International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:05 51

In-Cylinder Soot Particle Distribution in Squish Region of a Direct Injection Diesel Engine

Muhammad A. Zuber, Wan Mohd F. Wan Mahmood, Zulkhairi Zainol Abidin. and Zambri Harun., Department of Mechanical and Materials Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi, Malaysia

 Abstract— The size and distribution of in-cylinder soot [9], sulfation of building stones [10] and damage the engine. particles affect the sizes of soot particles emitted from exhaust Soot damage the engine by increase the engine wear by tailpipes as well as the soot in oil. The simulation work reported degrading the oil that reduces the flow ability of the oil and in this paper focuses on the study of soot formation and cause the need to change the oil frequently [11]-[14]. Thus it movement inside a diesel engine with in-depth analysis of soot particles in the squish region. Soot particles in the squish region is important to understand soot formation, behavior and have high potential to be deposited onto the cylinder wall, and movement inside the engine cylinder until emission so counter subsequently penetrate into engine lubrication system and measure action can be taken to reduce the soot and exhaust contaminate the oil. The prediction of a soot particle pathline and gases emission. size distribution was performed using post-processed in-cylinder Exhaust gas emission from a direct injection diesel engines combustion data from Kiva-3v computational fluid dynamics consist of carbon monoxide (CO), nitrogen oxide (NOx), (CFD) simulations with a series of Matlab routines. Only soot oxidation and soot surface growth process were considered in this sulfur dioxide (SO2), unburned hydrocarbons (UHC) [15] and study. Coagulation and agglomeration of soot particles were not particulate matter (PM) [16], [17]. Particulate matter taken into account. Soot particles were tracked from 8 crank composed of 10 % of fuel, 16 % of oil, 10 % of combination angle (CA) degree after top dead center (ATDC) as soot starts to sulfuric acid and water and, 64 % of soot [18]. form in high concentration until 120 CA degree ATDC at exhaust A modelling of soot formation with detailed chemistry and valve opening (EVO). The soot particle size and its distribution physics had been conducted by [1] and a series of model were analyzed at different crank angles. In the squish region, the most dominant soot particle size was 20-50 nm at earlier crank containing the gas-phase reaction, aromatic chemistry, soot angle and in 10-20 nm range at 120 CA ATDC. The percentage of particle coagulation, soot particle aggregation and surface soot loss in the squish region was analyzed to be 23.2 % and the growth were produce. According to [19] the soot formation soot loss was higher at earlier crank angle until 10 CA degree can divided to four major processes: homogeneous nucleation ATDC due to high rate of oxidation. of soot particles, particle coagulation, particle surface reactions and particle agglomeration. On the other hand, some Index Term— soot, particle tracking, squish region, in- cylinder soot size researcher focus on the soot properties [20], soot mechanism [2], [4], in-cylinder soot particle movement [3] and soot size I. INTRODUCTION [21]. While [22] characterize the soot formation reaction by The study and investigation of combustion and soot inside a four steps: 1.Particle nucleation, 2.Particle surface growth, diesel engine cylinder had been conducted by researcher via 3.Particle surface oxidation, 4.Particle coagulation 5.PAH experiment [1], [2] and simulation [3], [4]. This research deposition on the particle surface. gains the attention of the researcher due to the rule, restriction Various experimental studies had been conducted by [1], and regulation enforcement to reduce the exhaust gas emission [2], [20], [21], [23]-[25] to understand the formation and produce by diesel engine [5]. The exhaust gases produced can behavior of soot inside engine cylinder. An in-cylinder soot lead the severe health complication to human [6]-[8] and plant formation and oxidation had been carried out by [23] using the two-dimensional Laser-Induced Incandescence (LII) and the result showed that at 2° CA ATDC the soot start to form and This work was supported in part by the the Ministry of Higher Education soot concentration start to increase at 6° to 12° CA ATDC. of Malaysia and Universiti Kebangsaan Malaysia under FRGS/1/2013/TK01/UKM/02/2 and GGPM-2011-055 research grants. But after 12° CA ATDC soot concentration intensity start to Muhammad Ahmar Zuber. is with Department of Mechanical and decrease. An experiment was conducted to study the Materials Engineering, National University of Malaysia 43600 Bangi, hygroscopic properties of carbon and diesel soot particles by Malaysia (e-mail: [email protected]). Wan Mohd Faizal Wan Mahmood is with Department of Mechanical and [20]. They use diesel engine to produce soot particle and spark Materials Engineering, National University of Malaysia 43600 Bangi, discharge between two graphite electrodes to produce carbon Malaysia (e-mail: [email protected]). particle. The particle size found to be at 20-500 nm and the Zulkhairi Zainol Abidin is with Department of Mechanical and Materials Engineering, National University of Malaysia 43600 Bangi, Malaysia (e-mail: primary carbon particle at 10 nm and primary soot particle at [email protected]). 25 nm. Zambri Harun is with Department of Mechanical and Materials Soot particle mass, size and distribution were effected by Engineering, National University of Malaysia 43600 Bangi, Malaysia (e-mail: [email protected]). engine load, operation mode and type of fuel. A study by [21]

144105-2929-IJMME-IJENS © October 2014 IJENS I J E N S International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:05 52 on the particle size distribution during emission concluded that particle diameter found in the literature and typical diesel the engine that operate at higher load produce larger soot engine. At earlier combustion in engine, soot formation in the particle size with wider size distribution. This happened due to head of spray can be neglected due to high temperature and the nucleation process, condensation in exhaust emission, soot formation is limited to the beginning of diffusion burn coagulation and agglomeration of soot particle with water phase but after that oxidation will take place. content in exhaust gases. At higher engine load, more soot Work on the soot formation in the diesel engine and their were produce because of the increase of sulfur, ash, heavy interest is in the crevice near the cylinder wall were studied by hydrocarbon and aromatic content. While [24] recorded that [31], [33], [34]. At start of the ignition the soot and at the end soot embedding with hydrocarbon (e.g., Polycyclic Aromatic of expansion stroke the soot more likely to be transported to Hydrocarbon, PAH) can produce soot particle with smaller the wall liner and crevice region by the squish motion. The size. The different in-size of soot particle by different soot transport to the wall liner is depended on the soot density researcher is due to the different type of measurement and recirculation of charge this can be reduced by early techniques or machines use. Each measurement technique or injection of fuel [34]. machines has its own merit. The prediction the soot particle size and distribution in this As oppose to the experimental method, some of the paper was achieved by post processing the result obtained researcher [1], [3], [4], [22], [25]-[34] study the soot formation from simulation using CFD software, Kiva-3v. Kiva-3v and behavior by conducting a simulation with mathematical software was chosen due to its flexibility to be adapted and modeling. Pang et al. [30] conducted a simulation on the soot modified according to the user preference model. The Kiva-3v precursor formation mechanism using CFD software Ansys- CFD code has open architecture that allows researchers to Fluent with chemistry solver, Chemkin-CFD. A detailed understand, investigate and amend the codes [29]. Kiva-3v can chemistry soot models for internal combustion engine were be used to simulate air flow, fuel sprays, and combustion in used in a CFD simulation using Kiva reported by [1]. As PAH practical combustion devices. Originally, Kiva was intended was treated as a soot precursor in the simulation and the soot for three dimensions simulation for modelling flows in particle size was 2 nm with 667200 numbers of soot particle gasoline and diesel engine. It was then expanded on other were recorded at 60° CA ATDC. Another research by [4] was combustion devices such as turbines and furnaces. Kiva performed with various injections timing model result shown features the ability to calculate air flows in complex that the soot concentration is high at 0-30° CA ATDC for all geometries with fuel spray dynamics and evaporation, mixing cases and the multidimensional model they used was very of fuel and air, and combustion with resultant heat release and helpful. Puduppakkam et al. [28] use moment method with exhaust-product formation [35]. Hong et al. [36] used Kiva-3v FORTE CFD software to track soot formation and evolution to develop soot model using realistic physical and chemical inside a direct injection diesel engine. The findings showed equations as bases with reasonable cost and produced that the density of soot particle was peak at 10° CA ATDC excellent agreement with experiment. and decrease afterward. While at 30-40° CA ATDC soot with This simulation is in the limit of expansion stroke using a larger size were found and the size decrease afterward. The series of algorithm to predict it size and pathline. It is expected drop of soot density after 10° CA ATDC is contributed by from this paper that soot particle size distribution in the squish three factors, firstly lower soot nucleation after CA 10° that region at different crank angles can be determined so that decrease soot density, second is the soot coagulation occur further investigated on soot deposition onto the cylinder wall that reduce the soot particle number and lastly soot oxidation can be performed. occur that reduce the soot density. A simulation on soot formation characteristic using Kiva- II. METHOD 3v2 were conducted by [22] and state that soot density and The simulation of combustion inside the engine cylinder soot particle size significantly increase at earlier engine was perform by using Kiva-3v CFD software. The result of the combustion and drop down until it stabilize at certain number. simulation can be found on [29] as this paper is the extended Soot with smaller size in range 5-40 nm were produce at work from [3]. The details on the sub-model, mesh earlier engine combustion due to the pyrolysis reactions and configuration, fuel injector specification and test condition is polymerization of the hydrocarbon fuel. In middle engine available at [29]. The specification of engine used in Kiva-3v combustion the numbers of large size of soot particle increase as shown in Table I and type of bowl use in this simulation is rapidly due to the coagulation, condensation, surface growth bowl in type as in Fig. 1. All the important parameter from Kiva-3v result such as temperature, pressure, bulk gas and deposition of PAHs as PAHs contribute to increase of soot velocity, soot, diesel fuel and oxygen concentration were particle surface growth. At late engine combustion the size extracted to be used in Matlab routine to calculate soot distribution stabilizes at peak of 5-20 nm under the influence pathline and size. of continues oxidation reaction. Rao & Honnery [32] use a The prediction of soot pathline and size is limited to the multi-step soot model to predict the soot formation and domain of stroke expansion only. The domain is at inlet valve mechanism inside the diesel engine cylinder. They found both closing until before exhaust valve opening. soot particle number and diameter increase at earlier crank angle to the peak and start to decrease after that. They also found that average particle diameter is in the range of soot

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made are, soot particle to be in spherical shape with uniform TABLE I density of 2 g/cm3 for the entire time step and the soot mass SPECIFICATION OF THE ENGINE USE Parameter Specification spread uniformly onto the surface of existing soot particle Engine type 4 valve DI diesel considered as the surface growth process. The radius of soot Bore  Stroke 86. 0  86. 0 mm particle at a given time step can be obtained by rearranged the Squish height 1. 297140330 mm density formula as follows; Compression Ratio 18. 2 : 1 3 Displacement 500 cm (2) Piston Geometry Bowl-in-piston where is the soot particle radius, is the soot particle mass and is the soot particle density at that time step. The soot particle mass, , at each time step was calculated by using a combination of Hiroyasu’s soot formation and Nagle-Strickland Constable soot oxidation models. The rate for soot formation according to Hiroyasu’s model as below;

(3)

where is the concentration of soot formed and is the time step interval. is donated a soot particle formation multiplication factor. represents the concentration of fuel vapor, which was considered the source of soot formation, and is the pressure. Activation energy for soot formation is 12500 cal/mole denoted by , and is the temperature inside the engine cylinder with gas constant, = 1.987 cal/mole-K. The current time step is represented by i. Nagel-Strickland Constable (NSC) soot oxidation process equation can be written as follows;

Fig. 1. Half side of engine cylinder showing the bowl configuration (4)

A. Soot Pathline where, in this NSC formula, assumptions are made based on The assumptions made to calculate and predict the soot two types of side on the carbon surface, a more reactive side particle pathline are that the soot particle movement follows namely A, and a less reactive side, B. is the fraction of the velocity vector of bulk flow field at the point where the surface covered by A and 1- is the fraction covered by B. particle is located and the soot was massless. Since the soot is The following values are adopted for the constants [29]: assumed to be massless, the effect of gravity or drag forces can neglected. The position of soot is identify by crank angle (5) (CA) were calculated by using the velocity vector solved in

Kiva-3v. In the model used to calculate the soot particle (6) pathline, the fourth-order Runge-Kutta method and trilinear technique were employed for better accuracy. Equation for the (7) next time step soot particle position can be described as follows; (8)

(1) Similarly, mass loss due to surface oxidation was assumed to occur uniformly on the surface of soot particles. where donated as the current particle position and as the Soot particle size calculated in this paper depended on two current time step. represents the time interval parameters. The first parameter is the starting size of soot between the current time and the next time step. Soot particles particle radius and in this paper the value for soot particle size are counted as deposited at the cylinder wall at their last at 8° CA ATDC was taken as 10×10-9 m. This value was chose location if the calculated position to be out of the calculation as in literature it was in the size range [22]. The second domain. parameter is the soot particle formation multiplication factor and the value was set to 2×10-11 similar to the coefficient set B. Soot Particle Size by [29]. The value of 2×10-11 is an indication of the inverse In the calculation of soot particle size, the assumptions value of soot particle density (particle/cm3) as each tracked

144105-2929-IJMME-IJENS © October 2014 IJENS I J E N S International Journal of Mechanical & Mechatronics Engineering IJMME-IJENS Vol:14 No:05 54 soot particle was assumed to be a single particle among a B. Soot Particle Size cluster of soot particles in a cubic volume. The size of soot particles represents soot diameter in Fig. 2 showed the domains for selected starting point of nanometers (nm). Soot particles of different sizes were soot at 8° CA ATDC within the combustion volume engine classified to the respective size ranges with different bin chamber. The 8° CA was chose based on the high rate of soot numbers as shown in Table II. formation in the engine cylinder at this time frame [3]. The TABLE II shaded area with tones of grey to black represents the soot SIZE RANGE BIN concentration distribution from the result of CFD simulation. Bin number Size range (nm) The points selected inside the domain were calculated to 1 < 2 predict the pathline and size distribution. 2 2 – <10 3 10 – <20 4 20 – <50 5 50 – <100 6 >100 The size range bin used to classified the soot particle into group according to size for easier understanding.

Soot particle size distribution inside the engine cylinder as shown in Fig. 5 and 6. Fig. 5 showed the soot size distribution in the whole cylinder and Fig. 6 showed the soot size distribution in the squish region. From the Fig. 5 and 6, it can be observed that at 8° CA ATDC the concentration of soot is high and packed near the center of the cylinder following the spray profile. In the case of squish region in Fig. 6, the soot (a) particle spread out near the cylinder wall. After that soot concentration start to decrease and the soot dispersed out to the entire area in the cylinder due to the increasing combustion chamber volume. The decrease of soot concentration and particle number was influence by the soot oxidation that occurred afterward [22], [28], [32]. This can be further explained in Fig. 7 and 8. The surface growth in the whole cylinder (Fig. 7) and squish region (Fig. 8) showed that at earlier crank angle, the surface growth was dominant but was taken over by oxidation as early as 10° CA degree ATDC [22]. (b) As earlier as 30 ° CA ATDC it can be seen that smaller soot Fig. 2. Half side and full top view of the in-cylinder volume for the selection of starting points in at 8° CA ATDC particles went near to the wall boundary and these particles may be deposited at the cylinder wall boundary layer via soot deposition mechanism [3], [34]. About 59.1 % of soot III. RESULTS AND DISCUSSION transported near wall boundary was from bowl rim area and A. Soot Pathlines 40.9 % was from the inside the cylinder bowl area but very close to the bowl rim. The comparison between the whole Fig. 3 shows the pathlines of soot inside the cylinder and cylinder and squish region size distribution at later crank the pathlines were selected at one of the spray location as a angle, found that in squish region the soot particles were in representative to all the pathlines. It was too confusing to determine which particle of interest that went to the squish smaller size range near cylinder wall and larger soot particle region near cylinder wall by just looking at these pathlines. It size can be found in the bowl region or farther from cylinder is almost impossible to show all the pathlines in one figure as wall. This shows that in squish region the soot surface mass there are too many lines that mix together and become too was loss due to the high oxidation as explained before and as dense to distinguish from one another. The soot particle in shown in Fig. 8. Table III and IV show the distribution and pathlines followed the swirl direction in bulk gas motion. the percentage of soot particle quantities in each soot size bin In this paper squish region was defined as the region above with average soot particle size. Soot particle average size at and outside of the cylinder bowl near the cylinder wall. The start of the crank angle was 25 nm and increase to 40 nm squish region was defined as an area bigger than 3.4 cm radius through the combustion and reached 30 nm just before the from the engine cylinder central axis up to cylinder wall. The exhaust valve opening. Fig. 9 shows the soot particle size soot particle pathlines for the particles that travelled into the distribution at 8° CA ATDC and Fig. 10 shows the squish region was shown in Fig. 4. Soot that travelled to the squish region was observed to have originated from the cylinder bowl but most of soot came from the bowl rim area.

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Fig. 3. Soot particle pathline inside the engine cylinder at selected point and crank angle. The pathline shows soot particle movement through time.

Fig. 4 Soot particle pathline inside the engine cylinder at selected point and crank angle in the squish region. The pathline shows the soot movement inside the squish region

Fig. 5 Soot particle size distribution inside the whole engine cylinder at selected crank angle. The size range bin show at the bottom of the figure. Smaller particle size in small circle with black color and the size increase with color get lighter to grey. start of the crank angle was 25 nm and increased to 40 nm

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Fig. 6 Soot particle size distribution inside the squish region in the engine cylinder at selected crank angle. The size range bin show at the bottom of the figure. Smaller particle size in small circle with black color and the size increase with color get lighter to grey.

peak at size higher than 100 nm and shows a bimodal characteristic. About 47.54 % of soot particles were in size range of 10-20 nm at crank angle 20-120 and this can be considered as the primary size of soot [20]. The soot distribution range widens but the number of soot particle

TABLE III SOOT PARTICLE SIZE DISTRIBUTION IN THE WHOLE CYLINDER Percentage of soot size distribution in nm (%) Soot CA average size <2 2-10 10-20 20-50 50-100 >100 (nm) 8 0.00 0.33 24.74 69.29 5.25 0.39 25.8

Fig. 7 Surface growth rate and oxidation rate versus crank angle in whole 30 1.34 9.93 50.88 18.16 5.36 14.32 38.5 cylinder. The data recorded in this paper start at 8° to 120° CA ATDC. 60 3.47 16.85 48.68 13.59 5.83 11.58 33.7 90 5.47 17.56 48.76 12.51 5.40 10.31 30.9 120 6.72 17.63 47.54 12.72 6.14 9.25 29.8 Soot particle size distribution according to the size bin at selected crank angle. The value showed in percentage of particle number at that instant CA.

TABLE IV SOOT PARTICLE SIZE DISTRIBUTION IN THE SQUISH REGION Size (nm) (%) Average CA Size (nm) <2 2-<10 10-<20 20-<50 50-<100 >100

8 0.00 0.00 4.61 93.00 0.85 1.54 25.2 30 2.27 18.18 54.34 22.31 1.03 1.86 18.5

Fig. 8 Surface growth rate and oxidation rate versus crank angle in squish 60 8.81 20.04 50.22 19.60 0.88 0.44 15.5 region. The data recorded in this paper start at 8° to 120° CA ATDC. 90 7.32 22.62 50.11 19.07 0.44 0.44 15.1

distribution at 120° CA ATDC. In the whole cylinder the size 120 7.33 23.33 49.56 18.89 0.44 0.44 14.9 distribution at crank angle 8° the soot particle size shows Soot particle size distribution according to the size bin at selected crank Gaussian distribution characteristic with size peak at 20-50 nm angle. The value showed in percentage of particle number at that instant CA.

and about 69.29 % of soot particle were in the size range. At decreases as combustion progresses. the start of combustion, the soot particle size distribution fall To further understand the relation between soot particle near the initial diameter of soot particle set. As the crank angle number and time step or crank angle, Fig. 11 was provided. progress, the soot particle move around and experience The soot particle number started at around 3500 particles, then oxidation process. Thus at 20° CA ATDC to late crank angle the number dropped to below 1500 particles at 30° CA ATDC. the soot particle size peak shifted to 10-20 nm with second After that soot particle number slowly decreased until around

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1000 particle at 120° CA ATDC. For the squish region soot decreased slowly due to the slower rate of soot oxidation particle number started at around 500 particle and dropped process. The dominant soot size at the start was in the range of significantly to 120 particles at 120° CA ATDC. Fig. 12 20-50 nm and shifted to 2-10 nm at the end of the cycle. Soot explains the soot particle average size in the squish region and particles near the wall cylinder were observed to be in smaller in the entire cylinder. At earlier combustion process in the size range compare to other regions inside the engine cylinder. squish region, the soot oxidation rate increases rapidly after 8° The soot particles in the squish region have high possibilities CA ATDC [4] to overcome the surface growth of soot. Soot to be deposited onto the cylinder walls through one or various particle average size in the squish region slowly decreased transfer mechanisms. from 25 nm at 8 CA ATDC to 15 nm at exhaust valve opening (EVO). Soot particle size in the whole cylinder displayed a different result, where the soot particle average size at inlet valve closing (IVC) was 25 nm and increase to 40 nm at 30° CA ATDC. Beyond that the soot average size starts to decrease to 30 nm at EVO. The oxidation process started to dominate the overall soot formation process at higher crank angle, namely 30º CA ATDC, thus reduced the overall soot intensity, size and particles.

Fig. 11 Soot particle number in decimal against the crank angle. Soot particle numbers reduce as the crank angle increase.

Fig. 9 Soot particle size distribution at 8° CA ATDC. The distribution exhibit Gaussian distribution.

Fig. 12 Soot particle average size in nanometer against the crank angle. Whole cylinder and squish region exhibited different behavior.

A CKNOWLEDGMENT The authors would like to express their gratitude to the Ministry of Higher Education of Malaysia and National University of Malaysia for supporting this research through Fig. 10 Soot particle size distribution at 120° CA ATDC. The distribution exhibited bimodal distribution. their research grants of GGPM-2011-055 and FRGS/1/2013/TK01/UKM/02/2.

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