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Study on individual spray plume characteristics of multi-hole direct injection spark ignition (DISI) injector using cross-sectional area
Fuel 262 (2020) 116329
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Full Length Article
Study on individual spray plume characteristics of multi-hole direct injection spark ignition (DISI) injector using cross-sectional area Jeonghyun Parka, Jeong Hwan Parkb,c, Suhan Parkd,
T
⁎
a
Department of Mechanical Engineering, Graduate School of Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea Product Design Team 2, Hyundai-Kefico, Gunpo-Si, Republic of Korea c Department of Mechanical Convergence Engineering, Graduate School of Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea d School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Spray pattern Individual spray plume Cross-sectional area Spray center Individual spray angle
In a direct injection engine system, the fuel spray characteristics including the uniformity of air-fuel mixture significantly affects the combustion characteristics and the generation of harmful emissions. To optimize engine performance, a variety of studies on spray characteristics have been conducted in for years. However, it is difficult to identify the individual spray plumes characteristics of multi-hole GDI injectors due to interference from spray. In this study, the spray pattern was used to measure the uniformity and the accurate spray targeting of the individual spray. As the distance from the nozzle tip to the measurement section increases, the spray area increases due to fuel atomization and scattering. As a result, as the distance from the nozzle tip to the measurement section increases, the uniformity of the spray decreases while the deviation of the spray center increases. When the injection pressure is increased, the momentum of the spray droplet is increased, and the influence of the ambient air resistance is reduced, leading to improved spray uniformity. In addition, the method of measuring the spray angle of each individual spray plume was presented through the spray pattern image in the GDI injector with high interference between the spray plumes. High injection pressure can reduce the spray angle deviation of the individual spray plume, increasing spray uniformity between injector holes. These results can be used to establish a spray strategy with which to increase the accuracy of spray targeting in the combustion chamber and minimize collision with the combustion chamber and piston wall.
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Corresponding author. E-mail address: [email protected] (S. Park).
https://doi.org/10.1016/j.fuel.2019.116329 Received 14 June 2019; Received in revised form 30 August 2019; Accepted 1 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
Fuel 262 (2020) 116329
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using the multi-hole GDI injector by Schlieren and the backlight imaging method, while Wang et al. [11] analyzed the effect of deposits on the spray behavior in multi-hole GDI injectors with CFD and verified their findings using the PLIF technique. They reported that deposits would deform the spray structure as well as, increase the liquid velocity and Sauter mean diameter (SMD). Research on multi-hole injectors is actively underway in a variety of ways. However, each orifice (or hole) of multi-hole injectors has different internal and external flow characteristics, depending on the arrangement of the orifices (or holes). Thus, elucidating the flow characteristics of the individual spray plumes injected from each orifice is critical to understanding and controlling the macroscopic spray characteristics of the fuel injected from multihole injectors. In order to reduce the wall-wetting phenomenon in the combustion chamber, GDI injectors are designed to have relatively small injection angles [5,12]. These small injection angles lead to interference and collisions between neighboring spray plumes, making it difficult to visually separate each spray plume from the spray for easy analysis. In recent years, spray pattern visualization methods have mainly been used to understand the flow characteristics of individual spray. The measurement of spray pattern (spray cross-sectional area) enables the characterization of individual spray plumes even in injectors with narrow spray angles. Data acquisition for individual spray plumes will allow for a more accurate understanding of the distribution and spray behavior characteristics of the fuel according to the arrangement of the nozzle orifices. In addition, it is expected that the theoretical characteristics of the spray structure can be elucidated by grasping the fuel density and the influence of ambient air in the spray, which are difficult to identify in the general macroscopic spray visualization method. By applying these results to the shape design of the combustion chamber and the injector hole arrangement structure, it is possible to expect an increase in combustion efficiency through precise spray control. However, it is difficult to understand the existing macroscopic spray characteristics (spraying distance, spray angle) only by measuring the spray pattern, and there is a disadvantage that image processing is relatively complicated compared to macroscopic spray visualization. Wu et al.
Nomenclature Pinj Dtip tasoe θ SD
Injection pressure Measuring distance from nozzle tip Time after start of energizing Spray angle of individual spray plume Standard deviation
1. Introduction With the increasing number of gasoline engines (GDI type, gasoline direct injection) which directly inject fuel into the combustion chamber, such as diesel engines, interest in the control and injection strategy of the fuel spray has grown [1–4]. Precise control of the fuel–air mixture is necessary for the optimum combustion of direct injection engines [5,6]. The shape and geometry of the injector nozzle is one of the dominant factors affecting the formation of the fuel–air mixture. Various types of injector nozzles for internal combustion engines have been developed, such as pintle type, slit type, outwardly-opening type, multi-hole type injector, etc., for the efficient formation and atomization of fuel supplied into the chamber. Among them, multi-hole type injector, which have good characteristics of distribution, formation of mixture, and atomization performance of fuel, are adopted and distributed the most in the modern automobile market. The multi-hole injectors are adopted and used not only for gasoline engines but also for diesel direct injection engines, due to their convenience in terms of fuel distribution and economic benefits by changing the structure and arrangement of the orifice [7,8]. Hence, various studies on multi-hole injectors to control spray with the goal of optimum combustion have been conducted in many different parts of the world. For example, Mohan et al. [9] proposed a method of measuring the injection rate based on the momentum flux technique to identify the hydraulic characteristics of the multi-hole injector under high pressure injection conditions. In another study, Zhang et al. [10] observed the flash boiling phenomenon of propane
Fig. 1. Schematic diagram of spray pattern image measurement system.
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Fig. 2. Schematic diagram of test injector and position of the spray plume according to the specified hole number. Table 1 Properties of test fuels. Fuel
Gasoline
n-heptane
Molecular formula Molecular weight Density [g/cm3@20℃] Viscosity [cSt] Surface tension [mN/m]
– – 0.746 0.55 21.3
n-C7H16 100.2 0.682 0.689 20.53
Table 2 Experimental conditions. Conditions
Value
Test injector Fuel temperature [℃] Ambient air temperature [℃] Energizing duration [ms] Injection pressure [MPa] Ambient pressure [MPa] Measuring distance from injector tip [mm] Shoot timing after start injection [ms]
6-hole GDI injector 25 25 1.5 10, 20 Atmospheric 20–80 (increase by 5) 0.2–2.6
[13] and Wood [14] observed the collapse of the spray from the multihole injector at flash boiling conditions based on the spray pattern. They reported that the strong interaction of the fuel plume would lead to severe spray decay and that the spray structure would change significantly, resulting in a long spray tip penetration and a small spray angle [13]. Through CFD simulation analysis, Befuri et al. [15] verified the possibility of analyzing the spray targeting using the fuel behavior inside the injector and spray pattern (liquid foot print). In addition, Li
et al. developed a hybrid breakup model of GDI multi-hole injector using CFD to predict spray behavior [16]. Therefore, in this study, the individual spray plume behavior characteristics of multi-hole injectors with six- holes are analyzed. There has yet to be a study analyzing information on the individual spray of GDI injectors with multi-holes (movement of centroid, individual injection angles, spray dynamics, etc.) due to the difficulty associated with the measurement. In this sense, this study is expected to
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Fig. 3. Spray pattern image according to the variation of the shoot timing in each experimental condition.
provide very important basic data such as injector design, injection strategy establishment and combustion chamber shape design by analyzing the individual spray characteristics of multi-hole GDI injectors. In order to understand the characteristics of individual spray, the cross-section perpendicular to the center axis of the injector was first set up. The pattern of the section shape of the spray passing through this section was then visualized and analyzed. Information regarding the spray area, spray center, and injection angle of individual spray was obtained from the visualization image of the spray cross-section pattern, and the spray uniformity of each orifice, spray angle of individual spray plumes, and spray momentum were analyzed.
spray. Finally, quantitative data such as spray area, spray center, and injection angle were extracted using a program produced based on MATLAB. Fig. 2 shows a schematic diagram and the hole arrangement of the GDI injectors used in this study, sample spray visualization images, and a schematic diagram of the individual spray angle measurement. As shown in Fig. 2(a), the hole has been assigned a number of holes in a clockwise direction from 1 to 6. As shown in Fig. 2(b), interference is caused by two spray plumes according to the orifice arrangement designed to visualize the actual spray from the side of the injector. In addition, the angle at which the spray plume injected from each orifice is tilted from the center axis of nozzle is defined as the inclined angle. The injector used in this study was designed to have the smallest inclined angle in Holes 1 and 2 and the largest inclined angle in Holes 4 and 5, based on the vertical axis of the nozzle. The method used to calculate the individual spray angle is shown in Fig. 2(c). First, project the position of the injector tip to the measuring plane to create point 1. Create a virtual straight line connecting point1 and the center of the spray plume, and define the two points that intersect the spray boundary and virtual straight line as end point 1 and 2, respectively. Finally, after obtaining the distance of three straight lines using the coordinate positions of injector tip 1 and end points 1 and 2 in the three-dimensional space, the individual injection angle (θ) was obtained by using Eq. (1), calculated from the second law of cosine.
2. Experimental Set-Up and procedure 2.1. Experimental apparatus Fig. 1 shows a schematic diagram of the spray pattern measurement system. The spray pattern measuring device consists of a fuel supply unit, an image acquisition unit, and a signal control part. The fuel at the fuel supply unit was pressurized using a pneumatic pump (Haskel, DSF60), then the fuel was stored in the accumulator in order to maintain the fuel pressure. The injectors were controlled using the CompactRIO controller (NI, cRIO-9030), differential digital input (NI, 9411), and the injector controller (NI, 9751). The image acquisition part consists of a high-speed camera (FASTCAM, Mini AX-100), a lens (SIGMA, 105 mm f/1.8 DG MACRO HSM) and an Nd:YAG laser (Continuum, SL2-100) with a wavelength of 532 nm, and these are used to acquire a crosssection image of the spray through the combination of optic lenses. The signal control and synchronization of the high-speed camera, injector, and laser in the signal control unit were done using the signal generator (Berkeley Nuclear Technologies Corp., Model 577). The spray images obtained were processed in the following order; background removal, convert to grayscale, binarization, filtering. Background removal is the process of subtracting the background image from the spray image and removing the remaining areas except spray, and converts the image to grayscale. The presence and non-existing areas of the spray are then separated by '1′ and '0′ through a binary process using the specified threshold values. Filtering process removes noise remaining outside the
= arccos
L12 + L22 L32 2 × L1 × L 2
(1)
where,
L1 = |Injector tip L2 = |Injector tip
L3 = |End point 2
End point 1| End point 2|
End point 1|
2.2. Experimental condition and procedure In this study, n-heptane with physical properties similar to those of
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properties similar to those of gasoline along with easy data analysis. SAE (society of automotive engineers) also recommends n-heptane as a fuel for the spray test based on the spray measurement assessment of gasoline injectors [20]. The injector used a 6-hole GDI injector with a side-mount type, which is solenoid driven. Table 2 shows the experimental conditions used in this study. The energizing duration was fixed at 1.5 ms, which is the standard for the spray test of the gasoline injector [20], and the injection pressure was set to 10 MPa and 20 MPa. The distance from the injector tip to the spray cross-section was increased by 5 mm from 20 mm to 80 mm, and the shooting time of the camera after the spray was changed from 0.2 ms to 2.6 ms.
Spray cone angle [deg]
20
16
12
Pinj=10MPa Dtip=20mm
8
4
Hole 1 Hole 4 Trend line
0.30
0.32
0.34
Hole 2 Hole 5
0.36
0.38
Hole 3 Hole 6
3. Results and discussions The spray cross-sectional images according to the injection pressure and the distance from the nozzle tip to the measuring cross-section are shown in Fig. 3. As shown in the figure, as the distance from the nozzle tip to the measuring section increases, the spray develops, the spray area is increased, and the concentration of the spray liquid decreases, thereby reducing the brightness of the spray pattern image. When the injection pressure was increased during the same energizing duration (10MP → 20 MPa), the spray concentration and spray area were increased by the increased injection quantity, and finally the brightness of the spray image was increased as well. In addition, the shape of the spray cross-section varies with changes in the degree of spray development when the spray image is acquired, even if the distance and injection pressure conditions are the same from the nozzle tip. In other words, slowing down the shooting timing will make the spray develop, causing the liquid to disperse and scatter the light, thereby increasing the spray area. As the spray area increases under the same injection quantity condition, the concentration of the spray decreases and the brightness of the spray image decreases. Under these same experimental conditions, selecting the exact timing in spray cross-sectional images that change with the time taken is a crucial task that directly affects the understanding of the individual spray characteristics (particularly the individual spray angle results). In Fig. 4, the individual spray angles were compared according to the shooting timing in order to select the time to define the spray angle. As the shooting timing was delayed within the area where measuring the spray angle was possible, angle of individual spray plume increased at each injection orifice, and the trend lines for all orifices also tended to increase. Therefore, in this study, the spray angle was defined by selecting the medium shooting timing within which the spray angle could be measured. In addition, measuring the spray angle is somewhat difficult to define because the spray angle increases when the distance from the nozzle tip to the measuring section is short and the spray is unstable and not sufficiently developed. As shown in Fig. 3, when the distance from the nozzle tip to the measuring section is increased, the spray droplets lose their initial momentum and are scattered in all directions, thus reducing the accuracy of the injection angle. Therefore, in order to define the distance from the nozzle tip to the measurement cross-section, the spray angle was defined at 40 mm, where the spray is sufficiently developed and has good uniformity. The results of the visualized spray pattern experiment are shown in Figs. 5–7 by exposing the plane light made from a combination of an Nd:YAG laser and an optical lens in the vertical direction of the injector center axis. Fig. 5 shows the center of each individual spray plume with respect to the injection pressure and the distance from the nozzle tip. For each experimental condition, 200 repeated measurements were made and plotted on the graph. As shown in the figure, the 6-hole GDI injectors of the side-mount type can be found to be asymmetrical around the injector nozzle tip. As nozzle orifices are designed/made asymmetrically, the internal flow and cavitation characteristics of each hole will vary, and the behaviors of the spray plumes sprayed from each
0.40
Time after start of energizing [ms]
(a) Dtip=20mm
Spray cone angle [deg]
20
16
12
Pinj=10MPa Dtip=40mm
8
Hole 1 Hole 4 Trend line
4 0.6
0.7
0.8
Hole 2 Hole 5
0.9
1.0
Hole 3 Hole 6 1.1
Time after start of energizing [ms]
(b) Dtip=40mm
Spray cone angle [deg]
20
16
12
Pinj=10MPa Dtip=60mm
8
4
Hole 1 Hole 4 Trend line
1.5
1.6
1.7
Hole 2 Hole 5
1.8
1.9
Hole 3 Hole 6 2.0
Time after start of energizing [ms]
(c) Dtip=60mm Fig. 4. Variation of the spray cone angle of each spray plume according to the shoot-timing.
gasoline was used, and the detailed properties are shown in Table 1 [17–19]. The n-heptane has the advantage of having uniform spray characteristics as fuel for a single component as well as physical
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-50 Pinj=10MPa
20
40 50 -50
-40
-30
-20
2.23mm
-10
2.06mm
3.13mm
30
0
1.93mm
20
30
40
4.38mm
10
30
-40
-30
-20
-10
0
1.27mm
10
20
30
40
50
(b) Spray center at Pinj=20MPa & Dtip=20mm -50
Pinj=10MPa
-40
-30
-20
-10
0
10
20
30
40
-20 6.61mm 8.39mm
19.97mm
12.02mm
0 10 20 30 40 50 -50
50
11.07mm
-10
-40
20.89mm
37.72mm 9.84mm
-30
11.53mm
12.54mm
40
20.26mm
28.9mm
18.36mm
30
50 -50
5.55mm 28.43mm
16.17mm
15.28mm
9.26mm
5mm
0
5.86mm
Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6 Injector tip
-30
5mm
7.57mm
-10
Dtip=80mm
14.42mm
-20
Y-axial direction [mm]
Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6 Injector tip
-30
Pinj=20MPa
-40
Dtip=80mm
27.74mm
-40
26.18mm
-50
Y-axial direction [mm]
2.12mm
X-axial direction [mm]
(a) Spray center at Pinj=10MPa & Dtip=20mm
20
2.67mm
20
X-axial direction X-axial direction [mm] [mm]
10
1.12mm
0
50 -50
50
1.99mm
2.01mm
-10
40
10
Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6 Injector tip
1.07mm
2.15mm
10
-20
2.40mm
2.23mm
1.73mm
2.18mm
Dtip=20mm
-30
2.62mm
1.46mm
Y-axial direction [mm]
0
Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6 Injector tip
1.66mm
-10
1.48mm
1.34mm
-20
1.23mm
Pinj=20MPa
-40
Dtip=20mm
-30
1.75mm
Y-axial direction Y-axial direction [mm] [mm]
-40
-20
-10
0
10
20
30
40
50
X-axial direction [mm]
X-axial direction [mm]
(c) Spray center at Pinj=10MPa & Dtip=80mm
(d) Spray center at Pinj=20MPa & Dtip=80mm
Fig. 5. Injector tip and spray center of each spray plume at each condition.
hole will show differences as well. In addition, the cross-sectional area of Holes 4 and 5 with large tilt angles of spray targeting designed from the center axis of nozzle tip can be observed greatly. However, the position of the spray center was assumed to appear in the center of the spray plume separately from the spray area and the analysis was conducted. When the distance from the nozzle tip to the measuring crosssection was short (Fig. 5(a) and (c)), the spray droplets were shown to have relatively large momentum, so the center of spray and spray reached the designed position well. On the other hand, the farther the distance (Dtip) from the nozzle tip to the measuring section (Fig. 5(b) and (d)), the greater the variation in the spray center due to the scattering of the spray and the loss of the spray droplets’ momentum. That is, when the distance of the measuring section from the nozzle tip was short, the position deviation at the center of the spray plume of each hole was not significant. However, as the spray develops up to 80 mm, there is a noticeable difference in the spray center deviation of each hole. Holes 1 and 2 develop spray almost perpendicular to the measurement section, reducing the deviation of the center of the spray plume, and the spray centers of Holes 4 and 5 with the largest inclined angles have relatively large sides in both the X and Y directions. In addition, the increased injection pressure (10 MPa → 20 MPa) did not significantly affect the deviation of the spray center. However, even at the injection pressure of 20 MPa, the motion of the spray droplet decreases, indicating that the deviation in the center of the spray plume increases as the distance from the nozzle tip to the measuring section
increases. Fig. 6 shows a comparison of the variation in the spray center according to the distance from the nozzle tip, the hole position, and the X/ Y direction. The deviations were calculated by separating the spray center movement in the X- and Y-directions. In addition, the spray center deviation according to the distance from the nozzle tip to the measuring section is shown in Fig. 6(a) and (b), and the variation in the spray center according to each hole is shown in Fig. 6(c) and (d). As shown in the figure, the deviation in both the X and Y directions increased significantly as the distance from the nozzle tip increased, and the deviation in the spray center varied substantially from a minimum of 0.22 mm to a maximum of 2.76 mm. When the distance of the measuring section from the nozzle tip increased from 20 mm to 80 mm at the injection pressure of 10 MPa, the uniformity of the spray center decreased by increasing the deviation of the spray center by averages of 5.85 times in the X-direction and 3.89 times in the Y-direction. Small variations were shown in the spray centers of Holes 1 and 2 with small inclined angles, and relatively large deviations were shown in the spray centers of Holes 4 and 5 with large inclined angles. This is because the inclined angle increases, as does the movement of the droplets in the horizontal direction, resulting in a greater variation in the spray center. In addition, a sharp increase in the spray center deviation was observed where the distance from the nozzle tip exceeded 70 mm. This is thought to be the threshold at which the spray droplets significantly lose their initial momentum at an injection pressure of 10 MPa.
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3.5 3.0 2.5 2.0
Pinj=10MPa
Standard deviation of spray center [mm]
Standard deviation of spray center [mm]
4.0
x-direction Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6
1.5 1.0 0.5 0.0
20
30
40
50
60
70
80
Pinj=10MPa
3.5
Y-direction Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6
3.0 2.5 2.0 1.5 1.0 0.5 0.0
20
Measuring distance from nozzle tip [mm]
(a)X-direction depending on distance from nozzle
4.0
2.5
Standard deviation of spray center [mm]
Standard deviation of spray center [mm]
3.0
Pinj=10MPa
X-direction 20mm 35mm 50mm 65mm 80mm
25mm 40mm 55mm 70mm
30mm 45mm 60mm 75mm
2.0 1.5 1.0 0.5 0.0
Hole1
Hole2
Hole3
Hole4
Hole5
40
50
60
70
80
(b) Y-direction depending on distance from nozzle tip
4.0 3.5
30
Measuring distance from nozzle tip [mm]
Hole6
(c)X-direction depending on each holes
3.5 3.0 2.5
Pinj=10MPa
Y-direction 20mm 35mm 50mm 65mm 80mm
25mm 40mm 55mm 70mm
30mm 45mm 60mm 75mm
2.0 1.5 1.0 0.5 0.0
Hole1
Hole2
Hole3
Hole4
Hole5
Hole6
(d)Y-direction depending on each holes
Fig. 6. Standard deviation of the spray center along the distance from the nozzle tip to the measuring surfaces and each hole in the X- and Y-directions.
For a more detailed analysis of the uniformity of the spray, the deviation distributions of the spray centers are shown in Figs. 7 and 8. Fig. 7 shows the distribution comparison between Holes 1 and 4, which differs the most from the inclined angle. The distribution was compared at a point, 40 mm, where the spray is sufficiently developed and at a stable distance. At a high injection pressure of 10 MPa (Fig. 7(a)), Hole 1 with a small inclined angle was found to have substantially more distributions of small deviations than Hole 4. This allowed us to observe that the spray center deviation of the hole with a small inclined angle in Fig. 8 is small. However, when the injection pressure increases to 20 MPa, the spray droplets have a relatively large kinetic energy, reducing the spray center deviation of Hole 4 with a large inclined angle. In addition, the distribution of deviations according to Dtip changes is shown in Fig. 8. As a representative hole, Hole 1 was selected as the most stable spray plume. Both the set X- and Y-direction distributions are shown, and the distribution of deviations along the direction differed depending on the characteristics of the injector. When the injection pressure was 10 MPa, most were distributed in areas where the deviation was less than 1 mm from the short Dtip. In both the X- and Ydirections, the increase in Dtip resulted in an increased proportion of points with a deviation exceeding 1 mm. This led to a decrease in caused the uniformity of the spray as the Dtip increased. Similarly, the increase in Dtip at the injection pressure of 20 MPa resulted in a decrease in the spray uniformity. However, compared to 10 MPa, the rate
of large deviations and the band of spray deviation increased. At 10 MPa, the rate of deviations greater than 1 mm was about 18.68%, while at 20 MPa, it was about 37.92%. In other words, the rate of large deviations increased by about 2.02 times due to the increased injection pressure. Thus, it can be seen in Fig. 8, that with an increase in the injection pressure to 20 MPa, the difference between the holes is reduced, but the absolute value of the deviation is confirmed to increase as a whole. The linear distance from the injector tip to the spray center in each condition is shown in Fig. 9 for each hole. The spray center and its trend line are indicated in black, and the spray center prediction line using the initial spray center is shown in red. The results show that the spray plume develops differently according to the spray angle of each designed hole, and the inclination angle of the spray from the individual spray hole varies from about 10.6° to 26.6°. In addition, the spray center has not changed substantially, even though the injection pressure was increased by a factor of two. However, the spray dynamics characteristics, such as spray trajectory, identified by the spray-centered connections were slightly different. At the injection pressure of 10 MPa (Fig. 9(a)), it could be seen that the trajectories of the spray centers in Holes 1 and 2 with the small inclined angles were lower than the predicted spray centerline. However, with the injection pressure increasing to 20 MPa (Fig. 9(b)), the trajectory of the spray center could be seen to reach the prediction line
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60
Pinj=10MPa, Dtip= 40mm
Hole 1
50
X-direction Hole 1 Hole 4
Count
40 30 20
Hole 4
10 0
0
1
2
Absolute values of difference between average value and each measured value [mm]
3
(a) Pinj=10 MPa 60
Pinj=20MPa, Dtip= 40mm X-direction Hole 1 Hole 4
50
Count
40 30 20 10 0
0
1
2
Absolute values of difference between average value and each measured value [mm]
3
(b) Pinj=20 MPa Fig. 7. Comparison of deviation distribution between holes with largest differences in included angle.
Fig. 8. Comparison of deviation distribution due to Dtip change.
using the initial spray center. This is considered to be attributable to the fact that the spray droplets develop vertically under low injection pressure due to the influence of gravity, and at high injection pressure, the spray droplets from the nozzles have a large momentum, thereby overcoming the influence of gravity and maintaining the initial injection angle. In addition, the flow characteristics (cavitation, bubble generation and collapse) inside the nozzle orifice according to the tilt angle of the orifice in the nozzle are believed to have affected the external flow characteristics. However, currently, experimental studies involving internal flow visualization are extremely limited [21,22]. It is expected that correlation studies will continue through simultaneous visualization of internal and external flows using X-ray science and the CFD tool. The spray angle measurement and comparison results of each spray plume according to the injection pressure at 40 mm from the nozzle tip are shown in Fig. 10. After obtaining the 50 spray images, the individual spray angles were acquired using the average value. As shown in the figure, the spray angles of the individual spray plumes at the
injection pressure of 10 MPa were slightly different, with an average of 15°–18°. This means that the current method of measuring the spray angle is directly affected by the spray area and the spray width, and the inclined angles of each spray plume are judged to differ from each other. In the small inclined angle holes, the spray develops almost vertically, so the concentration of the spray becomes thick, and the uniformity increases, which can increase the spray area. In addition, the spray area can vary depending on the distance from the nozzle tip. Larger inclined angles can increase the spray area by increasing the distance from the nozzle tip, even with the same vertical distance. However, the amount of light due to scattering of the spray may decrease somewhat. Due to the effect of this complex spray development, the spray angle of the individual spray plumes can vary. However, the deviations in the individual spray angles were found to be small in Holes 1 and 2, similar to the spray center, and large in Holes 4 and 5. In addition, as the injection pressure increases to 20 MPa, the initial momentum of the spray droplets increases and the spray develops reliably, making the spray angle of each spray plume relatively stable,
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(a) Pinj=10 MPa
(b) Pinj=20 MPa Fig. 9. In each condition, the distance between the spray center and the injector tip (black point and line) and the predicted distance using the initial spray center.
similar, and increasing the injection quantity compared to 10 MPa, which consequently increases the spray angle. As was the case with the injection pressure of 10 MPa, it was found that the deviations of the individual spray angles of injection are large in Holes 4 and 5. The measurements result and analysis of individual spray angles can be used to reduce wall wetting and design the mounting position of injectors and piston shape for air–fuel mixture control [5]. To do this, the individual spray angles are measured in a variety of ways at the current stage, and continuous research is required to increase accuracy and predict individual spray angles.
2. A new method for measuring the spray angle of an individual spray plume using a spray pattern is presented. By using the injection angle measurement of the individual spray plume, the fuel distribution deviation of each individual spray plume can be checked and used to compare the fuel distribution between the experimental results and the designed orifice hole. Using these results, a more accurate and efficient nozzle orifice arrangement would be possible. 3. The deviation of the spray center, which can be considered as a measure of the uniformity and stability of each spray plume, increased in both the X- and Y-directions due to scattering and evaporation of the spray as the distance from the nozzle tip increased. In addition, the deviation increased rapidly after about 70 mm at the injection pressure of 10 MPa. 4. At the injection pressure of 10 MPa, the trajectory of the spray center became smaller than the angle of the initial designed orifice, making it closer to the vertical axis of injector. By contrast, when the injection pressure was increased to 20 MPa, the spray-centered trajectory maintained the inclined angle of the orifice. That is, when the injection pressure was high, the spray-centered trajectory remained straight for longer as the initial spray momentum increased. This means that with larger initial spray momentum due to the injection pressure, the spray center maintains the straight trajectory for longer. These results can be used to optimize the spray targeting of GDI injectors and the positions of the injectors as well as to establish injection strategies.
4. Conclusions In this study, the spray characteristics of the individual spray plumes from the multi-hole GDI injector were investigated and analyzed through a spray pattern experiment, which involved visualizing the cross-section of spray. The following important conclusions were drawn from the study. 1. The GDI injector with multi-holes show different fuel behavior within the nozzle orifice according to the designed orifice arrangement, and it also has different spray characteristics for each hole. The spray pattern was used to obtain the characteristics of each individual spray plume, which are difficult to separate by macroscopic spray images.
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(a) Pinj=10 MPa
(b) Pinj=20 MPa Fig. 10. Comparison of spray cone angle and deviation of individual spray plumes.
Acknowledgments
2009;88:1634–42. [6] Li Y, Bai XS, Tunér M, Im HG, Johansson B. Investigation on a high-stratified direct injection spark ignition (DISI) engine fueled with methanol under a high compression ratio. Appl Therm Eng 2019;148:352–62. [7] Zhao F, Harrington DL, Lai MC. Automotive gasoline direct-injection engines. SAE International; 2002. [8] Moon S, Komada K, Sato K, Yokohata H, Wada Y, Yasuda N. Ultrafast X-ray study of multi-hole GDI injector sprays: effects of nozzle hole length and number on initial spray formation. Exp Therm Fluid Sci 2015;68:68–81. [9] Mohana B, Du J, Sim J, Roberts WL. Hydraulic characterization of high-pressure gasoline multi-hole injector. Flow Meas Instrum 2018;64:133–41. [10] Zhang Z, Yanfei L, Ma X, Ding H, Xu H, Wang Z, et al. Characteristics of transcritical propane spray discharged from multi-hole GDI injector. Exp Therm Fluid Sci 2018;99:446–57. [11] Wang B, Badawy T, Jiang Y, Xu H, Ghafourian A, Zhang X. Investigation of deposit effect on multi-hole injector spray characteristics and air/fuel mixing process. Fuel 2017;191:10–24. [12] Mitroglou N, Nouri JM, Yan Y, Gavaises M, Arcoumaniks C. Spray Structure Generated by Multi-Hole Injectors for Gasoline Direct-Injection Engine. SAE Technical Paper 2017; 2007-01-1417. [13] Wu S, Yang S, Wooldridge M, Xu M. Experimental study of the spray collapse process of multi-hole gasoline fuel injection at flash boiling conditions. Fuel 2019;242:109–23. [14] Wood A. Optical investigations of the sprays generated by gasoline multi-hole injectors under novel operating conditions. Loughborough University; 2014. Doctoral Thesis. [15] Befrui B, Corbinelli G, D’Onofrio M, Varble D. GDI Multi-Hole Injector Internal Flow and Spray Analysis. SAE Technical Papers 2011; 2011-01-1211. [16] Li ZH, He BQ, Zhao H. Application of a hybrid breakup model for the spray simulation of a multi-hole injector used for a DISI gasoline engine. Appl Therm Eng 2014;65:282–92. [17] Wang L, Badra JA, Roberts WL, Fang T. Characteristics of spray from a GDI fuel injector for naphtha and surrogate fuels. Fuel 2017;190:113–28. [18] Aleiferis PG, Serras-Pereira J, Van Romunde Z, Caine J, Wirth M. Mechanisms of spray formation and combustion from a multi-hole injector with E85 and gasoline.
This study was financially supported by the Basic Science Research Program (2016R1D1A3B03935537) and Basic Research Laboratory Program (2015R1A4A1041746) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Korea). Also, this research was supported by the CEFV (Center for Environmentally Friendly Vehicle) as Global-Top Project of KMOE (Ministry of Environment, Korea). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116329. References [1] Reddy AA, Mallikarjuna JM. Parametric study on a gasoline direct injection engine – A CFD analysis. SAE paper 2017; 2017-26-0039. [2] Zhao F, Lai M-C, Harrington LL. Automotive spark-ignited direct-injection gasoline engines. Prog Energy Combust Sci 1999;25(5):437–562. [3] Storey J, Barone T, Thomas J, Huff S. Exhaust particle characterization for lean and stoichiometric DI vehicles operating on ethanol-gasoline blends. SAE Technical paper 2012; 2012-01-0437. [4] Bonatesta F, Chiappetta E, La Rocca A. Part-load particulate matter from a GDI engine and the connection with combustion characteristics. Appl Energy 2014;124:366–76. [5] Nishida K, Tian J, Sumoto Y, Long W, Sato K, Yamakawa M. An experimental and numerical study on sprays injected from two-hole nozzles for DISI engines. Fuel
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J. Park, et al. Combust Flame 2010;157(4):735–56. [19] Li T, Moon S, Sato K, Yokohata H. A comprehensive study on the factors affecting near-nozzle spray dynamics of multi-hole GDI injectors. Fuel 2010;190:292–302. [20] Hung David LS, Harrington David L, Gandhi Anand H, Markle Lee E, Parrish Scott E, Shakal Joseph S, et al. Gasoline fuel injector spray measurement and characterization – a new SAE J2715 recommended practice. SAE Int. J. Fuels Lubr. 2009;1(1):534–48.
[21] Marchitto L, Hampai D, Dabagov SB, Allocca L, Alfuso S, Polese C, et al. GDI spray structure analysis by polycapillary X-ray μ-tomography. Int J Multiph Flow 2015;70:15–21. [22] Zhang X, Moon S, Gao J, Dufresne EM, Fezzaa K, Wang J. Experimental study on the effect of nozzle hole-to-hole angle on the near-field spray of diesel injector using fast X-ray phase-contrast imaging. Fuel 2016;185:142–50.
11
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Full Length Article
Study on individual spray plume characteristics of multi-hole direct injection spark ignition (DISI) injector using cross-sectional area Jeonghyun Parka, Jeong Hwan Parkb,c, Suhan Parkd,
T
⁎
a
Department of Mechanical Engineering, Graduate School of Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea Product Design Team 2, Hyundai-Kefico, Gunpo-Si, Republic of Korea c Department of Mechanical Convergence Engineering, Graduate School of Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea d School of Mechanical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea b
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Spray pattern Individual spray plume Cross-sectional area Spray center Individual spray angle
In a direct injection engine system, the fuel spray characteristics including the uniformity of air-fuel mixture significantly affects the combustion characteristics and the generation of harmful emissions. To optimize engine performance, a variety of studies on spray characteristics have been conducted in for years. However, it is difficult to identify the individual spray plumes characteristics of multi-hole GDI injectors due to interference from spray. In this study, the spray pattern was used to measure the uniformity and the accurate spray targeting of the individual spray. As the distance from the nozzle tip to the measurement section increases, the spray area increases due to fuel atomization and scattering. As a result, as the distance from the nozzle tip to the measurement section increases, the uniformity of the spray decreases while the deviation of the spray center increases. When the injection pressure is increased, the momentum of the spray droplet is increased, and the influence of the ambient air resistance is reduced, leading to improved spray uniformity. In addition, the method of measuring the spray angle of each individual spray plume was presented through the spray pattern image in the GDI injector with high interference between the spray plumes. High injection pressure can reduce the spray angle deviation of the individual spray plume, increasing spray uniformity between injector holes. These results can be used to establish a spray strategy with which to increase the accuracy of spray targeting in the combustion chamber and minimize collision with the combustion chamber and piston wall.
⁎
Corresponding author. E-mail address: [email protected] (S. Park).
https://doi.org/10.1016/j.fuel.2019.116329 Received 14 June 2019; Received in revised form 30 August 2019; Accepted 1 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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using the multi-hole GDI injector by Schlieren and the backlight imaging method, while Wang et al. [11] analyzed the effect of deposits on the spray behavior in multi-hole GDI injectors with CFD and verified their findings using the PLIF technique. They reported that deposits would deform the spray structure as well as, increase the liquid velocity and Sauter mean diameter (SMD). Research on multi-hole injectors is actively underway in a variety of ways. However, each orifice (or hole) of multi-hole injectors has different internal and external flow characteristics, depending on the arrangement of the orifices (or holes). Thus, elucidating the flow characteristics of the individual spray plumes injected from each orifice is critical to understanding and controlling the macroscopic spray characteristics of the fuel injected from multihole injectors. In order to reduce the wall-wetting phenomenon in the combustion chamber, GDI injectors are designed to have relatively small injection angles [5,12]. These small injection angles lead to interference and collisions between neighboring spray plumes, making it difficult to visually separate each spray plume from the spray for easy analysis. In recent years, spray pattern visualization methods have mainly been used to understand the flow characteristics of individual spray. The measurement of spray pattern (spray cross-sectional area) enables the characterization of individual spray plumes even in injectors with narrow spray angles. Data acquisition for individual spray plumes will allow for a more accurate understanding of the distribution and spray behavior characteristics of the fuel according to the arrangement of the nozzle orifices. In addition, it is expected that the theoretical characteristics of the spray structure can be elucidated by grasping the fuel density and the influence of ambient air in the spray, which are difficult to identify in the general macroscopic spray visualization method. By applying these results to the shape design of the combustion chamber and the injector hole arrangement structure, it is possible to expect an increase in combustion efficiency through precise spray control. However, it is difficult to understand the existing macroscopic spray characteristics (spraying distance, spray angle) only by measuring the spray pattern, and there is a disadvantage that image processing is relatively complicated compared to macroscopic spray visualization. Wu et al.
Nomenclature Pinj Dtip tasoe θ SD
Injection pressure Measuring distance from nozzle tip Time after start of energizing Spray angle of individual spray plume Standard deviation
1. Introduction With the increasing number of gasoline engines (GDI type, gasoline direct injection) which directly inject fuel into the combustion chamber, such as diesel engines, interest in the control and injection strategy of the fuel spray has grown [1–4]. Precise control of the fuel–air mixture is necessary for the optimum combustion of direct injection engines [5,6]. The shape and geometry of the injector nozzle is one of the dominant factors affecting the formation of the fuel–air mixture. Various types of injector nozzles for internal combustion engines have been developed, such as pintle type, slit type, outwardly-opening type, multi-hole type injector, etc., for the efficient formation and atomization of fuel supplied into the chamber. Among them, multi-hole type injector, which have good characteristics of distribution, formation of mixture, and atomization performance of fuel, are adopted and distributed the most in the modern automobile market. The multi-hole injectors are adopted and used not only for gasoline engines but also for diesel direct injection engines, due to their convenience in terms of fuel distribution and economic benefits by changing the structure and arrangement of the orifice [7,8]. Hence, various studies on multi-hole injectors to control spray with the goal of optimum combustion have been conducted in many different parts of the world. For example, Mohan et al. [9] proposed a method of measuring the injection rate based on the momentum flux technique to identify the hydraulic characteristics of the multi-hole injector under high pressure injection conditions. In another study, Zhang et al. [10] observed the flash boiling phenomenon of propane
Fig. 1. Schematic diagram of spray pattern image measurement system.
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Fig. 2. Schematic diagram of test injector and position of the spray plume according to the specified hole number. Table 1 Properties of test fuels. Fuel
Gasoline
n-heptane
Molecular formula Molecular weight Density [g/cm3@20℃] Viscosity [cSt] Surface tension [mN/m]
– – 0.746 0.55 21.3
n-C7H16 100.2 0.682 0.689 20.53
Table 2 Experimental conditions. Conditions
Value
Test injector Fuel temperature [℃] Ambient air temperature [℃] Energizing duration [ms] Injection pressure [MPa] Ambient pressure [MPa] Measuring distance from injector tip [mm] Shoot timing after start injection [ms]
6-hole GDI injector 25 25 1.5 10, 20 Atmospheric 20–80 (increase by 5) 0.2–2.6
[13] and Wood [14] observed the collapse of the spray from the multihole injector at flash boiling conditions based on the spray pattern. They reported that the strong interaction of the fuel plume would lead to severe spray decay and that the spray structure would change significantly, resulting in a long spray tip penetration and a small spray angle [13]. Through CFD simulation analysis, Befuri et al. [15] verified the possibility of analyzing the spray targeting using the fuel behavior inside the injector and spray pattern (liquid foot print). In addition, Li
et al. developed a hybrid breakup model of GDI multi-hole injector using CFD to predict spray behavior [16]. Therefore, in this study, the individual spray plume behavior characteristics of multi-hole injectors with six- holes are analyzed. There has yet to be a study analyzing information on the individual spray of GDI injectors with multi-holes (movement of centroid, individual injection angles, spray dynamics, etc.) due to the difficulty associated with the measurement. In this sense, this study is expected to
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Fig. 3. Spray pattern image according to the variation of the shoot timing in each experimental condition.
provide very important basic data such as injector design, injection strategy establishment and combustion chamber shape design by analyzing the individual spray characteristics of multi-hole GDI injectors. In order to understand the characteristics of individual spray, the cross-section perpendicular to the center axis of the injector was first set up. The pattern of the section shape of the spray passing through this section was then visualized and analyzed. Information regarding the spray area, spray center, and injection angle of individual spray was obtained from the visualization image of the spray cross-section pattern, and the spray uniformity of each orifice, spray angle of individual spray plumes, and spray momentum were analyzed.
spray. Finally, quantitative data such as spray area, spray center, and injection angle were extracted using a program produced based on MATLAB. Fig. 2 shows a schematic diagram and the hole arrangement of the GDI injectors used in this study, sample spray visualization images, and a schematic diagram of the individual spray angle measurement. As shown in Fig. 2(a), the hole has been assigned a number of holes in a clockwise direction from 1 to 6. As shown in Fig. 2(b), interference is caused by two spray plumes according to the orifice arrangement designed to visualize the actual spray from the side of the injector. In addition, the angle at which the spray plume injected from each orifice is tilted from the center axis of nozzle is defined as the inclined angle. The injector used in this study was designed to have the smallest inclined angle in Holes 1 and 2 and the largest inclined angle in Holes 4 and 5, based on the vertical axis of the nozzle. The method used to calculate the individual spray angle is shown in Fig. 2(c). First, project the position of the injector tip to the measuring plane to create point 1. Create a virtual straight line connecting point1 and the center of the spray plume, and define the two points that intersect the spray boundary and virtual straight line as end point 1 and 2, respectively. Finally, after obtaining the distance of three straight lines using the coordinate positions of injector tip 1 and end points 1 and 2 in the three-dimensional space, the individual injection angle (θ) was obtained by using Eq. (1), calculated from the second law of cosine.
2. Experimental Set-Up and procedure 2.1. Experimental apparatus Fig. 1 shows a schematic diagram of the spray pattern measurement system. The spray pattern measuring device consists of a fuel supply unit, an image acquisition unit, and a signal control part. The fuel at the fuel supply unit was pressurized using a pneumatic pump (Haskel, DSF60), then the fuel was stored in the accumulator in order to maintain the fuel pressure. The injectors were controlled using the CompactRIO controller (NI, cRIO-9030), differential digital input (NI, 9411), and the injector controller (NI, 9751). The image acquisition part consists of a high-speed camera (FASTCAM, Mini AX-100), a lens (SIGMA, 105 mm f/1.8 DG MACRO HSM) and an Nd:YAG laser (Continuum, SL2-100) with a wavelength of 532 nm, and these are used to acquire a crosssection image of the spray through the combination of optic lenses. The signal control and synchronization of the high-speed camera, injector, and laser in the signal control unit were done using the signal generator (Berkeley Nuclear Technologies Corp., Model 577). The spray images obtained were processed in the following order; background removal, convert to grayscale, binarization, filtering. Background removal is the process of subtracting the background image from the spray image and removing the remaining areas except spray, and converts the image to grayscale. The presence and non-existing areas of the spray are then separated by '1′ and '0′ through a binary process using the specified threshold values. Filtering process removes noise remaining outside the
= arccos
L12 + L22 L32 2 × L1 × L 2
(1)
where,
L1 = |Injector tip L2 = |Injector tip
L3 = |End point 2
End point 1| End point 2|
End point 1|
2.2. Experimental condition and procedure In this study, n-heptane with physical properties similar to those of
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properties similar to those of gasoline along with easy data analysis. SAE (society of automotive engineers) also recommends n-heptane as a fuel for the spray test based on the spray measurement assessment of gasoline injectors [20]. The injector used a 6-hole GDI injector with a side-mount type, which is solenoid driven. Table 2 shows the experimental conditions used in this study. The energizing duration was fixed at 1.5 ms, which is the standard for the spray test of the gasoline injector [20], and the injection pressure was set to 10 MPa and 20 MPa. The distance from the injector tip to the spray cross-section was increased by 5 mm from 20 mm to 80 mm, and the shooting time of the camera after the spray was changed from 0.2 ms to 2.6 ms.
Spray cone angle [deg]
20
16
12
Pinj=10MPa Dtip=20mm
8
4
Hole 1 Hole 4 Trend line
0.30
0.32
0.34
Hole 2 Hole 5
0.36
0.38
Hole 3 Hole 6
3. Results and discussions The spray cross-sectional images according to the injection pressure and the distance from the nozzle tip to the measuring cross-section are shown in Fig. 3. As shown in the figure, as the distance from the nozzle tip to the measuring section increases, the spray develops, the spray area is increased, and the concentration of the spray liquid decreases, thereby reducing the brightness of the spray pattern image. When the injection pressure was increased during the same energizing duration (10MP → 20 MPa), the spray concentration and spray area were increased by the increased injection quantity, and finally the brightness of the spray image was increased as well. In addition, the shape of the spray cross-section varies with changes in the degree of spray development when the spray image is acquired, even if the distance and injection pressure conditions are the same from the nozzle tip. In other words, slowing down the shooting timing will make the spray develop, causing the liquid to disperse and scatter the light, thereby increasing the spray area. As the spray area increases under the same injection quantity condition, the concentration of the spray decreases and the brightness of the spray image decreases. Under these same experimental conditions, selecting the exact timing in spray cross-sectional images that change with the time taken is a crucial task that directly affects the understanding of the individual spray characteristics (particularly the individual spray angle results). In Fig. 4, the individual spray angles were compared according to the shooting timing in order to select the time to define the spray angle. As the shooting timing was delayed within the area where measuring the spray angle was possible, angle of individual spray plume increased at each injection orifice, and the trend lines for all orifices also tended to increase. Therefore, in this study, the spray angle was defined by selecting the medium shooting timing within which the spray angle could be measured. In addition, measuring the spray angle is somewhat difficult to define because the spray angle increases when the distance from the nozzle tip to the measuring section is short and the spray is unstable and not sufficiently developed. As shown in Fig. 3, when the distance from the nozzle tip to the measuring section is increased, the spray droplets lose their initial momentum and are scattered in all directions, thus reducing the accuracy of the injection angle. Therefore, in order to define the distance from the nozzle tip to the measurement cross-section, the spray angle was defined at 40 mm, where the spray is sufficiently developed and has good uniformity. The results of the visualized spray pattern experiment are shown in Figs. 5–7 by exposing the plane light made from a combination of an Nd:YAG laser and an optical lens in the vertical direction of the injector center axis. Fig. 5 shows the center of each individual spray plume with respect to the injection pressure and the distance from the nozzle tip. For each experimental condition, 200 repeated measurements were made and plotted on the graph. As shown in the figure, the 6-hole GDI injectors of the side-mount type can be found to be asymmetrical around the injector nozzle tip. As nozzle orifices are designed/made asymmetrically, the internal flow and cavitation characteristics of each hole will vary, and the behaviors of the spray plumes sprayed from each
0.40
Time after start of energizing [ms]
(a) Dtip=20mm
Spray cone angle [deg]
20
16
12
Pinj=10MPa Dtip=40mm
8
Hole 1 Hole 4 Trend line
4 0.6
0.7
0.8
Hole 2 Hole 5
0.9
1.0
Hole 3 Hole 6 1.1
Time after start of energizing [ms]
(b) Dtip=40mm
Spray cone angle [deg]
20
16
12
Pinj=10MPa Dtip=60mm
8
4
Hole 1 Hole 4 Trend line
1.5
1.6
1.7
Hole 2 Hole 5
1.8
1.9
Hole 3 Hole 6 2.0
Time after start of energizing [ms]
(c) Dtip=60mm Fig. 4. Variation of the spray cone angle of each spray plume according to the shoot-timing.
gasoline was used, and the detailed properties are shown in Table 1 [17–19]. The n-heptane has the advantage of having uniform spray characteristics as fuel for a single component as well as physical
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-50 Pinj=10MPa
20
40 50 -50
-40
-30
-20
2.23mm
-10
2.06mm
3.13mm
30
0
1.93mm
20
30
40
4.38mm
10
30
-40
-30
-20
-10
0
1.27mm
10
20
30
40
50
(b) Spray center at Pinj=20MPa & Dtip=20mm -50
Pinj=10MPa
-40
-30
-20
-10
0
10
20
30
40
-20 6.61mm 8.39mm
19.97mm
12.02mm
0 10 20 30 40 50 -50
50
11.07mm
-10
-40
20.89mm
37.72mm 9.84mm
-30
11.53mm
12.54mm
40
20.26mm
28.9mm
18.36mm
30
50 -50
5.55mm 28.43mm
16.17mm
15.28mm
9.26mm
5mm
0
5.86mm
Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6 Injector tip
-30
5mm
7.57mm
-10
Dtip=80mm
14.42mm
-20
Y-axial direction [mm]
Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6 Injector tip
-30
Pinj=20MPa
-40
Dtip=80mm
27.74mm
-40
26.18mm
-50
Y-axial direction [mm]
2.12mm
X-axial direction [mm]
(a) Spray center at Pinj=10MPa & Dtip=20mm
20
2.67mm
20
X-axial direction X-axial direction [mm] [mm]
10
1.12mm
0
50 -50
50
1.99mm
2.01mm
-10
40
10
Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6 Injector tip
1.07mm
2.15mm
10
-20
2.40mm
2.23mm
1.73mm
2.18mm
Dtip=20mm
-30
2.62mm
1.46mm
Y-axial direction [mm]
0
Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6 Injector tip
1.66mm
-10
1.48mm
1.34mm
-20
1.23mm
Pinj=20MPa
-40
Dtip=20mm
-30
1.75mm
Y-axial direction Y-axial direction [mm] [mm]
-40
-20
-10
0
10
20
30
40
50
X-axial direction [mm]
X-axial direction [mm]
(c) Spray center at Pinj=10MPa & Dtip=80mm
(d) Spray center at Pinj=20MPa & Dtip=80mm
Fig. 5. Injector tip and spray center of each spray plume at each condition.
hole will show differences as well. In addition, the cross-sectional area of Holes 4 and 5 with large tilt angles of spray targeting designed from the center axis of nozzle tip can be observed greatly. However, the position of the spray center was assumed to appear in the center of the spray plume separately from the spray area and the analysis was conducted. When the distance from the nozzle tip to the measuring crosssection was short (Fig. 5(a) and (c)), the spray droplets were shown to have relatively large momentum, so the center of spray and spray reached the designed position well. On the other hand, the farther the distance (Dtip) from the nozzle tip to the measuring section (Fig. 5(b) and (d)), the greater the variation in the spray center due to the scattering of the spray and the loss of the spray droplets’ momentum. That is, when the distance of the measuring section from the nozzle tip was short, the position deviation at the center of the spray plume of each hole was not significant. However, as the spray develops up to 80 mm, there is a noticeable difference in the spray center deviation of each hole. Holes 1 and 2 develop spray almost perpendicular to the measurement section, reducing the deviation of the center of the spray plume, and the spray centers of Holes 4 and 5 with the largest inclined angles have relatively large sides in both the X and Y directions. In addition, the increased injection pressure (10 MPa → 20 MPa) did not significantly affect the deviation of the spray center. However, even at the injection pressure of 20 MPa, the motion of the spray droplet decreases, indicating that the deviation in the center of the spray plume increases as the distance from the nozzle tip to the measuring section
increases. Fig. 6 shows a comparison of the variation in the spray center according to the distance from the nozzle tip, the hole position, and the X/ Y direction. The deviations were calculated by separating the spray center movement in the X- and Y-directions. In addition, the spray center deviation according to the distance from the nozzle tip to the measuring section is shown in Fig. 6(a) and (b), and the variation in the spray center according to each hole is shown in Fig. 6(c) and (d). As shown in the figure, the deviation in both the X and Y directions increased significantly as the distance from the nozzle tip increased, and the deviation in the spray center varied substantially from a minimum of 0.22 mm to a maximum of 2.76 mm. When the distance of the measuring section from the nozzle tip increased from 20 mm to 80 mm at the injection pressure of 10 MPa, the uniformity of the spray center decreased by increasing the deviation of the spray center by averages of 5.85 times in the X-direction and 3.89 times in the Y-direction. Small variations were shown in the spray centers of Holes 1 and 2 with small inclined angles, and relatively large deviations were shown in the spray centers of Holes 4 and 5 with large inclined angles. This is because the inclined angle increases, as does the movement of the droplets in the horizontal direction, resulting in a greater variation in the spray center. In addition, a sharp increase in the spray center deviation was observed where the distance from the nozzle tip exceeded 70 mm. This is thought to be the threshold at which the spray droplets significantly lose their initial momentum at an injection pressure of 10 MPa.
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3.5 3.0 2.5 2.0
Pinj=10MPa
Standard deviation of spray center [mm]
Standard deviation of spray center [mm]
4.0
x-direction Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6
1.5 1.0 0.5 0.0
20
30
40
50
60
70
80
Pinj=10MPa
3.5
Y-direction Hole 1 Hole 2 Hole 3 Hole 4 Hole 5 Hole 6
3.0 2.5 2.0 1.5 1.0 0.5 0.0
20
Measuring distance from nozzle tip [mm]
(a)X-direction depending on distance from nozzle
4.0
2.5
Standard deviation of spray center [mm]
Standard deviation of spray center [mm]
3.0
Pinj=10MPa
X-direction 20mm 35mm 50mm 65mm 80mm
25mm 40mm 55mm 70mm
30mm 45mm 60mm 75mm
2.0 1.5 1.0 0.5 0.0
Hole1
Hole2
Hole3
Hole4
Hole5
40
50
60
70
80
(b) Y-direction depending on distance from nozzle tip
4.0 3.5
30
Measuring distance from nozzle tip [mm]
Hole6
(c)X-direction depending on each holes
3.5 3.0 2.5
Pinj=10MPa
Y-direction 20mm 35mm 50mm 65mm 80mm
25mm 40mm 55mm 70mm
30mm 45mm 60mm 75mm
2.0 1.5 1.0 0.5 0.0
Hole1
Hole2
Hole3
Hole4
Hole5
Hole6
(d)Y-direction depending on each holes
Fig. 6. Standard deviation of the spray center along the distance from the nozzle tip to the measuring surfaces and each hole in the X- and Y-directions.
For a more detailed analysis of the uniformity of the spray, the deviation distributions of the spray centers are shown in Figs. 7 and 8. Fig. 7 shows the distribution comparison between Holes 1 and 4, which differs the most from the inclined angle. The distribution was compared at a point, 40 mm, where the spray is sufficiently developed and at a stable distance. At a high injection pressure of 10 MPa (Fig. 7(a)), Hole 1 with a small inclined angle was found to have substantially more distributions of small deviations than Hole 4. This allowed us to observe that the spray center deviation of the hole with a small inclined angle in Fig. 8 is small. However, when the injection pressure increases to 20 MPa, the spray droplets have a relatively large kinetic energy, reducing the spray center deviation of Hole 4 with a large inclined angle. In addition, the distribution of deviations according to Dtip changes is shown in Fig. 8. As a representative hole, Hole 1 was selected as the most stable spray plume. Both the set X- and Y-direction distributions are shown, and the distribution of deviations along the direction differed depending on the characteristics of the injector. When the injection pressure was 10 MPa, most were distributed in areas where the deviation was less than 1 mm from the short Dtip. In both the X- and Ydirections, the increase in Dtip resulted in an increased proportion of points with a deviation exceeding 1 mm. This led to a decrease in caused the uniformity of the spray as the Dtip increased. Similarly, the increase in Dtip at the injection pressure of 20 MPa resulted in a decrease in the spray uniformity. However, compared to 10 MPa, the rate
of large deviations and the band of spray deviation increased. At 10 MPa, the rate of deviations greater than 1 mm was about 18.68%, while at 20 MPa, it was about 37.92%. In other words, the rate of large deviations increased by about 2.02 times due to the increased injection pressure. Thus, it can be seen in Fig. 8, that with an increase in the injection pressure to 20 MPa, the difference between the holes is reduced, but the absolute value of the deviation is confirmed to increase as a whole. The linear distance from the injector tip to the spray center in each condition is shown in Fig. 9 for each hole. The spray center and its trend line are indicated in black, and the spray center prediction line using the initial spray center is shown in red. The results show that the spray plume develops differently according to the spray angle of each designed hole, and the inclination angle of the spray from the individual spray hole varies from about 10.6° to 26.6°. In addition, the spray center has not changed substantially, even though the injection pressure was increased by a factor of two. However, the spray dynamics characteristics, such as spray trajectory, identified by the spray-centered connections were slightly different. At the injection pressure of 10 MPa (Fig. 9(a)), it could be seen that the trajectories of the spray centers in Holes 1 and 2 with the small inclined angles were lower than the predicted spray centerline. However, with the injection pressure increasing to 20 MPa (Fig. 9(b)), the trajectory of the spray center could be seen to reach the prediction line
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60
Pinj=10MPa, Dtip= 40mm
Hole 1
50
X-direction Hole 1 Hole 4
Count
40 30 20
Hole 4
10 0
0
1
2
Absolute values of difference between average value and each measured value [mm]
3
(a) Pinj=10 MPa 60
Pinj=20MPa, Dtip= 40mm X-direction Hole 1 Hole 4
50
Count
40 30 20 10 0
0
1
2
Absolute values of difference between average value and each measured value [mm]
3
(b) Pinj=20 MPa Fig. 7. Comparison of deviation distribution between holes with largest differences in included angle.
Fig. 8. Comparison of deviation distribution due to Dtip change.
using the initial spray center. This is considered to be attributable to the fact that the spray droplets develop vertically under low injection pressure due to the influence of gravity, and at high injection pressure, the spray droplets from the nozzles have a large momentum, thereby overcoming the influence of gravity and maintaining the initial injection angle. In addition, the flow characteristics (cavitation, bubble generation and collapse) inside the nozzle orifice according to the tilt angle of the orifice in the nozzle are believed to have affected the external flow characteristics. However, currently, experimental studies involving internal flow visualization are extremely limited [21,22]. It is expected that correlation studies will continue through simultaneous visualization of internal and external flows using X-ray science and the CFD tool. The spray angle measurement and comparison results of each spray plume according to the injection pressure at 40 mm from the nozzle tip are shown in Fig. 10. After obtaining the 50 spray images, the individual spray angles were acquired using the average value. As shown in the figure, the spray angles of the individual spray plumes at the
injection pressure of 10 MPa were slightly different, with an average of 15°–18°. This means that the current method of measuring the spray angle is directly affected by the spray area and the spray width, and the inclined angles of each spray plume are judged to differ from each other. In the small inclined angle holes, the spray develops almost vertically, so the concentration of the spray becomes thick, and the uniformity increases, which can increase the spray area. In addition, the spray area can vary depending on the distance from the nozzle tip. Larger inclined angles can increase the spray area by increasing the distance from the nozzle tip, even with the same vertical distance. However, the amount of light due to scattering of the spray may decrease somewhat. Due to the effect of this complex spray development, the spray angle of the individual spray plumes can vary. However, the deviations in the individual spray angles were found to be small in Holes 1 and 2, similar to the spray center, and large in Holes 4 and 5. In addition, as the injection pressure increases to 20 MPa, the initial momentum of the spray droplets increases and the spray develops reliably, making the spray angle of each spray plume relatively stable,
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(a) Pinj=10 MPa
(b) Pinj=20 MPa Fig. 9. In each condition, the distance between the spray center and the injector tip (black point and line) and the predicted distance using the initial spray center.
similar, and increasing the injection quantity compared to 10 MPa, which consequently increases the spray angle. As was the case with the injection pressure of 10 MPa, it was found that the deviations of the individual spray angles of injection are large in Holes 4 and 5. The measurements result and analysis of individual spray angles can be used to reduce wall wetting and design the mounting position of injectors and piston shape for air–fuel mixture control [5]. To do this, the individual spray angles are measured in a variety of ways at the current stage, and continuous research is required to increase accuracy and predict individual spray angles.
2. A new method for measuring the spray angle of an individual spray plume using a spray pattern is presented. By using the injection angle measurement of the individual spray plume, the fuel distribution deviation of each individual spray plume can be checked and used to compare the fuel distribution between the experimental results and the designed orifice hole. Using these results, a more accurate and efficient nozzle orifice arrangement would be possible. 3. The deviation of the spray center, which can be considered as a measure of the uniformity and stability of each spray plume, increased in both the X- and Y-directions due to scattering and evaporation of the spray as the distance from the nozzle tip increased. In addition, the deviation increased rapidly after about 70 mm at the injection pressure of 10 MPa. 4. At the injection pressure of 10 MPa, the trajectory of the spray center became smaller than the angle of the initial designed orifice, making it closer to the vertical axis of injector. By contrast, when the injection pressure was increased to 20 MPa, the spray-centered trajectory maintained the inclined angle of the orifice. That is, when the injection pressure was high, the spray-centered trajectory remained straight for longer as the initial spray momentum increased. This means that with larger initial spray momentum due to the injection pressure, the spray center maintains the straight trajectory for longer. These results can be used to optimize the spray targeting of GDI injectors and the positions of the injectors as well as to establish injection strategies.
4. Conclusions In this study, the spray characteristics of the individual spray plumes from the multi-hole GDI injector were investigated and analyzed through a spray pattern experiment, which involved visualizing the cross-section of spray. The following important conclusions were drawn from the study. 1. The GDI injector with multi-holes show different fuel behavior within the nozzle orifice according to the designed orifice arrangement, and it also has different spray characteristics for each hole. The spray pattern was used to obtain the characteristics of each individual spray plume, which are difficult to separate by macroscopic spray images.
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(a) Pinj=10 MPa
(b) Pinj=20 MPa Fig. 10. Comparison of spray cone angle and deviation of individual spray plumes.
Acknowledgments
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