18 Internal Combustion Enginesis using the pressure piezoelectric transducer with big sensitivity and with high limit ofstatic pressure.For that casewe haveused the sensorPCBPiezotronic 106B51(USA)withthe following parameters:Measurementrange (for±5Voutput)35kPa9Maximumpressure (step)690 kPaMaximum pressure (static)3448 kPaSensitivity (±15%)145mV/kPa-Forthat sensor theamplifier Energocontrol VibAmpPA-3000 was used.The filling of thechamber with fixing of the spark plug and transducer is presented in Figure 9. Theadditional (medium)chamberwithcapacity200cm3isfilledundergivenpressure(shownon themanometer)from thepressure bottle.The caloricchamberis filled fromthismediumchamber by the special needle valves. After sparking the chamber was emptied by openingthe other needlevalve.The needlevalves were used in order todecreasethedead volume inthe pipes connecting the chamber.The total volume was measured by filling the chamber bywater and amounts 4,1cm?.HAdditionalvolmeV=200cmPowerandcontrolfrom ignition systemaouassadExhaustneedle valveneedlevalveBEesBsensorV=4,1cmPressuresignal [MI (to recordingsystem)Figure 9. Scheme of the direct measurement pressure in the caloric chamberThe target of the tests was to determine the amount of thermal energy delivered do thecharge in the chamber after the sparking; itmeans the measurements of the pressureincrementinfunctionof initial pressure.Foronepointofeachcharacteristicwecarried out10 measurements.Forthetests twotypes of electrodes were used:the normal with 2.8mmwidthandthe"thin"with25%cross-section of thefirsttype.Themeasurementswerecarried outinnitrogen and airatinitial pressureinthechambercorrespondedtoambientconditions (overpressure0bar)and at 25bars.Forthe"thin"electrodes there is observed abiggerincrementofthepressurethanwhileusingthesparkplugwithnormal electrodesboth atlow asathighinitial pressure,despite thedelivered energy from the secondarycircuit of the coil is almost the same. Increment of pressure inside the chamber caused by
18 Internal Combustion Engines is using the pressure piezoelectric transducer with big sensitivity and with high limit of static pressure. For that case we have used the sensor PCB Piezotronic 106B51 (USA) with the following parameters: Measurement range (for ±5V output) 35 kPa Maximum pressure (step) 690 kPa Maximum pressure (static) 3448 kPa Sensitivity (±15%) 145 mV/kPa For that sensor the amplifier Energocontrol VibAmp PA-3000 was used. The filling of the chamber with fixing of the spark plug and transducer is presented in Figure 9. The additional (medium) chamber with capacity 200 cm3 is filled under given pressure (shown on the manometer) from the pressure bottle. The caloric chamber is filled from this medium chamber by the special needle valves. After sparking the chamber was emptied by opening the other needle valve. The needle valves were used in order to decrease the dead volume in the pipes connecting the chamber. The total volume was measured by filling the chamber by water and amounts 4,1 cm3. Figure 9. Scheme of the direct measurement pressure in the caloric chamber The target of the tests was to determine the amount of thermal energy delivered do the charge in the chamber after the sparking; it means the measurements of the pressure increment in function of initial pressure. For one point of each characteristic we carried out 10 measurements. For the tests two types of electrodes were used: the normal with 2.8 mm width and the “thin” with 25% cross-section of the first type. The measurements were carried out in nitrogen and air at initial pressure in the chamber corresponded to ambient conditions (over pressure 0 bar) and at 25 bars. For the “thin” electrodes there is observed a bigger increment of the pressure than while using the spark plug with normal electrodes both at low as at high initial pressure, despite the delivered energy from the secondary circuit of the coil is almost the same. Increment of pressure inside the chamber caused by
Factors Determing Ignition andEfficient Combustion inModern Engines OperatingonGaseousFuels19energydelivered from spark plug is shown in Figure10 for initial pressure1bar and 25barsandbyapplicationof thesparkplugwith"thin"and"thick"electrodes.1000800[edeeninitialpressure-electrode6001bar-thin1bar-thick25 bar-thin40025bar -thick2000048121620time [ms]Figure10.Pressure increment in caloric chamberfilledby nitrogen at initial pressure1and 25bars byapplication of spark plug with"thin" and"thick"electrodesThe duration of the sparking lasted about 4 ms and after this time the decrement of thepressure is observed which is caused by heat exchange with walls of the caloric chamber. Ineverycaseattheendofignitionprocessthesuddenincreaseof secondaryvoltagetakesplace.Thecurrentinthesecondarycircuitof theignition coilincreasesrapidlytoabout80mAaftersignal of theignition andthendecreasesslowlyduring4mstozeroasoneshowsinFigure11forallconsideredcases.10080initialpressure-electrode[ renaeoon1bar-thin1 bar-thick6025bar-thin25bar-thick40920116812201time [ms]Figure 11. Secondary current in the coil during the ignition in the caloric chamber filled by nitrogen atinitialpressure1and25barbyapplicationof sparkplugwith"thin"and"thick"electrodesVariation of voltage in the secondary circuit is shown in Figure 12. For the consideredignition coil one reachesmaximum voltage3000 V in the caseof higher initial pressure 30bar. In every case at the end of ignition process the sudden increase of secondary voltage
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels 19 energy delivered from spark plug is shown in Figure 10 for initial pressure 1 bar and 25 bars and by application of the spark plug with “thin” and “thick” electrodes. Figure 10. Pressure increment in caloric chamber filled by nitrogen at initial pressure 1 and 25 bars by application of spark plug with “thin” and “thick” electrodes The duration of the sparking lasted about 4 ms and after this time the decrement of the pressure is observed which is caused by heat exchange with walls of the caloric chamber. In every case at the end of ignition process the sudden increase of secondary voltage takes place. The current in the secondary circuit of the ignition coil increases rapidly to about 80 mA after signal of the ignition and then decreases slowly during 4 ms to zero as one shows in Figure 11 for all considered cases. Figure 11. Secondary current in the coil during the ignition in the caloric chamber filled by nitrogen at initial pressure 1 and 25 bar by application of spark plug with “thin” and “thick” electrodes Variation of voltage in the secondary circuit is shown in Figure 12. For the considered ignition coil one reaches maximum voltage 3000 V in the case of higher initial pressure 30 bar. In every case at the end of ignition process the sudden increase of secondary voltage
20InternalCombustionEnginestakes place.Thermal energy delivered to the spark plug (in the secondary circuit)wasdetermined by integration of instant electricpower (multiplication of currentand voltage)with small time step.For the case with"thin"electrodes and at 1bar the thermal energyamountsonly0,89mJ and thus thethermal efficiencyisabout=1,29% (Figure13).Fornormal electrodes at the same pressure the thermal energy is very lower 0,36 mJ whichcausesasmallthermalefficiencymi=0,51%3000initialpressure-electrodeeonecoe1bar-thin20001bar-thick25 bar -thin25bar-thick10001216200A8time [ms]Figure 12. Secondary voltage in the coil during the ignition in the caloric chamber filled by nitrogen atinitial pressure 1 and 25 bars by application of spark plug with"thin"and "thick"electrodes-"thin"normal-"thin"normal91681471261050463422100002525Initial Pressure [bar]InitialPressure[bar]Figure 13. The comparison of the thermal energy and thermal efficiency for spark plug with normaland "thin"electrodes at two initial positive gauge pressuresThethermal energy and thermal efficiency increases with the increase of the initial pressure.For the case with "thin" electrodes of the spark plug the thermal efficiency amounts 13.49%
20 Internal Combustion Engines takes place. Thermal energy delivered to the spark plug (in the secondary circuit) was determined by integration of instant electric power (multiplication of current and voltage) with small time step. For the case with “thin” electrodes and at 1 bar the thermal energy amounts only 0,89 mJ and thus the thermal efficiency is about th = 1,29% (Figure 13). For normal electrodes at the same pressure the thermal energy is very lower 0,36 mJ which causes a small thermal efficiency th = 0,51%. Figure 12. Secondary voltage in the coil during the ignition in the caloric chamber filled by nitrogen at initial pressure 1 and 25 bars by application of spark plug with “thin” and “thick” electrodes Figure 13. The comparison of the thermal energy and thermal efficiency for spark plug with normal and "thin" electrodes at two initial positive gauge pressures The thermal energy and thermal efficiency increases with the increase of the initial pressure. For the case with “thin” electrodes of the spark plug the thermal efficiency amounts 13.49%, 0 1 2 3 4 5 6 7 8 9 0 25 Thermal Energy [mJ] Initial Pressure [bar] normal "thin" 0 2 4 6 8 10 12 14 16 0 25 Thermal Efficiency [%] Initial Pressure [bar] normal "thin
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels21on the other hand for normal electrodes only6.93%.The tests were done for fiveignitionsystems fromBERUat different initial pressure(0-25bars)and linear approximationvariations of the thermal efficiencies are shown in Figure 14. With increasing of the pressurein the caloric chambermuch more energy is delivered from the electric arc to the gas.Themeasurements of the pressure increase during spark ignition were carried out also for theair and the samepressures.Figure 16 presents the increase of secondary voltage in theignition coil with increasing of initial pressure in the caloric chamber. For nitrogen andleaner mixtures a higher secondary voltage in the coil was measured.ThermalEfficiency(linearapproximation)-Nitrogen16ColNo,1CoilNo.2ACoilNo.314CollNo.4XCollNo.5wpao80642o0510152025Pressureinthechamber[bar]Figure14.Thermal efficiency of five tested ignition systems30 adesreonee252015Nitrogen10·CNG^=1.4CNG^=1.0500231pressure in the chamber [MPa]Figure 15. Influence of initial pressure on secondary voltage in ignition coil measured in caloricchamberfilledbynitrogenandnaturalgas
Factors Determing Ignition and Efficient Combustion in Modern Engines Operating on Gaseous Fuels 21 on the other hand for normal electrodes only 6.93%. The tests were done for five ignition systems from BERU at different initial pressure (0 – 25 bars) and linear approximation variations of the thermal efficiencies are shown in Figure 14. With increasing of the pressure in the caloric chamber much more energy is delivered from the electric arc to the gas. The measurements of the pressure increase during spark ignition were carried out also for the air and the same pressures. Figure 16 presents the increase of secondary voltage in the ignition coil with increasing of initial pressure in the caloric chamber. For nitrogen and leaner mixtures a higher secondary voltage in the coil was measured. Figure 14. Thermal efficiency of five tested ignition systems Figure 15. Influence of initial pressure on secondary voltage in ignition coil measured in caloric chamber filled by nitrogen and natural gas
22InternalCombustionEngines7.Determination of energy losses during ignitionThe model of ignition process takes into account only a small part of thespark plug and isshowninFigure16p, TdElectrodePlasmahT>6000KuElectrodeReFigure16.Model of spark ignitionDuring the sparking the plasma is formed between two electrodes and it is assumed to besmaller than the thickness of these electrodes. After short time a pressure shock takes placeand the charge is moving on outer side with high velocity [1] [13]. The energy delivereddirectly to the charge is very low and therefore the energy losses should be assessed. As theexperimental test showed, only a small part of delivered energy is consumed to increase theinternal energyofthe charge (maximum10%).Theenergylosses during the ignitionprocesscan be divided into several kinds: radiation, breakdown, heat exchange with electrodes,kinetic energy which causes the turbulence,electromagnetic waves, flash and others.7.1.RadiationenergyofignitionThe part of the spark energy is consumed by radiation of the plasma kernel. Thetemperature T of plasma between two electrodes is above 6000K.At assumption of theBoltzman radiation constantk=5.67W/(m?K°) and the coefficient of emissivity of a greysubstance [9] for the ignition arc, the specific heat radiation e can be obtained from theformula:2=8.(35)100The emissivity of the light grey substance was defined by Ramos and Flyn [4] and theyamounted it in the range of 0.2 -0.4.For that case it was assumed that =0.3.The totalradiation energy is a function of the ignition core surface Aiand sparking time ti:
22 Internal Combustion Engines 7. Determination of energy losses during ignition The model of ignition process takes into account only a small part of the spark plug and is shown in Figure 16. Figure 16. Model of spark ignition During the sparking the plasma is formed between two electrodes and it is assumed to be smaller than the thickness of these electrodes. After short time a pressure shock takes place and the charge is moving on outer side with high velocity [1] [13]. The energy delivered directly to the charge is very low and therefore the energy losses should be assessed. As the experimental test showed, only a small part of delivered energy is consumed to increase the internal energy of the charge (maximum 10%). The energy losses during the ignition process can be divided into several kinds: radiation, breakdown, heat exchange with electrodes, kinetic energy which causes the turbulence, electromagnetic waves, flash and others. 7.1. Radiation energy of ignition The part of the spark energy is consumed by radiation of the plasma kernel. The temperature T of plasma between two electrodes is above 6000 K. At assumption of the Boltzman radiation constant k=5.67 W/(m2 K4) and the coefficient of emissivity of a grey substance [9] for the ignition arc, the specific heat radiation e can be obtained from the formula: 4 100 T e k (35) The emissivity of the light grey substance was defined by Ramos and Flyn [4] and they amounted it in the range of 0.2 – 0.4. For that case it was assumed that = 0.3. The total radiation energy is a function of the ignition core surface Ai and sparking time ti: