface (negativeelectrode). During the pre-breakdown phase, the gas temperature is still close to the initial value.

2 Breakdown

When sufficient electrons are produced, the impedance (frequency dependant resistance) of the gapdrops dramatically and the current increases within nanoseconds to hundreds of amperes until this islimited by the impedances of the discharge and electrical circuit close to the gap. At this stage, thevoltage across the gas reduces to hundreds of volts. The minimum energy required to initiatebreakdown at 1 bar and 1 mm spark gap is 0.3 mJ. A narrow (~40 m diameter) cylindrical ionisedgas channel is established and energy is transferred to this almost without loss. The temperature andpressure rise rapidly to values up to about 60 000 K and a few hundreds of atms. A strong shock wavepropagates outwards and, as a result, the temperature and pressure reduce. Although some 30% of theenergy is carried away by the shock, this is regained by the gas within a radius of about 2 mm.

The breakdown phase must always precede an arc or glow phase since it produces the requiredconductive path between the electrodes.

3 Arc

This is characterised by low voltages of less than 100 V and by currents in excess of 100 mA, which islimited by the external electric impedances. Voltage drops near the anode and cathode result insignificant heat losses to the electrodes as shown in Table 1. The arc requires a hot spot on thecathode, which produces a molten pool from which electrons are emitted to sustain the arc, andevaporation of the electrode takes place. The arc centre temperature is limited to about 6000 K.Energy transfer to the plasma surface is by heat and mass diffusion, which are slow (ms) processescompared with that of the breakdown phase.

Table 1 Energy balance for breakdown, arc and glow discharges

4 Glow

As the current reduces, and the voltage increases, the cathode cools such that electrons are liberated byion impact rather than by emission from a molten surface. Energy losses are high and peakequilibrium gas temperatures are about 3000 K.

Energy TransferAlthough as little as 0.2 mJ of energy is required to ignite a quiescent stoichiometric mixture, andturbulent lean mixtures require up to only 3 mJ, only a small fraction of the electrical energy istransmitted to the gas mixture. Hence, conventional ignition systems deliver about 30-50 mJ to thespark.


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Figure 3 shows the variation with supplied electrical energy of energy delivered to the spark plasma inthe breakdown, arc and glow phases. The transfer efficiency of all modes reduces with electricalenergy and the differences between the modes are, essentially due to the different heat losses to theelectrodes. The breakdown phase offers the highest efficiency and highest power level (~1 MW) butthe supplied energy is small (~0.5 mJ) due to its very short duration. The glow phase has the lowestpower level (~10 W) but the highest energy due to its long discharge time.

Figure 3 Figure 4

Shown in Fig 4 is the variation of activated volume diameter and its expansion velocity with time for acapacitive discharge ignition system with 3 mJ electrical energy and 100 s duration at 1 bar. TheSubscript 1 = the shock wave in air; 2 = the proceeding plasma in air; 3 = plasma in stoichiometricmethane-air. The initial shock wave attenuates rapidly to about the speed of sound. Up to about 1 s,there is little difference between v2 and v3, which indicates little combustion chemistry. Both v2 and v3

have a change in slope at about 100 s due to the transition from expansion by the breakdown inducedhigh pressure to that due to heat conduction and diffusion. At later times, combustion becomes moresignificant as indicated by the increasing difference between v2 and v3.

Shown in Fig. 5, are radial temperature profiles for discharges in air. The notation is:TB = breakdown discharge (30 mJ in 60 ns), TA = capacitive discharge (3 mJ in 100 s superimposedwith an arc of 30 mJ for 230 s), TG = capacitive discharge (3 mJ in 100 s superimposed with glowof 30 mJ for 770 s). All have, essentially, the same total electrical energy input of 33 mJ. The arcand glow modes were preceded by small discharge modes. Figure 5 clearly shows the advantage ofthe breakdown mode in producing a large diameter of hot plasma in a much shorter time than for theother modes. In addition, due to the rapid expansion, cold gas flows into the central region of the hotplasma, effectively insulating it from the cold electrodes.

Chemical reactions are initiated after a few nano-seconds by the high concentration of radicals in thebreakdown plasma. These consist of highly excited atoms (e.g. O, N, H) and ions. However, thetemperatures are too high for normal combustion products to exist. Therefore, combustion reactions(involving such molecules as OH, CH, C2, CO etc.) take place at the outer surface of the plasma werethe temperature is of the order of 1000-10 000 K. This chemical energy is added to that of the plasma.If one makes the approximation that combustion is initiated at a temperature of (say) 2000 K , then,from Fig. 5, a breakdown spark would give an initial flame radius of 2.8 mm at 50 s after sparkinitiation. In addition, the breakdown temperature profile is such that the heat energy is concentratedat the leading edge at which combustion chemistry takes place. The worst case is the glow discharge,which produces a flame of only 1 mm radius after 2 ms. Thus, the characteristics of the breakdownphase have the most impact on flame initiation. This is illustrated in Fig. 6 which shows, for the same


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energy input into breakdown, arc and glow discharges, the lean (>1, =1/) flammability limit ofmethane-air at 300 K and 4 atm.

Figure 5 Figure 6

Effect of flow fieldThe flow field has a significant effect on ignition because it can: transport the ignition kernel away from the spark gap, which might reduce heat losses to the

electrodes, increase the spark kernel volume which might produce a beneficially larger initial flame volume

or reduce the volumetric energy input to values too low for ignition; transport the ignition kernel towards or away from walls, which will effect heat transfer; transport the ignition kernel into a more favourable or less favourable location for combustion

(different turbulence).

Figure 7 Effect of velocity on a glowdischarge

Figure 8 Effect of cyclic variations of turbulence on theinitial flame kernel

On the time scale of a breakdown discharge (~10 ns) the flow field has no effect. However, during thelonger discharges of arcs and glows, the kernel is lengthened by the flow. Shown in Fig. 7 is the effectof mean flow velocity on a glow discharge. At velocities below 15 m/s the discharge channelincreases with velocity. This may be beneficial to subsequent combustion. As the spark channellengthens, the spark energy is distributed into numerous independent discharge channels. Initially, thiscan be beneficial as before. However, as the energy in each discharge reduces with increasingnumbers of channels, and as the total energy is spread over a greater volume, eventually, the energyper unit volume is insufficient to overcome the increasing heat transfer effects and ignition is notpossible. This is illustrated in Fig. 6 in which the flammability limit of a glow discharge is closer tostoichiometric than for the other modes, and ignition is possible only for lower velocities. Shown inFig. 8 is the effect of random cyclic variations in turbulence on initial flame kernel propagation. In


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one case the kernel is convected away from the spark plug, reducing heat loss to the electrodes. In theother case the kernel is convected towards the spark plug, increasing heat loss.

Ignition ModellingA number of ignition models are presented in the literature. A very simple one, presented by Ballaland Lefebvre (1997) is summarised here. One may wish to refer to the work of Maly for morecomprehensive models.

Most models are based on the idea that one must supply to the combustible mixture, sufficient energyto create a volume of hot gas such that the rate of heat generation by combustion just exceeds the rateof heat loss. Heat generation in a small flame kernel is a function of its volume, and heat loss is afunction of its surface area. Therefore, if the kernel radius is small, heat loss might be greater thanheat generation, but if it is large, the reverse may be true due to the increasing volume/area ratio withradius. Ballal and Lefebvre concluded that, for a spark kernel to survive and propagate unaided, itsminimum dimension should exceed the quenching distance, dq, as expressed by

uuc

kd

p

q

16.0

10

(1)

for low turbulence. Here, k, cp, and ul are the thermal conductivity, specific heat capacity, densityand laminar burning velocity of the reactants at pre-ignition conditions. In this Eq., k/cp is thethermal diffusivity and quantifies the rate at which heat diffuses from the ignition source. The laminarburning velocity gives a measure of heat release rate and this is modified by the increased heat transferdue to turbulence, quantified by u’.

For higher turbulence, it is more appropriate to use the turbulent burning velocity, ut, and Ballal andLefebvre suggested

uuc

kd

tp

q

63.0

10

(2)

They defined the minimum ignition energy Emin, as the amount of energy required to heat to itsadiabatic flame temperature, a sphere of gas whose diameter is equal to the quenching distance.Hence

3min

6qp dTcE

(3)

where T is the difference between the adiabatic flame and initial temperatures. Substituting Eqs. (1)or (2) into 3 gives

2

31

min

16.024.5

pc

uukTE

(4)

for low turbulence and

2

31

min

63.024.5

p

t

c

uukTE

(5)

for high turbulence.

Shown in Fig. 9 (from Ballal and Lefebvre) is the variation of measured minimum ignition energywith quenching distance as calculated by Eqs. (1) or (2) for a wide range of pressure, mean velocityequivalence ratios and u’ for propane and methane fuels. The straight line, drawn through the datapoints has a slope of 3, confirming the cubic relationship in Eqs (4) and (5). Equations (1) to (5) showthat quenching distance and minimum ignition energy increase with turbulence. This is confirmed byFig. 10 in which experimental results with stoichiometric mixtures are presented. In this Figure, thedifferent inert gases (replacing nitrogen) confirm the effects of thermal diffusivity. Shown in Fig. 11are typical modelled results of the effect of turbulence on minimum ignition energy for propane airmixtures at 0.17 bar. The curves show that the minimum ignition energy occurs at slightly rich of


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stoichiometric under quiescent conditions. The minimum energy increases rapidly as the mixture ismade leaner.

Inspection of Eqs. (1) to (5), which are supported by experiment, show that the quenching distance is

inversely proportional to pressure (since density is proportional to pressure) and 2min

PE . (This

explains why spark ignition engines use ignition energies of about 30 mJ while aero gas turbinesrequire several Joules. The ignition pressure in a SI engine might be of order 10 bar compared with0.2 bar at altitude relight in a GT. This gives a pressure ratio of 50 and 502 = 2500!).

Figure 9 Relationship between Emin and dq

for various quiescent and flowingmixtures velocity on a glow discharge

Figure 10 Effect of turbulence on quenching distance.N2 replaced by various inert gases.

Figure 11Effect of equivalence ratio and turbulence on minimum ignition energy

Other ignition sourcesMany conventional and non-conventional means of initiating combustion in engines have been used orproposed. In addition to the conventional spark plug the following have been examined. A discussionof these is beyond the scope of the present lecture.


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Use of several plug, use of higher power, higher energy or longer duration discharges, hightemperature reacting jet, plasma-jet and flame-jet ignition systems. A recent area of interest at Leedsis the use of laser beams to initiate combustion. Here, a pulsed laser beam is focused into the reactantsresults in a breakdown discharge. An advantage of laser ignition is that the breakdown can be directedto the most appropriate part of the chamber for ignition (even a location, which, at top dead centrewould be obstructed by the piston). This contrasts with a spark plug which is restricted by itsmounting arrangement and top dead centre chamber geometry. Also, heat losses to electrodes areeliminated by laser ignition. However, a problem with laser ignition in spark ignition engines is theneed for optical access. This might be through a window, which must be kept clean if adequate lasertransmission is to be maintained over the many thousands of combustion events between services.

BibliographyHeywood, J.B., ‘Internal combustion engine fundamentals’, McGraw-Hill, (1989)Lefebvre, A.H., ‘Gas turbine combustion’, Hemisphere, (1983).Maly, R., ‘Spark ignition: its physics and effect on the internal combustion engine’, in ‘Fuel Economyin road vehicles powered by spark ignition engines’, Eds. Hilliard and Springer, Plenum Press, pp.91-148, (1984)

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8 Spark Ignition