Rapid Meshing and Advanced Physical Modeling forGasoline DI Engine Application

R. Tatschl, H. Riediger, Ch. v. Künsberg Sarre, N. Putz and F. Kickinger

AVL LIST GmbH

A-8020 Graz

AUSTRIA

Gasoline direct injection engine development is significantly supported by advanced diagnostics and CFDsimulation techniques aiming at the optimization of injection and combustion system performance. With regard tothe simulation, the gasoline direct injection engine imposes severe requirements onto the performance of meshgeneration tools and the applicability of mixture formation and combustion models. Rapid meshing tools and theavailability of carefully validated models of mixture formation and combustion are required in order to enable theuse of CFD as a reliable tool within the engine development process. The present article is aimed at demonstratingthe current status of advanced rapid meshing technology and mixture formation / combustion modeling for theapplication to gasoline direct injection engines.

INTRODUCTION

Due to their promising performance with respect tofuel economy, SI engines with gasoline direct injectionare subject of intense world wide research anddevelopment [1]. The complex in-cylinder processes ofgasoline DI engines require the application ofadvanced development tools in order to meet therequired goals regarding injection and combustionsystem performance. Besides sophisticated opticaldiagnostic techniques [2], comprehensive CFD modelsof the governing flow, mixture formation andcombustion phenomena are needed in order to obtainthe desired insight into the complex in-cylinderprocesses.

In the engine development process, CFD modelingof DI gasoline systems is used to analyze theinteraction of fuel injection and combustion withcombustion chamber components and in-cylinder gasmotion. Obviously, the aim is to minimize the varietyof concepts and to identify potentially successfulsolutions in an early stage of system design.

The success in meeting these goals is first of allbased on the capability of performing a large numberof parametric studies, including intake port andcombustion chamber / piston bowl shape variations,

within the given time-frame in the engine developmentprocess. In this context only advanced meshing toolsenabling automatic generation of 3D computationalgrids of high geometrical complexity, includingmultiple moving parts, are capable of meeting thepresent demands with respect to short mesh generationtime and ease-of-use. The second prerequisite for asuccessful integration of CFD in the enginedevelopment process is the adequate modeling of theindividual processes governing mixture formation andcombustion.

The present article is aimed at demonstrating thecurrent status in the development and application of arapid IC engine meshing technology and state-of-the-art spray and combustion models to the simulation ofgasoline direct injection engines.

The basic features and philosophy of a flexible,automatic meshing environment, FAME and FAME-Engine, are introduced and demonstrated on the basisof meshing the AVL Direct Gasoline Injection engine.The mixture formation and combustion modelingprinciples adopted in FIRE are presented together withselected results of the experimental and numericalspray validation studies. Finally, representative resultsof the mixture formation and combustion optimizationcalculations are shown for the AVL Direct Gasoline

Injection engine. Where appropriate, the simulationresults are compared to experimental data.

ENGINE APPLICATION

In order to demonstrate the applicability of thepresent methodology to the numerical optimization ofgasoline DI engines, the processes of flow, mixtureformation and combustion are presented for the AVLDirect Gasoline Injection engine. The engine geometrydata and the operating conditions relevant to thepresent study are given in Table 1.

• AVL Direct Gasoline Injection Engine- bore x stroke 86 x 86 mm- compression ratio 11:1- 4-valve cylinder head- port deactivation- injector between inlet valves

• Operating Conditions- engine speed 2000 1/min- IMEP 2.9 bar- piston temperature 450 K- liner temperature 400 K- cylinder head temperature 450 K

Table 1 AVL Direct Gasoline Injection engine; enginegeometry data and operating conditions

MESHING

The method described here is valid for all types ofdiesel, gasoline (port- and direct-injection) and naturalgas engines. Two grid generation tools are used forthis method: FAME, a hex-based automatic meshingtool and FAME-Engine, a tool to automaticallygenerate different geometry positions for IC-engineapplications (steady state and transient).

The major steps of the mesh generation process are asfollows:

The procedure is begun with the loading of CADsurface data and a start topology into FAME. CADdata may be transferred from Pro/Engineer, CATIA,Unigraphics, I-DEAS or other systems. The starttopology is a simple volume mesh, which defines themajor directions and densities of the mesh. It can be

loaded from a set of engine specific start topologies,provided within FAME (Figure 1).

Figure 1 Start Topology

A volume mesh, referred to as the start mesh, isautomatically generated, using FAME, by deleting allcells of the start topology which lie outside of thegeometric domain defined by the CAD data. Betweenthe CAD surface and the resulting trimmed volumemesh a small gap remains. This gap is automaticallyfilled with body-fitted cell layers. Characteristic edgesare detected automatically and considered during themeshing process (Figure 2).

Figure 2 Start Mesh

In order to properly create meshes of differentcrank angle positions, different volume parts of thestart mesh are specified as moving, non-moving andbuffer regions within FAME-Engine (Figure 3).

Figure 3 Definition of Different Mesh VolumeConditions

The mesh is automatically moved according to thevalve lift diagrams and the piston kinematics. Duringthe movement the volume mesh quality is constantlyimproved by use of smoothing algorithms. Also duringthis process, the mesh quality is continuously checked(Figure 4).

Figure 4 Moving the Mesh

The intention of the process is to allow the user toautomatically generate the description of the completemovement on the basis of one start mesh. However,the procedure is flexible enough to enable manualinteractive manipulation to improve the mesh qualityat selected crank angle positions, or to use additionalstart geometries for the beginning of different crankangle ranges (Figure 5).

Figure 5 Details of the Moved Mesh

There are two major methods to deal with varyingengine geometries.a) Adapt the start topology and re-mesh with FAME.b) Split the part you want to exchange, mesh the new

part with FAME and adapt it to the rest.

The second method is used in Figure 6 to exchange anintake port.

Figure 6 Exchange of Intake Port

COMPUTATIONAL METHOD

The numerical method applied in the present workis the CFD code FIRE. A number of publications havedescribed the FIRE code and its application to enginerelated flow, mixture formation and combustionsimulation [3-8]. Hence, only a brief description of theapplied models relevant to the present study is givenhere.

Mixture Formation

The commonly used high pressure swirl injectorsfor gasoline DI applications are designed to create arotating hollow cone spray. The main characteristics ofthis type of injectors are the even fuel distribution, anearly breakup of the liquid sheet into individualdroplets and hence reduced penetration. Additionally,this type of injector provides the necessary variabilityof the spray geometry for different stratificationrequirements. The spray cone angle produced by theinjector depends on the fuel supply pressure and on thecylinder backpressure in a way that meets therequirements, i.e. the spray cone angle is reducedunder high backpressure conditions.

The applicability of a CFD spray model is henceclosely related to its capability to reflect thebackpressure influence on the spray shape, cone angleand penetration characteristics under engine operatingconditions. At AVL, experimental spray modelvalidation data under realistic engine operationconditions are obtained in an optically accessed sprayresearch engine. Different backpressure conditions areachieved by suitable crank-angle timing of theinjection process. The relevant data for validation ofthe CFD method, i.e. the spray shape evolution and itsdependence on the combustion chamber backpressure,are obtained by LIF spray imaging and statisticalevaluation of multiple injection events.

Figure 7 Chamber backpressure influence on sprayshape and cone angle; (a) CFD results, (b) statisticalevaluation of fuel LIF data (rail pressure 80 bar, 3 msafter SOI)

Figure 7 displays the simulated results of the spraypropagation process in the spray research engine for abackpressure variation from 1 to 6 bar together withthe corresponding experimental data. The imagesclearly show the strong dependence of the spray shapeand the maximum spray diameter on backpressure dueto earlier breakup of the stable liquid sheet. The earlierbreakup of the sheet leads to an increased droplet / gasinteraction, consequently also reducing the overallspray penetration length, as shown in Figure 8.

Figure 8 Chamber backpressure influence on spraypenetration length; calculation vs. measurement (railpressure 80 bar)

Combustion

Combustion in gasoline DI engines at part loadoperation no longer proceeds via a self sustainedpropagating flame front like under full load operationor in conventional gasoline engines, but occurs underpartially premixed and / or diffusion controlledconditions, depending on the local mixture andtemperature conditions in the fuel vapor plume.Additionally, finite-rate chemical kinetic effects playan important role on the lean side of the fuel / airmixture cloud.

In the present case, the different combustionregimes and finite-rate chemistry effects are accountedfor by a transported multi-scalar PDF method [6]. Thehydrocarbon oxidation process is modeled by a globalirreversible combustion reaction with an Arrheniusreaction rate expression reflecting the variation oflaminar burning velocity reasonably well for a widerange of equivalence ratio, including the lean and richflammability limits of φl=0.5 and φr=4.3, respectively.

Flame kernel formation is simulated throughprescription of the temporal variation of the reactionprogress variable and enthalpy pdf´s.

Engine Simulation Results

Part load mixture formation in gasoline DI engineshas to ensure proper supply of fuel at combustibleportions to the location of the spark plug in order toenable stable initiation of a self sustaining combustionprocess. The temporal evolution of the mixturepreparation behavior for the AVL DGI engineconfiguration is shown in Figure 9, together with thecorresponding LIF data. The depicted iso-levelsrepresent the local fuel vapor distribution.

a) b)

Figure 9 AVL Direct Gasoline Injection engine; (a)calculated fuel vapor distribution evolution, (b)statistical evaluation of fuel LIF data (section acrosscylinder and injector axis)

It can be seen from Figure 9 that the propagationbehavior of the fuel vapor cloud is predominantlydetermined by the induction and injection inducedflow pattern and its subsequent interaction with thepiston bowl. Hence, proper mixture formation is theresult of a large number of system parameters, such asintake port and piston bowl design, injectorcharacteristics and location as well as injection timing(Figures 10 and 11).

a) b)

Figure 10 AVL Direct Gasoline Injection engine;calculated fuel vapor distribution at SA=-33 deg. CAATDC, (a) effect of mean flow motion orientation, (b)impact of injection timing

a) b)

Figure 11 AVL Direct Gasoline Injection engine;calculated fuel vapor distribution at SA=-33 deg. CAATDC, (a) influence of injector characteristics, (b)effect of injector axis inclination

A typical example of the space and time resolvedcombustion evolution under gasoline DI operatingconditions is given in the sequence of pictures inFigure 12. Calculated results of the reaction productsmass fraction distribution in a section across thecylinder and fuel injector axis are presented togetherwith the corresponding statistically averaged fuel LIFimages.

Shortly after spark break-down (-33 deg. CAATDC) the flame kernel exhibits a rather small growthrate but significantly shows a preferential burndirection along the near stoichiometric fuelconcentration iso-surface. Fuel rich zones that canmainly be found in the vicinity of the piston bowl andwhich are considerably outside the flammability limitsremain nearly unburned during the first premixed-likecombustion mode. On the fuel lean side of the fuelvapor plume, however, the reaction speed ischemically kinetics limited. Due to the ongoingintermixing of fuel vapor with the surrounding air asthe combustion process proceeds, the local fuelportions at the periphery of the fuel containing zonebecome more and more leaned out until they finallyexceed the lean flammability limits. Flame quenchingin this area can finally lead to substantial formation ofunburned hydrocarbons.

a) b)

Figure 12 AVL Direct Gasoline Injection engine; (a)calculated reaction products distribution evolution, (b)statistical evaluation of flame LIF data (section acrosscylinder and injector axis); SA –33 deg. CA ATDC

The space / time resolved combustioncharacteristics also manifest themselves in a burn ratedifferent to the one obtained under conventionalpremixed conditions. The calculated mass fractionburned data in Figures 13 and 14 indicate a rapidburning of the fuel, with an overall main phasecombustion duration of approximately 15 degreescrank-angle. This is a reduction in the duration of themain combustion phase of about 50 % compared to aconventional SI engine.

Figure 13 AVL Direct Gasoline Injection engine;variation of injection timing

This behavior can be attributed on one hand to theremnant of the injection induced elevated turbulenceintensity level in the spray region, but is to a largerextend due to the short flame travelling paths withinthe compact fuel vapor containing area. The slowburning in the late combustion phase is then mainlygoverned by the diffusion type burnout of the initiallyfuel rich zones that get subsequently mixed with air. Acomparison of the calculated 2, 50 and 90% massfractions burned with the corresponding measured data(thermodynamic analysis of the cylinder pressuretrace) indicates good overall agreement for variationsin injection timing and spark advance (Figures 13 and14).

Figure 14 AVL Direct Gasoline Injection engine;variation of spark advance

SUMMARY AND CONCLUSIONS

The basic features of the new flexible, automaticmeshing environment, FAME and FAME-Engine,have been introduced. Its application to the rapidgeneration of highly complex computational meshesfor the AVL-DGI engine proves the superiorperformance when compared to conventionalsolutions. With FAME and FAME-Engine, meshingtimes can be reduced up to 70 %, assuming theavailability of reasonable CAD surface data.

A joint experimental / numerical approach has beenpresented for determination of the fuel spray initialconditions of hollow cone injectors in the FIREsimulations. Comparisons of the calculated results withexperimental data show good agreement with respect

to the overall spray shape, penetration lengths andradial spray extension.

The FIRE application to the full mixture formationand combustion simulation for the AVL-DGI engineclearly shows to adequately reflect the characteristicgasoline DI engine features and to provide a detailedand valuable insight into the complex processes offlow, spray propagation and combustion interaction.Locally resolved field quantities as well as globalmixture formation and combustion data show goodagreement with data extracted from in-cylinderpressure measurements. Parametric studies,systematically varying overall flow structure, injectorcharacteristics and positioning as well as injection andignition timing, have shown to coincide well withexperimental observations.

REFERENCES

[1] Wirth, M., Piock, W.F., Fraidl, G.K., “ActualTrends and Future Strategies for Gasoline DirectInjection“, IMechE S433/004/96

[2] Winklhofer, E., Fraidl, , G.K., Tatschl, R., “FlameVisualisation in Gasoline Engines - New Tools inEngine Development“, JSAE 9530850

[3] Tatschl, R., Wieser, K., Reitbauer, R.,“Multidimensional Simulation of Flow Evolution,Mixture Preparation and Combustion in a 4-ValveGasoline Engine“, COMODIA 94, pp. 139-149,Yokohama, 1994

[4] Cartellieri, W., Chmela, F., Kapus, P., Tatschl, R.,“Mechanisms Leading to Stable and EfficientCombustion in Lean Burn Gas Engines“,COMODIA 94, pp. 17-24, Yokohama, 1994

[5] Tatschl, R., Fuchs, H., Brandstätter, W.,“Experimentally Validated Multi-dimensionalSimulation of Mixture Formation and Combustionin Gasoline Engines“, IMechE C499/050/96

[6] Tatschl, R., Riediger, H., “PDF Modelling ofStratified Charge SI Engine Combustion“, SAE981464

[7] Tatschl, R., Pachler, K., Winklhofer, E., “AComprehensive DI Diesel Combustion Model forMultidimensional Engine Simulation“,COMODIA 98, pp. 141-148, Kyoto, 1998

[8] v. Kuensberg Sarre, Ch., Tatschl, R., “SprayModelling / Atomisation – Current Status ofBreak-Up Models“, Turbulent Combustion ofGases and Liquids, - Leading Edge Technologies,ImechE Seminar, Lincoln, 1998