MECHANISM ON DISTRIBUTION OF PILOT FUEL SPRAY AND COMPRESSING IGNITION IN PREMIXED NATURAL GAS ENGIN

CHINESE JOURNAL OF MECHANICAL ENGINEERINGVol.18, No.l, 2005.25.MECHANISM ON DISTRIBUTION OFPILOT FUEL SPRAY AND COMPRESS-Yao ChundeING IGNITION IN PREMIXEDYao GuangtaoNATURAL GAS ENGINE IGNITEDSong JinouBY PILOT DIESEL*Wang YinshanState Key Laboratory ofEngine Combustion,Abstract: Numerical simulations of pilot fuel spray and compressing ignition for pre-mixed naturalTianjin University,gas ignited by pilot diesel are described. By means of these modeling, the dual fuel and diesel fuelTanjin 30002, Chinaignition mechanism of some phenomena investigated on an optional engine by technology ofhigh-speed CCD is analyzed. It is demonstrated that the longer delay of ignition in dual fuel engine isnot mainly caused by change of the mixture thermodynamics parameters. The analysis results ilus-trate that the ignition of pre-mixed natural gas ignited by pilot diesel taking place in dual fuel engineis a process of homogenous charge compression ignition.Key words: Dual fuel Pilot diesel Modeling of spray and ignition Delay of ignition Spray fielddom orbit simulation, the transformation process of droplets (re-0 INTRODUCTIONgarding natural gas constituent when calculating various parame-Diesel engines are widely used on power equipments becauseters) is calculated directly fom equation groups based onof their higher eficiency, good fuel economy and durability. ButLagrange coordinate system, which are shown below.Fuel droplets track equations are given as followswith the energy shortage and environment pollution becomingmore and more serious, experiments have been done on alterna-tive fuel and new combustion methods to solve these problems.5=m1)dThe reserves of natural gas are very large. Meanwhile, due tohomogeneous mixture and better combustion, the particulateMovement equations are given as followsemission of the engines powered by natural gas will be completelybecompimproved. The lower f; lower fiame temperature of naural gas enginesd业_ 3CoP.(R_will also lead to less NOx emission. Moreover, the main constitu-出= 4.(v-K{u4+H-n,)+ent part of the natural gas is methane, the hydrocarbon with thesmallest C/H ratio. To give out the same amount of heat, the CO,d业_ 3CoPeIpproduced by buming methane is less than that produced by burn-di" 4d,P.-K(4+v'-y)+ing gasoline and diesel by 15%, which is very favorable for alle(2)viating the greenhouse efect. Therefore, natural gas is a perfectdw. 3CoPxp-[|(w+w'-w)alternative fucl to substitute for diesel. Much research work hasdr 4d,Abeen carried out on buming natural gas in engines. But due to thehigh ignition point of natural gas, when ignited by spark plugs, thespread speed of the flame is smaller, and the thermal efficiencylower. While ignited by pilot diesel, the thermal efficiiencyof dualfuel engines is higher, to which researchers atach more impor-tance. It is revealed by experiments that when pre-mixed naturalRe, =p.{(u+d'-以) +(D+v-n,j+(w+w-mj}"2d,gas is ignited by pilot diesel, the combustion process is more4%complex. The amount of pilot diesel, injection timing, injectionpressure, and air fuel ratio, etc, influence the ignition timing andEvaporation equations are as fllowspoints of pilot diesel, which in turn determines the quality ofcombustion process in dual fuel engines. Through modeling of2m”fuel spray and ignition of dual fuel engines, the mechanism of thephenomena observed in the experiments carried on an optical en-gine was explained, which provides useful theoretical and predictivehd._ 2(N,1 Le)n(+ B)(3)tools for further research on the combustion proess of premixed, nramirea(1+ B)"ie-1natural gas engines ignited by pilot dicsel.品=2N,1n(1+B). P。D1 MODELING OF MIXTURE FORMATION ANDIGNITION OF PILOT DIESEL[B=(45-_.)(l-Yrs)1.1 Modeling of pilot diesel spray中国煤化工3Through modeling of scattered droplets, regarding the influ-(4)ence of urbulene spread of droplets and mixture, and using ran-nd using ran-MYHCNMHGU.c1ONt、o(~s=l+ 12.2(e.s.c"""●This projet is spprted by National Natural Science Foundation of China(No.50076031). Received December 19, 2003; reeived in revised fom No-Heat transfer equationvember 1, 2004; accepted November 9, 2004Yao Chunde, et al: Mechanism on distribution of pilot fuel spray and compressing ignition●26.in premixed natural gas engine ignited by pilo dieseldTQ一Active intermittent productsm.Cps =πd、H(T。-7)-Nu+LgH- - -Fuel taking part in chemical reaction__C., dm,/dr5)kg,hk,fk,,fkp,fsk.fk,k,k- -Chemical reaction rateπd ANuconstantsThe heat release from chemical reaction iswhereu,v, wVelocity vectorQ,=k,qV[R*]-Diameters of fuel dropletsHeat loss rate is-Density-Fuel evaporation rateQ=φV(T-Tw) φ=hRe- -Reynolds numberLe一-Lewis numberWhen the temperature of a unit reaches 1 100 K or the tempera-Pr -Prandt numberture acceleration rate is higher than 10' Ksh, it can be regardedSc --Schmidt numberas the occurrence of ignition points.ip--Specitfic heat2 EXPERIMENTAL RESULTST一-TemperatureVu-Nusselt numberThe experiment is carried out on a single-cylinder optical en--Resistance coefficientginTo find the difference of ignition points between the combus-D--Fuel vapor distribution coefficientatof刁-Heat conduction cofficienttion of pre-mixed naturaln of pre-mixed natural gas ignited by pilot diesel and thatpure diesel, studies are crried out on the mixture formation andViscositycombustion in the cases of dual fuel, pure diesel with the same en--Fuel vaporization latent heatthalpy, and pure pilot diesel when the engine speed is 1 00 r .min-,g.-- Gravitational accelerationthe engine is running at full load, and the ijction advance angleis -16 CA(see Table 1 and Fig.1).-Mass concentration proportion of fuelvapor near the surface of the fuel dropletsTable 1 Data of igntion characteristies of dual fuel,Y.一diesel with the same enthalpy and pilot dieselNatural gas Proportion of Ignitionvapor surrounding the fuel dropletsamountpilot dieseltimingNp一-Heat transfer modificatory cofficientm/(g. cyc) m/(g. cye 1a/CA-Mass transfer modificatory coefficientDiesel0.066Dual fuel0.012189.6-Heat transfer rate modificatory factor0012Pilot diesel-0.2-Spalding transfer numberNote: Symbol“" reveals time before TDC.Cyclic heat release from chain reaction- -Superficial heat transfer cofficientTw---Wall temperature-Ratio between the surface area andvolume of combustion chamber-6.0 CA-4.8CAGroup of fuel droplets(a) Pure diesel-Gas phase property-Fuel vapor property1.2 Ignition modeling of pilot dieselThrough multi-stage chemical reaction dynamic modeling(shell modeling), regarding the influence of intake natural gas onoxygen concentration, and according to experimental results got9.6 CA10.8CA(b) Dual fuclfrom an optical engine with natural gas ignited by pilot diesel, thechemical reaction rate coefficient is adjusted.The reaction modeling is as follows:RH +O2→2R*R*→R*+P + Heatk,R*-R*+Bfk。-0.2CA2.2 CA(C) Pilot dieselR*→R*+Qfsk。Fig. 1_ Mixture formation and combustion process of dieselR*+Q→R*+BSk,and dual fuel with the same enthalpy and pilot dieselB→2R*kAs can be seen from Fig.1 and Table 1, with the same injec-R*- - The chain is brokenfktion adv中国煤化工2 R*→The chain is brokenkthe shortest, which iswhere R-Radical elementabout 1MYHCN M H Gme ethay is much-Combustion productslarger, which is 25.6 CA; While that of pilot diesel is interven-B一Simple chain branchient, which is 15.8 CA.CHINESE JOURNAL OF MECHANICAL ENGINEERING●27●(2) Comparison of the dstibution and number of ignition 0.7, the same as that in this experiment. Therefore, the volumepointsratio between pre-mixed natural gas and air is only 5.6%. As aIn the case of pure dicesel, ignition points occur in the process result, the decrease of the temperature and pressure of the com.of fuel injection, and near the injector. There are I or 2 ignitionpressed mixture caused by intake natural gas is limited. Then whatpoints near every fuel spray. In the case of dual fuel with the sameis the reason for the much larger ignition delay and the ignitionenthalpy, ignition points occur near the cylinder wall rather thanpoints moving towards cylinder wall, when buming natural gasthe fuel spray and injector, after the fuel injection process. Therewith its volume concentration is less than 6% and ignited by pilotare more ignition points which will increase with time passing. Indiesel!? What is the characteristic of in-cylinder spray distributionthe case of pilot diesel, the characteristic of the ignition points is when ignition points occur in the cases of dual fuel and diesel?intervenicnt.Table2 In-eyinder parameters at 16 CA BTDCin the cases of dual fuel and diesel3 THEORETICAL ANALYSISDensitTemperature3.1 Modeling verificationDieselTo dermonstrate the applicability of the model for systemati-Dual fuel27419.623 .cal analysis, the above mentioned experiments are studied usingthe model. The comparison between the measure value and mode(1) As a result of small amount of diesel, the injection proc-value of in-cylinder pressure of the three cases is shown in Fig.2.ess is shorter and fuel spray penetration distance is smaller, duringAs is revealed, the former is simila to the ltter. Therefore, the resutsthe combustion of natural gas ignited by pilot diesel and dieselcan be deemed highly credible from investigation on dual fuel mix-with the same amount. So the ijectiongoces; finishes before theture formation and ignition using fuel spray and ignition model.fuel spray reaches the cylinder wall. The fuel spray droplets aremainly scattered in the space of the combustion chamber beforethe end of fuel injection. Blown by the intake swirl, the fuel spray-Measure valuespreads rapidly. The distribution of the velocity of in-cylinder fuelprdroplets at the same time in the cases of dual fuel and diesel withthe same enthalpy is calculated using the models(see Fig.3). It isshown that a larger fuel spray distribution area is formed in thecombustion chamber in the case of dual fuel ignited by pilot diesel.But since a small amount of pilot diesel spreads faster in a largerarea, the concentration of fuel spray and concentration differencedecreases, which is unfavorable for the ignition of pilot diesel,-80--405一 80leading to a longer ignitin delay. Moreover, it can be known fromCrank angle a/CAchemical reaction dynamics, usually the element reaction of hy-drocarbon is non-single step one. The intake gas fuel decreases(a) Pure dieselthe oxygen concentration. Due to the exponential relationshipbetween the chemical reaction rate and the oxygen concentration- Modle valuewith the exponent bigger than I, a small change in oxygen con-- Measue valuecentration will lead to an obvious change in chemical reaction rate.And the intake natural gas hinders the ollision of pilot fuel withoxygen molecules. In conclusion, the decrease of concentrationand chemical reaction rate results in an obviously longer ignitiondelay. Within the shock limitation, the change of specific thermnalcapacity and concentration caused by a very limited amount of-80-440 8cnatural gas is not the major reason for the longer ignition delay ofdual fuel engines.(b) Dualuelnwg= 20.1 m/s“29.1 m/sThun=: 58.08 m/s- Model valuewn=22.4 m/s中10Ru- Measure value-80 一440 80(a) Pure diese!(b) Pilot dieselFig.3 In-cylinder spray velocity distributionat -10 CA(c) Pilot dieselin the cases of pure diesel and pilot disselFig.2 Comparison among the pressure traces in the cases ofdiesel, dual fuel and pilot diesel(2) The location of ignition points calculated through model-ing in the cases of diesel and dual fuel is shown in Fig.4. As canbe seen, when the pilot diesel of dual fuel is ignited, the ignition3.2 Analysis of calculation resultspoints are near the chamber wall. W hile the ignition points of pureThe mean value of in-cylinder pressure, temperature anddiesel are near the injectors. This is relevant to the characteristicconcentration before injection in the cases of dual fuel and dieselof in-cvlinder air and finelwith the same enthalpy calculated using models is shown in Table 2.Fig.中国煤化工vement near the com-As is shown in Table 2, the in-cylinder pressure, temperature=ile that in the center isand concentration before injection of pilot diesel in the case of| YHC N M H Gto longer injection du-dual fuel, is only slightly smaller compared to that of diesel withration, more injecuon amount ana o1gger spray velocity, the in-the same enthalpy. The main reason is that a shock limitation3]fluence of in-cylinder swirl on the spray distribution is smallerexists when burning natural gas, and the largest stoichiomnetic is than when burming dual fuel. Due to Smaller air swirl near theYao Chunde, et al: Mchanism on dsrnbutiono of pilo fuel spray and copessinsg igitionin premixed natural gas engine ignited by piliot dicseinjectors, the atomized fuel acumulates in the long process of ignitonoccustuoe.trniaota Theatomized fuelspreads all over thefuel ijection. Moreover, there is the highest compression tem- process has already tinishec. .h anoumeh coentratinvariation.perature in the center of combustion chamber. Therefor, igition combustion chamber and there is notmu Cooe Dremixedwill occur once there is favorable concentration. In the case ofThrefore, the combustion is similar to homogeneous premixeddual fuel, due to shorter pilot diesel injction process, the mixturecombustion, namely quasi- homogencous pre-mixed combustion.with favorable concentration will not occur near the injector. Tomakethings worse the central part of the lean and slow fuel sprayseriousiy afctedtby the circumferential swirl in the airfield.o.003r- atomied Tueluorropnrsmov cruertenially,y it will fly towards theWhen lhe dropletsberwalldeto cntfigal efet (shown in一- Ineciondurationcombustion chanber wal duc . cept thee0.002and ignition points occur there t00.Tu=725KTew- 830KTψ-1121KTuo=1124 K0.001Th- --20Crank angle a/CA .(a) Pure dieselTyetonizedfuel(b) Dual fuelFig4 In-yinder temperature field when igition occurs- - - Injectionin the cases of pure diesel and dual fuel0. 0o2-un=12.70m/swmn=3.52 m/s0. 901IoCrank angle a/CAFigs In-eylinder swidl speed ficldFig7 Amount of atomized fuel and injction duration(3) The concentaion dstribution, amount of atomized fuel,and injection duration in the cases of dual fuel and diesel are4 CONCLUSIONSshown respectively in Fig.6 and Fig.7. As can be scen, afer thepilot diesei of dual fuel is ignited, the atomized fuel is sattered all(1) Using the pilot diesel spray modeling and improved shellover the combustion chamber, and there is a larger area without ignition modeling, taking into account of the influence of swirl,much difference of concentration, except for the larger concentra- the mixture formation and ignition location in the cases of puretion near the right and lower part of the combustion chamber wall.ieselanddual fuel iefetveley predicted. And it provides ei-While in the case of pure diesel, due to smaller ignition delay,diese and dulftel is fectr poumaiouamu com-larger fuel amount, and higher spray velocity, the fuel spray iscient calculation methods for the optimn8re co withina smaller are, where the mean concenbustion system.causing the lner igion delayofdualfuel .moreconcentatea wehns L.smams asthatinthecaseofdualfuel.are as ollows: the small amount of pilot diesel evenly distributed(2) The factors causing thetration of aonized fue!is IipsmstststersertheiniectennAs can be seen from Fig.7, when burning pure diesel, the injection in the cylinder, lower concentration gradient, lower oxygen con-process lasts for 5.6 CA after ignition points occur. While in the centration, the obstacle imposed by natural gas molecules to pre-case of dual fuel, ignition points occur at 19 CA afer the injee.vent the molecules of atomized pilot diesel from cllision withtion process finishes. It is revealed that in the case of diesel, the oxygen molecules.fuel is buming during the njection process and the combustion (3) Within the shock limitation, the intake concentration de-takes place around the fuel spray. While in the case of dual fuce, when crease and specifie heat increase caused by intake natural gas islimited, therefore it is not the main reason for the longer ignitionPAo=0. 002 4 g/cm'Pun=0, 001 4 g/cm'delay of dual fuelPips =0.022 g/cm'Pip=0.0122 g/cm'(4) The combustion of dual fuel is similar to homogencouscompression combustion. And the ignition location is nearer thecylinder wall than that of pure diescl, which is caused by the lar-ger centrifugal force exerted by the in-cylinder swirl field on thePauatomized fuel droplets in the center of cylinder in the casc of-Pa atomizesmall中国煤化工MHCNMHG1n upuinauou o1 a hyrocarbon fel ignition; fuel before ignitionmodel for two single component surogates of diesel fuel. Combustion andDistritbnalottofieceladdualfeelFlame, 2003, 132(3); 433-450CHINESE JOURNAL OF MECHANICAL ENGINEERING●29●2 YaoC D, Liu Z Y, Cao C Q. Study on HCCI of pre -mixed natural gas ig felds of working process and exhaust emission reduction technology of inter-nal combustion engines, etc.ciety for intermal Combustion Engines), 2003, 21(1): 7~11 (In Chinese)Tel: +6-22-2740435-8010; E-mail: arcdyao@pulctptj.cnFeiS M. Modeling of combustion in. a dual fuel engine. Zhejiang DaxueYao Guangtao, male, bom in 1962, master, vice professor, PhD graduate stu-4 Cheikh Mansour, Abdelhamid Bounif, Abdelkader Aris, et al. Gasdiesel reseurch is mainly in the felds of working process simulation and experimet .(dual-fuel) modeling in diesel engine environment. Intemational Joumal ofstudy of intermal combustion engines. etc."Therm. Sci, 2001, 40 (4);: 409- 424Tel: +86 22 27404335- 8028; E-mail: yy9016@sina .com5 Abd Alla G H, Soliman H A, Badr 0 A, et al Combustion quasi-two zonepredictive model for dual fuel engines. Energy Conversion and Manage- Song Jinou, male, borm in 1965, vice pofessrs PhD. His rssarch is mainly inthe fields of wodking process simulation study of intemal combustion engines, elc.6 Denis Veymante, Luce Vrish Turbulet cobuston modling Prgess in Tel: 686-24245-8004 E-mail: snginouneyouou comEnergy and Combustion Science, 2002, 28 (3): 193-266Wang Yinshan, male, bom in 1966, master, vice professor, PhD graduate stu-Biographical notesdent of Mechanical Engineering Department in Tianjin University, China. HisYao Chunde, male, bom in 1995, PhD, professor. His research is mainly in theTesrearsh saiith felds of sudy on lmatie fuel engines control, etce中国煤化工MYHCNMHG