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SAE TECHNICALPAPER SERIES 2002-01-0893

Experimental Analysis on Soot FormationProcess in DI Diesel Combustion Chamber

by Use of Optical Diagnostics

Jiro Senda, Dae Choi, Makoto Iwamuro and Hajime FujimotoDoshisha Univ.

Go AsaiYanmar Diesel Engine Co., Ltd.

Reprinted From: Compression Ignition Combustion andIn-Cylinder Diesel Particulates and NOx Control

(SP1698)

SAE 2002 World CongressDetroit, Michigan

March 4-7, 2002

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2002-01-0893

Experimental Analysis on Soot Formation Process In DI Diesel Combustion Chamber

by Use of Optical Diagnostics Jiro Senda, Dae Choi, Makoto Iwamuro and Hajime Fujimoto

Doshisha Univ.

Go Asai Yanmar Diesel Engine Co., Ltd.

Copyright 2002 Society of Automotive Engineers, Inc.

ABSTRACT

Soot formation process inside the combustion chamber of an DI diesel engine is focused as a phenomenological basic scheme by using several optical diagnostics technique for the improvement of diesel exhaust emission. We have conducted the series of optical measurement research for the clarification of combustion field in an DI diesel engine. Then, this paper is a kind of review by adding the fuel vapor properties and particle image velocimetry (PIV) analysis with focusing the soot formation process. The experiments were carried out in a small sized high-speed DI diesel engine installed with an optical access view. The spray characteristics and its flow field in 2-D field were measured by laser sheet scattering (LIS) method and PIV scheme. Fuel vapor concentration field was also detected quasi-quantitatively as 2-D distribution by laser-induced exciplex fluorescence (LIEF) method, which is based on application of the TMPD/naphthalene exciplex system, relating to soot formation (initiation) process. Here, the heterogeneity of the vapor concentration should be discussed for the soot initiation, and the 2-D distribution of soot particles in several planes in the chamber was obtained with applying a laser-induced incandescence (LII) method. Further, measurement of the flow field inside the flame after an ignition was tried to clarify the effect of turbulence properties on soot formation and oxidation processes. Then, the effects of the spray properties, the vapor concentration, flow field characteristics on the soot formation process were discussed based on the experimental date obtained by optical diagnostics.

INTRODUCTION

In diesel engines, the trade relation for reducing both soot (particulate matter) and NO emissions can not be avoided. Thus, the way to reduce soot or PM is contrary to the reduction of NO in diesel combustion process. Then, several combustion systems concerning with the

injection system, such as high pressure fuel injection and HCCI combustion process, have been applied in recent years to reduce exhaust emissions with keeping a thermal efficiency. However, these attempts seem to be insufficient for simultaneous reduction of both soot and NO with keeping the thermal efficiency relating to lower CO2 emission. Accordingly, it is considered that the soot formation and oxidation process should be focused basically during the combustion process inside the chamber by applying the several optical diagnostic measurements and a physical-chemical modeling approach.

In general, many studies for soot reduction in a diesel engine may be classified into three approaches: improvement of the combustion characteristics [1-4], application of alternative fuels [5,6], and investigation of soot formation and oxidation mechanism based on the understanding of unsteady spray diffusion flame in a DI diesel engine [7-12].

A phenomenological or empirical model of DI diesel engine combustion is very useful to understand its emission characteristics. Dec [10] has proposed a phenomenological model or conceptual model of how DI diesel combustion occurs. The model has been derived from various laser-sheet imaging diagnostics and other optical measurements [13-17]. Although the effects of wall interaction and swirl flow which were unavoidable in small-sized high speed DI diesel engines were not considered, the model could provide the new insight into the most likely mechanisms for soot formation and oxidation. Furthermore, it is considered that this kind of phenomenological model could provide a framework for interpreting experimental measurements and guide the development of numerical modeling of a combusting diesel fuel jet. Kazakov and Foster [12] have implemented their phenomenological model of soot formation in KIVA-II CFD code, and confirmed that the analysis of soot formation process

Fuel Injection

Mixture Formation Process

Ignition and Combustion

Other Factor

Fuel Properties

Ambient Air Temperature and Pressure

?Injection Pressure?Injection Timing

Direct Photography

Analysis of Heat release rate

Elastic (Mie) Scattering for Fuel Droplet

Laser Shadowgraph Method

Particle Image Velocimetry (PIV)

Elastic(Mie) Scattering for Soot Particle

Flame Characteristics

Combustion Properties

Fuel Spray Behavior

Soot Formation Process

Flow Field in the Spray

Fuel Vapor Distribution and its Heterogeneity

Atomization

Vaporization

?Injection Rate?Injection Duration

Mixing

Ignition Delay

Ignition

Combustion

Turbulent Mixing Condition

Droplet SizeDroplet DistributionSpray Penetration

Fuel Vapor Concentration

In-Cylinder Flow- Swirl / Squish

?Fuel Pyrolysis

Engine Design Specification

?Nucleation?Surface Growth Coagulation

Soot Formation

?Low Temperature Oxidation?High Temperature Oxidation?Molecular Growth Reaction

Figure 1. Diesel combustion process

Soot

Fuel Injection

Mixture Formation Process

Ignition and Combustion

Other Factor

Fuel Properties

Ambient Air Temperature and Pressure

Injection PressureInjection Timing

Direct Photography

Analysis of Heat release rate

Elastic (Mie) Scattering for Fuel Droplet

Laser Shadowgraph Method

Particle Image Velocimetry (PIV)

Elastic(Mie) Scattering for Soot Particle

Flame Characteristics

Combustion Properties

Fuel Spray Behavior

Soot Formation Process

Flow Field in the Spray

Fuel Vapor Distribution and its Heterogeneity

Atomization

Vaporization

Injection RateInjection Duration

Mixing

Ignition Delay

Ignition

Combustion

Turbulent Mixing Condition

Droplet SizeDroplet DistributionSpray Penetration

Fuel Vapor Concentration

In-Cylinder Flow- Swirl / Squish

Fuel Pyrolysis

Engine Design Specification

NucleationSurface Growth Coagulation

Soot Formation

Low Temperature OxidationHigh Temperature OxidationMolecular Growth Reaction

MEASUREMENT

Soot

based on model predictions indicates consistency with the experimental observations derived from laser-sheet imaging studies.

On the other hand, according to the work on the two step semi-empirical model of Hiroyasu and Kadota [18] and Fusco et al. [19], initial soot formation and auto-ignition have been thought to be strongly related to the fuel vapor concentration. Then, the fuel vapor concentration field including its heterogeneity should be considered as one of key factors as well as the temperature and the turbulent flame fields for the soot formation properties.

FOCUS OF THE STUDY

In general, diesel combustion process is classified into several physical and chemical processes as shown in Fig. 1. In the combustion chamber, diesel combustion is proceeding through the spray dispersion, the droplet evaporation, its turbulent mixing with ambient air, the mixture formation and fuel vapor distribution (spatially homogeneity or heterogeneity), auto-ignition, formation of the combustion products (including soot and NO), under the high-pressure, high-temperature and high-turbulent fields.

The present authors have conducted several experimental studies using optical measurements for the purpose of developing a phenomenological combustion model of a typical small-sized DI diesel engine. In previous studies [20-24] the temporal and spatial characteristics related to soot formation and oxidation

have been investigated in the following terms: a semi-quantitative measurement of soot particle size and number density by simultaneous measurements of laser-induced incandescence (LII) and elastic (Mie) scattering [20-22], the characteristics of self emissions of the OH radical as a soot oxidant [21], flame temperature and the characteristics of soot particles by the combination of two color method and cross correlation velocimetry [22], 2-D soot distribution on mid-plane of a fuel jet by LII [23,24], and liquid and vapor fuel distribution by elastic scattering and laser shadowgraphy [24]. Further, in the previous paper [25], spatial distribution of soot particles, obtained by LII, was compared to experimental results obtained by high-speed direct photography of self flame emission and by laser shadowhraphy. To investigate the effect of fuel vapor concentration on the process of initial combustion and soot formation, laser-induced exciplex fluorescence (LIEF), developed by Melton [26,27], was applied to unfiring conditions. In this paper, correlations by combinations of experimental results were investigated, and the effect of fuel vapor concentration on auto-ignition and initial soot formation is discussed.

This paper is evidently a review of optical diagnostic research in DI diesel combustion by our institute in Doshisha University with focusing the soot formation process. In particular, spatial distribution of fuel vapor and its heterogeneity by LIEF, and the flow field by PIV method are discussed relating to the soot formation process.

Laser Induced Exciplex Fluorescence (LIEF)

Laser Induced incadescence (LII)

Turbulent Mixing between Spray and Ambient Air

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EXPERIMENTAL DISCRIPTION

OPTICAL ACCESS ENGINE

The optical access engine used in the present study is a single cylinder, water-cooled, 4-stroke, and small-sized direct injection diesel engine based on the Yanmar TF120 production engine. Although the test engine has been changed into a research engine with a classic bottom view type via design change for various optical measurements, most of the original parts have been maintained. For several optical measurements, this test engine utilizes an extended cylinder piston and liner with piston crown quartz window. In addition, two additional quartz windows are located at the top of extended piston and cylinder liner provide the orthogonal access required for two dimensional laser imaging diagnostics.

Furthermore, as can be seen from Fig. 2, these additional quartz windows allow the incident laser sheet to enter the combustion chamber parallel to the cylinder head and along the axis of the fuel jet, facilitating visualization of the 2-D development processes of fuel jet and flame development. Figure 2 presents the schematic diagram of a test engine, and its specifications are summarized in Table 1. The test engine has the following specifications: a bore of 92 mm, a stroke of 96 mm, a stroke volume of 0.638 liters, and a compression ratio of 18.4. This test engine has a Bosch-type injection pump. A fuel is injected through a 4-hole nozzle tip. The nozzle hole diameter is 0.26 mm and the declination of the fuel jet axis is about 16 degrees from the cylinder head as shown in the figure.

OPERATING CONDITIONS AND PROCEDURE

The engine operation conditions are listed in Table 2. All the data and optical image sets obtained in the present study were taken at an engine speed of 1000 rpm. This engine speed was selected to minimize the cycle-to-cycle variation. For stable data acquisition during motoring and firing cycles, the engine speed was kept at a certain one by a dynamometer, which is directly linked to the test engine. To

warm the engine prior to experiments, coolant water heated by an electric heater was circulated in the water jacket until the test engine reached 363 K. A fuel was injected through a sub-nozzle system that allows fuel injection to occur at a given injection timing. For data acquisition, the fuel was injected into the combustion chamber through a main injection nozzle by injection onset signal from control unit. Otherwise the fuel returned to the fuel tank through a sub-nozzle. The injection period was 10 degrees (from 10BTDC to TDC) as confirmed not only by the needle lift profile detected by the needle lift sensor equipped at the fuel injector but also by comparison between sets of laser shadowgraph and crank angle plate images obtained by simultaneous visualization. At that time, the injection quantity was 22 mg.

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accurately inside the spray and near the shearing region. The time resolution is 1 [deg. CA] of the engine due to the limitation of the control unit in the measurement. Comparing to the images of the evaporative spray obtained by laser shadowgraphy, it is found that the liquid phase of the spray is not affected well by the swirl flow in the chamber.

SPRAY DROPLET VELOCITY FIELD BY PIV

In this study, the combustion chamber was divided into three regions to investigate the spatial dependence of the spray characteristics and the vapor concentration field on initial combustion properties, as shown Fig. 5. Then, PIV scheme was applied to the images as shown in Fig. 4 to identify the droplet velocity field, and the spatial .distributions of an vorticity and turbulence intensity.

According to the results of combustion process described later, an auto-ignition of the spray occurs around -4ATDC under this operating conditions. Therefore, a period from -6ATDC to -2ATDC is focused to analyze the flow field. Fig. 6 demonstrates the representative results of Mie scattering image, a velocity vector, vorticity, and

turbulence intensity in the spray at -6ATDC, -4ATDC and -2ATDC. In the velocity vector results, the main jet region near the spray axis has a state of meandering structure with higher velocity. On the contrary, it seems that the droplets in shearing region have several dispersing directions with lower velocity attribute the vortex like structure and the shearing effect with the ambient air.

Figure 7 represents the change in mean droplet velocity against the spray penetrating direction. Here, the mean velocity was defined at each distance position from the nozzle. Actually speaking, the detection of the droplet velocity in region A of near the nozzle is pretty difficult owing to the dense droplet number density. Then, the

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accuracy of the results in region A is not so good. In regions B and C, the mean velocities are remarkably small compared to the injection velocity at the nozzle hole, and they are keeping the relatively constant value over the penetrating distance. This result seems to be attributed to the combined effect of the velocity reduction along the distance and the swirl flow. And it should be noticed that the wall interaction effect is included in the results of region C.

Further, velocity range distribution at each crank angle is

shown in Fig. 8 for each region A, B and C in the chamber.

Next, spatial distribution of the vorticity and turbulence intensity inside the spray are discussed. According to the results in Fig. 6, vorticies initiate at the region near the spray axis at -4ATDC, and those are growing up along the downstream region with a state of left and right alternatively to the spray axis as the time proceeds. Thus, it is found that the mixing near the shearing region is promoted at -4ATDC and -2ATDC. And, higher

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and C are normalized by the highest fluorescence obtained by image processing in region A. The fuel vapor concentration in the upper plane A is higher than that in any other plane. As a result, the first ignition takes place surely at this region where the relatively high concentration of vapor phase fuel is prepared subsequent to the active turbulent mixing process. The other marked matters are that the normalized fluorescence intensity on the mid-plane B has almost the same trend as that on the upper plane A and there is little change in this intensity on the lower plane C. Namely, there is some capability of the first ignition on the plane B, however, there is no capability on the plane C.

DIFFUSION CONDITION OF VAPOR PHASE FUEL

The entropy indicates the probability of state in the statistical dynamics. The entropy tends to be high with the progress of uniformity in the mixing process. Figure 17 presents the diffusion condition over the whole image of vapor phase fuel. Considering the physical meaning of S*, it becomes larger obviously as the diffusion process is progressing. The calculation of entropy S* was carried out for every images obtained from three different locations of the incident laser sheet, that is, the upper plane A, the mid-plane B and the lower plane C. All the entropy values is the ensemble-averaged over 10 images. The temporal history of S* shows Gaussian-like profile except for the case of the lower plane C. It is noted that S* at around ignition point shadowed in the figure is higher than that in the other zone of crank angle. And it is noteworthy that S* at around ignition timing in the case of the upper plane A is higher than S* in that of the mid-plane B. From these facts, it is very clear that the diffusion state just before and/or after the ignition is much more progressing in this area. In the late crank angle, it is found out that there is a great deal of fluctuation in the behavior of diesel flame and in that of signal intensities of laser induced incandescence related to the spatial distribution of soot particles [23, 25]. With respect to these facts, it is probable that the unstable diffusion condition of vapor phase fuel might be also responsible for the cycle-to-cycle variation. The averaged S* in the case of the plane A is the highest and that in the case of the plane C is the smallest. Namely, the capability of the ignition in the latter case is very small, and the plane B has this capability although it is smaller than that in the case of plane A.

Figure 18 summarizes entropy S* evaluated on each plane in regions A, B, and C mentioned in Fig. 5 at -4ATDC just at the break out of the ignition. The region A shows the most diffused condition due to the vapor phase fuel. And averaged S* in the case of the region A is much higher than those in the other regions. Therefore, the most stable diffusion condition with the vaporized fuel takes place on the upper plane A at the region A. Consequently, it is able to be speculated that the ignition phenomena are very close to the diffusion state of vapor phase fuel.

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Method, SAE Paper No. 970313, 1997.

2. Suzuki, H., Koike, N., Ishii, H., and Odaka, M., Exhaust Purification of Diesel Engines by Homogeneous Charge with Compression Ignition Part 2: Analysis of Combustion Phenomena and NOx Formation by Numerical Simulation with Experiment, SAE Paper No. 970315, 1997.

3. Senda, J., Ikeda, M., Yamamoto, M., Kawaguchi, B., and Fujimoto, H., Low Emission Diesel Combustion System by Use of Reformulated Fuel with Liquefied CO2 and n-Tridecane, SAE Paper No. 1999-01-1136, 1999.

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6. Kitamura, T., Ito, T., Senda, J., and Fujimoto, H., Detailed Chemical Kinetic Modeling of Diesel Spray Combustion with Oxygenated Fuels, SAE Paper No. 2001-01-1262, 2001.

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982463, 1998.

13. Dec, J. E., Soot Distribution in a DI Diesel Engine Using 2-D Imaging of Laser-Induced Incandescence, Elastic Scattering, and Flame Luminosity, SAE Paper No. 920115, 1992.

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19. Fusco, A., Knox-Kelecy, A. L., and Foster, D. E., Application of a Phenomenological Soot Model for Diesel Engine Combustion, The 3rd International Symposium on Diagnostics and Modeling of Combustion in Internal Combustion Engines, COMODIA 94, pp. 571-576, 1994.

20. Asai, G., Kurata, K., Yokoyama, T., Senda, J., and Fujimoto, H., Soot Diagnostics in Diesel Combustion by means of Laser Induced Scattering and Laser Induced Incandescence Method, JSAE Transaction Paper No. 9736636, Vol. 28, No. 3, July, 1997.

21. Hajime Fujimoto, Kazuo Kurata, Go Asai, and Jiro Senda, OH Radical Generation and Soot Formation / Oxidation in DI Diesel Engine, SAE Paper No. 982630, 1998.

22. Choi, D., Enami, M., Kurata, K., Asai, G., Senda, J., and Fujimoto, H., Soot Formation and Oxidation Process in a DI Diesel Engine by Use of LII/LIS Technique, The 15th Internal Combustion Engine Symposium (Intl.) in Seoul, Paper No. 9935590, pp. 315-320, 1999.

23. Choi, D., Enami, M., Shima, Y., Senda, J., and Fujimoto, H., Soot Formation / Oxidation and Fuel-Vapor Concentration in a DI Diesel Engine Using Laser-Sheet Imaging Method, Seoul 2000

FISITA World Automotive Congress, Paper No. F2000A074, 2000.

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25. Choi, D., Iwamuro, M., Shima, Y., Senda, J., and Fujimoto, H., The Effect of Fuel-Vapor Concentration On Process of Initial Combustion and Soot Formation In a DI Diesel Engine Using LII and LIEF, SAE Paper No. 2001-01-1255, 2001.

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29. Fujimoto, H., and Sato, G T., Investigation on combustion in medium-speed marine diesel engines using model chambers, Proc. Of 12th CIMAC (Tokyo) B-23 pp. 1-48, 1977.

CONTACT

Jiro Senda

Department of Mechanical Engineerig Doshisha University 1-3, Miyakodani, Tatara, Kyotanabe, Kyoto 610-0321, JAPAN TEL : .+81-774-65-6405 FAX : +81-774-65-6405

E-mail : jsenda@mail.doshisha.ac.jp