Modeling Low-Pressure Injections in Diesel HCCI Engines

Transcription

1 Modelng Low-Pressure Injectons n Desel HCCI Engnes Yong Sun * and Rolf D. Retz Engne Research Center, Department of Mechancal Engneerng Unversty of Wsconsn-Madson Madson, WI USA Abstract Homogeneous Charge Compresson Ignton (HCCI) combuston s beng consdered as an alternatve to conventonal engne combuston systems due to ts hgh effcency and low engne-out emssons. To prepare a homogeneous mxture for desel HCCI combuston, two types of low pressure (5MPa~2MPa) njectors were consdered: a swrl njector and a mult-hole njector. A modfed verson of the KIVA-3V R2 code, was used to smulate the two types of njectons. The Kelvn-Helmholtz and Raylegh-Taylor (KH-RT) hybrd breakup model, whch s often used to smulate droplet breakup processes of hgh-pressure (5MPa~3MPa) desel njectons, was recalbrated and extended for low-pressure, mult-hole njecton applcatons. Two technques were used to mprove the predcton of spray behavor: use of an ndependent collson mesh wth random rotaton, and couplng the gas and lqud phases usng polar nterpolaton. The numercal models were valdated by comparng smulaton results wth experments under dfferent condtons. The smulaton results show that the spray structure of the swrl nozzle njecton s senstve to the ntake flow feld and n-cylnder gas densty, whle the spray structure of a mult-hole nozzle njecton s less nfluenced by the n-cylnder flow and gas densty. The smulaton results also show that swrl njectors are more sutable for low ambent pressure (<.3MPa) condtons because at hgh pressures (>.3MPa), the hollow-cone spray collapses nto a sold-cone spray. Mult-hole njectors are more sutable for hgh ambent pressure condtons because at low pressures, the spray penetraton s too long, whch can cause spray-wall mpngement. * Correspondng author

2 Introducton Future desel engne technologes wll need to ncorporate advanced combuston strateges for achevng low emssons whle mantanng good fuel economy and power densty. It wll be necessary to operate seamlessly over broad load and speed ranges when condtons change between dfferent combuston regmes. HCCI combuston s beng consdered as an alternatve to conventonal engne combuston systems. It has the potental to elmnate noxous engne-out emssons whle producng hgher engne effcences. To prepare a homogeneous mxture, fuel should be njected early nto the cylnder to allow enough tme for fuel/ar mxng. However, when drect njecton of fuel nto the cylnder durng the ntake or early compresson stroke s used for mxture preparaton, the use of conventonal hgh pressure common-ral njecton systems s lmted by the relatvely low n-cylnder gas densty due to spray mpngement on the cylnder walls [1]. Too much wall mpngement can not only deterorate the qualty of the charge mxture, ncrease fuel consumpton and unburned hydrocarbon emssons, but also lead to lubrcaton ol contamnaton. Therefore, t s of nterest to consder low-pressure njecton systems as an alternatve. In ths study, the KIVA-3V Release2 code [2] wth mproved numercal models was used to explore the charge preparaton n a desel HCCI engne usng two types of low-pressure njectors: a swrl njector and a mult-hole njector. Several numercal models n KIVA were updated and the mproved models were valdated by comparng smulaton results to experments. An nhomogenety concept s also proposed to evaluate the effects of njector type and SOI tmng on desel HCCI charge preparaton. Numercal Approach The CFD code used n the smulatons was a verson of the KIVA-3V Release2 code wth mprovements n varous physcal and chemstry models developed at the Engne Research Center, Unversty of Wsconsn-Madson. The RNG k-ε model [3] was used for the ncylnder turbulence smulaton. For hollow-cone sprays from swrl njectors, the Lnearzed Instablty Sheet Atomzaton (LISA) breakup model [4] was used to calculate the prmary breakup process of the fuel droplets, and the Taylor Analogy Breakup (TAB) model [5] was used for the secondary breakup calculaton. For low-pressure mult-hole sprays, the KH-RT hybrd breakup model, whch s used to smulate the droplet breakup process of hgh-pressure desel njectons, was recalbrated and extended for low-pressure njecton applcatons An advanced unsteady vaporzaton model [6] was appled to predct the droplet evaporaton process. A droplet collson model based on the stochastc partcle method [7] was used to calculate the droplet collson and coalescence. The effects assocated wth spray/wall nteractons ncludng droplet splash, flm spreadng due to mpngement forces and moton due to flm nerta were consdered n the wall-flm model [8]. Engne Specfcatons and Operaton Condtons The engne consdered was a Caterpllar 341E sngle cylnder ol test engne (SCOTE). The specfcatons of the engne are lsted n Table 1. Engne type Caterpllar 341E Bore Stroke (mm) Engne valve tmng ( CA)* EVC=-355, IVC=-143, EVO= 13, IVO= 335 Swrl Rato 1. Table 1. Engne specfcatons * In ths paper, CA means CA ATDC Sx engne modes have been proposed to smulate the Federal Test Protocol (FTP) cycle [9]. Mode 2 (25% load, 821rev/mn, naturally asprated) and Mode 5 (57% load, 1737rev/mn, boosted) were selected for the study. Two types of njectors were consdered n ths study. The swrl njector was a Semens hollow-cone spray njector wth 9 spray cone angle and 25 mcron nozzle dameter. The mult-hole njector was a Bosch 6-hole sold-cone spray njector wth 9 ncluded angle and 17 mcron nozzle dameter. 1MPa njecton pressure was used for both njectors. Hollow-Cone Spray at Hgh Ambent Pressure One way to prepare a homogeneous mxture s to nject the fuel early nto the cylnder usng a lowpressure hollow-cone spray njector. However, t was found that an unrealstc spray structure was predcted usng KIVA at hgh ambent pressure (.3MPa) usng a Cartesan mesh, as shown n Fgure 1(a). To understand the reasons, for smplcty, a hollow-cone njecton nto a quescent hgh-pressure atmosphere was tested usng a regular Cartesan mesh. In ths case, a four-leaf clover spray was predcted as shown n Fgure 1(b). However, ths problem does not appear n a cylndrcal mesh or wth njectons at atmospherc pressure. Ths s due to the grd dependency of the O Rourke collson model, and also due to the gas and lqud phase couplng method used n KIVA, accordng to Nordn, Schmdt and other researchers [1, 11]. Ths problem was solved usng a combnaton of technques also proposed by these researchers: use of a cylndrcal collson mesh and by couplng the gas and lqud phases usng polar nterpolaton. These technques were apopted n ths study and the detals are descrbed as follows.

3 (a) (b) Fgure 1. Unrealstc structures of hollow-cone sprays at hgh ambent pressure (.3MPa) n Cartesan mesh computatons usng KIVA Cylndrcal collson mesh wth random rotaton The four-leaf clover spray structure shown n Fgure 1(b) was found to be manly due to the mesh dependency of the O Rourke collson model [7] used n KIVA. O Rourke assumes that the probablty of a droplet,, colldng wth any other droplet, j, s gven by: σ, jv, j t p, j= (1) V Only droplet parcels located n the same computatonal cell are allowed to collde, and the ncdence of collson s proportonal to the relatve velocty v,,j between the par parcels, as shown n Eq. (1). Those parcels whch have veloctes perpendcular to each other have the hghest probablty to collde f the nozzle locaton concdes wth a cell node. Ths s llustrated n Fgure 2, where parcel d s not n the same computatonal cell as parcel a and b, so t s not allowed to collde wth them. Parcels a and b are n the same cell and ther veloctes are perpendcular, so they have the hghest probablty to collde and f coalescence occurs, the resultng parcel c moves n the drecton shown n the fgure, accordng momentum conservaton. Ths explans why the four-leaf clover structure s observed. At low ambent pressure, such as atmospherc pressure, the problem stll exsts. However, t s not so notceable. Ths s probably because at low pressure, the spray s not as dense as t s at hgh pressure, so droplet collson doesn t occur as frequently as at hgh pressure. In a cylndrcal mesh, the problem should also exst and an n-leaf clover structure s predcted. However, when there s enough resoluton n the azmuthal drecton (.e., n s bg enough), the problem also becomes unnotceable even at hgh ambent pressure. Therefore, a pseudo cylndrcal collson mesh, whch s coaxal wth the spray and ndependent of the KIVA mesh, was used. An example of the collson mesh s shown n Fgure 3 [11]. The cylndrcal collson mesh s based on a cylndrcal coordnate system that s coaxal wth the spray. It s called a pseudo mesh because the mesh s not actually generated n the code. The mesh s establshed only hypothetcally, and each cell can be dentfed by an dentfcaton array (, j, k), where, j, and k are ntegers that correspond to the three cylndrcal coordnates. Usng the pseudo mesh concept, not only s computer memory saved, but also the programmng effort and computatonal tme s reduced. Another mert of usng ths concept s that the mesh has no boundary and t s not needed to calculate the extent that the spray reaches every tme step. To further reduce non-axsymmetry of the predcted spray structure, the collson mesh was rotated at a random angle around the spray axs every tme step. Smlar to the standard O Rourke collson model, only parcels n the same collson cell, nstead of a KIVA cell, are allowed to collde. Use of a randomly rotated cylndrcal collson mesh sgnfcantly mproves the spatal resoluton of the spray and reduces grd dependency. However, snce the method s based on the assumpton that there s only one spray, t must be refned for multple sprays. Indeed, for multple sprays the method has been found to work better than computatons performed wthout an ndependent collson mesh. Fgure 2. Parcels whch have veloctes perpendcular to each other n the same njecton cell have the hghest probablty to collde f the nozzle s located on a cell node Fgure 3. A cylndrcal collson mesh s used that s ndependent of the Cartesan gas phase mesh [11]

4 Gas and lqud phase couplng polar nterpolaton After ntroducng the randomly rotated cylndrcal collson mesh, the results were found to be greatly mproved. However, there was stll some nonaxsymmetry predcted n the spray structure. The remanng grd dependency was found to be manly due to numercal errors assocated wth the gas and lqud phase couplng method used n KIVA [1, 11]. Therefore, an nterpolaton method was ntroduced to mprove the accuracy of the calculaton. The goal of the nterpolaton s to calculate the momentum transfer between the gas and lqud phases more accurately by provdng more accurate gas phase veloctes at the pont where the parcel s located. In KIVA, the gas phase velocty of the cell node nearest to a parcel s smply taken as the gas phase velocty at the pont where the parcel s located, and t s used to calculate the relatve velocty between the gas and lqud phases. Ths s a smple but potentally naccurate method. A better approach s to take a weghted average (nterpolaton) of the gas phase veloctes from all eght nodes of the gas phase cell n whch the droplet resdes, as shown n Eq. (2) [1, 11]. The veloctes are weghted by the nverse of the dstance from the nodes to the drop locaton rased to the power n, where n was selected to be 3 (mass weghtng): V = 8 = 1 8 Vr = 1 n r n (2) Ths velocty s used to calculate the momentum transfer from the gas phase to the lqud phase. The same weghtngs are used to dstrbute the momentum transfer from the lqud phase to the gas phase (source terms). Ths nterpolaton can be done n a Cartesan coordnate system,.e., the weghted averages of the Cartesan components of the veloctes are calculated. However, snce the spray s polar n nature, t s better to use polar nterpolaton [11] nstead of Cartesan nterpolaton. In the polar nterpolaton method, a cylndrcal coordnate system coaxal wth the spray s establshed frst. Cartesan components of the veloctes of the eght nodes are transformed to the correspondng polar components. Then the weghted averages of the eght nodes polar veloctes are calculated. Fnally, the averaged polar veloctes are transformed back to Cartesan veloctes, whch are the Cartesan components of the gas phase velocty at the droplet locaton. The advantage of usng polar nterpolaton over Cartesan nterpolaton can be llustrated usng Fgure 4. For smplcty, a 2-D mesh s depcted. The left bottom spot represents the spray axs of symmetry and the upper spot represents a droplet parcel. Consderng a spray njected nto a quescent atmosphere, snce the spray s axsymmetrc, the gas phase entraned velocty at any locaton should pont at the spray axs, as shown n Fgure 4. Usng Cartesan nterpolaton, the resultng velocty may not pont at the axs, as the arrow b shows. However, usng polar nterpolaton, all the nterpolated veloctes have no tangental component, whch guarantees that the resultng velocty also has no tangental component,.e., the resultng velocty also ponts at the axs, as the arrow a shows. Snce polar nterpolaton represents the nature of the spray better than Cartesan nterpolaton, t s suggested and used n ths study. Fgure 4. Advantage of usng polar nterpolaton over Cartesan nterpolaton (a: polar nterpolaton result. b: Cartesan nterpolaton result) Improved results Smulaton results usng the combnaton of the above two technques: collson mesh and polar nterpolaton, are shown n Fgure 5. Compared to the results n Fgure 1, sgnfcant mprovements were acheved usng these technques, and the results are seen to be approxmately axsymmetrc and physcally reasonable. (a) (b) Fgure 5. Structures of hollow-cone sprays at hgh ambent pressure (.3MPa) n Cartesan meshes usng KIVA wth collson mesh and polar nterpolaton.

5 Low-Pressure Mult-Hole Injecton Low-pressure mult-hole njectors have been manly developed for use on Gasolne Drect Injecton (GDI) engnes to replace swrl njectors as spray-guded GDI combuston systems are recevng more attenton than wall-guded GDI combuston systems. One major advantage of the mult-hole njector over a swrl njector s that the spray does not collapse at hgh ambent pressure condtons, as does the swrl spray. Model Descrpton Low-pressure mult-hole njecton has a smlar spray pattern as compared to conventonal hghpressure desel njecton sprays. The man dfference s the fuel njecton pressure, whch s relatvely low (5~2MPa) compared to conventonal desel commonral njectons (5~3MPa). Therefore, the atomzaton processes are dfferent for these two types of njectons and dfferent droplet breakup models must be potentally appled. In conventonal desel njecton, the hgh njecton pressure leads to hgh droplet veloctes and therefore ncluson of RT acceleratve nstabltes s necessary. In hgh-pressure njecton, most of the droplet breakup occurs n the catastrophc breakup regme (We>1) and breakup models based on a competton between KH and RT nstabltes should be consdered. Whle for low-pressure njecton, RT nstablty s not as mportant because of the low njecton veloctes and acceleratons. Most breakup occurs n the strppng regme (1<We<1) where KH nstablty plays the major role n droplet breakup [12]. Accordngly, the KH-RT hybrd breakup model, whch has been used for hgh speed jets, was recalbrated to extend ts applcaton to low speed jets n the present sutdy. Though RT nstablty s stll ncluded n the model, t was observed that t rarely occurs n low speed njectons, whch s consstent wth the argument above. Therefore, only the KH breakup model constants were recalbrated to extend the applcaton of the KH model from hgh speed jets to low speed jets, and to match Nauwerck and Mtroglou s expermental data [13, 14]. The wave length constant of the KH model was recalbrated to be.2 (vs..6 for hgh speed jets), and the breakup tme constant was recalbrated to be 15 (vs. 4 for hgh speed jets). The polar nterpolaton technque was also found to be mportant for low-pressure mult-hole njecton smulatons. Fgure 6 shows mprovements of the predcton of the spray structure of low-pressure mult-hole njectons after usng the polar nterpolaton technque. Unrealstc spray structures (shown crcled n Fgure 6(a)) are elmnated by usng polar nterpolaton, as shown n Fgure 6(b). At hgh ambent pressures, a sold-cone spray s much denser and collson occurs more frequently than at low ambent pressures. In a cylndrcal collson mesh, the collson mesh sze around the spray axs can be too small, whch prevents collsons from occurrng and ths causes an under-predcton of SMD. A Radus of Influence (ROI) method [15] was used to prevent ths problem. The radus of nfluence for the collson calculaton was chosen to be 2mm. In the ROI method, one parcel s allowed to collde wth another only f ths parcel resdes wthn the radus of nfluence of the other one. Ths ensures that the effectve collson mesh sze remans adequate. (a) Wthout polar nterpolaton (b) Wth polar nterpolaton Fgure 6. Comparson of structures of low-pressure mult-hole sprays n engne smulatons wthout and wth polar nterpolaton (Mode 2, 3 CA ASOI, SOI=- 8 CA) Model valdaton Snce several models were updated, the new code was valdated usng the expermental data of Sun [16] and Mtroglou [13] and Nauwerck [14]. Model valdaton for swrl njecton Smulaton results of hollow-cone sprays at dfferent tmes after the Start-of-Injecton (SOI) (1, 2, 3, 4, and 5 ms), under dfferent ambent pressures (.1,.3,.9 and 1.1 MPa) and usng dfferent njecton pressures (4.3, 5.3 and 7.7 MPa) are compared wth the experments n Fgure 7. It can be seen that both the spray structures and the spray tp penetratons predcted by the smulatons match the experments very well at the dfferent ambent and njecton pressures. Both experment and smulaton shows that, as the ambent pressure ncreases, the spray collapses, and the spray angle and the spray penetraton decrease. As the njecton pressure ncreases, the spray becomes denser, the spray angle decreases and the spray penetraton ncreases. Wth ths valdaton by the experments, t was determned that the code can be used n swrl njecton smulatons wth more confdence.

6 (a) P amb =.1MPa, P nj =5.3MPa (c) P amb =.9MPa, P nj =5.3MPa (b) P amb =.3MPa, P nj =5.3MPa (d) P amb =1.1MPa, P nj =5.3MPa

7 (e) P amb =.1MPa, P nj =4.3MPa Model valdaton for mult-hole njecton Fgures 8 to 1 show the correspondng valdaton results of the low-pressure mult-hole spray model. Fgure 8 shows a comparson of SMD between experments [14] and smulatons at dfferent njecton pressures. It can be seen that both experments and smulatons show decreased drop sze wth ncreased njecton pressure. The predcted SMD matches the experments very well at the dfferent njecton pressures. Fgure 9 shows a comparson of the lqud spray tp penetraton length between the experments [13] and smulatons at dfferent njecton and ambent pressures. Fgure 1 compares the spray structures of the cases shown n Fgure 9. The results show that not only are the spray structures well predcted under the dfferent condtons, but the smulated spray penetraton also matches the experments very well. It can be seen that as the njecton pressure ncreases, the spray penetraton also ncreases due to the ncreased spray momentum. Unlke swrl njecton sprays, whch collapse at hgh ambent pressures, the spray structure of a mult-hole nozzle njecton almost does not change and only the spray penetraton s reduced sgnfcantly wth ncreased ambent pressure. Ths s a favorable feature for desel HCCI operaton, because spray-wall mpngement mght be reduced or avoded by operatng the engne under boosted condtons, or by njectng the fuel durng the compresson stroke. From the results shown n Fgures 8 to 1, t can be concluded that the recalbrated KH model predcts the behavor of low-pressure mult-hole nozzle njectons very well under a wde range of operatng condtons Experments Smulaton SMD [µm] (f) P amb =.1MPa, P nj =7.7MPa Fgure 7. Comparson of hollow-cone sprays between experments and smulatons under dfferent njecton and ambent pressures Injecton pressure [MPa] Fgure 8. Comparson of SMD between experments and smulatons at dfferent njecton pressures

8 Lqud penetraton [cm] p12p1sm p12p12sm p2p1sm p2p12sm p12p1expt p12p12expt p2p1expt p2p12expt.5ms 1.5ms 2.5ms ASOI [ms] Fgure 9. Comparson of lqud length between experments (expt) and smulatons (sm) at dfferent njecton and ambent pressures (p12p1 s njecton pressure 12MPa, ambent gas pressure.1mpa, and so on).5ms 1.5ms 2.5ms 4.ms (a) P amb =1.2MPa, P nj =2MPa.5ms 1.5ms 2.5ms 4.ms (b) P amb =.1MPa, P nj =2MPa (c) P amb =.1MPa, P nj =12MPa Fgure 1. Comparson of mult-hole sprays between experments and smulatons under dfferent njecton and ambent pressures Inhomogenety An nhomogenety concept was proposed to evaluate the qualty of fuel/ar mxtures n the engne smulatons. In ths case, the fuel chemstry s deactvated,.e., only fuel spray evaporaton and mxng s consdered. The calculaton ends at -1 CA, whch s close to when gnton normally occurs n HCCI engne combuston. At -1 CA, the equvalence rato n each computatonal cell s calculated usng the concentraton of C, H and O atoms; as 2[ C] +.5[ H] Φ= [ O] (3) Then the statstcal mean equvalence rato n the whole computatonal doman s calculated usng the gas phase mass n each cell as a weghtng factor Φ= # cells # cells Φ δ m δ m (4) The standard devaton of the equvalence rato n the computatonal doman s defned as SD = # cells 4.ms 2 ( Φ Φ) δ m # cells δ m (5)

9 Fnally, the normalzed standard devaton of the equvalence rato, whch s called nhomogenety, s calculated by SD NSD = Φ (6) The nhomogenety value (NSD) s thus an ndcator of the mxture qualty: the hgher the nhomogenety, the less homogeneous the mxture. Snce fuel chemstry s not consdered, computer tme s greatly reduced and the approach can be used for mxture preparaton optmzaton, especally for HCCI combuston, snce, to acheve HCCI combuston, charge nhomogenetes should be mnmzed. Accordng to the authors prevous experence of usng the nhomogenety defnton, when ts value reaches below about.25, an adequately homogeneous mxture s acheved n the cylnder and HCCI combuston s optmzed. SOI Sweep Study Results and Dscusson The performance of both the swrl njector and the mult-hole njector was evaluated at the Mode 5 and Mode 2 operatng condtons through several SOI tmng sweep studes. Only sngle njectons were consdered n ths study. SOI sweep at Mode 5 Mode 5 s a boosted, hgh-speed and medum-load engne operatng mode. The n-cylnder pressure at the SOI and the EOI for both njectors s plotted n Fgure 11 as a functon of the SOI tmng. In-cylnder pressure at SOI [bar] SOI [Deg. ATDC] Fgure 11. In-cylnder pressure at the SOI and the EOI at Mode In-cylnder pressure at EOI [bar] 1. SOI sweep of swrl njecton The effects of SOI tmng on desel HCCI charge preparaton were nvestgated at Mode 5 usng the swrl njector. The SOI tmng was vared from -33ºCA to - 12ºCA. Wall-flm fuel fracton [%] Inhomogenety (NSD) SOI [Deg. ATDC] Fgure 12. Wall-flm fuel amount and nhomogenety as a functon of SOI tmng usng the swrl njector at Mode 5 Fgure 12 shows the wall-flm fuel fracton and nhomogenety as a functon of SOI tmng. The wall-flm fuel fracton was defned as drops lqud that have mpnged on wall surfaces and are located n wall cells, dvded by the total amount of fuel njected. Both wallflm fuel fracton and nhomogenety were calculated at -1ºCA. From Fgure 12, both varables are seen to decrease as the SOI tmng s retarded from ntake TDC, to reach a mnmum value at SOI tmngs around BDC, and to ncrease agan wth further delay of the SOI tmng. There s thus an optmum SOI tmng around BDC for the mnmzaton of spray-wall wettng and charge nhomogenety. When the fuel s njected durng the early ntake stroke, as shown n Fgure 13(a), the ntake flow s very strong and the spray s blown off course, and hts the cylnder lner. As the SOI tmng s delayed to around BDC, as shown n Fgure 13(b), the ntake flow s not so strong because the valves are about to close and the pston velocty s close to zero. The spray s less deflected by the ntake flow. The mnmum nhomogenety s acheved and there s relatvely lttle wall-flm fuel (3.3%). However, the nhomogenety (.81) s stll well above the desred value (.25) for HCCI combuston. When the fuel s njected durng the compresson stroke, as shown n Fgure 13(c), though the ntake flow effects are mnmal, the spray collapses due to the hgher gas densty caused by the pston compresson and fnally the spray hts the pston. The collapsng spray also confnes the fuel droplets n a small spatal range n the cylnder, whch deterorates the fuel/ar mxng. From Fgures 11 to 13, t can be seen that when the ambent pressure s hgher than about.3mpa, apparent collapse of a swrl nozzle spray can be observed, whch leads to spray-wall mpngement and deteroraton of fuel/ar mxng. Therefore, swrl njectors are

10 only sutable for low ambent pressure condtons (<.3MPa). (a) Crank angle: -23ºCA, SOI: -27ºCA (b) Crank angle: -14ºCA, SOI: -18ºCA and -14ºCA wth the mxture nhomogenety close to the suggested value (.25). Fgure 15 shows the structure of the sprays wth the dfferent SOI tmngs. Unlke for swrl njecton, multhole njecton does not have a deflected spray when the SOI occurs n the ntake stroke (Fgure 15(a)) and the spray does not collapse when the SOI occurs n the compresson stroke (Fgure 15(c)). The spray structure s almost preserved at the dfferent SOI tmngs, though the spray penetraton of the dfferent spray plumes s not the same due to the nfluence of the ntake flow, as depcted n Fgure 15(a). Due to the tumble moton whch s formed durng the ntake stroke, the spray plume on the left of the mage has a longer tp penetraton than the one on the rght. However, the tumble gradually dsappears durng the compresson stroke. Therefore, the effect becomes less obvous for fuel njecton durng the compresson stroke (Fgure 15(c)). Durng the ntake stroke, both the n-cylnder pressure and the spray structure does not vary much, and therefore the wall-flm fuel amount does not change much. Durng the compresson stroke, as the SOI tmng s retarded, the n-cylnder pressure ncreases very quckly (Fgure 11), and the spray penetraton s reduced, so the wall-flm fuel amount decreases very quckly. When the SOI tmng s later than -12 ºCA, the n-cylnder pressure s hgher than.3mpa durng the entre njecton process and the wall-flm fuel amount becomes neglgble. From the above dscusson, mult-hole njectors are sutable for njectons under hgh ambent pressure (>.3MPa) condtons. Comparng Fgure 12 and Fgure 14, t can also be seen that, although the wall-fuel amount s comparable for the two njectors, the nhomogenetes of the mxture generated by the mult-hole njector s sgnfcantly lower than that generated by the swrl njector due to the avodance of a collapsng spray. 3.9 (c) Crank angle: -6ºCA, SOI: -12ºCA Fgure 13. Spray structures of swrl njectons at Mode 5 2. SOI sweep of mult-hole njecton A smlar SOI tmng sweep study was conducted usng the mult-hole njector at Mode 5 and the results are shown n Fgures 14 and 15. From Fgure 14, when the SOI tmng s before BDC, the wall-flm fuel amount does not change much. As the SOI tmng s delayed n the compresson stroke, the wall-flm fuel amount decreases very quckly and the nhomogenety frst decreases and then ncreases agan. The optmum SOI tmng s between -12ºCA Wall-flm fuel fracton [%] Inhomogenety (NSD) SOI [Deg. ATDC] Fgure 14. Wall-flm fuel amount and nhomogenety as a functon of SOI tmng usng the mult-hole njector at Mode 5

11 (a) Crank angle: -21ºCA, SOI: -27ºCA (Tumble flow s depcted wth the arrow) (b) Crank angle: -13ºCA, SOI: -18ºCA c) Crank angle: -6ºCA, SOI: -12ºCA Fgure 15. Spray structures of mult-hole njectons at Mode 5 SOI sweep at Mode 2 Mode 2 s a naturally asprated, low-speed and low-load engne operatng mode. The n-cylnder pressures at the SOI and the EOI for both njectors are plotted n Fgure 16 as a functon of SOI tmng. Smlar SOI sweep studes were conducted usng both njectors at Mode 2 as at Mode 5. However, only SOI tmngs after the stock engne IVC were consdered at Mode 2 because the duraton-of-njecton (DOI) at Mode 2 s sgnfcantly lower than that at Mode 5 (n crank angle). In-cylnder pressure at SOI [bar] SOI [Deg. ATDC] Fgure 16. In-cylnder gas pressure at the SOI and the EOI at Mode In-cylnder pressure at EOI [bar] 1. SOI sweep of swrl njecton A 2-D mesh was used to save computer tme snce the nfluence of the flow feld has been found to be small at these condtons. The SOI sweep results are shown n Fgure 17 and 18. As expected, when the SOI s later than -11 ºCA and the n-cylnder pressure reaches above.3mpa before the EOI (see Fgure 16), the spray collapses, as shown n Fgure 18(b), and the spray-wall mpngement amount and nhomogenety ncreases very quckly. An SOI around -12 ºCA seems to be the optmum wth almost no wall-flm fuel and wth an nhomogenety only about.3. From Fgure 18(a), t can be seen that the ar entranment effect assocated wth vertex at the spray tp helps the swrl njecton to reduce the spray penetraton. These results further prove the concluson that swrl njectors are sutable only for low ambent pressure condtons (<.3MPa). Wall-flm fuel fracton [%] Inhomogenety SOI [Deg. ATDC] Fgure 17. Wall-flm fuel amount and nhomogenety as a functon of SOI tmng usng the swrl njector at Mode 2

12 2. SOI sweep of mult-hole njecton A 6º sector mesh was used n ths study consderng the fact that the nozzle had sx holes. Fgures 19 and 2 show the predcted SOI tmng sweep results usng the mult-hole njector at Mode 2. As can be seen, an optmum njecton tmng was found to be around - 8 CA, where both the wall-flm fuel amount and nhomogenety was relatvely low. However, from Fgure 19, t can be seen that even the lowest wall-flm fuel amount s above 3%. Wth relatvely early njecton, the spray hts the cylnder lner (Fgure 2(a)) due to the low gas densty. As the SOI tmng s delayed, the spray penetraton s reduced due to the ncreased n-cylnder pressure. As the SOI tmng s later than -8 CA, the n-cylnder pressure durng njecton becomes hgher than.3mpa. However, wth the delayed SOI, the maxmum allowed spray penetraton wthout wall-wettng s also decreased because the spray starts httng the pston surface (Fgure 2(b) and 2(c)). That s why the wall-flm fuel amount does not change much wth SOI tmngs later than -1 CA, whle t mght be expected to decrease monotoncally. At mode 2, all the cases smulated had sgnfcant amounts of spay-wall mpngement. Ths supports the concluson that mult-hole njectors are sutable for njectons under hgh ambent pressure (>.3MPa) condtons. However, the spray-wall mpngement amount could be reduced at Mode 2 by optmzng the njecton or engne operatng parameters, such as usng lower njecton pressure and/or hgher boost pressure, as shown n Fgure 21. Fgure 21 shows that usng a reduced njecton pressure (5MPa) and a hgher boost pressure (.15MPa), spray-wall mpngement could be avoded at an optmum SOI tmng around -1 CA wth a relatvely low nhomogenety (.4) (a) Crank angle: -93ºCA, SOI: -12ºCA (b) Crank angle: -63ºCA, SOI: -9ºCA Fgure 18. Spray structures of swrl njectons at Mode 2 Wall-flm fuel fracton [%] Inhomogenety SOI [Deg. ATDC] Fgure 19. Wall-flm fuel amount and nhomogenety as a functon of SOI tmng usng the mult-hole njector at Mode 2 (a) Crank angle: -113ºCA, SOI: -14ºCA

13 (b) Crank angle: -53ºCA, SOI: -8ºCA (c) Crank angle: -33ºCA, SOI: -6ºCA Fgure 2. Spray structures of mult-hole njectons at Mode 2 Wall-flm fuel fracton [%] Inhomogenety SOI [Deg. ATDC] Fgure 21. Wall-flm fuel amount and nhomogenety as a functon of SOI tmng usng the mult-hole njector at Mode 2 wth reduced njecton pressure (5MPa) and hgher boost pressure (.15MPa) Conclusons In ths study, numercal smulatons of two types of low-pressure sprays, swrl and mult-hole nozzle sprays, were conducted usng the KIVA-3V Release2 code wth mproved numercal models. An nhomogenety concept s ntroduced to evaluate the effects of njector type and SOI tmng on charge preparaton n a desel HCCI engne. The followng conclusons can be drawn from the smulaton results: 1. To mprove the numercal accuracy of spray predctons, spray models must be mproved to remove mesh dependences. In the present study, the models were also valdated usng expermental data. 2. The spray structure of swrl nozzle njectons s senstve to the ntake flow detals and the n-cylnder gas densty, whle the spray structure of a mult-hole nozzle njecton s less nfluenced by the n-cylnder flow and gas densty. 3. Swrl njectors are sutable for low ambent pressure (<.3MPa) condtons because at hgh pressures (>.3MPa), the spray collapses. Mult-hole njectors are sutable for hgh ambent pressure condtons because at low pressures, the spray penetraton s too long, whch causes spray-wall mpngement. Acknowledgements Ths project has receved gudance and fnancal support from Caterpllar Inc. and the US Department of Energy (DOE) HCCI contract # DE-FC4-2AL Addtonal fnancal support was provded by the Engne Research Center s Desel Emssons Reducton Consortum (DERC) member companes. Nomenclature p probablty of collson between two parcels σ collson cross-secton between two parcels v relatve velocty between two parcels t tme step V volume V velocty vector r dstance between a droplet and a cell node Φ equvalence rato Φ average equvalence rato δm gas phase mass n a computatonal cell SD standard devaton of equvalence rato NSD normalzed SD of equvalence rato Subscrpts,j parcel or cell ndex References 1. Iwabuch, Y., Kawa, K., Shoj, T., and Takeda, Y., Tral of New Concept Desel Combuston System Premxed Compresson-Ignted Combuston, SAE Paper , Amsden, A.A., KIVA-3V, Release 2, Improvements to KIVA-3V. LA-UR Han, Z., and Retz, R.D., Turbulence Modelng of Internal Combuston Engnes Usng RNG k-e models, Comb. Sc. Tech., Vol. 16, pp., Schmdt, D.P., Nouar, I., Senecal, P.K., Rutland, C.J., Martn, J.K., and Retz, R.D., Pressure-Swrl Atomzaton n the Near Feld, SAE Paper , O Rourke PJ and Amsden, A.A, The TAB Method for Numercal. Calculaton of Spray Droplet Breakup, SAE Paper 87289, Ra, Y. and Retz, R.D., The Applcaton of Mult- Component Droplet Vaporzaton Model to Gasolne Drect Injecton Engnes, Int. Journal of Engne Research, Vol. 4, No. 3, pp , 23.

14 7. O Rourke, P.J., Collectve Drop Effects on Vaporzng Lqud Sprays, Ph.D. Thess, Department of Mechancal and Aerospace Engneerng, Prnceton Unversty, O Rourke, P.J., and Amsden, A.A., A Spray/Wall nteracton Submodel for the KIVA-3 Wall Flm Model, SAE Paper , Montgomery, D.T., and Retz, R.D., Sx-Mode Cycle Evaluaton of the Effect of EGR and Multple Injectons on Partculate and NOx Emssons from a D.I. Desel Engne, SAE Paper 96316, Nordn, N., Complex Chemstry Modelng of Desel Spray Combuston, Ph.D. thess, Chalmers Unversty of Technology, Schmdt, D. and Senecal, P.K., Improvng the numercal Accuracy of Spray Smulatons, SAE Paper , Rotond, R., Bella, G., Grmald, C., and Postrot, L., Atomzaton of Hgh-Pressure Desel Spray: Expermental Valdaton of a New Breakup Model, SAE Paper , Mtroglou, N., Nour, J.M., Gavases, M., and Arcoumans, C., Spray Characterstcs of a Mult- Hole Injector for Drect-Injecton Gasolne Engnes, Int. Journal of Engne Research, Vol. 7, No. 3, pp , Nauwerck, A., Pfel, J., Velj, A., Spcher, U., and Rchter, B., A Basc Expermental Study of Gasolne Drect Injecton at Sgnfcantly Hgh Injecton Pressures, SAE Aban, N., Munnanur, A., and Retz, R.D., Reducton of Numercal Parameter Dependences n Desel Spray Models, Proceedngs ILASS-26, Toronto, Canada, May Sun, Y., Numercal Smulaton of Mxture Formaton and Combuston n Gasolne Drect Injecton Engnes, Master s Thess, Tsnghua Unversty, 23.