Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels23dE,=A,-de.dt=A,-&-k(36)At assumption that the temperatureT of the arc increases proportionally with time from6000K to300Kthetotal radiation energycan be calculated asfollows:(Tmax -T)-tF((Tmax -T2)-tdt=A,-8-k--dt+(37)E,=A,--k.(100t,-100t,-100Assuming the radial shape of the core equal the radius d/2 of the electrodes and its height hequal thegapofelectrodes and also thatmaximumtemperatureof thearcamounts6000Kaftertr=20usand then decreasesto800Kaftertz=2ms,we cancalculatethepart of thecoilenergyas a loss of theradiation energy.Because 20 μs is comparably small with2ms thentheequation (14)canberewrittenasfollows:E, = 4-6-k.(Cmx-T).4(38)5108whereAi-thesurfaceoftheplasmacoreamounts A,=-d-h7.2.IonizationenergyOur experimentwas carried out in nitrogen and on the basis of the literature data there arethree ionization energies [7]: eil = 1402.3kJ/mol, ez =2856.0 kJ/mol, es = 4578.0 kJ/mol. Theenergy required to breakdown of the spark is an ionization energy that can form later thearc.Total ionization energy can be calculated for n moles of the gas (nitrogen) in the core ofplasmaas:r.d.h.pp.Vi(39)E,=n-e,=e,(MR)-T=e)4-(MR)TThe initial temperature T amounts 300 K and universal gas constant (MR)= 8314 J/mol.Forhigher pressure, proportionally the higher ionization energy is required and the same is forlowertemperature.Howevertheplasmaisformedwithsmallerradius,theionizationtakesplace in a higher volumewith radius two timesbigger.7.3.Heat transfer to electrodesA certain part of the energy delivered by the secondary circuit is consumed on the heatingoftheelectrodes.Inasmall timeofthesparkingtheheattransfertakesplaceonthesmallarea approximately equal the cross section of the electrodes with diameter d.The maintarget is to determine the specific heat conductivity a between the gas and metal. This valueα can be obtain from the Nusselt number Nu [2],gas conductivity p and a characteristicflowdimension,inthiscasethediameteroftheelectrode:
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels 23 4 dd d 100 ri i T dE A e t A k t (36) At assumption that the temperature T of the arc increases proportionally with time from 6000 K to 300 K the total radiation energy can be calculated as follows: 4 4 4 max 2 max 2 00 0 () () d 100 100 100 ii i tt t ri i i i T T Tt T Tt E A k t A k dt dt t t (37) Assuming the radial shape of the core equal the radius d/2 of the electrodes and its height h equal the gap of electrodes and also that maximum temperature of the arc amounts 6000 K after t1=20s and then decreases to 800 K after t2=2 ms, we can calculate the part of the coil energy as a loss of the radiation energy. Because 20 s is comparably small with 2 ms then the equation (14) can be rewritten as follows: max 2 8 ( ) 10 5 i i r A kT T t E (38) where Ai - the surface of the plasma core amounts Ai d h . 7.2. Ionization energy Our experiment was carried out in nitrogen and on the basis of the literature data there are three ionization energies [7]: ei1 = 1402.3 kJ/mol, ei2 = 2856.0 kJ/mol, ei3 = 4578.0 kJ/mol. The energy required to breakdown of the spark is an ionization energy that can form later the arc. Total ionization energy can be calculated for n moles of the gas (nitrogen) in the core of plasma as: 2 ( ) 4( ) i i ii i p V d h p E ne e e MR T MR T (39) The initial temperature T amounts 300 K and universal gas constant (MR) = 8314 J/mol. For higher pressure, proportionally the higher ionization energy is required and the same is for lower temperature. However the plasma is formed with smaller radius, the ionization takes place in a higher volume with radius two times bigger. 7.3. Heat transfer to electrodes A certain part of the energy delivered by the secondary circuit is consumed on the heating of the electrodes. In a small time of the sparking the heat transfer takes place on the small area approximately equal the cross section of the electrodes with diameter d. The main target is to determine the specific heat conductivity between the gas and metal. This value can be obtain from the Nusselt number Nu [2], gas conductivity p and a characteristic flow dimension, in this case the diameter of the electrode:
24InternalCombustionEnginesNu-Ap(40)αdwhere Nu is obtained from Reynolds number Re and Prandtl number Pr.However Ballaland Lefebvre [6] accounted for heattransfer the following expression for Nusselt number:00.46dNu=0,61- Re.46 =0,61(41)where u,is gas velocity along the wall and μiskinematic viscosity ofgas. On the other handthekinematic viscosity of the gas depends on the temperature T and density p according tothe formula:μ= 5,18-107. 70.62[m2 /s](42)pThe conductivity of the gas is calculated based on the basis of Woschni [3] formula:,=3,654·10-T0,748(43)[W / (mk)]Finallythe cooling energy iscalculated from theequation(44)-α-(T-T.)-tin·d?En =7.4. Kinetic energyLiu et al. [9] assumed that some fraction of the input energy is converted into kinetic energyoftheturbulenceaccordingtothefollowingformula(45)"Pu-u3E,=4·元where p is density,u is the entrainment velocity and d is thekernel diameter.Using thisequationthekineticenergycanbecalculatedforgivenvalues:p=1,403kg/mandforwavepressure moving with mean velocity u [m/s]. During ignition time ti (less than 2 ms) the totalkineticenergyamounts:(46)JEdt=E-tE,=08. Ignition efficiencyElectric efficiency of the ignition systems define also the thermal resistance of these devices,becauselowerefficiencyvaluedecidesabouthigherheatingofthecoilbodyandtakeseffect
24 Internal Combustion Engines Nu p d (40) where Nu is obtained from Reynolds number Re and Prandtl number Pr. However Ballal and Lefebvre [6] accounted for heat transfer the following expression for Nusselt number: 0,46 0,46 Nu 0,61 Re 0,61 g u d (41) where u is gas velocity along the wall and is kinematic viscosity of gas. On the other hand the kinematic viscosity of the gas depends on the temperature T and density according to the formula: 0,62 7 2 5,18 10 m / s T (42) The conductivity of the gas is calculated based on the basis of Woschni [3] formula: 4 0,748 p 3,654 10 W / m K T (43) Finally the cooling energy is calculated from the equation: 1 2 ( ) 2 h wi E d TT t (44) 7.4. Kinetic energy Liu et al. [9] assumed that some fraction of the input energy is converted into kinetic energy of the turbulence according to the following formula: 2 3 4 2 k u d E u (45) where u is density , u is the entrainment velocity and d is the kernel diameter. Using this equation the kinetic energy can be calculated for given values: u = 1,403 kg/m3 and for wave pressure moving with mean velocity u [m/s]. During ignition time tl (less than 2 ms) the total kinetic energy amounts: it 0 d k k ki E Et Et (46) 8. Ignition efficiency Electric efficiency of the ignition systems define also the thermal resistance of these devices, because lower efficiency value decides about higher heating of the coil body and takes effect
Factors Determing Ignition andEfficient Combustion inModern Engines OperatingonGaseousFuels25on their durability.On the basis of conducted tests by measurements of the primary (state1)and secondary (state 2) current and voltage, it is possible to calculate the total electricefficiency of the ignition systems.Thetotal electric efficiency can bedefined as follows:u,l,dtE2_ m(47)neE,[u,I,dtSThe electric efficiency for the ignition system with transistor ignition coil from Beru No0040102002 is shown in Figure 17.The test of energy efficiency was done for 6 probes foreverypoint of measurements.The electric efficiencyisverysmall and at assumed initialpressures does not exceed 30%.The rest of energy goes into the surroundings ina form ofheat.Lower efficiency is observed for nitrogen as the neutral gas.The same input energy forall consideredcasesamounted210.74mj.transistorignitioncoil00401020020.350.3 oueaeaee0.250.2testedmixtures元=1.0元=1.40.15nitrogen0.1010253051520initial pressureinchamber[bar]Figure17.Electric efficiency of ignition systemfortwo mixturesand nitrogen9.EnergybalanceduringignitionOnthebasis of the carried out experimental testsand thetheoretical considerations thebalance of theenergies delivered to the chamber from the secondary circuit of the coil can bedonebySankeychart.Thecarried outcalculationsdeterminethefollowingvaluesofheatlossesfor thecasep=25bars and spark plugwith the normal electrodes:1)radiation-E, =7.8 mj,2) ionization -Ei=7.2mj,3) heat transfer -Eh=31 mJ, 4) kinetic energy-Ek=9mj.Calculated total losses amount 55 mJ and measurements show that the thermal energydelivered to the charge En amounts only 4.23mj.On the other hand themeasured energydeliveredbythesecondarycircuitamountsE2=61.05mJ.Theothernon-consideredheatlossesamountE=1.82mj.Thegraphical presentation of the participation of particular
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels 25 on their durability. On the basis of conducted tests by measurements of the primary (state 1) and secondary (state 2) current and voltage, it is possible to calculate the total electric efficiency of the ignition systems. The total electric efficiency can be defined as follows: 2 02 1 01 2 2 2 1 1 1 t t e t t U I dt E E U I dt (47) The electric efficiency for the ignition system with transistor ignition coil from Beru No 0040102002 is shown in Figure 17. The test of energy efficiency was done for 6 probes for every point of measurements. The electric efficiency is very small and at assumed initial pressures does not exceed 30%. The rest of energy goes into the surroundings in a form of heat. Lower efficiency is observed for nitrogen as the neutral gas. The same input energy for all considered cases amounted 210.74 mJ. Figure 17. Electric efficiency of ignition system for two mixtures and nitrogen 9. Energy balance during ignition On the basis of the carried out experimental tests and the theoretical considerations the balance of the energies delivered to the chamber from the secondary circuit of the coil can be done by Sankey chart. The carried out calculations determine the following values of heat losses for the case p = 25 bars and spark plug with the normal electrodes: 1) radiation - Er = 7.8 mJ, 2) ionization - Ei = 7.2 mJ, 3) heat transfer - Eh = 31 mJ, 4) kinetic energy - Ek = 9 mJ. Calculated total losses amount 55 mJ and measurements show that the thermal energy delivered to the charge Eth amounts only 4.23 mJ. On the other hand the measured energy delivered by the secondary circuit amounts E2 = 61.05 mJ. The other non-considered heat losses amount Ec = 1.82 mJ. The graphical presentation of the participation of particular
26 Internal Combustion Enginesenergies for the spark plug with normal electrodes and with‘thin'electrodes is shown onthe Sankey diagram (Figure 18).The energetic balance shows that the heat transfer to the electrodes consumes a half ofdeliveredenergyduringthesparkingprocess.Decreaseofthecross-sectionoftheelectrodesto 25% of their initial value causes the increase of the thermal efficiency almost twice withdecreaseoftheheattransfertotheelectrodes.TheworkdonebyLiuetal [5]showsthedischargeefficiencyof differentignitionsystemandforconventional sparkignitionsystemthisefficiency is below0.1 (10%)despite thebiggercoil energy (above100m).Normal electrodesThin"electrodesThermal (useful)er6,93%13,5%Heatransfe50.8%Radiation 12,7%lonization (breakdown)11.8%.Kinetic energy14,7%Kinetic energy 14,7%Others 3,07%Others 3.07%Figure 18. Balance of energy in the conventional ignition system for 2 types of the electrodes10.CFD simulationofignitionand combustion processofCNG mixturesPropagation offlame(temperature and gas velocity)depends on the temporarygasmotionnearthesparkplug.TheignitionprocessinSIgaseousengineswassimulatedinCFDprograms (KIVA and Phoenics).Setting of the electrodes in direction of gas motioninfluencesonspreadingoftheflameinthecombustionchamber.10.1.Propagationof ignitionkernelThe propagation of thetemperature during ignition process depends on the gas velocitybetweenthesparkelectrodes.Theexperimentaltestsshowanabsenceofthecombustionprocess in the engine without gas motion. The combustion process can be extended with abig amount of hydrocarbons in the exhaustgases.Thepropagation of thetemperature nearthe spark electrodes was simulated by use of Phoenics code for horizontal gas velocityamounted 10 m/s with taking into account the heat exchange, radiation, ionization andincrease of the internal energy.The model of the spark ignition contained 40x40x1 cells withtwosolidblocksaselectrodesandoneblockoftheplasmakernel.Theelectrodeswereheated during 1 ms with energy equal 8 mJ as it was determined during experimental tests.Propagation of the temperature near spark electrodes is shown in Figure 20 for two times 0.4and0.8ms,respectively
26 Internal Combustion Engines energies for the spark plug with normal electrodes and with ‘thin’ electrodes is shown on the Sankey diagram (Figure 18). The energetic balance shows that the heat transfer to the electrodes consumes a half of delivered energy during the sparking process. Decrease of the cross-section of the electrodes to 25% of their initial value causes the increase of the thermal efficiency almost twice with decrease of the heat transfer to the electrodes. The work done by Liu et al [5] shows the discharge efficiency of different ignition system and for conventional spark ignition system this efficiency is below 0.1 (10%) despite the bigger coil energy (above 100 mJ). Figure 18. Balance of energy in the conventional ignition system for 2 types of the electrodes 10. CFD simulation of ignition and combustion process of CNG mixtures Propagation of flame (temperature and gas velocity) depends on the temporary gas motion near the spark plug. The ignition process in SI gaseous engines was simulated in CFD programs (KIVA and Phoenics). Setting of the electrodes in direction of gas motion influences on spreading of the flame in the combustion chamber. 10.1. Propagation of ignition kernel The propagation of the temperature during ignition process depends on the gas velocity between the spark electrodes. The experimental tests show an absence of the combustion process in the engine without gas motion. The combustion process can be extended with a big amount of hydrocarbons in the exhaust gases. The propagation of the temperature near the spark electrodes was simulated by use of Phoenics code for horizontal gas velocity amounted 10 m/s with taking into account the heat exchange, radiation, ionization and increase of the internal energy. The model of the spark ignition contained 40x40x1 cells with two solid blocks as electrodes and one block of the plasma kernel. The electrodes were heated during 1 ms with energy equal 8 mJ as it was determined during experimental tests. Propagation of the temperature near spark electrodes is shown in Figure 20 for two times 0.4 and 0.8 ms, respectively
Factors Determing Ignition andEfficient Combustion inModern Engines OperatingonGaseousFuels27Figure 19. Temperature in the charge during ignition after 0.4 and 0.8 msThetemperatureinsidetheplasmagrowsasafunctionofthepowerofthesecondarycircuitinthe coil and the velocity of the charge causes propagation of the temperature from the sparkingarc outsideof theplasma.Temperature insidetheplasma kernel reachesvalue about 13000K10.2.CNGignitionprocessincaloricchamberThefirststepoftheexperimental testswasanobservationoftheignitionofthemixtureofCNG and the air in the caloric chamber and the second step by use the simulation. Thecylindermodel has diameterD=34 mm andheightB=22mm.Volumeofthechambercorresponds to the minimal volume of the combustion chamber in the engines ofdisplacement260cm3andcompressionratio14.burning offuel dose200160[veqg]m,=0,035g120nssajm,=0,04gm,=0.045g10time [ms]Figure 2o. Increment of the pressure during combustion in the caloric chamberPredictionofthemixtureparametersinthechamberduringcombustionprocesswascarriedoutbyusingtheopensourcecodeofKlVA3V[4].Thecomplextestwasconductedfor3dose of CNG:0.035,0.04 and 0.045g,which corresponds to air excess coefficients X:1.58, 1.38and 1.23,respectivelyat initial pressure40 bars and temperature600K.At assumptionofthe high compression pressure in the caloric chamber it was obtained very high level of final
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels 27 Figure 19. Temperature in the charge during ignition after 0.4 and 0.8 ms The temperature inside the plasma grows as a function of the power of the secondary circuit in the coil and the velocity of the charge causes propagation of the temperature from the sparking arc outside of the plasma. Temperature inside the plasma kernel reaches value about 13000 K. 10.2. CNG ignition process in caloric chamber The first step of the experimental tests was an observation of the ignition of the mixture of CNG and the air in the caloric chamber and the second step by use the simulation. The cylinder model has diameter D=34 mm and height B=22 mm. Volume of the chamber corresponds to the minimal volume of the combustion chamber in the engines of displacement 260 cm3 and compression ratio 14. Figure 20. Increment of the pressure during combustion in the caloric chamber Prediction of the mixture parameters in the chamber during combustion process was carried out by using the open source code of KIVA3V [4]. The complex test was conducted for 3 dose of CNG: 0.035, 0.04 and 0.045g, which corresponds to air excess coefficients : 1.58, 1.38 and 1.23, respectively at initial pressure 40 bars and temperature 600 K. At assumption of the high compression pressure in the caloric chamber it was obtained very high level of final This image cannot currently be displayed