Understanding the Effects of Fuel Type and Injection Conditions on

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Understanding the Effects of Fuel Type and Injection Conditions on Spray Evaporation Using Optical Diagnostics

2015-01-0926 Published 04/14/2015

Tianyun Li, Min Xu, David Hung, Shengqi Wu, and Siqi Cheng Shanghai Jiao Tong Univ

CITATION: Li, T., Xu, M., Hung, D., Wu, S. et al., "Understanding the Effects of Fuel Type and Injection Conditions on Spray Evaporation Using Optical Diagnostics," SAE Technical Paper 2015-01-0926, 2015, doi:10.4271/2015-01-0926. Copyright © 2015 SAE International

Abstract Comparing with port-fuel-injection (PFI) engine, the fuel sprays in spark-ignition direct-injection (SIDI) engines play more important roles since they significantly influence the combustion stability, engine efficiency as well as emission formations. In order to design higher efficiency and cleaner engines, further research is needed to understand and optimize the fuel spray atomization and vaporization. This paper investigates the atomization and evaporation of n-pentane, gasoline and surrogate fuels sprays under realistic SIDI engine conditions. An optical diagnostic technique combining high-speed Mie scattering and Schlieren imaging has been applied to study the characteristics of liquid and vapor phases inside a constant volume chamber under various operating conditions. The effects of ambient temperature, fuel temperature, and fuel type on spray atomization and vaporization are analyzed by quantitative comparisons of spray characteristics. Experimental results indicate that the evaporation of n-pentane, gasoline, light surrogate fuel and heavy surrogate fuel is affected by fuel temperature and ambient temperature. Under the same condition, the evaporation of n-pentane is strongest, and the evaporation of gasoline and light surrogate fuel spray is stronger than that of heavy surrogate fuel spray. Moreover, the fuel temperature and ambient temperature have significant effects on spray penetration length of liquid phase as well as the two-phase interaction between liquid and vapor. Therefore, changing the environment temperature and fuel temperature are effective ways to control the spray penetration and evaporation.

Introduction Since fuel is injected directly into the combustion chamber of spark ignition direct injection (SIDI) engines, fuel spray plays a very important role in controlling the combustion stability, engine efficiency as well as emission formations. It is well known that the fuel injection process and in particular the characteristics of the fuel spray significantly influence the fuel economy and emissions of the SIDI engines [1, 2]. Hence, to generate a better spray for combustion improvement and emissions reduction, further study to understand the atomization and vaporization characteristics of spray is needed.

Due to the different physical properties of liquid fuels, such as density, viscosity, surface tension, etc., liquid fuel type has a strong influence on the spray characteristics. Specifically, the forces acting on the spray jet and droplets, such as inertia force, viscous force, surface tension force, and air drag force, govern the breakup and atomization processes of fuel and they play the main roles in the liquid fuel spray formation and characteristics [3, 4]. In the literature, extensive research on the single component fuel is available. In our previous study, the spray characteristics of single component fuels, such as alcohol fuels and alkanes, were investigated [4, 5, 6, 7]. However, in SIDI engines, gasoline is a complex mixture in nature and it has a very different performance when compared to the single component fuel representation under various operating conditions [8, 9]. Hence, surrogate fuels which decrease the chemical and/or physical complexity of gasoline can be used to enhance the understanding of fundamental processes involved in the interaction between fuels and internal combustion engines [10]. Besides, it has been demonstrated that in engine simulations, the surrogate fuel models with multiple components are critical to enhance the accuracy of the model in predicting the fuel air mixture preparation process, especially under cold start condition in gasoline engines [8]. Therefore, the characteristics of multi-component fuel spray should be fully investigated based upon an accurate surrogate fuel model. However, most of the surrogate fuels were only studied numerically, and far less studies have been focused on the spray characteristics with experimental verifications. In this work, the spray characteristics of n-pentane, gasoline and surrogate fuels with different mass fractions of three components were experimentally studied to understand the fuel spray atomization and vaporization. Xu et al. [8] utilized a three-component fuel recipe to represent the actual gasoline fuel depending on the similar distillation of the three-component fuel and gasoline. Hence, in this study, n-pentane, iso-octane and n-decane have been chosen as the components to study the spray characteristics of surrogate fuel. This study focuses on the influences of fuel temperature and ambient temperature on the spray geometry from a multi-hole injector.


Experimental Techniques and Apparatus Schlieren and Mie Scattering Imaging Techniques Mie scattering is the elastic scattering of light from particles that are much larger than the wavelength of the incident light [5]. Therefore, Mie scattering imaging is only sensitive to the liquid phase of fuel spray since the scattering only occurs on the surface of droplets.

20,000Hz. The injector was installed on the top of the chamber. The injector used in this study was a solenoid actuated, SVCO (steppedhole valve covered orifice) style, 8-hole injector with the nozzle diameter of 0.15mm. The nominal spray angle of the injector was 60 degrees.

Schlieren imaging technique is a sensitive and relatively simple optical diagnostic which has been used for decades [1]. It depends on the different medium and density which leads to different refractive index values [11]. Since Schlieren imaging is much sensitive to the gradients of refractive index, when liquid fuel is injected to a chamber, the fuel spray, which can be considered as a multiphase flow of liquid droplet with surrounding ambient air, changes the refractive index of the test area. Hence, part light is deflected when parallel light passes through the fuel spray. The liquid and vapor phases of the spray can be captured by the Schlieren imaging system. However, the liquid and vapor phases cannot be definitively distinguished with Schlieren imaging technique. In the Schlieren imaging system, a high speed camera is used to capture the fuel spray. Since the ambient gas flow is influenced by the gas admission and non-uniform temperature exists inside the test chamber, the ambient gas flow, even a weak flow, can be captured due to the high sensitivity of Schlieren imaging system. It will interfere with the extraction of the spray boundaries from the images. However, with the high speed imaging, the ambient flow does not move significantly between the successive frames. As a result, the artifacts in the background of spray images could be effectively eliminated with the high speed Schlieren imaging. Hence, high speed Mie scattering imaging was employed to capture the liquid phase of the spray and the high speed Schlieren imaging was employed to capture the liquid and vapor phases, especially the vapor phase. By combining Schlieren imaging and Mie scattering imaging, the spray atomization and vaporization characteristics can be elucidated effectively.

Figure 1. Experimental system for Mie scattering imaging

With the same xenon arc lamp and high speed camera, the Schlieren imaging system is shown in Figure 2. A 300W xenon arc lamp was used as the light source of Schlieren imaging system. The lamp provided continuous light with an outlet flange 1.5 inch in diameter. The output from the lamp was focused on the slit by the condenser lens to produce a point light source. Then the point light source was collimated by a parabolic mirror to produce a beam approximately 150 mm in diameter. The focal length of parabolic mirror was 914.40 mm. The plane mirror was used to direct the collimated beam into the chamber through the window. Therefore, the fuel spray in the volume chamber could be illuminated by the collimated beam.

Experimental Apparatus Figure 1 shows the experimental system that was used to capture the Mie scattering images. The system consisted of a constant volume chamber, high speed camera, xenon arc lamp, fuel supply system, fluid temperature control system and other accessory equipment. The constant volume chamber with high pressure and high temperature capability (HPHT chamber) was utilized to contain the spray and provided the ambient pressure and temperature. In the chamber, the ambient gas temperature was measured with a thermocouple which was mounted near the injector tip. The ambient pressure from 20kPa (absolute) up to 8MPa and ambient temperature from room temperature to 1000K could be achieved. Fuel injection pressure up to 20MPa was supplied by the fuel system which consisted of the nitrogen gas bottle, accumulator, filter and rigid fuel lines. A fluid temperature control system was used to regulate the fuel temperature between 293K and 363K. The camera used in the experiments was a Phantom V1210 (full resolution: 1280×800 @12,600Hz). In this experiments, the resolution of the camera was set to 512×512 with a frequency of

Figure 2. Schlieren imaging system

The plane mirror on the receiving side was used to reflect the collimated beam to the other parabolic mirror. The beam was focused on the focal plane of the parabolic mirror. A knife edge was installed at the focus of the beam between the parabolic mirror and the high speed camera. The knife edge was used to cut off the beam to improve the sensitivity of the Schlieren imaging system to the gradients of refractive index. If more light was cut off, the sensitivity


of the Schlieren imaging system would be higher, however, the light intensity would become weaker. Therefore it was necessary to expand the slit to improve the light intensity. However, a smaller slit could produce a better collimated beam. Therefore, a balance between the cutting off and the size of slit was needed. Besides, in order to obtain the background of spray images with a uniform light intensity, the knife edge and slit were mounted at the foci of the beam on the receiving and transmitting sides.

Experimental Conditions and Test Fuels The experimental conditions are shown in Table 1. This study of the spray characteristics is focused on the fuel types and the temperature of fuel and environment. The spray images were captured with fuel temperatures of 298K, 328K and 358K. The fuel temperatures were chosen to represent the fuel temperature of engine cold start, part load, and heavy load conditions. For each fuel temperature, three ambient temperatures were studied. The ambient pressure was maintained at 101kPa, and the fuel injection pressure was 10MPa with an injection pulse width of 1.1ms. Table 1. Test Conditions

In this study, n-pentane, 95# gasoline (Ron 95), light surrogate fuel (LSF), and heavy surrogate fuel (HSF) were chosen as the test fuels. The compositions of surrogate fuels are shown in Table 2. Compared to light surrogate fuel, the heavy surrogate fuel has a higher fraction of n-decane. The distillation curves of surrogate fuels and gasoline are shown in Figure 3, which were measured using the American Society for Testing and Materials (ASTM) method. It indicates that when the temperature is below 380K, gasoline evaporates more easily when compared to the surrogate fuels, and the evaporation of light surrogate fuel is stronger than the heavy surrogate fuel. Besides, the physical properties of those single-component fuels and gasoline are listed in Table 3.

Figure 3. Distillation curves of the light surrogate fuel, heavy surrogate fuel and gasoline (Ron 95)

Table 2. Fuel Compositions of Surrogate Fuels

Table 3. Physical Properties of Fuel

The definitions of spray penetration and spray angle are shown in Figure 4. In order to obtain accurate results from recorded spray images (12-bit), the predefined image threshold was very critical to distinguish the spray from its background. Since the highest intensity was approximately 3500, Schlieren images threshold was set to 90. To maintain the same thresholding proportion, the Mie scattering image threshold was set to 50 for the highest intensity of approximately 2000. A total of 40 injection spray cycles were recorded to calculate the ensemble image. Spray data were extracted from the ensemble images.

Figure 4. Spray penetration and spray angle definition

Results and Discussion In this investigation, the effects of fuel type, fuel temperature and ambient temperature on spray characteristic were studied. The spray penetration and angle were calculated. It is worth mentioning that the light traversed the spray from the right to left side in Mie scattering experiment. All images were captured based on the acquisition time after the start of fuel (ASOF). It represents the actual time of the fuel exiting from the injector tip, and provides a consistent basis to compare the initial characteristics of the liquid fuel emerging from the injector tip. Figure 5 shows the Mie scattering and Schlieren images of n-pentane at the time ranging from 0.1ms to 2.0ms ASOF. The images were captured at a fuel temperature of 358K and ambient temperature of 298K. Comparing the Mie scattering and Schlieren images, it shows that at early injection (before 0.5ms ASOF), the sprays in Mie scattering images and Schlieren images developed similarly since


only small amount of fuel was evaporated in a very limited time. As the spray continued to form, much more vapor was produced and the spray in Schlieren images expanded continuously. However, the liquid spray structure in Mie scattering images gradually reduced due to the evaporation of fuel. At 2.0ms ASOF, almost all the fuel evaporated into vapor.

Effect of Fuel Temperature

Figure 5. Mie scattering and Schlieren images of n-Pentane. Ambient Temperature: 298K, Fuel Temperature: 358K, Injection Pressure: 10MPa, Ambient Pressure: 101kPa.

Figure 6. Schlieren images of various fuel types under different fuel temperatures. Ambient Temperature: 298K, Injection Pressure: 10MPa, Ambient Pressure: 101kPa, Time: 0.8ms ASOF.

Figure 6 shows the raw Schlieren images of various fuels. Each row corresponds to one different fuel and each column corresponds to a different fuel temperature. Comparing the images of n-pentane under different fuel temperatures, the spray under the fuel temperature of 358K collapsed to the centerline. The difference between the fuel temperature and the local boiling point under the ambient pressure was defined as the fuel superheated degree (SD) [12]. The superheat degree of n-pentane under the fuel temperature of 358K was 49K. Hence, n-pentane with 358K fuel temperature boiled immediately when it was injected into the environment with the ambient pressure of 101kPa. This resulted in lower pressure than the ambient pressure along the centerline of the flash boiling spray [13]. The pressure difference between the inner and outer of the spray led to the collapse of plumes altogether. However, for gasoline, light surrogate fuel and heavy surrogate fuel spray, only some part of fuel components boiled under the fuel temperature at 358K, the spray plumes became a little wider, but they did not collapse altogether.


Figure 7. Penetration curves of Schlieren images and Mie scattering images. (a), (b), (c), (d) are the penetration curves under various fuel temperatures, (e), (f) are the penetration curves of various fuel types. Ambient Temperature: 298K, Injection Pressure: 10MPa, Ambient Pressure: 101kPa.

In Figure 6, it shows that the area of vapor phase increases with increasing fuel temperature. It indicates that the evaporation is accelerated. Compared to the images of different fuels, the vapor phase area of n-pentane was largest, followed by gasoline and light surrogate fuel. The smallest one was associated with the heavy surrogate fuel. As expected, the evaporation of n-pentane was the strongest whereas the evaporation of heavy surrogate fuel was the weakest due to the larger portion of n-decane present in it. The penetration curves of Schlieren and Mie scattering images under different fuel temperatures are depicted in Figure 7. The penetration of Mie scattering images represents the liquid penetration, and the penetration of Schlieren images demonstrates the vapor penetration since the liquid phase is surrounded by the vapor phase. It shows that both the liquid penetration and vapor penetration reduced when increasing the fuel temperature. Comparing the penetration curves of n-pentane, increasing fuel temperature from 298K to 328K enhanced the evaporation of n-pentane, leading to a larger penetration difference between the liquid and vapor. However, further increasing the fuel temperature to 358K, the penetration difference reduced significantly. This was caused by the collapse of spray plumes, as shown in Figure 8. The liquid phase of spray collapsed to the central axis of the spray. This allowed the fuel to go forward together and reduced influence of the air drag force. However, since the vapor lost the velocity more easily than liquid, the vapor at the spray tip moved to the outside of spray. Therefore, the liquid phase of spray kept up with the vapor phase at the fuel temperature of 358K. In Figure 7 (e), it can be found that the vapor penetrations of the four fuel are very similar due to the weak evaporation at a fuel temperature of 298K. When the fuel temperature was increased from 298K to 358K, the vapor penetration of n-pentane at 0.8ms ASOF reduced by 31.7%, however, the vapor penetration of heavy surrogate

fuel only reduced by 6.4%. It indicates that the effect of fuel temperature on evaporation of n-pentane was stronger than that of heavy surrogate fuel. Compared with the penetration curves of different fuel types in Figure 7 (e) and (f), the penetrations were similar at the fuel temperature of 298K due to weak evaporation of the fuel. However, at 358K, the vapor penetration of n-pentane was the smallest, the vapor penetration of heavy surrogate fuel was the largest. This indicates that due to the low boiling point, the evaporation of n-pentane was much stronger than that of gasoline and surrogate fuels, and the evaporation of gasoline was stronger than that of surrogate fuels.

Figure 8. Mie scattering image (left) and Schlieren image (right) of n-pentane. Fuel Temperature: 358K, Ambient Temperature: 298K, Injection Pressure: 10MPa, Ambient Pressure: 101kPa, Time: 0.8ms ASOF.

The fuel temperature effect on spray angle was investigated under the ambient temperature of 298K, as shown in Figure 9. It shows that the vapor spray angles were much larger than the liquid spray angle since the vapor phase surrounded the liquid phase of spray. When heating the fuel from 298K to 358K, the vapor spray angle of n-pentane increased by 10.8%, but its liquid spray angle decreased by 30.3%. It is because the high fuel temperature enhanced the evaporation and


caused the liquid phase of spray to collapse altogether. However, the spray angles of gasoline and surrogate fuels were similar when increasing the fuel temperature. Hence, the effect of fuel temperatures between 298K and 358K on spray angle of gasoline and surrogate fuels was minor.

surrogate fuel, and the minimum one was that of gasoline. Due to the collapse of n-pentane spray plumes, the vapor penetration of n-pentane was slightly larger than that of gasoline. Therefore, compared with the evaporation characteristic of gasoline and surrogate fuels, the evaporation of gasoline was the strongest. In order to make the spray characteristic of surrogate fuel similar to that of gasoline, the mass fraction of n-pentane in light surrogate fuel should be increased.

Figure 9. Spray angle under different fuel temperature. Ambient Temperature: 298K, Injection Pressure: 10MPa, Ambient Pressure: 101kPa, Time; 0.8ms ASOF.

Effect of Ambient Temperature The raw Schlieren images under various ambient temperature are shown in Figure 10. Comparing the Schlieren images of n-pentane, the spray under 433K ambient temperature collapsed more seriously than that under 298K ambient temperature. This phenomenon can also be found in the images of gasoline and surrogate fuels. It indicates that increasing ambient temperature enhanced the flash boiling phenomenon of liquid spray. Besides, at a higher ambient temperature, the projected area of vapor from the spray image increased. Hence, the evaporation of fuel spray was enhanced by increasing the ambient temperature. Comparing the images of different fuels, the spray of n-pentane collapsed the most, followed by the spray of gasoline and light surrogate fuel, and the least one was the spray of heavy surrogate fuel with very minor collapse. Figure 11 shows the penetration curves of vapor and liquid phases under different ambient temperature. It shows that both the liquid penetration and vapor penetration of gasoline and surrogate fuels decreased when increasing the ambient temperature, since a higher ambient temperature provided more thermal energy for the evaporation of fuel spray. Increasing the ambient temperature from 298K to 433K, the vapor penetration of gasoline was decreased by 14.7% at 0.8ms ASOF, and the vapor penetration of light surrogate fuel decreased by 11.3%. However, the vapor penetration of heavy surrogate fuel only decreased by 5.3%. For the n-pentane spray, due to the more serious collapse and the decrease of ambient air density, the vapor penetration increased observably with increasing ambient temperature. Comparing the difference between vapor penetration and liquid penetration at the 433K ambient temperature in Figure 11(e), the difference of n-pentane was the largest, and the difference of heavy surrogate fuel was almost zero. Compared the vapor penetration of gasoline and surrogate fuels, the vapor penetration of heavy surrogate fuel was the largest, followed by the vapor penetration of light

Figure 10. Schlieren images of various fuel types under different ambient temperature. Fuel Temperature: 358K, Injection Pressure: 10MPa, Ambient Pressure: 101kPa, Time: 0.8ms ASOF.

In Figure 12, the spray angles of various fuels under various ambient temperature were studied. It shows that the vapor spray angles of n-pentane and gasoline increased with increasing the ambient temperature due to the stronger evaporation. However, the liquid spray angles decreased due to the evaporation and collapse of spray. Both the liquid and vapor spray angles of light and heavy surrogate fuels remained almost the same with increasing ambient temperature. Hence, the evaporation of gasoline was stronger than that of light and heavy surrogate fuels.


Figure 11. Penetration curves of Schlieren images and Mie scattering images. (a), (b), (c), (d) are the penetration curves under various ambient temperatures, (e) are the penetration curves of various fuel types. Fuel Temperature: 358K, Injection Pressure: 10MPa, Ambient Pressure: 101kPa.

However, due to the high boiling components of gasoline and surrogates fuels, the liquid and vapor spray angles remained similar under the fuel temperature ranging between 298K and 358K. 2.

For gasoline and surrogate fuel sprays, high ambient temperature increased the evaporation and reduced the liquid and vapor penetration. At 0.8ms ASOF, the vapor penetrations of gasoline, light surrogate fuel and heavy surrogate fuel decreased by 14.7%, 11.3%, and 5.3%, respectively when increasing the ambient temperature from298K to 433K. However, due to the collapse of spray, the vapor penetration of n-pentane increased by 13.6%.

3.

Due to the low boiling point, the evaporation of n-pentane spray was strongest. Compared with gasoline and surrogate fuels, the evaporation of gasoline spray was stronger than that of surrogate fuels, especially under the high fuel temperature and high ambient temperature conditions. Due to a higher mass fraction of n-decane, the evaporation of heavy surrogate fuel spray was somewhat weaker than that of light surrogate fuel spray.

4.

The spray characteristics of gasoline and three-component fuels with a suitable mass fraction are similar. Increasing the mass fraction of n-pentane in light surrogate fuel can make the spray characteristics much closer to that of gasoline.

Figure 12. Spray angle under different ambient pressure. Fuel Temperature: 358K, Injection Pressure: 10MPa, Ambient Pressure: 101kPa, Time: 0.8ms ASOF.

Conclusions In this work, the spray characteristics of n-pentane, gasoline and two surrogate fuels were investigated using Schlieren and Mie scattering imaging technique. The effects of fuel temperature and ambient temperature on spray penetrations and angles of liquid and vapor were studied. The fuel injection pressure and ambient pressure in the study were fixed at 10MPa and 101kPa. The main conclusions are as follows: 1.

A higher fuel temperature increased the evaporation of fuel spray and reduced the liquid and vapor penetration. For the n-pentane spray under a higher fuel temperature, the vapor spray angle increased significantly due to the stronger evaporation, and its liquid spray angle reduced due to the collapse of spray.

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Contact Information Prof. Min Xu Institute of Automotive Engineering, Shanghai Jiao Tong University [email protected] Mr. Tianyun Li Institute of Automotive Engineering, Shanghai Jiao Tong University [email protected]

Acknowledgments The research was sponsored by the National Natural Science Foundation Committee (project number: 51376119), and was carried out at National Engineering Laboratory for Automotive Electronic Control Technology in Shanghai Jiao Tong University.

Definitions/Abbreviations ASOF - After start of fuel ASTM - American Society for Testing and Materials GDI - gasoline direct injection HSF - heavy surrogate fuel LSF - light surrogate fuel PFI - port-fuel-injection SD - superheated degree SIDI - spark-ignition direct-injection

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