!

Indian J oumal of Engineering & Materials SciencesVol. I,June 1994,pp.135-148

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I: Three-dimensional modelling of fluid flow in a four-stroke compression

ignition engineD Mohan Krishna, K Rajagopal, P Srinivasa Rao & V Ganesan

Department of Mechanical Engineering, Indian Institute of T~chnology, Madras 600 036, India

Received 5 June 1993; accepted 4 January 1994

This paper presents the analysis of three-dimensional calculations of flow field development in afour-stroke compression ignition (CI) engine during the compression process. The calculations are

.performed with a modified multidimensional arbitrary mesh using finite difference hydrodynamicm?del. A post. processo.r has been ~eveloped to generate in-cylinder flow characteristics. Comput-~tions are camed out WIth a computing mesh of 4152 cells for a crank angle duration of 900 requir-

, ill~ 17000 .s.of CPU time on ~ S~MENS ~2 bit mainframe computer. Results reveal an increase insWIrl velocItIes and decrease ill axIal velocIty of the fluid. Formation of squish induced vortices are

~ observed near lDC. A comparison of average cylinder pressure obtained through three-dimensionalcomputations and experimental values from a motored engine showed good agreement validating theprediction.

In a four-stroke CI engine the combustion pro- knowledge from these traditional as well as adv-cess directly influences not only the performance anced experimental techniques is time-consuming.and emissions of the engine but also many other Expensive experiments and specific engine de-thermal and mechanical details of engine charac- signs do not lead to similarity laws for predictingteristics. There is a continuous interest in the the characteristics of a different design. Therefore,studies of combustion phenomena so that delicate with the advent of computational tools, the fieldbalance between fuel economy, emissions and of computer simulation of engine flow processesperformance could be achieved. is now developing rapidly. The present work

j The process of combustion is always controlled deals with the three-dimensional simulation of in-'~'. by the fuel-air mixing and the flow field develop- cylinder flows during the compression process in

ment established during compression near roC. a four-stroke CI engine.The compression process, combined with theconfiguration of the combustion chamber helps in Numerical Modeldeveloping a turbulent flow field and to enhance The air flow within the cylinder is extremelythe fluid motion at a point in the engine cycle complex and three-dimensional. They are un-where it is mostly needed, the point of ignition. steady and turbulent in nature as a result of reci-This turbulent flow field enables proper mixing of procating piston movement. In addition, when thethe fuel with air and prepares a mixture environ- liquid fuel is injected into the cylinder phasement that will be conducive for ignition. The flow change is also involved. The flow becomes reac-patterns and the temperature distribution within tive when combustion takes place. Simulation ofthe cylinder also affect heat transfer to cylinder the above flows through solving concerned partialwalls and piston crown. Hence a good under- differential equations demands excessive amounts

! standing of the fluid dynamics within the cylinder of computing resources. In addition to these limit-II'! during the compression process is essential in de- ations, problems do exist in terms of numericalr-"- signing the ~n~es with m?s! attractive perform- accuracy and ability to represent the me~hanismsI ance and effilSSIon charactenstIcs. for turbulence, fuel droplet evaporatIon and

The research in understanding engine flows chemical kinetics during combustion.comes from both experimental measurements and In the present work, KN A, a time-marchingnumerical computations. Experimental techniques three-dimensional, finite difference program deve-include applications of hot-wire anemometryl- 4 loped by Amsden and others8,9 is modified andand laserDoppleranemometryS-7 for flow analysis used to study the flow field during the compres-at specific locations in engine cylinder. To obtain sion process. This program solves the three-di-

136.- INDlANJ.ENG.MATER.SCI.,JUNE1994'-

mensional, unsteady equations of motion of a The momentum equation for the fluid mixturechemically reactive mixture. of ideal gases. It can isalso solve for the dynamics of liquid fuel sprayan~ the couplin~ betw~en the sp~ay and the gas. i (,au) + V .(puu) = -V p + V .0'+ F, ...(7)This program IS specifically wntten for super atcomputers and makes extensive use of vector pro- h .th fl' d ..cessor. The following sections present the basic were p IS e UI. pressure, 0' IS the VI~COUS

...stress tens6r and F IS the momentum per urnt vo-goverrnng equations and a scheme of solution. I .t t.st f d f thume per urn Ime rans erre rom e spray

Governing equations droplets to the fluid (not considered in the pres-.The governing equations are written in vector ent work).

notation. The unit vectors in the x-, y- and z-di- The stress tensor is given byrections. ~re denoted. by i, j, and k respectively. 0'= p.[(Vu) + (VU)T] + )'(V .u)l ...(8)The posItion vector x IS defined by

..1 where p. and ). are the first and seCbnd viscosityx = XI + YJ + Z k ...() coefficients, 1 is the unit dyadic and superscript T

The vector operator V is given by denotes the transpose.

a a a The int~rnal energy equation is

V=i-+j-+k- ...(2)ax ay az a-(pl)+V. (p/u) = -pV. u+O':Vu-V.Jand the fluid velocity vector u is given by at

u=u(x,y,z,t)i+v(x,y,z,t)j+w(x,y,z,t)k ...(3) +Q,+Q,+QT .., (9)

where t is the time. It is to be noted that x~ Ixl where I is the specific internal energy of theand u~ lul. fluid (exclusive of the chemical energy), J is the

Since the program solves the equations of mo- heat flux vector, Qc and Qs are the source termstion for fluid along with those for spray droplets associated with the chemical reactions and sprayand chemical kinetics, equations of motion for gas droplets (not considered), and QT is a source termphase alone are presented here. However, the de- associated with the turbulence model (specified intails of the spray dynamics and chemical reactions turbulence model).are as given by Amsden and others8.9. The heat flux is given by

Equations of motion for the fluid phase-Thetotal mass density P of t~e .fluid (excluding the J = -kVT- D I h V ( ,,/ ) ...(10)mass of the spray droplets) IS given by P m m P P

-I (4) where k is the thermal conductivity, T is the ab-P -Pm ...solute temperature, and hm is the partial specific

m .enthalpy of species m.

where Pm IS the partial mass density of the species The state relations in the model are assumed tom. be those of an ideal gas mixture, Hence

The continuity equation for species m is

apm .,. p= RgTI (p,,/Wm) ..; (11)-a/+V. (Pmu)=V. [pDV(PnIP)]+Pm+P,{)ml m

where D is the species diffusivity (assu~;d(;~ I(T)=I(p,,/p)lm(T) ...(12)be same for all species), p~; is the rate of change m

of Pm due to chemical reactions (not considered -13in the present work), Ps is the rate of change of PI C,,( T ) -I (Pml p) C vm ( T ) ...( )due to spray evaporation or condensation (not m

cons~dered), ~jiistheKroneckerdelta,and lis the h( T)=l (T)+R TIW ...(14)specIes of which the spray droplets are composed. m m g m

The total fluid density P satisfies the equationwhere Im( T) is the partial specific internal ener-

~ + V .( u) = (6) gy for species ni, Cvm( T). is the s~ecific h~at atat P p, constant volume for species m, Cv IS the mIXture

,. J KRISHNA et al,: FLUID FLOW IN A FOUR-STROKE CI ENGINE 137

\\ specific heat at constan! volume, Rg is the univer- The turbulence source term QT in the internal

sal gas constant, and Wm is the molecular weight energy equations is given by the following equa,"'1 of species m. tion:

Turbulence model-The effects of turbulence Q =.-1o(b L -I 3/2- o.Vu) (18)are represented by a subgrid scale turbulence T p q. ...

model which is a generalisation of the original where Ao = 1 when the turbulence model is in usealgebraic subgrid scale model. This model uses md Ao = 0 otherwise.

the same above given laminar equations with suit- The dynamics of atomized fuel sprays are rep-able averaging. In the process of averaging, flow resented by a Monte Carlo based discrete particlevariables are separated into mean and fluctuating technique, by Ducowiczlo. The spray is consid-components, The averaged equations are consid- ered to be composed of discrete computationalerably simpler if tbe mean values are mass particles each of which represents a group ofweighed. The fluctuation terms are generally mo- droplets of similar physical properties. The dis-delled by the gradient-flux approximation. tribution function in droplet size, velocity, spray

In this approximation the averaged turbulent pattern and temperature by the fuel injector isequations become identical in form to be laminar statistically sampled and the resulting particles are

i ones; the transport coefficients (i.e. viscosity, ther- followed as they locally interact and exchangemal conductivity and species diffusivity) are sim- mass, momentum and energy with the gas. Theply replaced by the appropriate turbulent values. model accounts for turbulence effects and interac-

~ Hence the above equations are used when the tions between droplets but other thick spray ef-

, flow is turbulent but with turbulent contributions fects are neglected.added to the laminar values of the transport coef- Chemical reactions that can be considered

\ ficients. The transport coefficients in the model through the KIVA are arbitrary in number and: are as follows: are limited only by computer memory and time,1 = + .+ Chemical reactions are treated by a procedure" ,u pVo ,ualr ,u, that identifies between slow reactions which pro-) K = ,u C pi Pr ceed kinetically and fast reactions which are as-

;i D=,uI(pSc) ". (15) sumedtobeinequilibrium.'! However, in the present analysis only flow mo-l where ~ I.S called tant '~ b k d delling during compression process is considered.,] 0 cons UDllorm ac groun H . d ' d .

d1 turbulent diffusivi ty II = A T 3/2/( T+ ..i ) -h ence equatIons regar mg spray ynamJcs an,ralr I ~were h ' l . d . h.

Al and A2 are constants, ,u, is a subgrid scale c emlca reactIons are not presente m t IS pa-I '<-- turbulent viscosity defined below, Pr is the per.I Prandtl number, and Sc is the Schmidt number. .

Numencal schemeBoth Prand Scare taken as constants. Th d I I h fi ' d."I e mo e so ves t e rute Illerence approx-, The subgrid scale turbulent viscosity ,u, is given imations to the governing equations ]nentioned ini by the earlier sections, In this part of the paper some

,u =ApLql/2 (16) general features of the numerical scheme are ex-, , ..plained briefly. The complete information of the

..governing equations, their difference equationswhere A !s.a constant of the or~er of 0.05, L IS a and their solutions are available in KIVA Reportcha,racten~tIc l~ngth of order, twice the ~ength of a by Amsden and othersH.9.I typIcal finite-dIfference cell sIde and q IS the sub- ., .grid scale turbulent kinetic energy. The latter sa- , T~mporal differen~mg- The temporal rlifferenc-

! tisfies the transport equation given below. mg IS performed with respect to a sequence of.discrete times, (cycles), f' (n=O, 1,2 ...). The time

i) interval L\tn= f'+ 1-'n is the time step and the inte-..ar(pq)+V. (pqu) = -(2/3)pqV' u+ u:V. u ger n is the cycle number, The value of each de-

pendent variable of each cycle is calculated from+ V .(,uV q) -bpL -( q3/2 + w. ...(17) that of the previous cycle. The temporal differ-

-encing scheme is explicit, which uses an acousticwhere D is a constant of order unity. w. is the subcycling method 11 for calculating flows efficient-

\. ~ource term (not considered in the present work) ly at low Mach numbers, The cycle number is dis-~ due to the interaction between spray droplets and played as a superscript so that Qn denotes the

g~s. difference approximation to the quantity Q at

\ .""":J~l. ..

138 INDIANJ.ENG.MATERSCI.,JUNE 1994

time r. The difference approximation to the de- form.rivativeiJQ .Boundary conditionsa IS Two types of boundary conditions are required

: t to be provided to calculate the flow field compu-

(Qn+J -Qn)/At ~tional domain. T~~y are temperature and velo-City boundary conditIons. In the present work the

..law of the wall boundary conditI' ons'8 I' S consl'd-A cycle that IS the advancement of variable fromtime tn to time tn+ I, is performed in three stages. ered to .resolve ~undary laye~~ near the solidPhase A'is the calculation of thc effects of most walls while calculatIng the velOCItIes as shown be-

. th .. th h low:terms m e govemmg equatIons 0 er t an pres-sure and convection terms. Phase B is a Lagran- (i) when the wall is flat (as in the case of flatgian subcycling calculation in which the vortices portion of a piston, or the surface of the cylinder

' I are temporarily assumed to move with the local head), the components of tangential velocity areI fluid velocity. This phase is also concerned with calculated.

the calculation of the terms related to acoustic (ii) when the wall is having curvature in one di-waves (pressure forces and pdV1work terms) and rection (as in the case of the cylinder liner), thethe drag forces between spray particles and fluid. component of tangential velocity in the directionPhase -C is the rezone phase. In this phase the of no curvature, is set equal to the same compo-convective transport associated with moving the nent at the next closest cell comer (but not on thevertices from their phase B locations to their final wall). Thf other component of the tangential ve-locations is calculated. This transport arises be- locity, i.e., in the direction of curvature is set tocause of the motion of the fluid relative to mesh. zero.

Spatial differencing-Spatial differencing is (iii) when the wall is curved in both principalformed with respect to, a generalized three-dimen- directions (like in the case of hemispherical bowl)sional finite difference mesh that subdivides the i?oth components of tangential velocity are set toregion of interest into a number of small cells. zero.The comers of the cells (vertices) may be arbi- ..trarily specified as functions of time, thereby al- Th~ normal velocity IS. always set equal to thelowing a Lagrangian, Eulerian or a mixed descrip- -velocity of t~e corre~pondmg wall. ..tionJ2.JJ. This feature is particularly useful for rep- From this. velocity boundary conditIon wallresenting curve/moving boundaries. Spatial differ- shear stress IS deduced at the grid points closestence approximations are constructed by the con- to the wall. Then wall heat lo~s is obtained usingtrol-volume or integral ba;iance approachl2-14 Reynolds analogy form~l~. With fixed. wall tem-which largely preserves the local conservation pro- perature boundary condl~lon ~eat flux IS calculat-perties of the differential equations. The proce- ed and coupled to energy equation.

dure that is used for spatial differencing is to dif-ference the basic equations in integral form, with Computer Programthe volume of ~ typical cell used as control. vo- The computer program consists of a set of pri-lume and the divergence terms transformed mto mary and secondary subroutines controlled by asurface integral using divergence theoremJ4. short main program. This program is written for

State relations-The internal energy of species use in CRAY-l 64 bit .super cQmputer using theis obtained from the JANAF tablesJ5 are stored CRAY Fortran Compiler (Cff). The Fortranin tabular form at intervals of 100 K. A simple code. contains. st~tements peculiar to CFT thatlinear interpolation is used to determine the spe- permits vectonzatIo~ of the code to run very !ast.cies internal energy at temperatures within the Th~ code als~' requires many syste":l subroutInesrange of the tables. Specific heats at constant vo- dunng ~xecutIon. For post processm~ the locallume for species are approximated by the differ- field. v~nables the .system uses a Graphic Softwareences between adjacent tabular values of species that It mtegrated WIth Fortran code.i~ternal energy-.at corresponding temperatures di- Mo&ficationso of the code-The code, that isvlded by 100 K. specifically developed for the above super compu-

For one of the species, liquid octane fuel, data ter is fully implemented in the SIEMENS-32 Bitlike latent heat of fuel, internal energy and equi- Computer. This system, operating under timelibrium vapour pressure are stored in the tabular sharing environment provides a FORTRAN com-

.

KRISHNA et al,: FWID FLOW IN A FOUR-STROKE CI ENGINE 139

piler. However, the BS 2000 operating system of In arriving at the best possible grid size, theSIEMENS does not offer any postprocessing uti- grid size is varied in two directions, namely axialtities, and tangential direction of the computing mesh,

...,. The code was fully modified so that it will be In the first case where the grid size is varied onlysuitable to the SIEMENS environment. The mo- in axial direction by varying the number of cells,difications include a complete change in the struc- NZ, as 10, 13 and 16, The number of cells in ra-ture of the common blocks, provision of system dial direction, Nx, and tangential direction, NY,subroutines, logical routines that are available on, are kept constant at 13 and 16, respectively, InCRAY, the second case, the number of cells in tangential

In addition to this, provision is made to run the direction (NY) is changed from 12 to 16, NX andprogram in segments, Since the program is used NZ are kept constant at 13 and 19, respectively,

~ to solve time-marching three-dimensional flow It may be noted that the number of cells is vari-problems and the system is non-CRAY one, the ed in the NZ direction in which the piston move-run cannot be completed in a single day. Hence ment takes place, Variation of cells in tangentialmodules are developed to dump the common direction, NY, is to account for three-dimensionalblock and equivalence variables at a required time effect. In the radial direction the number of cells,(or crank angle position), For subsequent calcul- NX, is chosen to keep the truncation errors toations the run can be restarted from the previous the minimum,crank angle position, For the grid independence test the comput-

~ Development of post-processor-The prediction ations are commenced at 90° bmC and are con-~ procedure generates a huge amount of data at any tinued for a crank angle duration of 10°, i,e., upto

position of the piston. The global parameters like 80° bmC during the compression process withaverage cylinder pressure, !otal angular momen- bowl in piston configuration,turn, etc., can be analysed for through ordinary An examination of the local field variations in-x-y plots. However, the analysis of local field var- dicated that there is no variation in the velocityiables (like velocities in x-, y- and z-directions, field16, The differences in maximum velocity com-temperatures, TKE) at each position of the piston ponents are of higher magnitude when grid size iscan be carried out only through a specifically varied in axial direction, Increasing the grid sizedeveloped post-processor, also reduced the peak values of velocity compo-

Th ' ft (pps) ' d nents,e post processmg so ware is eve- , ,I d 16 b't hi k tati' hi h Table 1 shows the computatIonal tIme that wasope on a i grap c wor s on w c op-

erates under UNIX 2 operating environment, The consumed fo~ each test case, It can be obsef':ed, -, , that the gnd refinement demands exceSSive

~ post processor developed m this work usmg the f CPU ' W 'th th . ts ' ' d, "V'bo tI'l 'tI' all " d I ttm' th ~ I amount 0 tlOle, i ese pom m mm ,

a ve u i es ows Vlewmg a.R po g e 10 -, , , .I ' ' b b th ' h ' tal d 'al I f from all Sizes of gnds considered m both cases,owmg ~o s <:> m ,onzon ~ axl panes 0 i,e, 13 x 16 x 10; 13 x 16 x 13; 13 x 16 x 16 (case

th~ en~e cylm?er, (i) ComputIng mes~ repr~sen- 1) and 13 x 12 x 19; 13 x 16 x 19 the grid chosentatIon m two-dimensional and three-dimensional, 13 16 19 Thi ' 'd '

I, ( " ) " I ' fi Id ' ( '.. ) is X X .s computatIons gn is emp oy-Vlews, 11 ve OCity vector e representatIon, III d ~ th 3 D ' f '

, " e lor e -computatIons 0 companson pro-Contour mappmg of scalar vanables hke tempera-TKE d ( ' ) S . I d .' b cess,ture, , etc. an IV pray partIc e istn u- D 'l 'l' '.1' .tI ' ' tI ' etal s oJ computatIons oJ compressIon

on proJec on. Th . b' . f h kprocess- e mam 0 jectIve 0 t e present wor

is to study the in-cylinder fluid mechanics dUrulgSelection of Computational Grid compression process in a CI engine. The piston is

In fluid flow computations, the grid size will beemployed to approximate spatial variations in ..., .

, I d ' 'II I ' rtant I Table 1-CPU time history for dIfferent gnd SIZescomputatIona omaln Wi p ay an impo ro e .., .in arriving at plausible results. By using high dens- Gnde sIze Computational time" d ' I ' b NXx NYx NZ s

.,. Ity gn S numenca truncatIon errors can e re-, duced to acceptable levels, But increasing the grid 13 x 16 x 10 1091,1

\ density increases computational time and space 13 x 16 x 13 1479.6considerably, So a certain compromisation is re- 13 x 16 x 16 1738,6

\ quired between the computing resources and the 13 x 12 x 19 1564,0

..; s~lection of best possible grid size is three-dimen- 13 x 16 x 19 2153,2

"!"~ SiOns~ ' :.~ ';

: ---~.I .IIII~

140 INDIANJ.ENG. MATER. SCI.,JUNE 1994

Table 2-Engine details J: 5

Engine Configuration Piston Configuration rBore 8.75 cm Hemispherical Bowl-in-piston YStroke 11.00 cm Bowl radius 2.53 cm

JSquish 1.50 cm Operating ConditionsConnecting Engine speed 1500 rom j:9 1:14,J:1rod length 23.20 cm Swirl 3 (clock-

wise)

provided with a fuel injection or ignition pro-cesses are considered in this work. The engineconfiguration, operating conditions, bowl confi- J:13guratlon and other details considered for this (A) Plan

study are given in Table 2.During compression process, actual compres-

sion begins at 1460 broC (inlet valve closure)and proceeds upto roC. However, initial part of J:9 1:1 -x -1:14,J:I,K'20

Icompression (upto 900 broC) produces a smallchange in in-cylinder as well as global propertieslike pressure, etc. The stage of compression from900 broC to roc is an important one where fu- -zel injection and ignition will take place. Due tothis reason the significant part of compressionprocess, i.e., from 900 broC to roc is consid-ered for the present study. K:6 K:8

The other important point in calculating three. J1-rdimensional flows, particularly of engine flows, is ' .j Ithat it consumes excessive amount of computing ~L

time and memory. For the present calculations,i.e., from 900 broC to roc it takes 4.716 h of (8) ElevationCpu time. This is the' other reason why the com-putations are carried out only for this significantstage of compression.

The calculations are performed with an initialair swirl ratio (at 900 broC) of 3, with a chargetemperature of 400 K and wall temperature of400 K. Fig. 1 shows the top view, elevation andpictorial view of the computational grid at this 'point (89.840 broC). The results are analyzed in Iterms of local field parameters (velocity vectors f,and contour plots for temperature and TKE bothin horizontal and axial planes of the engine cylin-der), which were generated through PPS. In addi- jtion to these parameters, ,ital global parameterslike average cylinder pressure and total angular I ,

momentum are also considered for analysis.An attempt has been made to compare the av- ~ .

erage cylinder pressure history with that of exper- 1I-~r, ~imental one, both qualitatively and quantitatively. -~~

~

Results and DiscussionsThe local and global variables are stored at ev- (C) Pictorial View

ery 100 crank angle duration and processed fordata analysis through PPS. Flow parameters are Fig. I-Details of the computational grid.

KRISHNA et al.: FLUID FWW IN A FOUR-STROKE CI ENGINE 141

analyzed at 800, 600, 200 and 100 before and very Velocity field development duringclose to roc (0.020). compression- Fig. 2 show the plan view of veloc-

In cylinder flow predictions and analysis are iiy vectors in K6 plane which is within the bowl.~ done at the same plane irrespective of the piston In this plane, at 800 broC, the average velocity

movement in order to make the comparison mea- increases linearly from bowl center to bowl rim.ningful. Horizontal planes K6'and K8 and vertical As the piston moves towards roc, the magnitudeplanes J1 and J9 (diametrically opposite planes in of U (velocity component in x-direction) and Velevation) are selected for the flow analysis. Im- (velocity in y-direction) increase rapidly due toportant point is that as the piston moves towards decrease in the combustion chamber v~lume. .Asroc the position of K8 plane (the plane that is expected the Umax and V max start mcreasmgimmediately above the piston surface) also varies. as the piston moves towards roc, showing an in-But it is still close to the pis~on surface. crease in swirl (an organised rotation).

/--/'

'" :: ,.--'".. ...~,' ~.,'- \., ,~.., /"",._,\\'r '."~.' ,\ /,"~--., 1 \" ,.' II' , ( "II I I I 1 , I .) .I I I I r : I \ II

I 1,,'.'.'.'1 11',,~,'," "", 'I , \ \' "-, "I.."-" \~'--.' /,'-_I'" \., c' ..-'" ,.

\" .--

I -, ., ,j(~) Crank angle 80'C bTDC -Umax = 10.144 mis, (b) Cra(lk angle 60'billC ( Umax = 11.845 mis, Vmax = 11.845m1s)

Vmax= W.144 mls)

.' -: ::-,. -,

', "-...~ -..'p ,. ~ '\ ""-, \.I ..."..-, \ /, ",'-,,''I, ~,..,..'\ \ " .'-~.' \\

"1/ (0 ') '" 1111"( -\'\\, .".;' 11IJ \ \\" ,',11/1

I 1_"__',,"" ,','-",',\' ' -, , ..-'" /\ ,'-- , ,. .-"- "'"

(c) Crank angle 20'billC ( Umax = 17.571 mis, Vmax = 17.708m1s) (d) Crank angie 10'billC (Umax = 25.13 mis, Vmax~ 25.349 mls)

+~~-~/..-""If / ..-':.",'1, ',"" \\1f " ...\'/1: (.) : \ ' I I\\ "",_.., +,,~.,' "" '

I"'..-""" .,"' "'"~--/'r +-~

,

(e) Crank angle 0.02' aillC (Urnax ~ 33.719 mis, Vrnax ~ 33.835

mls)

Fig. 2-Velocity vectors in K6 horizontal plane

142 INDIAN J. ENG. MATER. SCI.,JUNE 1994

Fig. 3 gives the details of the velocity field at K8 Fig. 4 shows the variation of total angular mo-plane. This plane is just above the piston surface. mentum with respect to crank angle during theFrom this figure, the flow field within the piston compression process. It clearly shows the angular

! .bowl can also be observed. At 800 (Fig.3a) momentum decays continuously as the compres-i 1 .there seems to be a uniform rotation of the flow sion progresses. This is due to the friction at walls, field. Here, the velocity profile increases gradually and turbulence decay within the fluid 17. To con-

from center to the cylinder wall. As the piston serve angular momentum, the swirl velocities in-moves towards roc it is clearly seen that swirl crease. In bowl-in-pistons the increase in swirlvelocity within the bowl increases towards the velocities is greater compared to flat pistoDs. Thewall and outside the bowl the velocity decreases decay in total angular momentum is more fromand the air is squeezed into the bowl. 400 broC onwards showing rapid increase in

." '"'" "..-, ," '..-" \" ..., ~ -, \ \., ..' \.-."" ' --,,\ ,I

" , '"'.. I I'. ' ".' , I 1

1111111"::'(.).::"'1 1 1 1111", ".' ..., I \ ..., .'. ..I

I I ...", ""., ,1 ," " -, ",.\ \" ..""

-,,-:.- I, " '\ -:: -~,.~ , , -,.~ , ,... '" .-'"

'"

(a) Crank angle 80° bTDC (Umax= 19.54 MOs, (b)Crankangl e600bTDC(11 =27.742m1s ~ = 8.01. mls

)Vmax = 19.552 mls) max , max

.-, "., -.,~ -.-" '---, \~ ,I" , --,' ,\ "

'/, ' " " "I, .'. .." .' \ I I I

.' ..., , , I .III , ." , ', .','. :... I 1 I I , , I ,I , \ .' ..,' .I I

, " ,," .', ",/-.' \,. --, , I, ~.. ., '" \ '- ---.--, ...'-

(c)Crankangie200bTDC(Umax=47.086m1s, Vmax = 48.027m1s) (d) Crank angle 100bTDC( Umax = 57.980 mis, Vmax = 58.661 mls)

(e) Crank angle 0.02° aTDC (Umax=49.381 mis, Vmax=49.502mls)

Fig. 3-velocity vectors in K8 horizontal plane

KRISHNA et al.: FLUID FLOW IN A FOUR-STROKE CI ENGINE 143

I swirl velocities from 200 bfiC onwards. ; 0 Fig. 5 shows the variation of velocity vectors in I ., " .I , ., , ., ., .., , , , I ' , ,

Jl dJ9 .all S 'hfl .rtt "1""""""""""'"--1 an ax! panes. qws ow,anllllpoan l'II""I"il'I!'I'!"'"

feature of the flow during compression, can be I I , , , I I , , , I , , , , I I , , I , I I I ,observed from these figures. Squish flow is de- I , , I I I I I , I I , , , , , , I I , r I , , ,

.. d II I 1" I I I , I I , I I I I r , , , , , "fined as the radIally ~war or transver.se gas mo- , , , , I , , 1 , , " '" I , , , , , , , ,tion that occurs dunng the compressIon stroke. , , , 1 1 1 I I I I' ' , , , , , I IAt 800 bfiC (Fig. 5a) it is observed that the ve- I , , , , , , t I , , , , , , , I

locity ~ectors move along with pis.ton. ~t 600 ~ } ) ) ) , \I! \ ' 1f ' ( ( ( ( :bfiC It can be seen that the flow IS tendmg to I

1enter the bowl. At 200 bfiC (Fig. 5c), due to the Ipiston movement towards the cylinder head, less 11' ,11 1 , I r, r 1space is available and the fl~id is push~ in~o the 1 , l' r , rbowl. At 100 bfiC formation of vortices m the 1 t r rbowl is clearly observed. These vortices are same -in size, symmetric about the cylinder axis and are (a) Crank angle 80° billC (Umax=2.304 mIS,opposite in direction. Increased velocities near the Wmax = 11.501 mls)

bowl rim are also observed. Thus the formationof vortices as a result of swirl-squish interaction : : : : : : : : : : : : : : : : : : : : : : : : :are clearly observed. The vortex size is observed : : : : : : : : : : : : : : : : : : : : : : : : :to be larger by about 20. The axial component of : : : ~ ~ : : : : : : : : : : : : : : : : : : : :

I .III d ~ II . th ., , I , , ., , , , , Ive OClty, ""max' ecreases 10 oWIng e piston I , , I , , , ~ , , , , 1 I. fiC " , , , , ".. , , , \ 1 1 I

motion towards dunng compression. I I I f I .., , -, , ~ , I I :" I '/ """"""""' \\\Temperature field development- J.emperature , / _A_.,.,.., ---, I

contours plots are drawn at the same crank : : : : : ~ : ~angle positions where velocity are presented. Fig.6 show the temperature contours in K8 horizontalplane of the engine cylinder during compression.At 800 bfiC (Fig, 6a) the highecst and lowesttemperatures shown are 427 and 425K with a (b)Crankangle600billC(U = 13.166 mis, Wmax=8.361 mls)difference of 2K. This temperature difference in- Max

creases as the piston moves towards roC. This ;:: ,increase is moderate at 600 bfiC (8K). From 200 -

bfiC to 0.020 afiC (Fig. 6e) this difference in-creases from 15 to 66K. At any position of the

-0.22 (c) Crank angle 20° billC(Umax = 15.320m/s, Wmax = 8.340mls)I.

NE :$~=t:=r ! t t ( I JJ;nq==!:-~

~ 0.21 ' , .., , , , I ...IN" , , .., , , , , , ..,OS? -.I I , , \ .-,x '

E"C 0.20..g (d) Crank angle 100billC(Umax= 16.034 mis, Wmax=7.223m1s)E

! 0.19 ~\:~~ I.'t1;'-r-"=2 \ '-d L//

~ 0.18 1 92 61 1,2 17 8

Cronk Angle,degr.esbTDC (e) Crank angle 0.02° aTDC (Umax= 11.486 mis, Wmax=7.647mls)

Fig. 4-Variation of angular momentum Fig. 5-Velocity vectors in JI-J9 axial planes

144 INDIAN J. ENG. MATER. SCI., JUNE 1994

II) (a) Crank angle 800bffiC (0-425, 0-426, A-427) (bi Crankanglc 600bffiC (0-494,0-495, A-496)

(c) Crank angle 200bffiC(0-760, 0- 765,A- 770, + -772,0- 775) (d) Crank angle 19°bffiC(0-826, 0-840,A-850, + -860, 0-865)

.II

(e) Crank angle 0.02° affiC (0-835, 0-850, A-865, + -885, 0- 890, x -899)Fig. 6- Temperature contours in K8 horizontal plane

piston the temperature contours are symmetric peak temperatures always occur within the bowl.about the cylinder axis. Maximum temperature is At roc a peak temperature of 902 K is ob-located at the center of the bowl and reduces served. It is also observed that the temperaturegradually towards cylinder wall. The .Jbservations contours obtained for J5 and J13 (diametricallyforK6andK7planesalsoindicateaverysimilartrend opposite) axial planes ind.icate the same trend inin temperature contours16. their variation as to that of J1 and J9 axial planes.

In axiai planes, Fig. 7, the temperature contours The nature of temperature profiles in axialexhibit the same behavio'-r as seen in respect of planes and their trend support the fact that com-horizontal planes. It is very clearly seen that the pression results in higher temperatures and pro-

1

KRISHNA et al.: FWID FLOW IN A FOUR-STROKE CI ENGINE 145

a Fig. 8 shows the turbulent kinetic energy con-tours during the cornpression process in K8plane. At 800 broC the center of the cylinder has

,." ~ the lowest TKE level. As the radius increases/' TKE also increases and reaches a rnaximurn near

the cylinder wall. The same trend can be ob-served even at 600 broC (Fig. 8b).

Frorn 200 broC onwards the peak values ofTKE can be found near the bowl rim region.Here, cornpared to swirl the squish plays a rnoredominant role which is evident frorn the corre-sponding velocity vector plots. Due to squish, the

(a) Crank angie 800bmC(0-424, 0-425,A-426, +-427, flow. enters the bowl resul~g in TKE r~aching a<>-428) rnaxImum value due to high shear actIon. The

same trend is observed at 100 broC and at 0.020broC. This is due to flow field affect near roC.

TKE contours in J1 and J9 axial planes (Fig. 9)at 800 bTDC show tongue formation near the cy-

t linder head. At the bowl-rim, the TKE contoursfollow the shape of the piston surface. Near thecylinder head there will be a drop in velocitiesdue to the friction resulting in tongue formationwith lower TKE levels. Due to the effect of swirl-ing flow, peak TKEs are observed near the cylin-

(b)Crankangie600bTDC(0-493,0-494,A-495, +-496,<>-497) der walls at 800 broC and 600 broC (Fig. 9b).E: ~~~:~~r:=:i) Frorn 200 broC to roc the squish flow seerns

to be predominant. It is observed that peak TKEalways located at the bowl rim. This is due to theincreased velocities of squish as -a---result of shear

D near the bowl rim. As the cornpression progresses(c)Crankangle200bmC(0-715 0-760 A-776 +-778 <>-780) lowest TKE contours rnove towards the axis of

, , , , the cylinder. Frorn 200 broC onwards the in-

1I:==~~rl)l= crease in TKE is very high. This shows that the.flow field variation near roc is of utrnost im-

portance. The TKE plots obtained for J5 and J13axial planes are found to be symmetric andidentical as those obtained for J1 and J9 planes.

(d)Crankangie 100bmC(0- 725, 0-830,A-862, + -867, <>-868)Comparison with experimental data- The pro-

e== '~f~)~=~ gram is capable of predicting the pressure field

also. Frorn the pressure field it is possible to corn-pute the average pressure. Since the va;}idation ofvelocity field is not possible with the existing set-

(e) Crank angle 0.02° aillC (0-725, 0-800, A-850, +-885, <>- up the validation of the pressure history obtained

900, x -903) frorn the three-dimensional cornputations by corn-paring with experirnental values is carried out.

Fig. 7-Temperature contours in JI-J9 axial planes The rnotored curve frorn a test engine of the...sarne configuration as used in the cornputations is

VIde a good envIronment for proper evaporatIon bt . d th gh .al al At 1 t df th .. t d fu 1 0 ame rou a SIgn an yser. se ec e

0 emJec e e. . f h .. al d .~

b [, ha t . t . Th tu b 1 t h pOInts 0 t e motorIng curve expenrnent ata ISJ ur u ence c rac ens ICS- e r u en c ar- ...

t . t . f fl fi ld al t d th gh cornpared wIth average cylInder pressure obtaIned

ac ens ICS 0 ow e are ev ua e rou b 3 D t tI.1 ... f b 1 k .Y -cornpu a ons.

so vmg transportatIon equatIon or tur u ent m-etic energy (TKE). Contours of turbulent kinetic Fig. 10 shows the variation of average cylinderenergy are plotted both in horizontal and axial pressure with crank angle obtained frorn both ex-planes of the engines during cornpression process. perirnental and cornputational cases during the

,

146 INDIANJ.ENG. MATER SCI., JUNE 1994

(a)~~~~~~~;)800bTDC(0-0.50,0-0.75,~-I, +-5, (b)Crankangie600bTDC(o-o.50,0-5.~-10, +-15)

(c) Crank angie 20° bTDC (0-2, 0-5,"~-10, +-20, <>-30, x -40) (d)Crankangie 100bTDC (0-10, 0-50, ~-70. +-90, <>-120)

" --..r" v, ."" ';~r'!;-2( V

','»,',;I'! ;') I

"t,," :~nrJ?IY') c!

iYJ(ilt;;rl, ,;(Ii: rrbo~Q() ,. ,rnf;'j '::, c;.. :l.: '1'/

j ' ,(' I, ..;., ,.", .'" ., .."11 1,., '; 1 -, '.\.'

' 11.!, ..> '.. II'; f

"', ,

(e) Crank angle 0.020 aTDC (0-5, 0-30, ~-60, +-90, <>-120,x-155)

Fig. 8- TKE contours in K8 horizontal plane

KRISHNA et al.: FLUID FLOW IN A FOUR-STROKE CI ENGINE 147

I.

-COMPUT E 0 VALUES 0

I 0 EXPERIMENTAL VALUES 0~ J

I 3

I0

0-J :J:

.2..~~....~

,~0-

1

(a)Ciank angle 800blDC (0-0.45, 0-0.50, A.. 1, + -5, 0-10,0

X-13) 92 67 1.2 17 TDS 8

Crank angte,degre.s bTOS

Fig. 10-Comparison of cylinder pressure from experimentsand computations

",~ compression process from 900 broC to roC.The graph indicates that the predicted values ofaverage cylinder pressure show a very goodagreement with measured values. The computedpressure has slightly lower values compared to ex-perimental values. This difference increases thecompression process progresses and reaches a

° 50 1 A 1 10 0 18) maximum of 3.5 per cent of experimental values.rankan e60 bmC 0-0. 0- .u- +- , -.(b)C gi (" The reason for computed values bemg lower may

~ ~~~~::~~1~::d:J be attributed to the input value of the initiaJH -charge temperature.

ConclusionsThree-dimensionaJ analysis of the flow field is

~ '- carried out during the compression process of afour-stroke compression ignition engine. The flow

(c)Crankangie200bmC(0-2,0-5,~-10, +-20,0-30, X-40) field is analysed in terms of velocity vectors, tem-

perature and turbulent kinetic energy contours.= ='\~[~~~,== Some of the important global parameters like av-

erage cylinder pressure and totaJangular momentuma~e al~o considered for ~alysis. ~omputed pressurehistones are compared WIth expenmental data. From

.the foregomg anaJysis the following conclusions are(d) Crank angie10oblDC (0-3, 0-10, A-80, +-100,0-120) drawn:

1 The flow field development from 900 broCto roc during compression can be divided into

~ =.~~~~~~~~1=~ two phases. The first phase can be observed frQm900 broC to 400 broC. During this phase theswirl velocities increase moderately. Within thebowl, the charge in lower hori~ontal planes rotatewith IQwer speed compared to the upper planes.

° Above the bowl, from piston surface to cylinder(e)CrankangieO.02 amC(0-14.5,0-30,A-60,+-90,0-120, head swirl velocities reduce. In axial planes the ef-)( -155) fect of bowl is clearly seen. The axial velocity of

the flow field reduces considerably as the pistonFig. 9- TKE contours in JI-J9 axial planes towards the cylinder head.

148 INDIAN J. ENG. MATER. SCI.,JUNE 1994

2 The temperature contours show the expected 2 Subramaniyam S, Sampath S, Srinivasa Rao P & Ganesantrend of high temperature at the core and compa- V, JExp Methods Fluids, 9(1990) 167-174.ratively lower temperature near the cylinder wall. 3 Sub:amaniy~ S'. G~esan V & Srinivas~ ~o. P, An e~-Turbulent kinetic energy contours reflect the perlmental investigatIon of flow characterIStiCS In the swIrl

...chamber of a CI engine, SAE Paper No. 910480,1990.pro~erty of sWIr~g flows along WIth tongue for- 4 El Tahry S H, Khalighi B & Kuzian Jr. W R, Unsteadymatlon near the cylinder head. flow velocity measurements around an intake valve of a

3 Total angular momentum varies approximate- reciprocating engine, SAE PaperNo. 870593, 1987.ly as the compression progresses from 900 broC 5 Dy~r T M: New experimental techniques for in-cylinderto 400 broC engine studies, SAE Paper No. 850395, 1985.

.0 6 Arcoumanis C, Green H G & Whitelaw J H, The applica-4 In the second phase from 40 broC to roc tion of laser Rayleigh scanering to a reciprocating engine

.that can be termed as near roc region, the local mode~ SAE Paper No. 840376, 1984.and global parameters change rapidly. 7 Jhonson S C, Precombustion fueVair distribution in a

5 During this phase the swirl velocities in- stratified charge engine using laser Raman spectroscopy,crease drastically due to the presence of bowl in SAE Paper No. 790433,1979.the piston. Squish flow is observed to be domi- 8 Amsden A A, Ramshaw J D, O'Rourke P J & Ducowic~

. thi h S . 1 d . h ...J K, KIVA: A computer program for two- and three-d,..nant m s p ase. WIr an SqUIS mteractlon IS mensional fluid flows with chemical reactions and fuelalso clearly seen. sprays, Los Alamos National Laboratory Report No. LA-

6 The temperature of the charge increases ra- 10245-MS,1985.pidly and reaches a maximum. The variation of 9 Amsden A A, Ramshaw J D, Cloutman J D & 0' Rourketemperature within the bowl is less compared to P J, I~provements and .extensions to KIVA computer pro-that in the cylinder. Due to squish, the maximum gram, Los Alamos National Laboratory Report No. LA-TKE . al b d b th b 1 .10534-MS, 1985.

IS ways 0 serve to e near e ow nm. 10 Dukowicz J K, J Comput Phys, 35 (1980) 229.7 Total angular momentum decays rapidly as 11 Haselman L C. TDC-A computer code for calculating

the piston reaches roC. chemically reacting hydrodynamic flows in twa-dimen--8 The predicted average cylinder pressure his- sions, Lawrence Livermore Laboratory, Report UCRL-

t . d 1 11 .th th .52931, 1980.ones compare mo erate y we WI e expen- .mental data 12 HIrt C W, Amsden A A & Cook J L, JComput Phys, 11

...(1974)227.It IS m the second phase during which the local 13 PrachtWE,JComputPhys,17(1975)132.

and global parameters of the flow field vary stee- 14 Stein L R, Gentry R A & Hilt C W, Comput Methodsply. This creates an environment that is more Appl Mech Eng, 11 (1977) 57.convenient for fuel-air mning, evaporation and 15 Stull D R & Prophet H, JANAF thermochemical tables,combustion. 2nd Edition, (U S Department of CommercelNational

Bureau of Standards), 1971.R ~ 16 Mohan Krishna D, Multidimensional modelling of fluid

e erences flow in a four-stroke CI engine, M S Thesis, liT, Madras,1 Williams T J & Tindal M J, Gas flow studies in direct in- 1990.

jection diesel engines with re-entrant combustion cham- 17 Heywood J B, Internal combustion engine fundamentalshers, SAE Paper No. SOO027, 1980. (McGraw Hill Series, New York), 1989.

~.'d:~I~~fl.t XII;;'!) ;0'

.-o,p'."".O)':xITs.S:UO ")1;1~ iris1;) b)~,"~.' .i~t!..)(

I 'I.. &'I&IO1O()'J :1XT..ii! 3"4