8InternalCombustionEnginesof the natural gas lasted about 80 deg CA and its ignition began at TDC. In the real enginethe diesel oil injection started at 38 deg CA BTDC.n=2200rpm;mcwo=0,0436g:mgo=0.0173g0.050.0400.03total mass CNG0burning mass CNG0.020.010320340360380400420440460480500crank angle [deg]Figure 2. Mass variation of natural gas in Andoria 1HC102 diesel engine fuelled by CNG and ignitiondose of diesel oil (index do- diesel oil, CNG - natural gas)n=2200 rpm;m=0,0436g;ma=0,0173gCNG250020005Heat releaseMtotal heatCNGON10005000330360390420450480510crank angle [deg]Figure 3. Heat release in dual fuel Andoria 1HC102 diesel engine fuelled by CNG and ignition dose ofdiesel oil (index do- diesel oil, CNG - natural gas)Heatreleasefromthebothfuels(CNGanddiesel oil)isshowninFigure3forthesameengineatrotational speed2200rpm.Totalheatreleased duringcombustionprocessresultsmainly on higher burningmass of thenatural gas.The ignitionprocess inthegas dieselengines with the ignition dose of diesel oil differs from other systems applied in modifiedengines fuelled by natural gas delivered into the inlet pipe and next ignited by the sparkplug.The initiation of combustion process in CNG diesel engines with spark ignition isalmostthesameasinthesparkignitionengines
8 Internal Combustion Engines of the natural gas lasted about 80 deg CA and its ignition began at TDC. In the real engine the diesel oil injection started at 38 deg CA BTDC. Figure 2. Mass variation of natural gas in Andoria 1HC102 diesel engine fuelled by CNG and ignition dose of diesel oil (index do- diesel oil, CNG – natural gas) Figure 3. Heat release in dual fuel Andoria 1HC102 diesel engine fuelled by CNG and ignition dose of diesel oil (index do- diesel oil, CNG – natural gas) Heat release from the both fuels (CNG and diesel oil) is shown in Figure 3 for the same engine at rotational speed 2200 rpm. Total heat released during combustion process results mainly on higher burning mass of the natural gas. The ignition process in the gas diesel engines with the ignition dose of diesel oil differs from other systems applied in modified engines fuelled by natural gas delivered into the inlet pipe and next ignited by the spark plug. The initiation of combustion process in CNG diesel engines with spark ignition is almost the same as in the spark ignition engines
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels94.IgnitionconditionsofnaturalgasmixturesThe flammability of thenatural gas is much lower thangasoline vapours or diesel oil in thesame temperature.At higher pressure the spark-over is more difficulty than at lowerpressure.During the compression stroke the charge near the spark plug can be determinedby certain internal energy and turbulence energy. Additional energy given by the sparkplug at short time about 2 ms increases the total energy of the mixture near the spark plug.The flammability of the mixture depends on the concentration of the gaseous fuel andturbulenceofthe chargenear thespark plug.Maximum of pressure and velocityofcombustion process in the cylinder for given rotational speed depend on the ignition angleadvancebeforeTDC(Figure4).TDCMmaanoIgnitionadvanceCrank angleFigure 4. Influence of ignition angle advance on the engine torqueThe beginning of the mixture combustion follows after several crank anglerotation.Whilethisperiod certain chemical reactionsfollowin themixturetoformtheradicals,which caninducethecombustionprocess.Theenergyinthesparkprovidedalocalriseintemperatureof several thousand degrees Kelvin,which cause any fuel vapourpresentto be raised aboveits auto-ignition temperature.Theauto-ignition temperature determinesthepossibility ofthe break of the hydrocarbon chains and the charge has sufficient internal energy to oxidizethe carbon into CO2 and water in the vapour state.Immediately,after the beginning ofcombustion(ignitionpoint)theinitialflamefrontclosetothesparkplugmovesinaradialdirectionintothespaceofthecombustionchamberandheatstheunburnedlayersofair-fuelmixturesurroundingit.For the direct injection of CNG for small loads of the engine in stratified charge mode theburningofthemixturedependsonthepressurevalueattheendofcompressionstrokeandon the relative air-fuel ratio.These dependencies of the CNG burning for different mixturecomposition and compressionratio arepresented inFigure5[15].Theburningof CNGmixture can occur in very small range of the compression pressure and lean mixturecomposition andmaximumcombustionpressurereachesnear 200bars.Forveryleanmixtures and higher compression ratios the misfire occurs, ontheotherhand forrich
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels 9 4. Ignition conditions of natural gas mixtures The flammability of the natural gas is much lower than gasoline vapours or diesel oil in the same temperature. At higher pressure the spark-over is more difficulty than at lower pressure. During the compression stroke the charge near the spark plug can be determined by certain internal energy and turbulence energy. Additional energy given by the spark plug at short time about 2 ms increases the total energy of the mixture near the spark plug. The flammability of the mixture depends on the concentration of the gaseous fuel and turbulence of the charge near the spark plug. Maximum of pressure and velocity of combustion process in the cylinder for given rotational speed depend on the ignition angle advance before TDC (Figure 4). Figure 4. Influence of ignition angle advance on the engine torque The beginning of the mixture combustion follows after several crank angle rotation. While this period certain chemical reactions follow in the mixture to form the radicals, which can induce the combustion process. The energy in the spark provided a local rise in temperature of several thousand degrees Kelvin, which cause any fuel vapour present to be raised above its auto-ignition temperature. The auto-ignition temperature determines the possibility of the break of the hydrocarbon chains and the charge has sufficient internal energy to oxidize the carbon into CO2 and water in the vapour state. Immediately, after the beginning of combustion (ignition point) the initial flame front close to the spark plug moves in a radial direction into the space of the combustion chamber and heats the unburned layers of air-fuel mixture surrounding it. For the direct injection of CNG for small loads of the engine in stratified charge mode the burning of the mixture depends on the pressure value at the end of compression stroke and on the relative air-fuel ratio. These dependencies of the CNG burning for different mixture composition and compression ratio are presented in Figure 5 [15]. The burning of CNG mixture can occur in very small range of the compression pressure and lean mixture composition and maximum combustion pressure reaches near 200 bars. For very lean mixtures and higher compression ratios the misfire occurs, on the other hand for rich
10InternalCombustionEnginesmixtures and high compression ratios the detonation is observed. During the cold start-uptheignition process of theCNG mixture is much easier than with gasolinemixturebecauseof whole fuel is in the gaseous state.Today in the new ignition systems with electronic orcapacitor discharge the secondary voltage can reach value 40kV in somemicroseconds..FASTBURN"18200barW16BesTexh≤500c<800°℃Texh14=150barPmax-I!1210.SLOWBURNRealoperationrange1>821.21.8A1.41.62Figure 5. The range of combustion limits for lean CNG mixture [3]The higher voltage in the secondary circuit of the transformer and the faster spark riseenable that the sparking has occurred even when the spark plug is covered by liquidgasoline.With fuelling of the engine by CNG the sparkingprocess should occur in everycondition of the engine loads and speeds.However,at higher compression ratio and higherenginechargingthefinal chargepressureincreases dramaticallyinthemoment of ignitionand thisphenomenon influenceson thesparkingprocess.5.Electric and thermal parameters of ignitionOn the observation and test done before on the conventional ignition systems, the higherpressure of the charge in the cylinder requires also higher sparking energy or less the gap ofthe electrodes in the spark plug.The chemical delay of the mixture burning is a function ofthepressure, temperature and properties of the mixtureand was performedbySpadaccini[12] in theform:At,=2.43·10-°p-2.exp[41560 / (R·T)](3)where: p- pressure [bar], T-temperature [K] and R-gas constant [(bar cm")/(mol K)]The simplest definition of this delay was given by Arrhenius on the basis of a semi-empirical dependence:
10 Internal Combustion Engines mixtures and high compression ratios the detonation is observed. During the cold start-up the ignition process of the CNG mixture is much easier than with gasoline mixture because of whole fuel is in the gaseous state. Today in the new ignition systems with electronic or capacitor discharge the secondary voltage can reach value 40 kV in some microseconds. Figure 5. The range of combustion limits for lean CNG mixture [3] The higher voltage in the secondary circuit of the transformer and the faster spark rise enable that the sparking has occurred even when the spark plug is covered by liquid gasoline. With fuelling of the engine by CNG the sparking process should occur in every condition of the engine loads and speeds. However, at higher compression ratio and higher engine charging the final charge pressure increases dramatically in the moment of ignition and this phenomenon influences on the sparking process. 5. Electric and thermal parameters of ignition On the observation and test done before on the conventional ignition systems, the higher pressure of the charge in the cylinder requires also higher sparking energy or less the gap of the electrodes in the spark plug. The chemical delay of the mixture burning is a function of the pressure, temperature and properties of the mixture and was performed by Spadaccini [12] in the form: 9 2 2.43 10 exp[41560 / ( )] z p RT (3) where: p - pressure [bar], T - temperature [K] and R - gas constant [(bar cm3)/(mol K)]. The simplest definition of this delay was given by Arrhenius on the basis of a semiempirical dependence:
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels114650At, =0.44.10-3.p-1.19 exp(4)where p is the charge pressure at the end of the compression process [daN/cm?].Experimental and theoretical studies divide the spark ignition into three phases:breakdown, arc and glow discharge.They all haveparticular electrical properties.Theplasma of temperatureabove6000K and diameterequal thediameterof theelectrodescauses a shock pressure wave during several microseconds. At an early stage a cylindricalchannel ofionization about 40 μm in diameterdevelops, together with a pressure jump anda rapid temperature rise. Maly and Vogel [10] showed that an increase in breakdown energydoes not manifest itself a higher kernel temperatures, instead the channel diameter causingalargeractivated-gas volume.Since the ratiobetween the initial temperature of themixtureand the temperature of the spark channel is much smaller than unity,the diameterd of thecylindrical channel is given approximately by thefollowing expression:1/Epd(5)d = 2]y-元·h-pwhere y is ratio of the specific heats, h is the spark plug gap and p pressure, Ebd representsthebreakdown energy to produce the plasmakernel.Ballal and Lefebvre[6] considered thefollowing expression for the breakdown voltage Ubd and total spark energy Er-2,8-105-p-hE, =[v.I-dt(6)Uba= 5,5-In(p-d)0One assumed, that the charge is isentropic conductive and the field attains a quasi-steadystate (no time influence).Knowing the potential of the electromagnetic field @ and electricalconductivity the following equation can be used [12]:(7)div(g·gradob)=0Afteraformingoftheplasmabetweentheelectrodestheheatsourceqinthemixturecanbe calculated directly from the electrical current in the secondary coil circuitI,whichchanges duringwithtime:12(8)9e =[2元ro(r,z)drwhere r and z are the coordinates of the ionization volume.At leaner homogenous mixture the discharging ofthe energyby spark plug leads sometimestothemisfireand increasing of thehydrocarbons emission.At stratified chargeforthesame
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels 11 3 1.19 4650 0.44 10 exp z p T (4) where p is the charge pressure at the end of the compression process [daN/cm2]. Experimental and theoretical studies divide the spark ignition into three phases: breakdown, arc and glow discharge. They all have particular electrical properties. The plasma of temperature above 6000 K and diameter equal the diameter of the electrodes causes a shock pressure wave during several microseconds. At an early stage a cylindrical channel of ionization about 40 m in diameter develops, together with a pressure jump and a rapid temperature rise. Maly and Vogel [10] showed that an increase in breakdown energy does not manifest itself a higher kernel temperatures, instead the channel diameter causing a larger activated-gas volume. Since the ratio between the initial temperature of the mixture and the temperature of the spark channel is much smaller than unity, the diameter d of the cylindrical channel is given approximately by the following expression: 1 2 2 1 bd E d h p (5) where is ratio of the specific heats, h is the spark plug gap and p pressure. Ebd represents the breakdown energy to produce the plasma kernel. Ballal and Lefebvre [6] considered the following expression for the breakdown voltage Ubd and total spark energy Et: 5 2,8 10 5,5 ln( ) bd p h U p d 0 it t E V I dt (6) One assumed, that the charge is isentropic conductive and the field attains a quasi-steady state (no time influence). Knowing the potential of the electromagnetic field and electrical conductivity the following equation can be used [12]: div( grad ) 0 (7) After a forming of the plasma between the electrodes the heat source e q in the mixture can be calculated directly from the electrical current in the secondary coil circuit I, which changes during with time: 2 2 0 2 (,) e R I q r r z dr (8) where r and z are the coordinates of the ionization volume. At leaner homogenous mixture the discharging of the energy by spark plug leads sometimes to the misfire and increasing of the hydrocarbons emission. At stratified charge for the same
12InternalCombustionEnginestotal air-fuel ratio the sparking of the mixture can be improved by turning the injected fueldirectly near spark plug at strictly defined crank angle rotation depending on the enginespeed. The energy involved from the spark plug is delivered to the small volume near sparkplug. The total energy, which is induced by the spark plug is a function of the voltage andcurrent values in the secondary circuit of the ignition coil and time of the discharge. On theother hand, values of voltage U and current I change in the discharge time and total energyinduced by the coil can be expressed as a integral of voltage U, current I and timet:Eig-ju-Idt(9)E0where t is the time of current discharge by the secondary circuit of the ignition coil.Integration of the measurement values of voltage and current in the secondary circuit of thecoil gives the total electric energy to the mixture charge near spark plug.The total internalenergy of the mixture near the spark plug increases in the period t = 0..t and according tothe energy balance in the small volumethe temperature of the charge in this regioncontinuouslyincreases.The modern conventional ignition system can give the burning energy eburn = 60 mJ at thesecondary voltage 30 kV and burning current iburn =70mA during 1.8 ms.In practice arequired value of the secondary voltage of the ignition system is calculated from thefollowing formula:U, = 4700-(a-s)0.718(10)where:U2-secondaryvoltage [V],a-gapbetweenelectrodesofthesparkplug,-compression ratio.80000a=0,3mma=0.4mmoneprorspeinboa=0.5mm60000a=0,7mma=0.9mm40000200001020304050Ucyinder pressure [bar]Figure 6. The secondary voltage as a function of compression pressure and electrode's gap
12 Internal Combustion Engines total air-fuel ratio the sparking of the mixture can be improved by turning the injected fuel directly near spark plug at strictly defined crank angle rotation depending on the engine speed. The energy involved from the spark plug is delivered to the small volume near spark plug. The total energy, which is induced by the spark plug is a function of the voltage and current values in the secondary circuit of the ignition coil and time of the discharge. On the other hand, values of voltage U and current I change in the discharge time and total energy induced by the coil can be expressed as a integral of voltage U, current I and time t: 0 ign E U I dt (9) where is the time of current discharge by the secondary circuit of the ignition coil. Integration of the measurement values of voltage and current in the secondary circuit of the coil gives the total electric energy to the mixture charge near spark plug. The total internal energy of the mixture near the spark plug increases in the period t = 0. and according to the energy balance in the small volume the temperature of the charge in this region continuously increases. The modern conventional ignition system can give the burning energy eburn = 60 mJ at the secondary voltage 30 kV and burning current iburn = 70 mA during 1.8 ms. In practice a required value of the secondary voltage of the ignition system is calculated from the following formula: 0.718 2 U a 4700 (10) where: U2 - secondary voltage [V], a - gap between electrodes of the spark plug, - compression ratio. Figure 6. The secondary voltage as a function of compression pressure and electrode’s gap