Experimental Study on Macroscopic Spray and Fuel Film Characteristics of E40 in a Constant Volume Chamber

Abstract

In the modern industrial field, there is a strong emphasis on energy-saving and emission reduction. Increasing the amount of ethanol in ethanol–gasoline blends has the potential to replace fossil fuel gasoline more effectively, improving energy efficiency and lowering emissions. The interaction between liquid fuel film generation on the piston crown and spray impingement in the combustion chamber in the setting of GDI engines has a substantial impact on particle emissions and engine combustion. In this study, 92# gasoline and ethanol by volume are combined to create the ethanol–gasoline blend E40. The spray characteristics and film properties of both gasoline and the intermediate proportion ethanol–gasoline blend E40 were researched utilizing a constant volume combustion platform and the schlieren method and refractive index matching (RIM) approach. The results show that, for 0.1–25 operating conditions, gasoline consistently displays greater macroscopic spray characteristic parameters than E40. This shows that gasoline fuel spray evaporation is superior to E40. Similar results are seen in the analysis of wall-attached fuel films, where the volume and thickness of the gasoline film are less than those of the E40 film under the given operating conditions. In contrast, E40 consistently exhibits stronger macroscopic spray characteristic values than gasoline under the 0.1–150 and 0.4–150 operating conditions, along with lower film thickness and volume. As a result, under these two operating conditions, E40 fuel performs better during spray evaporation.

1. Introduction

In recent years, the two primary directions of industrial development have been energy conservation and emission reduction. In the realm of transportation, gasoline direct injection (GDI) engines have emerged as the predominant powertrain for passenger vehicles. The utilization of direct gasoline injection into the cylinder facilitates more precise combustion processes, leading to reduced emissions. Furthermore, the evaporation of gasoline within the cylinder enhances the engine’s resistance to knocking, enabling higher engine compression ratios and resulting in improved power output and fuel efficiency [1]. However, relative to port fuel injection (PFI) engines, GDI engines have been observed to generate elevated levels of particulate matter, encompassing fine and ultrafine particles, primarily due to localized enrichment of the air–fuel mixture and wall-wetting phenomena [2]. Extensive research in the field of pathophysiology has demonstrated that exposure to exhaust particulates emitted by internal combustion engines heightens the risk of respiratory ailments, cardiopulmonary mortality, and other potential health complications [3]. In response to concerns surrounding air quality and human health, there has been a surge in regulations imposing increasingly stringent limits on particulate matter emissions from GDI engines [4].

Biofuels serve as a primary source of renewable energy, offering a potential supplement to diminishing fossil fuel resources. Finding a clean alternative fuel has a lot of potential for lowering engine particulate matter (PM) emissions and reducing the use of fossil fuels. Among the various biofuels, ethanol is one of the most suitable due to its combustion characteristics being similar to those of gasoline [5]. In fact, extensive research has already been conducted on the combustion processes and particulate matter emissions of ethanol–gasoline blends in spark ignition (SI) engines. Moreover, some countries have already embraced the use of low-percentage ethanol–gasoline blends (E10, i.e., gasoline which is 10% ethanol) as substitutes for conventional gasoline fuel. Generally, ethanol-blended fuels exhibit lower PM emissions in SI engine applications compared to gasoline and other alternative fuels [6,7,8]. However, some studies have also reported an increase in concentration and mass of PM for gasoline engine fuels with the addition of ethanol [9,10,11]. Maricq researched the effect of ethanol blending on GDI engine PM emissions. It was found that the PM emission reduction potential of low-proportion ethanol–gasoline blends (<20 vol%) is uncertain, while the PM emission decreased significantly for high-proportion ethanol-gasoline blends (>30 vol%). Engine particulate matter emissions may be reduced more efficiently by using a medium ratio ethanol–gasoline mixture [12].

In the context of GDI engines, the occurrence of wall-wetting is highly probable, particularly during delayed injection events, which has detrimental effects on combustion efficiency and can lead to increased particulate matter (PM) emissions. The combustion characteristics of the blended fuel, as well as its propensity for particle emissions, are influenced by both the physical mechanisms of the fuel, such as atomization and evaporation, and the underlying chemical processes involving reaction kinetics and oxygen content. Ethanol, when compared to gasoline, exhibits higher liquid fuel density, surface tension, and viscosity. Moreover, the enthalpy of ethanol evaporation is twice that of gasoline. These inherent properties can potentially result in delayed atomization and larger droplet sizes, thereby adversely impacting engine combustion and contributing to elevated PM emissions [13,14]. Consequently, it becomes imperative to thoroughly investigate the characteristics associated with wall-wetting in order to gain a comprehensive understanding of its influence on combustion and particulate emissions. Furthermore, in the real-world environment of combustion chambers, significant variations in temperature and pressure are experienced. To effectively address this, quantitative and continuous time-resolved measurements under diverse temperature and pressure conditions are essential to facilitate a deeper comprehension of the formation and evaporation dynamics of liquid fuel films. Drake and Fansler introduced the Refractive Index Matching (RIM) method, leveraging the similarity between the refractive index of the fuel and quartz, to precisely measure parameters such as fuel film thickness, area, and volume [15]. Prior studies have consistently demonstrated the robustness of RIM as a powerful technique for conducting quantitative, time-resolved measurements of fuel films [16,17,18,19,20].

Currently, the research focus on ethanol–gasoline blended fuels primarily revolves around engine dynamometer tests, mainly examining low and high ethanol blending ratios. However, there is a dearth of research on the spray characteristics and the formation and evaporation of liquid fuel films specifically for moderate ethanol–gasoline blends. This study utilized a constant-volume combustion bomb optical measurement platform and employed the schlieren technique and the RIM method to comprehensively investigate the macroscopic characteristics of free sprays and the distribution of fuel films after spray impingement. The investigation encompassed gasoline and E40 (a blend comprising 40% ethanol and 60% gasoline) under diverse operating conditions. The influences of environmental conditions and ethanol blending on the spray characteristics and deposited fuel film, namely penetration length, angle, and fuel film volume, were clarified. The fuel-air mixing process is greatly influenced by the spray structure and its flow field, which lays the groundwork for future research and improvements to the combustion phase and PM emissions. The research findings not only provide a reference for the application of medium-proportion ethanol–gasoline mixtures, but can also be used to calibrate engine simulation models.

2. Experimental Setup

2.1. Experimental Apparatus

This study developed a visualized constant volume combustion chamber system. The system features three circular windows with a diameter of 100 mm, and its primary combustion chamber is made up of two intersecting cylinders that constitute its internal cavity. To enable the effective visualization and data acquisition of spray and hot jet phenomena, transparent quartz glass is positioned on both sides of the constant volume chamber, facilitating the utilization of a schlieren system. The single-hole GDI injector is mounted on top of the chamber system. When measuring fuel film based on the RIM method, a bracket is installed on the side of the chamber, the quartz glass is fixed on the bracket, and the distance between the nozzle and the upper surface of the quartz glass is 35 mm.

Figure 1 shows the schematic of the experimental setup. A Phantom 7.3 high-speed camera is used with a Nikon telephoto lens to capture the macroscopic forms of free spray. The shooting speed is 10,000 frames/s and the image resolution is 512 × 512 pixels. A v611 phantom camera is used with a Nikon 105 mm fixed-focus lens to capture the development process of the deposited fuel film through the 45° reflector under the chamber system. At this time, the schlieren light path is not used, and the light source position is adjusted to the axis of the windows on both sides of the fixed-volume bomb to match the horizontal plane, which is about 20 degrees incident from above.

2.2. Experimental Conditions

In this paper, pure gasoline (E0) and gasoline–ethanol blended fuels which contained 40% ethanol by volume (E40) were investigated. The fundamental physical and chemical properties of gasoline and ethanol are summarized in

Table 1. Physical and chemical properties of ethanol and gasoline.

Fuel	Ethanol	Gasoline
Chemical formula	C2H5OH	C5H12–C12H26
Boiling point (°C)	78.0	25–215
Density (g/cm3) @ 20 °C, 1 bar	0.79	0.72–0.75
Dynamic viscosity @ 1 bar, 25 °C (mPa s)	1.104	0.65
Heat of vaporization (kJ/kg) @ 20 °C	904	310–340
Research octane number (RON)	109	88–99
Stoichiometric air–fuel ratio (kg/kg)	9	14.7
Lower heating value (MJ/kg)	26.8	43.5
Surface tension(mN/m)	22.6	21.5
Laminar burning velocity @ 25 °C, λ = 1 (cm/s)	45	35

The properties of fuel vary under different environmental conditions. “@” indicates that under this condition, the fuel properties are the values in the table.

. A single-hole gasoline injector was used throughout the experiment, maintaining a constant fuel injection pressure of 8 MPa. To realistically simulate the impact of internal cylinder pressure and high-temperature environments on the spray process during fuel combustion, two distinct environmental back pressures (0.1 MPa and 0.4 MPa) and two environmental temperatures (25 °C and 150 °C) were strategically chosen for investigation. It is vital to set the same spray pulse width when comparing sprays produced under various working situations. The study used a jet pulse width of 1.5 ms for testing the macroscopic properties of free spray, and a jet pulse width of 0.5 ms for the wall-attached fuel film test that follows the spray’s impact on the wall. Each operating condition was meticulously repeated at least five times to ensure data reliability, and the resultant average values were considered as the outcomes. The specific operating conditions are denoted in the “environment pressure + temperature” format. For example, the operational condition with an environmental pressure of 0.1 MPa and an environmental temperature of 25 °C is represented as 0.1–25.

2.3. RIM Calibration

The RIM method operates based on the refractive index disparity between rough quartz glass and air. When liquid fuel is injected into the quartz glass, the surface of the rough quartz glass is effectively “repaired,” facilitating the measurement of the liquid film thickness. The quartz glass, functioning as a piston surrogate, features a smooth bottom surface and a frosted top surface with an average surface roughness of 2.5 µm. The propagation of light is visually depicted in Figure 2. The presence of a liquid film on the quartz glass surface enhances the transmitted light while attenuating the corresponding scattered light. The RIM method effectively calculates the thickness of the liquid film by utilizing this subdued scattered light, employing a calibration process.

Drake initially discovered the correlation between changes in light intensity captured by the camera and the thickness of the oil film [15]. This relationship can be expressed as follows:

$$\mathsf{\Delta }I(\mathrm{x},\mathrm{y})=1-\frac{I_{wet}(x,y)}{I_{ref}(x,y)}$$

$$h(\mathrm{x},\mathrm{y})=f\left(\mathsf{\Delta }I\right)$$

here, $\mathsf{\Delta }I(\mathrm{x},\mathrm{y})$ is the fluctuations in scattered light intensity, $I_{ref}(x,y)$ corresponds to the scattered light intensity observed on dry quartz glass, $I_{wet}(x,y)$ represents the scattered light intensity detected on quartz glass coated with a fuel film, and $h(\mathrm{x},\mathrm{y})$ denotes the local thickness of the oil film at position $(\mathrm{x},\mathrm{y})$.

To establish the connection between transmittance and oil film thickness, a calibration procedure is undertaken. This process necessitates a substantial discrepancy in the evaporation rates between heavier and lighter components, with each component evaporating independently at room temperature. Isooctane and dodecane are chosen as calibration oils due to their ability to fulfill these requirements. During the calibration experiments, a microsyringe is employed to deposit mixtures of isooctane and dodecane, varying the volume ratios, onto the surface of the quartz window. An experimental program is developed using MATLAB 2021a. Initially, a reference image capturing the background without fuel attachment serves as the calibration baseline. By subtracting the background image from the image captured at the calibration point, the attached oil film image at the calibration point is obtained. Subsequent image processing steps, including grayscale conversion, binarization, boundary extraction, and area calculation, are performed to determine the oil film area at the calibration point. Varied volume fractions of isooctane and dodecane mixtures are prepared as calibration fuels throughout the calibration process. By calculating the fuel volume and corresponding area, the average thickness of the attached oil film can be determined. The data obtained from different calibration points are fitted to establish a functional relationship between the oil film thickness and transmittance [21,22].

During the calibration process, the error in the titration solution volume is 3%; the light intensity at each point in the wall-coated oil film area is averaged during calculation, and the resulting error is in the range from 8% to 10% [17]; dodecane will evaporate early during the isooctane evaporation stage, causing an error of about 10%; and the final measurement system error is 15%.

3. Results and Discussions

In this study, the processing of spray images was performed using MATLAB 2021a. When examining the macroscopic characteristics of the free spray, according to the Society of Automotive Engineers (SAE) J2715 recommended practice guidelines, spray penetration and angle were calculated [23]. Moreover, the concept of spray area is also introduced, which corresponds to the projected area of the spray on the image plane. This parameter offers an intuitive indication of the diffusion rate and extent of the spray (Figure 3).

3.1. Spray Morphology

The spray penetration distance serves as a valuable indicator of the spray’s axial development trend, facilitating an understanding of its spatial distribution and propensity to impinge on surfaces. Figure 4 and Figure 5a depict the macroscopic morphology and spray penetration distances of gasoline and E40 under three distinct operating conditions. It can be seen that, for the same kind of fuel, under the same environmental back pressure, the spray penetration distance increases with the increase in ambient temperature. This behavior can be attributed to the temperature-induced reduction in surrounding air density, accompanied by an increase in fuel molecule kinetic energy. Consequently, the decreased resistance to spray penetration and augmented impact velocity contribute to an amplified spray penetration distance. Conversely, under identical ambient temperatures, an increase in ambient back pressure yields a significant decrease in the spray penetration distance. This phenomenon arises from the heightened density of the ambient atmosphere, which intensifies the resistance to spray penetration, diminishes spray momentum, and consequently reduces the spray penetration distance. Within the operating range of 0.1–25, gasoline demonstrates a slightly greater spray penetration distance compared to E40. This disparity can be attributed to the lower viscosity of gasoline, resulting in higher injection velocities and, consequently, an extended spray penetration distance. However, in the operating range of 0.1–150, both gasoline and E40 exhibit similar spray penetration distances, suggesting that fuel characteristics exert minimal influence on spray penetration distance under higher ambient temperatures and lower ambient pressures. Remarkably, within the range of 0.4–150, E40 showcases a slightly larger spray penetration distance than gasoline. This outcome stems from the slower evaporation of less volatile components in gasoline, which leads to a higher concentration of large liquid droplets in the spray. Consequently, gasoline sprays with an abundance of large liquid droplets experience a more pronounced reduction in spray penetration distance under high ambient back pressure, highlighting the heightened resistance to spray penetration. In contrast, E40 demonstrates a comparatively smaller reduction in spray penetration distance under these conditions, owing to its lower concentration of large liquid droplets.

The spray cone angle can reflect the degree of spray divergence to some extent, further influencing its atomization quality. Figure 4 and Figure 5b display the spray cone angles of gasoline and E40, respectively, under three different operating conditions. For the same fuel, as the ambient temperature increases, the spray cone angle significantly decreases. Under the same ambient temperature, the spray cone angle of the fuel slightly decreases with the increase in ambient pressure. Under the low ambient temperature and low ambient pressure condition of 0.1–25, the spray cone angle of gasoline is larger than that of E40. Gasoline is a mixture of multiple components, and the presence of high-volatility components in gasoline makes its evaporation slightly better than E40 under low ambient temperatures. This might be because the evaporation of high-volatility components in gasoline increases the interaction between the spray edge droplets and the surrounding air molecules, accelerates the fragmentation and evaporation of the spray edge droplets, and hence increases the spray cone angle. Under the conditions of 0.1–150 and 0.4–150, the spray cone angle of E40 is slightly larger than that of gasoline. Due to the better evaporation of E40, during the development of the spray, the aerodynamic interaction between the spray edge droplets and the surrounding air will increase the spray cone angle.

Understanding and analyzing the characteristic parameters of the spray area can help understand the development process of the spray from the perspective of the overall morphological changes of the spray. Figure 4 and Figure 5c show the development trend of the spray area of gasoline and E40, respectively. For the same fuel, as the ambient temperature increases, its spray area increases; under the same ambient temperature, the spray cone angle of the fuel significantly decreases with the increase in ambient pressure. Under the condition of 0.1–25, the spray area of gasoline is slightly larger than that of E40; the evaporation of high-volatility components in gasoline increases the spray area. Under the conditions of 0.1–150 and 0.4–150, the spray area of E40 is higher than that of gasoline. Due to the higher ambient temperature, the evaporation characteristics of E40 are better than those of gasoline, and stronger evaporation makes the spray area larger. However, under the condition of 0.4–150, the difference between the spray areas of the two is very small. This is because, under high ambient pressure, the development of spray along the axial and radial directions is suppressed.

3.2. Characteristics of Fuel Film Deposited after Spray Impingement

Figure 6 presents the variations in the deposited fuel film morphology of gasoline and E40 fuels under diverse operational conditions. Notably, the impact of ambient temperature on the deposited fuel characteristics outweighs that of ambient density. Higher ambient temperatures induce a substantial reduction in the wall film area. Figure 7 provides a quantitative depiction of the temporal evolution of the deposited fuel for both gasoline and E40 across three distinct operating conditions. As can be seen from Figure 7a,b, it becomes evident that an elevation in ambient temperature leads to noticeable decreases in both the area and volume of the deposited fuel film. The observed phenomenon can be attributed to two primary factors. Firstly, the fuel mist discharged by the injector undergoes heat exchange with the surrounding air medium before impingement, raising the fuel temperature as the ambient temperature rises. Because of the rapid evaporation that results, less fuel impinges on the wall, resulting in a reduced peak area and volume of the fuel film. Secondly, the rise in ambient temperature corresponds to an increase in the temperature of the quartz glass, intensifying heat exchange and expediting the evaporation of the wall film. The combined influence of these factors significantly diminishes the peak volume of the wall film and expedites the evaporation rate under high ambient temperatures. At the same ambient temperature, as the ambient pressure increases, the fuel film area and volume both increase slightly. The reason for this is that when the ambient pressure is low, the gas resistance has little impact on the spray, and the droplets remain after the spray hits the wall. With a certain momentum, splashing will occur, so that some of the fuel droplets splash out of the quartz glass and gradually break and evaporate, no longer falling back to the quartz glass. When the environmental pressure is high, the high back pressure hinders the splashing of the droplets, and some of the droplets will fall back into quartz glass.

It can be seen from Figure 7c,d that the maximum thickness and average thickness of the deposited fuel film of the two fuels under the three working conditions are both around 0.6 and 0.3μm, which is slightly different, but the difference is not significant. During the development process of the spray, the interaction with the ambient air and heat exchange mainly occurs in the edge area, and the spray droplets in the center area of the spray still maintain a certain momentum to hit the quartz glass. In addition, the maximum thickness of the fuel film and the average thickness of the oil film of the two test fuels in the 0.1–25 operating conditions both increased slightly over time. Research has shown that during the outward expansion of the wall-coated oil film formed by spray hitting the wall, the expansion rate gradually increases to a peak value and then decreases. When the expansion area reaches the maximum value, under the action of surface tension, the direction of movement changes, causing the oil film at the edge to move toward the center as the oil film develops outwardly from the center of the spray wall. When the temperature of the ring mirror is low, the oil film on the wall evaporates slowly, and the oil film at the edges moves toward the center under the action of surface tension, making the oil film thicker [24].

In the 0.1–25 working conditions, it is observed that E40 exhibits a greater maximum thickness, and average thickness, area, and volume of the wall film compared to gasoline. This disparity can be attributed to the high-volatility components of gasoline evaporating before spraying impingement at low ambient temperatures, thereby enhancing its evaporation characteristics under these specific operating conditions. However, during the 0.1–150 and 0.4–150 operating conditions, the maximum thickness, average thickness, area, and volume of the E40 wall film are smaller than those of gasoline. This discrepancy arises due to the superior evaporation characteristics of E40 compared to gasoline under high ambient temperatures. Specifically, in the 0.1–150 operating condition, the thickness and area of the E40 film decrease rapidly over time, indicating accelerated evaporation.

4. Conclusions

In this study, images of spray morphology and fuel film deposited after the spray hits the wall under various operating conditions were compared to demonstrate how ambient temperature, ambient pressure and fuel characteristics affect spray characteristics and fuel films resulting from spray-wall impingement. Based on the current investigation, the following specific findings can be made:

(1)

For the same test fuel, spray penetration distance and spray area increase when ambient temperature rises. However, spray penetration distance, spray cone angle, and spray area all considerably decrease as environmental pressure rises. The spray macro-characteristic parameters of gasoline are greater than those of E40 in the 0.1–25 working conditions, demonstrating that gasoline fuel spray evaporates better when compared to E40. The spray macro-characteristic parameters of E40 are greater than gasoline in both the 0.1–150 and 0.4–150 working conditions, demonstrating that E40 fuel spray evaporates better in these two operating conditions.

(2)

As the ambient temperature increases, the area and volume of the deposited fuel film decrease significantly. Under the same ambient temperature, as the ambient back pressure increases, the oil film area and volume both increase slightly. In the 0.1–25 working condition, the maximum oil film thickness, average oil film thickness, oil film area, and oil film volume of E40 are larger than that of gasoline. In the 0.1–150 and 0.4–150 working conditions, the maximum oil film thickness, average oil film thickness, oil film area of E40, and oil film volume are both smaller than gasoline. The evaporation characteristics of fuel have a great influence on its oil film properties.

For GDI engines, the research data on spray characteristics and deposited fuel film in this study can help optimize injection strategies, improve in-cylinder combustion and particulate matter emissions. Ethanol is a renewable alternative fuel that is generally recognized by researchers and has been practically used in engines. Further increasing the proportion of ethanol in ethanol–gasoline mixed fuel can increase the gasoline substitution rate. The research in this study provides a reference for the application of E40, which is positive for environmental protection.