二沖程發(fā)動機掃氣計算

1、Copyright © 2003 Fluent Inc.EX204 Page 1 of 3A P P L I C A T I O N B R I E F S F R O M F L U E N TIn two-stroke internal combustion(IC engines, each outward strokeof the piston is a power stroke. Toachieve this operating cycle, afresh charge of air and fuel mustbe supplied to the engine cylinde

2、rat a high enough pressure todisplace the burned gases from theprevious cycle. The combinedintake and exhaust process thatclears the cylinder of burned gasesand fills it with a fresh mixture (ofair and fuel is called scavenging.An analysis of scavenging is veryimportant as it plays a major rolein de

3、termining the efficiency of anIC engine.To study the scavenging process,FLUENT's dynamic mesh featureis used to model the motion of thetwo-stroke engine during a typicalcycle. The dynamic mesh modelrequires as input an initial meshalong with a specification of themotion of the moving parts. In t

4、hecase of an IC engine, theprescribed motion of the piston isrequired. The solver moves thecomponents accordingly, and thenautomatically reconstructs themesh, employing one, or acombination of two or three of thethree available remeshingschemes: dynamic layering, springEX204are assumed to be compres

5、sible.The geometry of the diesel engineand its components is shown inFigure 1, with the piston in red.There are three intake portsspaced 120 degrees apart, and onesmoothing, and local remeshing. To track the flow of the two gases,a simulation involving two non-reacting species is used. Onespecies re

6、presents the reactionproducts, and the other representsthe fresh air-fuel mixture.This strategyhelps identifythe fraction ofburned gasesleft in thecombustion chamber afterthe scavengingprocess iscomplete. It cantherefore beused to improvethe design ofthe engine andestimate itsefficiency. Theproperti

7、es of thetwo speciescould bedifferent, butfor simplicity,both are giventhe sameproperties asthose of freshair. Both gasesScavenging in a Two-Stroke IC EngineIn this example, the scavenging process in a two-stroke marine engine is modeledusing the dynamic mesh feature in FLUENT 6.1. Species transport

8、 is used toassess the ability of air injected into the combustion chamber to displace theburned gases from the previous engine cycle. Results show that the process isworking as expected.Figure 1: The geometryof the engine and itscomponentsFigure 2: The surfacegrid, showing the celllayering that resu

9、lts fromthe piston motionCopyright © 2003 Fluent Inc.EX204 Page 2 of 3of these is labeled in the figure.There is one exhaust port, whichis just above the intake ports andjust below the piston in theposition shown. The outlet andinlet are shown in blue and green,respectively.To define the mesh m

10、otion,periodic rigid body motion isprescribed in the fluid zone abovethe piston using a built-in user-defined function (UDF, withparameters that are specified bythe engine manufacturer. Tominimize the cell count andensure proper resolution of theflow, the dynamic layeringtechnique is used as the pis

11、tonmoves. In Figure 2, layers ofprisms are stretched and/or addedto the gray region as the pistonmoves to bottom dead center(BDC, and the cells are collapsedand/or removed from this regionas the piston moves toward topdead center (TDC. The lowerpart of the geometry is meshedwith hexahedral cells. In

12、 theremainder of the geometry, atetrahedral mesh is used. Theoverall mesh size varies from141,000 cells when the piston is attop dead center to 180,000 cells atbottom dead center.In the flow calculation, pressureboundary conditions are applied atthe inlet and outlet boundaries.The exhaust port and t

13、hecombustion chamber (above thepiston in Figure 1 are initializedwith burned gas whereas the inletports and crank-case (shown inpink in Figure 2 are initializedwith fresh air. The standard k-model is employed to captureturbulence. Figure 3 shows aseries of snapshots of contours ofmass fraction of bu

14、rned gas on aplane cut through the center of theengine. As seen from the figure,the upper part of the engine isinitially filled with burned gasesFigure 3 : Contours of mass-fraction of burned gas on a plane through the center of the engine at nine times during the engine cycleCopyright © 2003 F

15、luent Inc.E204 Page 3 of 3(red and the lower part with freshair (blue. As the piston movesfrom top to bottom dead center,uncovering the intake ports, theburned gases are pushed into theexhaust port by the incomingfresh air.Figure 4 shows a series ofsnapshots of velocity vectors onthe same plane. Exa

16、mination ofthese in sequence shows how theincoming flow from the intakeports pushes the burned gas into the exhaust port. The momentumFigure 4 : Velocity Vectors on a plane through the center of the engine at nine times during the engine cyclefrom the inlet ports is first evidentin the top right fra

17、me, the third inthe series. At this point in time,the opening just begins to form,and as a result the flow in thecombustion chamber changesdramatically. The push of burnedgases into the exhaust duct isevident during the next severalframes shown, as the piston dropsdown and begins its return motionback to TDC.In summary, the scavengingprocess in a two-stroke IC enginehas been simulated using thedynamic mesh feature inFLUENT 6.1. The results nicelyillustrate the ability of the inletflow to purge the combus