Research on Predicting Fuel Spray Characteristics of Diesel Engine

Hu Mingjiang1,Shen Chaoying2 1.Henan University of Urban Construction, Pingdingshan ,Henan,467036,China; 2. Shangqiu Vocational & Technical

College,Shangqiu,Henan ,476000,China humingjiang@tom.com

Abstract—Based on the particular structure of the ω combustion chamber, the three-dimensional dynamic CFD model of the diesel engine fuel injection characteristics was established, the changing mechanism on the fuel mixture flow field and pressure field in the combustion chamber were predicted by the commercial Fluent software. The results showed that after the fuel began to spray, the oil-beam-driven vortex flow was essential to maintain. Late in the combustion, fuel droplet particles sprayed into the wall, and fuel components concentrated near the wall in the combustion chamber. The reverse squish was formatted in the initial expansion stroke.

Keywords-DieselEengine;CombustionChamber;Extrusion Stream;Numerical Calculation.

I. INTRODUCTION Diesel engine is an extremely widely used heat engines,

the combustion will have a direct decision of the diesel engine performance and emissions. In recent years, the energy and environmental issues become more prominent, the performance of internal combustion engines have become increasingly demanding. As the diesel engine combustion is a strong and transient process, so it is very important on researching on the combustion in designing the diesel engine. For the direct injection diesel engine, the combustion chamber shape has the direct impact on the fuel mixture flowing and forming in the cylinder, and ultimately affects the burning effect. As the cylinder temperature, pressure and mixture ingredients have been changed very rapidly with the space and time, so the traditional performance testing methods can only provide limited information on the fuel mixture flow, the combustion and emission performance of the diesel engine[1～3].

With the development of the technologies, such as, the computer, the image processing and the laser diagnostics, the flow field visualization and numerical simulation techniques have made important development in the diesel engine cylinder. The numerical simulation has several merits on the short cycle, the low cost, informative, research on the varying parameters of the diesel engine, etc. So it is the important means on studying the fuel mixture flowing and combustion process of the diesel engine.

In paper, the two-valve 1115 combustion chamber as the research object, the combustion model on ω combustion chamber of the diesel engine was established by the numerical simulation methods; the flow field and the pressure field in ω combustion chamber were calculated at

the calibrating working conditions of the diesel engine by the Fluent simulation software; the changing rules of the flow field movement and the fuel mixture distribution were discussed in the fuel injection process .which provide some theoretical basis on predicting the fuel distribution characteristics.

II. THEORETICAL ANALYSIS Calculation process involves the gas flow in the airway

and the in-cylinder, the constraint equation is as follows: ①Mass conservation equation (continuity equation) is:

0)()()( =∂

∂+∂

∂+∂

∂+∂∂

zw

yv

xu

tρρρρ （1）

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

②Momentum conservation equation (N-S equation) is:

)()( uudivtu ρρ +

∂∂

xzxyxxx F

zyxxp +

∂∂

+∂

∂+

∂∂

+∂∂−=

τττ (2a)

)()( uvdivtv ρρ +

∂∂

yzyyyxy Fzyxy

p +∂

∂+

∂∂

+∂

∂+

∂∂−=

τττ （2b）

)()( uwdivtw ρρ +

∂∂

zzzyzxz F

zyxzp +

∂∂

+∂

∂+

∂∂

+∂∂−=

τττ （2c）

In (2), p is the fluid pressure on the micro-unit; τxx, τxy and τxz are the weight of the viscous stress acting on the surface of the micro-unit; FX, FY and FZ are the volume force on the micro-unit.

③Energy conservation equation is ( )

( ) ( )cT

p

kTd iv uT d iv grad T S

t cρ

ρ∂

+ = +∂

（3）

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

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

])[()()(

jk

t

ji

i

xk

xxku

tk

∂∂+

∂∂=

∂∂

+∂

∂σμμρρ

kMbk SYGG +−−++ ρε (4a)

2010 International Conference on Intelligent Computation Technology and Automation

978-0-7695-4077-1/10 $26.00 © 2010 IEEE

DOI 10.1109/ICICTA.2010.304

69

])[()()(

j

t

ji

i

xxxu

t ∂∂+

∂∂=

∂∂

+∂

∂ εσμμρερε

ε

εεεε

ερε Sk

CGCGk

C bk +−++2

231 )( (4b)

In (4), Gk is the production item of the rapid kinetic energy (k) caused by the average gradient; Gb is the production item of the rapid kinetic energy (k) caused by the flotage; YM is the fluctuation expansion effects of compressible turbulence; C1ε, C2ε and C3ε are the empirical constants; σk and σε are the respectively Prandtl numbers corresponding to the rapid kinetic energy (k) and the dissipation rate (ε); Sk and Sε are the source terms on the user-defined;xi, xj and ui are the respectively tensors 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 of the uniform four holes, in order to save computing time in the simulation, the geometric model only took one quarter of the cylinder shape. Due to the computational domain of the combustion chamber are changed along with the piston movement, so the simulation method of the combustion chamber is the dynamic grids technique, namely, the grids are changed along with crank angle. The model and its simulation calculation 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 plotting out sizing grid ;it is exported by the MSH format, and imported into Gambit software in order to setting the regional and the border. The grid of the crank angle 180 ° CA is shown in Figure 1.

Figure 1. Grid model in the end point

(3) The grid is imported into the fluent software, the part grids of the combustion chamber are set, and the criteria model on the k-ε turbulence is selected. The working fluid is used the compressible ideal gas, using the import and export conditions of pressure, and setting the gas temperature in the cylinder, the import and export pressure and temperature are respectively the measured data by the tests. It is calculated by the solver [6].

Figure 2 is the result comparison between the cylinder pressure test and the simulation at the operating conditions on the 1900r/min and full load of the diesel engine. From figure 2, the error is existed between the cylinder pressure test and the simulation, but the trend is basically identical, the error is also within the acceptable range. So the model simulation is credible.

Figure 2. results comparison on experimental and simulation

IV. SIMULATION RESULTS AND ANALYSIS

A. Flow Field Analysis Figure 3 is the velocity field of fuel injection process on

the calibration operating conditions. From figure 3, When the fuel are begun to spray, the airflow around the fuel is moved to the oil-beam injection direction acting by the fuel injection speed, it is shown in the Figure 3(a). At the same time, the previously formed squeeze flow and swirl are sprayed to the oil from two directions on the upper and lower, the clear spray vortex is formed in the upper part of the fuel, which made the fuel particles to mix commendably with the air mixture. After the Burning is begun, the air-driven vortex flow are basically maintained in the spray cylinder, it is beneficial that the flame spread to the direction of the oil-beam injection, in order to take away the fuel evaporated components and mix and evaporate with the fuel Particle inside the spray. It is shown in the Figure 3(b) and Figure 3(c). At the initial time of the expansion, due to the piston is run downward, the gas in the combustion chamber are flowed rapidly outward the annular space (Figure 3d), reverse squeeze flow is formed, which is beneficial that air mixture inside the combustion cylinder are flowed out, so that it could be further mixed with the air inside the combustion cylinder. It is shown in the Figure 3(d).

(a) 350 º CA

70

(b) Fire starting point (356.5 º CA)

(c) 360 º CA

(d) Maximum combustion pressure point (366 º CA)

Figure 3. standard fixed-point diesel combustion velocity vector

B. Analysis of Fuel Injection Spray Figure 4 is the particulate droplets of in the cylinder of

the diesel engine running at the standard fixed-point, From Figure 4, the corresponding value of particles exist in the time is seconds. Figure 5 are the mass distribution of the diesel components in the cylinder on the diesel engine and dual-fuel diesel engine, From Figure 5, the corresponding value are the gas component mass fraction.

From Figure 4 and Figure 5, after the diesel engine continuously injects the fuel about 15 º CA, the oil particles run into the wall. As the vortexes are acted in the cylinder, the oil particles run tend to flow direction on the gas swirling particles, which are shown in Figure 4(b) and Figure4(c). From Figure 5, at the later of the combustion, the oil particles sprayed into the wall, the fuel components concentrated near the wall of the combustion chamber. This time, the fuel combustion is based on the diffusion combustion on the oil particles of the wall.

(a) after the injection 5 º CA

(b) after the injection 15 º CA

(c) after the injection 15 º CA (Overlooking)

(d) After the injection 30 º CA

Figure 4. fixed-point fuel injection diesel engine particulate droplets

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(a) 5 º CA

(b) 15 º CA

(c) 30 º CA

(d) 400 º CA

Figure 5. mass fraction of component in longitudinal cross-section

Through simulating the spraying process of the oil particles by the CFD method, the great deal of visual information are obtained in order to analyze familiarly the development process of the oil particles, which provide a theoretical basis on designing and improving the diesel engine. The compression air of the combustion chamber has a great impact on the fuel mixture process, the pit makes the flow field of the cylinder to generate the strong radial

squeeze flow at the compress end, the strong radial squeeze makes not only the swirl in the combustion chamber greatly increase, but also have a very strong local turbulent field at the entrance of the pit top.

Therefore, as the compression is progressed, the impact of the initial the flow field generated by the intake process becomes smaller and smaller, the turbulence characteristics in the combustion chamber are largely determined by the geometry of the combustion chamber, the turbulent structure is non-uniform and anisotropic.

V. CONCLUSIONS Based on the particular structure of the ω combustion

chamber, the three-dimensional dynamic CFD model of the diesel engine fuel injection characteristics was established, the changing mechanism on the fuel mixture flow field and pressure field in the combustion chamber were predicted by the commercial FLUENT software. From the simulation results, the main conclusions are the following:

(1) After the fuel began to spray, the oil-beam-driven vortex flow was essential to maintain. Late in the combustion, fuel droplet particles sprayed into the wall, and fuel components concentrated near the wall in the combustion chamber. The reverse squish was formatted in the initial expansion stroke.

(2) After the diesel engine continuously injects the fuel about 15 º CA, the oil particles run into the wall, the oil particles run tend to flow direction on the gas swirling particles.

(3) At the later of the combustion, the oil particles sprayed into the wall, the fuel components concentrated near the wall of the combustion chamber. This time, the fuel combustion is based on the diffusion combustion on the oil particles of the wall.

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dimensional Simulation of Effect of Combustion Chamber Geometry on Emission in DI Diesel Engine,Chinese Internal Combustion Engine Engineering, 2007, 28:11-15.

[2] YU Yu-song,LI Guo-xiu,and LIU Jian-ying.Three-Dimension Models of the Effects of Combustion Chamber Geometry on Combustion Characteristics in a DI Diesel Engine,JOURNAL OF BEIJING JIAOTONG UNIVERSITY, 2007, 31:115-119.

[3] ZHAO Chang-pu, SONG Chong-lin,and LIXiao-jua. Effects of Fuel Injection Tim ing and Combustion Chamber Geometry on Combustion Process and Emissions of Diesel Engine,Journal ofCombustion Science and Technology,2009,15:393-398.

[4] Lin Xuedong,Wang Haijun,and Wan Xiaodong.Evaluation for Configuration Characteristic of Reentrant Combustion Chamber on Heavy-duty DI Diesel Engine, Transactions of the Chinese Society for Agricultural Machinery, 2007,38:56-60.

[5] Yu Jianghong,Xiao Jinsheng, and Zhu Zhongbo.Parallel Multi-grid Algorithm and Its Performance of Fluent Software,Journal of Wuhan University of Technology, 2009,33:133-136.

[6] JIANG Guang-jun, GAO Guo-zhen, and WEN hua,Three-Dimensional Numerical Simulation of Fuel Flow in Injector of Diesel Engines,Journal of Nanchang University,2008,30:57-59.

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