Effect of RME Fuel on Combustion in Optical Engine Operating in HCCI Mode

Transcription

1 Effect of RME Fuel on Comustion in Opticl Engine Operting in HCCI Mode E. Mncruso nd B. M. Vglieco Istituto Motori - C.N.R., Npoli ITALY 1. Introduction In the lst thirty yers it hs een growing increse of the production nd sle of motor vehicles in the most importnt world community: North Americ, Europe nd Jpn. The high request drives the Stte systems to regulte oth the fuel consumption nd pollutnt emissions of the internl comustion engines. As n exmple, in Europe from the 2009, on Septemer, could e rtified only the vehicles equipped with diesel engines tht fulfil EURO V stndrd emission: mximum concentrtion of oth prticulte mtter (PM) nd nitrogen oxides (NOx) of 0.025g/km [1]. These severe emission limits drive oth the motor vehicles mkers nd the reserchers to ly out new comustion systems, to use synthetic nd io fuels, to optimize the engine system, nd to found new technologicl tools to get the est out of the ctul motor vehicles. One good nd chep solution proposed y the utomotive reserchers is the doption of the homogeneous chrge compression ignition (HCCI) in the production engine. In prticulr, HCCI comustion promises to reduce the PM nd NO x emission from diesel engines without penlize the performnces [2-3]. In the HCCI comustion, homogeneous mixing utoignites in the sme time nd in the whole chmer. Moreover, the comustion develops with low temperture nd urns premixed ir/fuel mixture, thus the production of NOx nd PM is very low. On the other hnd, the gseous unurned emissions, like CO nd HC, increse ecuse the low temperture doesn t fvour their complete oxidtion in the cylinder. The fuel mixing with the ir cn occur either in the intke mnifold or into the cylinder. In this cse the homogeneous mixture formtion is fvoured y the internl flows of the engine like swirl nd tumle motions. In every cse, for oth mixture formtion systems, the min prolem of the HCCI/LTC comustion is the control of the mixture formtion nd utoignition. It hs een oserved tht too rich mixture produces fst comustion, cuses knocking phenomen nd increses the NOx production; insted, too len mixture produces incomplete nd not regulr comustion, HC nd CO emission increses, nd low comustion efficiency. In the HCCI engines the utoignition cn e controlled y the mixture composition nd the temperture in the comustion chmer; then, the comustion development is due to the kinetic of the chemicl rections menly. The comustion process nd the pollutnts formtion depend lso on: the compression rtio, the qulity of fuel, the intke ir temperture, the use of externl nd internl exhust gs recircultion (EGR), nd the oxygen concentrtion. Another solution proposed is the use of iodiesel fuel in order to improve the pollutnts emissions from the engines. The enefits of the iodiesel re well known: higher cetne numer, low sulphur content, reduction of HC nd CO emission, reduction of PM emission, nd reduction of CO 2 net emission on life cycle sis. On the other hnd it hs ny concerns like: mterils comptiility, impct on low-temperture operility, increse in NO x emission, reduction of power nd fuel economy, nd higher cost. For these reson the usge of wste vegetle oils nd niml fts for the production of iodiesel is one of the promising wys to reduce the cost price of iodiesel [4]. The improvement of low temperture chrcteristics is solved y pplying fuel dditives [4]. As result, the multicomponentl iodiesel fuels re creted. The se of new types of iodiesels consists of trditionl types of V-5, 1

2 iodiesel: rpeseed methyl ester (RME), soyen methyl ester (SME), plm-oil methyl ester (PME). The new iocomponents chnge properties of iodiesel nd influence differently the comustion process of diesel engine [4]. The gol of the performed pper ws to evlute the development of the HCCI comustion in n opticlly ccessile diesel engine vrying the injection pressure with oth diesel nd RME fuels. The chemicl species involved like OH, HCO, CO, CH were determined y mens of chemiluminescence mesurements nd UV-visile digitl imging. The pollutnts formtion nd their emission in the exhust were nlyzed nd correlted to OH rdicl tht is the dominnt species in the comustion chmer. 2. Experimentl pprtus nd mesurement procedures 2.1. Opticl engine The opticlly ccessile engine used during experiments ws single cylinder, direct injection, four-stroke, diesel engine, with EURO IV multi vlves production hed. Tle I reports specifictions of the engine. Tle I Engine specifiction The production hed ws designed for the four cylinders engine thus it ws necessry to modify it for the single cylinder reserch engine with prticulr ttention to the wter-cooling nd luricting oil hed ducts. The design of the engine utilizes clssic extended piston with piston crown window (dimeter of 46 mm). The piston crown window provides full view of the comustion owl y locting n pproprite 45 UV-visile mirror inside the extended piston. All windows re mde of UV-grde fused silic. The engine is equipped with Common Ril injection system cple of mximum injection pressure of 1800 r. The CR system consists of rdil three-piston high-pressure pump, which supplies ril common to the cylinders of the engine. The fuel ws injected in the chmer y mens of n injector with solenoid vlve. The injector is locted centrlly nd hs the sme cylinder xis. The nozzle hs seven holes symmetriclly distriuted nd ech hole hs convergent-divergent shpe. A fully flexile Electronic Control Unit (ECU) mnges the injector for comustion optimiztion. The ECU controls the injection pressure, through pressure regultor locted on the pump, nd rrnges the numer of injections (up to 5) per cycle, the strt nd the durtion of injection s well s the dwell time etween the consecutive injections. To nlyse the injection signls, Hll-effect sensor ws pplied to the line of the solenoid current nd piezoelectric pressure trnsducer ws locted in the injection line etween the ril nd the injector. Moreover, to cquire the cylinder pressure in motored nd fired condition, piezoelectric pressure trnsducer ws set in the glow plug set of the engine hed. Opcity nd gseous emissions (HC, CO, CO 2, NO x, nd O 2 ) were mesured with conventionl systems y mens of proes plced in the exhust pipe. The engine is motored nd its speed controlled y 43 kw (58.5 hp) dynmometer. An ir compressor supplies pressurized intke ir tht is heted, dehumidified, nd highly filtered. V-5, 2

3 2.2. Engine operting conditions The engine rn t chrcteristic point of the ECE-EUDC mp with fixed engine speed of 1500 rpm nd using oth commercil diesel fuel s reference (REF) nd rpe seed methyl ester (RME). At motored condition, intke ir temperture nd pressure were set to 312 K nd 1.06 r, respectively. At TDC temperture nd density were 787 K nd 14.8 kg/m 3, respectively, ssuming polytropic coefficient of 1.36 [5]. This operting condition voided knocking nd misfire phenomen during fired condition. In order to relize good homogeneity of the mixture, four erly injections were crried out during the compression stroke. The injections hd very short durtion in order to reduce the mount of wll film fuel tht resulted from the spry wll impingement. Moreover, in order to equlize the ril pressure (P ril ) t the strt of injection (SOI) nd to perform uniform injections, high dwell time (Δt) etween the injections ws set. In tle II the investigted strtegies re reported for REF nd RME. At fixed injection pressure ll the injections hd the sme durtion clled energizing time (ET). Incresing the injection pressure, the durtion ws chnged in order to inject the sme fuel mount. Due to the low energy content of the RME fuel, the injection durtions were vried in order to rech the sme rek men effective pressure (BMEP=1.6 r). Tle II Engine operting conditions for diesel (REF) nd rpe seed methyl ester (RME) fuel Opticl pprtus Fig. 1 Opticl engine nd experimentl pprtus for spectroscopic nd imging mesurements Imging nd chemiluminescence mesurements from ultrviolet (UV) to visile were performed y mens of the opticl set up shown in figure 1. In prticulr, two different CCD cmers, with different cpility, were used. The first (ICCD) hd high sensiility oth in the UV nd visile rnge. It ws intensified nd hd chrge coupled device with mtrix of 512 x 512 pixels nd with every pixel size of 24 µm x 24 µm. The second one (CCD) ws sensile in the visile nd hd mtrix of 640 x 480 pixels with every pixel size of 9.9 µm x 9.9 µm. Imges were recorded through the piston crown window nd 45 mirror, plced in the elongted piston. CCD cmer equipped with 50 mm f/5.6 ojective ws used to investigte the temporl nd sptil evolution of visile comustion due to its etter sptil resolution. V-5, 3

4 Two color pyrometry method ws crried out y using n d hoc filter. On the other side, in order to detect the light emitted during the first stges of comustion due to the ultrviolet light, the intensified CCD ws used, nd it ws coupled with n UV ojective (Nikon 78 mm f/3.8) nd filter t 310 nm with ndwidth of 10 nm. Synchroniztion of engine with ICCD nd CCD cmer ws otined y the unit dely connected to the engine shft encoder. Spectr nd imges were cquired in sets of 10 or 20, from 10 or 20 seprte comustion cycles. Spectr nd imge reported were selected in order to e more representtive of their respective set. 3 Results nd discussion Fig. 2 Histories of cylinder pressure, rte of het relese nd drive injector current for HCCI comustion process: ) diesel reference (REF); ) rpe seed methyl ester (RME) fuel In figures 2 nd 2, the cylinder pressures nd the rte of het relese curves vrying the injection pressures re reported for reference diesel fuel nd rpe seed methyl ester, respectively. In oth figures, the drive injector current is referred to the strtegy with 400 r injection pressure in order to void the overlp of the signls nd to hve confused figure. The other current signls hve the sme strt of injection nd shorter durtions s reported in tles II. It cn e noted tht the typicl rte of het relese for HCCI comustion hs two well resolvle peks not correlted with the injections. The first is chrcteristic of low temperture rections tht occur in the chmer t utoignition; nd the second one is due to the development of high temperture rections [3]. In figure 2 nd 2, no significnt vrition in comustion pressure development is noted vrying the injection pressure. A smll vrition is oserved in the comustion pressures intensity pek for RME fuel (fig.2), due to the little mount of injected fuel in order to hve the sme strt of comustion nd the sme BMEP of the REF fuel. HCCI comustion ignites homogeneous len chrge over the entire comustion chmer producing very low soot concentrtion. The smll mount oserved is due to prt of fuel impinging on the cylinder nd piston wlls. Also the NO x concentrtion is very low t the exhust ecuse the temperture in the chmer is low. In figures 3 nd 3, the exhust emissions for the investigted strtegies re reported. Incresing the injection pressures, oth the PM mount nd the NO x concentrtion decrese due to the improving tomiztion, nd etter vporiztion nd homogeniztion s oserved y imging mesurements crried out in the comustion chmer. Similr trends re oserved for RME fuel where further reduction of the PM mount nd HC concentrtion occurs s detected y other uthors [4]. The O 2 nd CO 2 concentrtions re constnts nd equls to the vlues mesured for the REF fuel. V-5, 4

5 Fig. 3 Exhust pollutnt emissions: ) diesel reference (REF) fuel ) nd rpe seed methyl ester (RME) fuel In order to etter understnd the production of these pollutnt emissions, opticl dignostics consisting in UV nd visile imging were performed. By the nlysis of the nturl flme emission pplying two color pyrometry methods it is possile to mesure the soot temperture nd mss concentrtion in the comustion chmer. By mens of the UV imging with d hoc filter t 310 nm it is possile to detect the nturl emission of the OH rdicl during the comustion evolution. In prticulr, s reported in previous pper [3] this rdicl ws firstly detected y spectroscopic nlysis nd ws identified during the HCCI comustion s good mrker of the premixed comustion nd the soot oxidtion nd NO x formtion process. Fig. 4 Soot KL fctor: ) diesel reference (REF); fuel ) rpe seed methyl ester (RME) fuel In figures 4 nd 4 the integrl soot KL fctors detected in the comustion chmer re reported. They re computed y the visile emission intensities nd re proportionl to the soot mss concentrtions. In figures 4 nd 4 the integrl KL fctors for oth fuels hve durtions longer thn the rte of the het relese reported in figure 2. In prticulr, the integrl KL fctor using REF fuel increses quickly from the strt of the comustion nd depends on the injection pressures. As it cn e noted the KL fctor t 400 r injection pressures hs the highest vlue nd rte nd reches the mximum vlue round 1100 μs. Moreover, it decreses in the chmer incresing the injection pressure nd this result is in good greement with the PM concentrtions detected in the exhust. Similr trends nd durtions re oserved in figure 4 for the soot evolution produced y the operting conditions urning RME fuel. In these cses the detected mss concentrtions re lower thn the previous ones. Also these results re in good greement with those reported in figure 3. V-5, 5

6 Fig. 5 Integrl OH concentrtion: ) diesel reference (REF) fuel ) rpe seed methyl ester (RME) fuel In figures 5 nd 5, the integrl OH concentrtions re reported for oth fuels investigted. In figure 5 the OH reches the mximum vlue with the sme rte tht does not depend on injection pressures. On the other hnd, the intensities decrese incresing the injection pressure. Considering the trends of fig. 4 this indictes tht the OH rdicl prticiptes to the oxidtion of the soot produced during the comustion [5]. Moreover, it my e responsile of the NO formtion in the chmer [5] ecuse it results higher in the engine condition t higher NOx exhust emission. In figure 5, the OH intensities for the RME cses result lower nd shorter thn previous ones showing the sme trends. In this cse, the oxygen content of the fuel contriutes strongly to the reduction of the soot out 23% nd produces lso the reduction of HC emission with respect the REF fuel. 4 Conclusion HCCI comustion strtegy t 1500 rpm engine speed vrying the injection pressures were investigted in n opticlly ccessile diesel engine y mens of UV-visile imging spectroscopy. RME nd diesel commercil fuel were tested. The in-cylinder mesurements nd exhust emission were correlted. Soot nd OH concentrtion were evluted in the comustion chmer. The positive effect in the use of RME fuel for the reduction of pollutnt emission ws oserved. In prticulr, the use of RME fuel decreses further the PM nd NO x concentrtions oth in chmer nd exhust with respect to those of REF fuel. The OH ehviour in chmer ws strictly correlted to formtion-oxidtion of PM-NO x. 5 References 1 Diesel Net - Emission stndrds: 2 Assnis, D. N., Njt, P. M., Dec J. E., Eng J. A., Asmus T. N., Zho F.: Homogeneous Chrge Compression Ignition Engines. SAE Interntionl Wrrendle, PA (2003). 3 Mncruso, E., Merol, S. S., Vglieco, B. M.: SAE pper: , (2007). 4. Leedevs, S., Vicekusks, A., Suškov, P.: Trnsport 2007, Vol XXII, No 2, , (2007). 5 Heywood, J. B., Internl Comustion Engine Fundmentls, Mc Grw-Hill, NewYork, (1988). V-5, 6