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Predicting NOX Emissions of Diesel Engine Based on

Fluent

Yang Gao-feng

Department of Thermal Energy EngineeringHenan University of Urban Construction

Pingdingshan, China

[email protected]

Hu Ming-jiang

Department of Thermal Energy EngineeringHenan University of Urban Construction

Pingdingshan, China

[email protected]

Abstractthe calculation model was carried out by use of

commercially software Fluent. The distribution of temperature

and NOx consistency field under different crank angle was

calculated. The results of the analysis found that there was a

greater temperature gradient near the cylinder wall. A larger

temperature difference in the oil-beam external and internal and

a high-temperature region formatted near the nozzle after the

fuel spray and before the start of combustion. In the rapid

combustion phase, combustion region was mainly focused on the

oil-spray periphery. Post-combustion; the main combustion zonenear the wall, Areas of high consistency for NOx was

concentrated in high-temperature and oxygen-rich region. First,

continue to increase, and then declined, and finally remains

unchanged were the lows of the total NOx changing in cylinder.

Keywords-Diesel Eengine; NOx consistency; Temperature

Field; Numerical Calculation

I. INTRODUCTION

Diesel engine is an extremely widely used heat engine; thecombustion will have a direct decision of the diesel engineperformance and emissions. In recent years, the energy andenvironmental issues become more prominent, the performance

of internal combustion engines have become increasinglydemanding. As the diesel engine combustion is a strong andtransient process, so it is very important to research on thecombustion in designing the diesel engine. For the directinjection diesel engine, the combustion chamber shape has thedirect impact on the fuel mixture flowing and forming in thecylinder, and ultimately affects the burning effect. As thecylinder temperature, pressure and mixture ingredients havebeen changed very rapidly with the space and time, so thetraditional performance testing methods can only providelimited information on the fuel mixture flow, the combustion

and emission performance of the diesel engine[13].

With the development of the technologies, such as, the

computer, the image processing and the laser diagnostics, theflow field visualization and numerical simulation techniqueshave made important development in the diesel enginecylinder. The numerical simulation has several merits on theshort cycle, the low cost, informative, research on the varyingparameters of the diesel engine, etc. So it is the importantmeans on studying the fuel mixture flowing and combustionprocess of the diesel engine.

In this paper, take the two-valve 1115 combustion chamber

as the research object, the combustion model on combustion

chamber of the diesel engine was established by the numerical

simulation methods; the flow field and the pressure field in combustion chamber were calculated at the calibrating workingconditions of the diesel engine by the Fluent simulationsoftware; the changing rules of the flow field movement andthe fuel mixture distribution were discussed in the fuelinjection process .which provide some theoretical basis onpredicting the fuel distribution characteristics.

II. THEORETICAL ANALYSIS

Calculation process involves the gas flow in the airway andthe in-cylinder, the constraint equation is as follows:

Mass conservation equation (continuity equation) is:

( ) ( ) ( )0

u v w

t x y z

+ + + =

1

In (1), is the density; t is time; u, v and w are the weightof the velocity vector on the x, y, and z direction.

Momentum conservation equation (N-S equation) is:

( )

( )yxxx zx

x

u pdiv u u F

t x x y z

+ = + + + +

G

(2a)

( )( )

xy yy zy

y

v pd iv vu F

t y x y z

+ = + + + +

G

2b

( )( )

yzxz zz

z

w pdiv w u F

t z x y z

+ = + + + +

G

2c

In (2), p is the fluid pressure on the micro-unit; xx, xy andxz are the weight of the viscous stress acting on the surface ofthe micro-unit; FX, FY and FZ are the volume force on themicro-unit.

Energy conservation equation is

( )( ) ( )c

T

p

kTd iv uT d iv g ra dT S

t c

+ = +

G 3

In (3), cp is the specific heat capacity; T is the temperature;kc is the fluid diathermanous coefficient; ST is the viscousdissipation energy.

The standard k- turbulence model is selected in thecalculations. Its model equations are [4-5]:

978-1-4244-7161-4/10/$26.00 2010 IEEE


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( )( )[( ) ]i t

i j k j

k b M k

kuk k

t x x x

G G Y S

+ = +

+ + +

(4a)

2

1 3 2

( )( )[( ) ]

( )

i t

i j j

k b

u

t x x x

C G C G C S

k k

+ = +

+ + +

(4b)

In (4), Gk is the production item of the rapid kinetic energy(k) caused by the average gradient; Gb is the production item ofthe rapid kinetic energy (k) caused by the flotage; YM is thefluctuation expansion effects of compressible turbulence; C1,C2 and C3 are the empirical constants; k and are therespectively Prandtl numbers corresponding to the rapid kineticenergy (k) and the dissipation rate (); Sk and S are the sourceterms on the user-defined;xi,xj and ui are the respectivelytensors on the coordinate x and velocity (i, j = 1,2,3).

III. MODELING

The numerical simulation of the whole process is from the

crank angle (180 CA) to the crank angle (540 CA), namely,the intake TDC crank angle is 0 CA. the injector is made ofthe uniform four holes, in order to save computing time in thesimulation, the geometric model only takes one quarter of thecylinder shape. Due to the situation that the computationaldomain of the combustion chamber is changed along with thepiston movement, so the simulation method of the combustionchamber is the dynamic grids technique, namely, the grids arechanged along with crank angle. The model and its simulationcalculation steps are as follows:

(1) Based on the PRO-E software, when crank angle is(180 CA), the three-dimensional model is established.

(2) The three-dimensional model is converted into STEP

format, it is imported in Hyper Mesh software in order to plotout the sizing grid; it is exported by the MSH format, andimported into Gambit software in order to set the regional andthe border. The grid of the crank angle 180 CA is shown inFigure 1.

Figure 1. Grid model in the end point

(3) The grid is imported into the fluent software, the partgrids of the combustion chamber are set, and the criteria modelon the k- turbulence is selected. The working fluid used thecompressible ideal gas, the import and export conditions ofpressure, and set the gas temperature in the cylinder; the importand export pressure and temperature are the measured data bythe tests respectively. It is calculated by the solver [6].

Figure 2 is the result comparison between the cylinderpressure test and the simulation at the operating conditions onthe 1900r/min and full load of the diesel engine. From figure 2,the error exists between the cylinder pressure test and thesimulation, but the trend is basically identical, the error is alsowithin the acceptable range. So the model simulation iscredible.

Figure 2. Results comparison on experimental and simulation

IV. SIMULATION RESULTS AND ANALYSIS

A. Temperature Field Analysis

Figure 3 is the temperature distribution of the cylinder onthe speed of 1500r/min of the full-load for diesel engine (thevalue unit of the figure is K). From figure 3, in the compressionprocess, except having in larger gradient, the temperaturedistribution in the cylinder wall is more symmetrical, beforethe fuel is fired, due to the fuel droplets that are evaporated byabsorbing heat, the lower temperature fuel and the mixture inthe cylinder Mass is diathermanous and mixed, the sprayingfuel temperature difference between external and internal islarger, it is shown in the Figure.3 (b).

Through a series of physical and chemical preparations, theevaporating fuel mixture components external the oil are self-ignition, in the nozzle, the gaseous components near the firedfuel is most in order to form the high temperature region, it isshown in the Figure.3(c).

In the rapid combustion phase of the diesel engine,combustion areas are mainly concentrated in the peripheral fuelbundles; the main reflected features are in the proliferatingcombustion, which are shown in the Figure.3 (d) and Figure3(e).

At the post-combustion, the main combustion area is nearthe wall region, which is shown in the Figure.3 (f).From theFigure.3 (f), the main reflected features are in the proliferatingcombustion.

(a) 340 CA


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(b) 350CA

(c) Fire starting point (356.5 CA)

(d) 360 CA

(e) Maximum combustion pressure point (366 CA)

(f) 400 CA

Figure 3. Temperature distribution in the cylinder of the diesel engine

B. Consistency Distribution of NOXand O2

Figure 4 is the consistency distribution of NOX and O2 onthe speed of 1500r/min of the full-load for diesel engine (thevalue corresponding to contour map is the gas component massfraction).

From figure 4, the NOX formation mainly occurred in therapid combustion period. The consistency map of NOX and O2is compared with the temperature map; the high consistency ofNOX is the region on the high temperature and the rich O2 .

(a)NOx of the maximum combustion pressure point (366 CA)

(b) O2of the maximum combustion pressure point (366 CA)

(c)NOx of the pressure point (400 CA)

(d)NOx of the pressure point (485 CA)

Figure 4. consistency distribution of NOx and O2in the cylinder

Figure 5 is the cumulative value of NOX at different crankangle on the speed of 1500r/min of the full-load for dieselengine.

From figure 5, the consistency map of NOX is comparedwith the temperature map; the consistency distribution andtemperature field distribution of the NOX have great

similarities, which show that the temperature field has a crucialimpact on the NOX distribution.

In the early ignition, NOX maximum consistency is 0.4263 10-4, which is corresponding to the temperature region morethan 1800K. With the fuel burnt and the piston moved upward,the combusted zone temperature has been increased rapidly,NOx consistencies also increased sharply.


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Figure 5. Cumulative value of NOx at different crank angle

As the burning progressed, at first, the mixed gas in theburning flame reaction zone is burnt out, the temperature ofthis part area began to decline, and the regional of the NOXlargest consistency is transferred from the flame to the wall.The highest consistency regions of the figure are the maximumtemperature zone (above 1800K). To the TDC, the piston movedown, the temperature in the cylinder and the NOXconsistencies began to decline, but the NOx total is raising dueto the fact that generated amount of the NOX in the cylinder aregathered in the time and space.

According to the calculation of the amount of NOXgenerated in the cylinder, all the generated amount of the NOX

from the ATDC16 to ATDC20 begun to decline at theATDC30 , the most gas temperature in the cylinder declinedto 1500K. At this time, the highest consistency and the totalgenerated volume of the NOx in the cylinder are continuouslydescended after the ATDC60, the total generated volume ofthe NOx remained basically unchanged.

Through simulating the spraying process of the oil particlesby the CFD method, the great deal of visual information isobtained in order to analyze the familiar development processof the oil particles, which provides a theoretical basis ondesigning and improving the diesel engine. The compressionair of the combustion chamber has a great impact on the fuelmixture process, the pit makes the flow field of the cylinder togenerate the strong radial squeeze flow at the compress end, thestrong radial squeeze makes not only the swirl in thecombustion chamber greatly increase, but also have a verystrong local turbulent field at the entrance of the pit top.

Therefore, as the compression is progressed, the impact ofthe initial the flow field generated by the intake processbecomes smaller and smaller, the turbulence characteristics inthe combustion chamber are largely determined by thegeometry of the combustion chamber, the turbulent structure isnon-uniform and anisotropic.

V. CONCLUSIONS

Based on analyzing the temperature field and NOX

consistency field in the combustion chamber of the dieselengine, the results show that:

(1) in the compression process, except having in largergradient, the temperature distribution in the cylinder wall ismore symmetrical, before the fuel is fired, due to the fact thatfuel droplets are evaporated by absorbing heat, the lowertemperature fuel and the mixture in the cylinder Mass isdiathermanous and mixed, the spraying fuel temperature

difference between external and internal is larger; In the rapidcombustion phase of the diesel engine, combustion areas aremainly concentrated in the peripheral fuel bundles; At the post-combustion, the main combustion area is near the wall region,the main reflected features are in the proliferating combustion.

(2) The distribution of the temperature field has a crucialimpact on the NOX formation, the high consistencies of NOX isthe region on the high temperature and the rich O2.

(3) The variation of the total NOX in the cylinder is that it isincreased continually, then declined, and finally remainedunchanged.

ACKNOWLEDGMENT

This work was supported by Natural Science Foundation ofEducation Department of Henan Province (No. 2008A470008and 2010B470003).The contents of this paper reflect the viewof the authors who are responsible for the facts and accuracy ofthe data presented in herein. The contents do not necessarilyreflect the official views or policies of Henan University ofUrban Construction. This paper does not constitute a standard,specification or regulation.

REFERENCES

[1] Lin Xuedong,Wang Haijun,and Wan Xiaodon. Evaluation forConfiguration Characteristic of Reentrant Combustion Chamber onHeavy-duty DI Diesel Engine, Chinese Internal Combustion EngineEngineering. vol. 28, pp. 5660, Mar 2007.

[2] LV Ji-zu,BAI Min-li. Reaearch on Influence of Fuel Spray Atomizationand Combustion on Air Flow in Cylinder, Chinese Internal CombustionEngine Engineering. vol.28,pp. 2529, Jun 2007.

[3] YE jian, LIANG Gui-hua, and LIU Li-hu. Simulation of the effect onthe combustion capability of135diesel engines by changing thecombustion chamber, Journal of Dalian Maritime University. vol. 34,pp. 6167, Jun 2008.

[4] ZHAO Chang-pu,SONG Chong-lin,and LI Xiao-juan. Effects of FuelInjection Timing and Combustion Chamber Geometry on CombustionProcess and Emissions of Diesel Engine, Journal of CombustionScience and Technology. vol. 15, pp. 393398, Oct 2009.

[5] LIN Xuedong,CONG Chunmei,and CHEN Xuemei. Geometriccharacteristic of DI diesel engine shallow reentrant combustion chamberand its effect on emissions, Journal of Jilin University (Engineering andTechnology Edition). vol. 35, pp. 1823, Jan 2005.

[6] YANG Xu-yuan, ZUO Cheng-ji. Influence of Offset of CombustionChamber on Combustion Processes of Diesel Engine,I.C.E&Powerplant. vol. 30, pp. 13, Feb 2009

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