Influence of Blending Ratio on Spray Characteristics of Gasoline–Hydrogenated Catalytic Biodiesel Blended Fuel

Abstract

Blending gasoline with hydrogenated catalytic biodiesel has the potential to improve combustion problems of gasoline direct-injection compression combustion, and the spray characteristics of the blending fuel can directly affect the combustion effect. In order to understand the spray characteristics of a gasoline–hydrocatalyzed catalytic biodiesel mixture, a numerical spray model of constant volume combustion chamber was established, and the accuracy of the model was verified by experimental data in the literature. Based on this model, the spray penetration, sauter mean diameter, spray velocity field and concentration field of gasoline–hydrocatalyzed catalytic biodiesel at different blending ratios were studied. The results show that under the conditions of 850 K ambient temperature, 5 MPa ambient pressure, and 80 MPa injection pressure, as the proportion of hydrogenated catalytic biodiesel in the blending fuel increases, the spray penetration increases, the sauter mean diameter decreases slightly, and the area of high velocity and high concentration at the spray center increases. The results of this study will contribute to the development of blended fuels for superior combustion performance and reduced pollutant emissions at appropriate blending ratios.

1. Introduction

Internal combustion engines are widely used in passenger cars, engineering machinery, ships, defense equipment, and other fields and are an important support for the development of the national economy. According to statistics, the fuel consumed by internal combustion engines accounts for two-thirds of the total oil consumption, and the exhaust emissions of motor vehicles powered by internal combustion engines account for nearly 30% of air pollution [1]. With the increasingly stringent emission regulations and the rapid development of fuel cell technology [2], continuously improving the thermal efficiency of engines and reducing pollutant emissions are the main directions and urgent needs for the development of new combustion theories and technologies for internal combustion engines. Among them, clean alternative fuels coupled with new combustion modes have become one of the important measures for energy conservation, emission reduction, and improvement in thermal efficiency of internal combustion engines and have attracted widespread attention.

Among the many new combustion modes, the compression ignition mode can significantly increase the compression ratio, and its indicated thermal efficiency can reach 60%, which is much higher than the 45% indicated thermal efficiency of the ignition mode [3]. However, due to the high carbon number and poor volatility of diesel fuel, it is difficult to form a uniform premixed gas, and its carbon smoke emissions are high [4]. Gasoline fuel has good volatility and can achieve full mixing before ignition, ensuring a large proportion of premixed fuel, making the combustion process cleaner [5]. Therefore, the new combustion mode (gasoline compression ignition, GCI) has attracted widespread attention from scholars at home and abroad [5,6].

In 2006, Kalghatgi et al. [7] conducted a gasoline compression ignition test on a single-cylinder engine with a compression ratio of 14 and a displacement of 2L compared with diesel; the NOx and soot content in the exhaust gas were lower when using gasoline compression ignition. The GCI combustion mode takes advantage of the good volatility and long ignition delay of gasoline to form a better combustible mixture, as well as the high compression ratio of diesel engines, to achieve lower nitrogen oxide and soot emissions and higher thermal efficiency [8]. However, the high octane number and high auto-ignition temperature of gasoline lead to cold-start difficulties, unstable operation under low load, and excessive pressure rise rate under high load in the GCI combustion mode [9]. In response to the high-load problem, many scientific researchers have basically explored effective solutions through the study of control strategies [10,11,12,13]. However, achieving efficient and stable combustion under low load conditions is still a key bottleneck problem for gasoline compression ignition. Among them, gasoline-based fuels mixed with diesel or biodiesel have become an effective way of achieving gasoline compression ignition. Because diesel or biodiesel has a low octane number and has good ignition performance, the mixed fuel formed after being mixed with gasoline has a low octane number and improves the ignition performance. Compared with diesel, biodiesel is a renewable, clean energy source with low sulfur content and less pollution [14]. Therefore, it is more suitable for improving GCI combustion problems. Moreover, the promotion and use of biodiesel can not only reduce the consumption of fossil fuels. Dependence can also reduce pollutant emissions [15]. At present, there are three generations of biodiesel. The raw materials of the first-generation biodiesel mainly come from crops, which are not easy to use and promote in large quantities, and the main component is fatty acid methyl ester, which is significantly different from diesel; the second-generation biodiesel, also known as hydrocatalytic biodiesel (hydrogenated catalytic biodiesel, HCB). The manufacturing process of waste gas oil is relatively mature. Biodiesel synthesis from waste materials and animal fats has also been widely studied [16]. Its main component is a normal saturated alkane structure, which is highly similar to diesel, and has similar physical and chemical properties, as shown in

Table 1. Main physical and chemical properties of HCB and diesel.

Fuel
Fuel	Density (20 °C) /(Kg/m3)	Kinematic Viscosity (20 °C /(mm2/s)	Sulfur Content /(mg/Kg)	Oxygen Content /%	Distillation Temperature (90%)/°C	Cetane Number	Low Calorific Value /(MJ/Kg)	Solidifying Point /°C
Diesel	839	5.8	≤10	0	355	49	38	−5
HCB	781.6	6.08	≤2	0	296	103	44	14

[17]. The manufacturing process of third-generation biodiesel is complex and is still in the research and development stage, making it difficult to mass-produce.

Adams et al. investigated the GCI combustion of gasoline–biodiesel blends at 5% and 10% biodiesel blending ratios [18]. This study focused on the reduction in intake air temperature by using gasoline–biodiesel blends. Their results confirmed the advantages of LTC in reducing particulate emissions by blending gasoline with biodiesel. Putrasari and Lim [19] conducted an experimental study to investigate the effect of adding biodiesel to gasoline and found that IDT could be reduced by pre-injection of gasoline–biodiesel blends. However, due to the excess oxygen in the blends, blending biodiesel with gasoline blends can lead to severe NOx emissions.

Compared with the first- and third-generation biodiesel, it is more reasonable to use and promote HCB at present, so the blending of HCB in GCI has received more and more attention and research. Zhong et al. [20] conducted a study on gasoline mixed with HCB in a constant-volume combustion bomb. The study found that with the increase in the HCB mixing ratio, the ignition delay period and flame floating length were shortened. Leng et al. [21] studied the effect of spray section injection strategy on gasoline–HCB combustion characteristics when the gasoline–HCB mass ratio was 7:3. The results showed that multi-stage injection can reduce the pressure increase rate in the cylinder and improve the combustion and explosion problem. The study also comprehensively considered the engine’s new energy and emissions and gave better injection strategy suggestions. Zhan et al. [22] conducted a study on the effect of different gasoline–HCB mixing ratios on engine combustion and emission characteristics on a common-rail single-cylinder diesel engine. The study found that with the increase in HCB ratio, the fuel ignition performance was improved, and the emission of CO (unburned hydrocarbons) was effectively reduced. Yuan et al. [23] studied the effect of ambient temperature on the spray characteristics of gasoline–HCB and found that at 323 K, there was almost no difference in the spray penetration distances of gasoline–HCB mixed fuel, diesel, and biodiesel. However, when the ambient temperature rose to 423 K and 523 K, the spray penetration distance of gasoline–HCB mixed fuel was the shortest.

Zhong et al. [20] performed in a fixed capacity combustion chamber using different gasoline–HCB mixtures. The results showed that the ignition delay time and take-off length became shorter as the HCB ratio increased, while the opposite was true for the spray length. Yuan et al. [23] experimentally investigated the spray characteristics of HCB and gasoline–HCB mixtures under different environmental conditions. They found that the spray penetration of gasoline–HCB mixtures was low, which indicated that wall-crashing would not occur in small-bore engines. And recently, the soot formation of different gasoline–HCB blends was investigated by using the High-Frequency Diffusion Background Illumination Extinction (HFDBIE) technique [24], and it was found that the higher the HCB blending ratio, the stronger the soot formation. Zhang et al. [25] investigated the effect of HCB blending in 95# gasoline on the combustion characteristics of heavy-duty diesel engine. They reported a significant improvement in ignition characteristics as the percentage of HCB in the blend increased. Xu et al. [26] investigated hydrogenated catalytic biodiesel as a pilot fuel for gasoline by means of an optical engine and showed that a graded injection strategy using P30M70 (30% main injection and 70% pilot injection) has superior heat dissipation characteristics and can improve the cold-start performance of diesel engines. Overall, blending gasoline with HCB is a promising approach for alleviating the limitations of GCI engines and improve engine combustion and emissions.

In summary, gasoline’s higher auto-ignition temperature hinders its start-up in compression ignition mode, and biodiesel possesses a lower octane number and excellent ignition properties. Blends of the two would be promising for compression combustion engines for reducing fossil fuel use and pollutant emissions. The current literature on such studies is not extensive and the related combustion theories are not well developed. To gain a deeper understanding of how fuel composition affects the spray characteristics of blended fuels, this study employs numerical simulations to analyze the impact of different gasoline–HCB blending ratios on spray penetration distances, Sauter mean diameters (SMDs), velocity fields, and concentration fields. This study will help to optimize the fuel efficiency of gasoline–HCB blends, improve combustion stability and reduce emissions, and provide a scientific basis for fuel formulation to facilitate the promotion and application of HCB.

2. Simulation Model Establishment and Verification

2.1. Bullet-Capacity Model

This study develops a simulation model using the spray test data from Li et al. [27] and the parameters of a constant-volume combustion bomb. The spray penetration distance data from the experiment are employed to validate the accuracy of the proposed model. The physical model of the combustion bomb is illustrated in Figure 1. The diameter of the bomb is 60 mm, and the height is 80 mm. The nozzle is set at the center of the top surface of the bomb, and the nozzle diameter is 0.176 mm. The test fuel is a mixture of gasoline and HCB in a mass ratio of 7:3 (G70H30), and it was found that good ignition reliability and combustion stability at low loads could be achieved using a 70%:30% gasoline–HCB fuel blend. The temperature is 850 K, the initial pressure in the constant volume combustion bomb is 5 MPa, the injection pressure is 80 MPa, the injection pulse width is 1787 μs, the injection amount is 8.4 g, and the oxygen concentration in the ambient gas in the bomb is 21%.

2.2. Gasoline and HCB Component Selection

The actual fuel composition is relatively complex. In numerical calculations, it is common to use a few or a single component to represent the actual fuel and to predict the actual combustion process from the combustion of these components [28]. In the numerical calculation of spray, toluene reference fuel (TRF) with 63% iso-octane, 17% n-heptane, and 20% volumetric toluene was selected as the alternative fuel for gasoline to reduce the calculation cost, and n-hexadecane was the ideal alternative fuel for HCB in the study of Zhong et al. [29].

2.3. Spray Model

In the numerical model of this paper, the k-ε model is used as the turbulence model, the KH-RT model is used as the breakup model, and the Frossling model is used as the evaporation model.

The crushing mechanism of the KH-RT model is as follows: The fuel droplets are ejected from the nozzle, and the high-density gas interaction produces two types of unstable wave disturbances: One is the KH instability wave, which is generated based on the disturbance of the flow direction of gas and droplets; the other is the RT instability wave, which is generated by the density difference in the normal direction of the gas and droplet interface [30]. In the KH-RT spray breakup model, the first breakup is the breakup of the fuel beam into droplets, and the droplet radius is expressed as follows:

$$r_c=B_0{\mathsf{\Lambda }}_{KH}$$

(1)

In the formula, r_c is the droplet radius, B₀ is the breakup model constant, and Λ_KH is the wavelength of the fastest-growing wave on the jet surface.

The second breakup is the droplet atomization process, and the breakup length is expressed as follows:

$$L_b=C_{bl}{d}_0\sqrt{{\displaystyle \frac{{\rho }_l}{{\rho }_g}}}$$

(2)

In the formula, L_b is the breakup length, C_bl is the breakup length coefficient, d₀ is the nozzle diameter, ρ_l is the spray liquid density, and ρ_g is the spray meteorological density. When the distance between the droplet and the nozzle is greater than L_b, secondary breakup is considered to have occurred.

The Frossling evaporation model and the Chiang evaporation model are currently commonly used evaporation models. The Frossling evaporation model proposed by Amsden et al. [31] can predict the entire process of droplet conversion from liquid to gas and determine the rate of change in droplet size over time well. This paper selects the Frossling model to predict the evaporation process of droplets. In the Frossling evaporation model, the radius of the droplet changes continuously with time during the evaporation process, which is expressed by the following equation:

$${\displaystyle \frac{d_{r_0}}{dt}}={\displaystyle \frac{{\alpha }_{spray}{\rho }_{g}D}{2{\rho }_l{r}_0}}{B}_{d}S{h}_d$$

(3)

In the formula, α_spray is the heat transfer coefficient proportional factor; B_d is the vapor diffusion coefficient; $\mathrm{S}{h}_d$ is the Sherwood number.

2.4. Mesh Independence Verification and Mesh Density Selection

In this paper, grids of 1 mm, 2 mm, and 4 mm are selected to mesh the containment model, and numerical simulation is performed with the parameters set in Section 1. Figure 2 is a comparison of the effects of different mesh sizes on the penetration mechanism. It can be seen from the figure that when the mesh size is 1 mm and 2 mm, the variation curves of the penetration distance are basically the same, while the penetration distance curve when the mesh size is 4 mm is significantly different from the former. It can be considered that the mesh size has an impact on the calculation results of this model, but when the mesh density reaches a certain level, there is no obvious difference in the calculation results. Considering the computational time cost and computational accuracy, this paper selects a grid size of 2 mm.

2.5. Model Verification

Figure 3 shows the variation in spray penetration distance between the simulation and test with time. In the 0~0.25 ms period, the spray penetration distance increases rapidly. After 0.25 ms, the spray penetration tends to be stable, which is basically maintained around 13 mm. The simulation results in this paper accurately predict the changing trend of spray penetration and have a high degree of agreement with the experimental values, which shows that the model established in this paper is accurate.

2.6. Simulation Condition Setting

In order to investigate the effect of the HCB blend ratio on the spray characteristics of the blended fuel over a wide range of operating conditions, HCB was blended with gasoline at 0% (G100), 30% (G70H30), 50% (G50H50), and 70% (G30H70) by mass.

3. Analysis and Discussion of the Results

3.1. Effect of Mixing Ratio on Spray Penetration Distance

Figure 4 shows the spray penetration distance of different mixed fuels, as can be seen from the figure, at the initial stage of the spray, about 0~0.1 ms. The spray penetration distance is almost the same, and the proportion of mixed HCB has little effect on the spray in the initial test stage. After 0.1 ms, the spray penetration distance increases with the increase in HCB content. Because in the initial stage, the time is short, the hindrance of the ambient gas to the development of the droplet is not obvious, and the spray develops rapidly. As droplets experience resistance from the ambient gas, their spray penetration distance stabilizes after a certain period. Variations in the hydrocarbon blend (HCB) mixing ratio result in notable changes in the spray penetration distance. The primary factor contributing to this disparity is the higher density of HCB compared to that of gasoline [32]. Consequently, an increased HCB content in the mixture results in heavier mixed droplets, which possess greater inertia for forward propulsion. Additionally, the higher volatility of gasoline facilitates a faster evaporation rate of the droplets. Based on these analyses, it is posited that a higher HCB content in the mixture extends the spray penetration distance.

3.2. Effect of Mixing Ratio on SMD

Figure 5 shows the SMD of different mixed fuels. It can be seen from the figure that the SMD change trend of each mixed fuel is roughly the same. During the period of 0~0.05 ms, the SMD decreases rapidly because the newly ejected droplets are large and the initial test speed is fast, and the friction with the ambient gas is large, which is conducive to breakage. After 0.07 ms, the SMD tends to be stable. During the period of 0~0.05 ms, the change rate of G100 is the largest, and the SMD of G100 is the smallest at the same time. As the proportion of HCB in the mixed fuel increases, the SMD increases. This is because the viscosity of HCB is smaller than that of gasoline. After gasoline is mixed with HCB, the viscosity increases, and the mutual force during the droplet breakup process is stronger, which is not conducive to droplet breakup. After 0.05 ms, the SMD of G100 and G90H10 tended to be stable, but the SMD of the other mixed fuels continued to decrease slightly. The final SMD size was G100 > G70H30 > G50H50 > G30H70. This was mainly because the higher the HCB content, the longer the spray penetration distance, the longer the friction distance between the droplets and the ambient gas, and therefore the better the degree of fragmentation and the smaller the SMD, but the difference was not large.

3.3. Effect of Mixing Ratio on Velocity Field

Figure 6 shows the velocity field distribution of different mixed fuels. It can be seen from the figure that the distribution law of the velocity field is basically the same. The color is the darkest and the speed is the highest in the center of the spray, and the color gradually becomes lighter and the speed decreases from the center to the outside. At the same time, there are slight differences in the red areas in the center of the mist beams of the four fuels G100, G70H30, G50H50, and G30H70. As the proportion of HCB increases, the area of the central red area increases slightly, indicating that as the proportion of HCB mixed in gasoline increases, the velocity of the droplets will increase accordingly. On the one hand, as the proportion of HCB in the mixture increases, the density of the mixture increases, which increases the initial kinetic energy of the droplets and the velocity; on the other hand, as the viscosity increases with the proportion of HCB, high viscosity will hinder the breakup and evaporation of droplets, resulting in slower energy loss [24].

3.4. Effect of Mixing Ratio on Concentration Field

Figure 7 shows the concentration field distribution of different mixed fuels. It can be seen from the figure that the concentration field distribution is generally that the concentration is highest in the center of the mist beam, and the concentration gradually decreases from the inside to the outside. Because the interaction force between the droplets on the periphery of the mist beam and the ambient gas is relatively strong, the atomization and evaporation effects are better than those in the center of the mist beam. In addition, by comparing the central concentrations of different fuels, it is found that as the proportion of HCB in the mixed fuel increases, the area of the central red area increases slightly, that is, the high-concentration area in the center increases. This is because as the proportion of HCB increases, the viscosity and surface tension of the mixed fuel increase, the interaction between the droplets increases, and the mist beam is hindered from developing outward, so the concentration in the center is higher.

4. Conclusions

This paper analyses the spray characteristics under gasoline–HCB blends at different blending ratios. The conclusions of this study are as follows:

(1) The density of hydrogenated catalytic biodiesel is higher than that of gasoline. Consequently, as the proportion of hydrogenated catalytic biodiesel in the fuel mixture increases, the density of the mixed fuel also rises. This leads to an increase in the mass flow rate and imparts higher kinetic energy to the droplets. The enhanced interaction between the droplets and the surrounding gas results in increased spray penetration distance, a decrease in the Sauter mean diameter, and an expansion of the high-speed region at the spray center.

(2) The viscosity of hydrogenated catalytic biodiesel is relatively high. Consequently, as the proportion of hydrogenated catalytic biodiesel in the mixed fuel increases, the overall viscosity of the mixture also rises. This results in a higher-concentration region at the center of the spray.

As the HCB mix ratio increases, longer spray run-through distances contribute to more uniform mixture formation, which is favorable for ignition. The increased area of the high velocity region at the center of the spray contributes to enhanced mixture formation and combustion stability. An increase in the area of the high-concentration region at the center of the spray has the potential to increase carbon smoke emissions, which is detrimental to the application of blended fuels. The results suggest that increasing the HCB blend ratio can help improve ignition and combustion but may have a detrimental effect on pollutant emissions. Future work will be devoted to investigating the effect of gasoline–HCB blend ratios on combustion and emissions of dual fuels under different injection strategies in an engine.