# Effect of In-Cylinder Low-Pressure Direct Injection Strategy on the Atomization and Combustion Process of a Small-Scaled Gasoline Wankel Rotary Engine

Authors: Yuan Li; Jinxiang Liu; Weiqing Huang; Nana Wang
Journal: International Journal of Energy Research
 (2023)

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## Abstract

To solve the problem of poor atomization and combustion in a small-scaled gasoline Wankel rotary engine (WRE) with low-pressure port injection mode, the numerical simulation was used to optimize the injection strategy. Firstly, the effects of in-cylinder temperature and pressure on gasoline atomization characteristics were studied, and the optimal injection timing was determined. Based on this, the influence of injection position, injection angle, and installation direction on the atomization, fuel-air mixing, and combustion processes of low-pressure direct injection (DI) small-scaled gasoline WRE was investigated. The results show that the injection angle is a key factor in determining the gasoline atomization characteristics. Injecting along the direction of rotor rotation causes the impingement between fuel bundle and combustion chamber pocket, resulting in the smaller Sauter mean diameter (SMD) and liquid penetration length (LPL). The installation direction of nozzle plays an important role in the airflow movement. When the nozzle is vertical-installed, the airflow repeatedly crosses to form multiple eddies, making the fuel to move more easily towards the front of combustion chamber. When the nozzle is parallel-installed at the lower edge of the installing zone and injecting along the direction of rotor rotation, the peak in-cylinder pressure is the largest and increased by 21% compared to the original port injection. By this injection strategy, the problem of incomplete combustion for the studied small-scaled gasoline WRE could be almost completely solved.


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## Body

## 1. Introduction

Compared with conventional reciprocating engines, Wankel rotary engines (WREs) have many advantages, such as simple structure, compact design, light weight, high power-to-weight ratio, and low noise [1–3]. Owing to the above advantages, WREs have been widely used in military and civilian areas in recent years, including mobile generators, expander power, and small-scaled unmanned aerial vehicle (UAV) [4, 5]. Although WREs have been widely used in many fields, there are still some disadvantages that limit the further expansion of its application. The most prominent problem is the incomplete combustion in the rear of combustion chamber. This problem is mainly due to its narrow combustion chamber structure, resulting in a unidirectional airflow that flows from the rear to the front of combustion chamber [6–8]. The unidirectional airflow causes the flame to propagate forward, thus hindering the complete combustion in the rear of combustion chamber [9–11].At present, many new solutions have been proposed to improve the incomplete combustion of WREs [12]. For example, the design of combustion chamber was optimized, the direct injection (DI) technology was adopted, and the flame propagation speed was improved by burning gas fuels such as hydrogen [13–15] and natural gas [16, 17]. In addition, the lean burn and chamber optimization promise many advantages to improve the performance and combustion characteristics of WREs [18, 19]. Among them, DI technology is considered as a very effective way to solve the problem of incomplete combustion for WREs [20, 21]. Due to the relatively low compression ratio of WREs, spark ignition is a more feasible approach compared to compression ignition [12]. DI technology could provide a flexible and precise control of fuel in the cylinder to ensure that the mixture is distributed near the spark plugs, thereby improving the combustion process of WREs [22]. Researchers have carried out many studies on the DI strategy for WREs on the basis of premixing or port injection fuel supply methods [23–26]. Fan et al. numerically simulated the effect of injection strategy on the fuel-air mixture and combustion process of a DI natural gas Wankel rotary engine (WRE) with a cylinder width of 40 mm and a generating radius of 69 mm. The results showed that compared to the premixed method, the in-cylinder pressure of DI strategy increased by 29.7% [20]. Chen et al. applied the high-pressure DI strategy to a diesel WRE with a cylinder width of 79.2 mm and a generation radius of 103.5 mm. The effects of injection timing and injection angle on the fuel-air mixing as well as the combustion processes of this diesel WRE were investigated [12]. The results demonstrate that a reasonable injection strategy can improve the combustion process of the engine and increase the peak in-cylinder pressure. The variation of peak in-cylinder pressure between different DI strategies was as high as 16.82%.It can be concluded that DI technology has become a very important development direction to improve the combustion efficiency of WREs. The current research works are only related to the WREs adopting high-pressure DI injection strategy. Because under the higher injection pressure, the jet mixing area is larger, which is more conducive to improving the atomization quality [12]. In contrast, small-scaled WREs are generally used in equipment with stringent size and weight requirements [27]. High-pressure DI technology, which would increase the size and weight of the attachment, is not suitable for small-scaled WREs. More importantly, under the limitation of small combustion chamber volume of small-scaled WREs, it is not conducive to the development of fuel bundles. Under the action of high pressure, the fuel is prone to cause strong impingement with the wall, which results in wall-attached combustion of the fuel, reducing the combustion quality [28]. Based on this, it is inferred that the low-pressure DI strategy becomes a possible option for small-scaled WREs. The low-pressure DI is inherently detrimental to the fuel atomization compared to high-pressure DI injection strategy. Especially for small-scaled WREs, the development of fuel bundles is more sensitive to the injection parameters due to the small space of combustion chamber. Therefore, under the conditions of low-pressure DI and small combustion chamber space, how to reasonably match the injection parameters to obtain good mixture distribution and thus improve the combustion efficiency in the small-scaled WREs is an urgent issue to be investigated.In this paper, a three-dimensional simulation model of a small-scaled gasoline WRE with a generating radius of 47.25 mm was established and validated by experimental data. The atomization process of gasoline at different injection timing was studied. The effect of injection position and injection direction on fuel atomization, fuel-air mixture, and combustion process of low-pressure gasoline WRE was investigated. Based on this, the optimal low-pressure DI injection strategy was determined. This study could provide a foundation for the determination of optimal injection strategy for small-scaled WREs and could also be used as a reference for the application of low-pressure devices in DI injection strategy.


## 2. Establishment of Simulation Model

### 2.1. Engine Basic Parameters

In this study, an air-cooled single-rotor small-scaled gasoline WRE with peripheral-ported was used. The engine parameters are given in Table1, where BTDC is the acronym of before top dead center, ATDC is the acronym of after top dead center, ABDC is the acronym of after bottom dead center, and BBDC is the acronym of before bottom dead center. The simulation analysis was carried out at a speed of 5000 r/min, with the throttle widely open and the equivalent ratio of 1.0. In the fuel supply system, a double-hole nozzle with a hole diameter of 0.22 mm was used, and the cone angle of the holes is 12°. An injection pressure of 0.3 MPa was employed. The engine is equipped with two spark plugs, i.e., the leading spark plug (LSP) and the trailing spark plug (TSP). The two spark plugs are set as the same ignition timing, which is 30°EA BTDC. The schematic diagram of the studied engine is shown in Figure 1.Table 1
Specifications of the studied gasoline WRE.
Engine parametersValueGenerating radius/R (mm)47.25Eccentricity/e (mm)6.75Chamber width/B (mm)28Number of rotorsOneDisplacement/Vh (L)0.05Compression ratio8.5Intake and exhaust methodPeripheral-portedIgnition sourceTwo spark plugsIgnition timing30°EA BTDCIntake timing80°EA ATDC/40°EA ABDCExhaust timing60°EA BBDC/70°EA ATDCFigure 1
The schematic of the studied gasoline WRE.


### 2.2. Mesh Generation

A small-scaled WRE model including three combustion chambers, intake, and exhaust ports was established using the software SolidWorks. After the model was established, meshing was performed. Considering the complex eccentric motion of the rotor and the variation of the combustion chamber geometry, the flow field in the cylinder of WRE is very specific. In order to accurately reflect the phenomenon in the cylinder of WRE, the commercial CFD software CONVERGE with adaptive mesh refinement (AMR) and fixed embedding (FE) was used in this paper. Four different meshing schemes, 1 mm, 2 mm, 2.5 mm+AMR, and 3 mm + AMR, were used to eliminate the influence of mesh size on the simulation results. A comparative analysis of the simulation results was performed to determine the appropriate meshing method. The variation of average in-cylinder pressure for different grid sizes is illustrated in Figure2.Figure 2
Comparison of in-cylinder pressure under different grid sizes.As can be seen from Figure2, the simulation results of in-cylinder pressure with 2.5 mm+AMR are very close to those with the basic grid size of 1 mm. This indicates that these two grid size schemes have less influence on the simulation results. The AMR technique significantly refines the grid only for regions where the computational field variables are large enough. Compared with the meshing mode with the base grid size of 1 mm, the grid number in the 2.5 mm+AMR meshing mode is much less. In addition, under the meshing mode of AMR, the FE function was also used at the rotor walls, injector, and spark plugs to ensure that the mesh accuracy can meet the information capture. Thus, the 2.5 mm+AMR meshing mode provides a balance between computational accuracy and computational cost. In this study, the meshing scheme for all subsequent computational models is 2.5 mm+AMR. With this meshing mode, the minimum mesh size is less than 0.625 mm. Studies have shown that the flow field and flame could be accurately captured using this grid size for engines of the same size [4, 29]. The mesh model generated during the calculation is shown in Figure 3. The meshes number for a single working chamber is about 10,000 to 120,000. Due to the special centrosymmetric triangular rotor structure, the three chambers of WRE have similar operating processes. Hence, simulation of any one of these three working chambers can generally represent the overall engine performance [30]. The working chamber III was chosen as the computing object in this study.Figure 3
The grid model of studied WRE in the simulation process.


### 2.3. Boundary Conditions and Simulation Models

In this paper, the studied WRE is naturally aspirated. The boundary condition is classified into two categories. The first category is the inlet and outlet boundary conditions gotten from reference [9]. In the simulation work, the boundary of inlet and outlet ports was defined as the pressure boundary. The pressure at the inlet port was set to 0.1 MPa and the pressure at the exhaust port was set as 1 atm. The rotor was set as a motion boundary by providing a WRE motion file. The second category is the wall boundary conditions of the simulation model. The wall boundary conditions adopted in this study are set according to the one-dimensional simulation results and comprehensive consideration of references [4, 8, 31]. The conditions of the calculation area except for the intake port, exhaust port, and the rotor were fixed wall boundary. The temperature of the fuel and air was decided to be 300 K, which is in accordance with the normal atmospheric temperature. The wall temperature was set to 400 K.The main models involved in this study are listed in Table2. It needs to be mentioned that for the downsize rotary engine at high speed, the combustion tends to be in the distributed reaction zone regime. The SAGE-detailed chemical kinetics solver provides a versatile modeling of combustion, which is more suitable for rotary engine. All the models selected in this paper have been widely used in the simulation analysis of WREs, and its effectiveness has been widely verified. Validation of the model in this paper will be carried out in the next subsection.Table 2
Mathematical models and chemical mechanisms.
ObjectSelection of model and mechanismSpray modelKH-RT [32, 33]Reaction kineticsPRF skeletal mechanism [34, 35]TurbulenceRNG k-ε model [4, 36]IgnitionSpherical selection module [37, 38]CombustionSAGE model [8, 39]


### 2.4. Model Verification

To verify the reliability of the three-dimensional simulation model in this study, the adaptability of models, initial, and boundary condition were verified. Firstly, in order to complete the validation of the spray model, the numerical simulations of gasoline spray process were carried out under the same experimental conditions as in reference [40]. In the experiment, the ambient temperature and pressure were 298 K and 0.14 MPa, respectively. In recent years, the constant volume combustion chamber (CVCC) has been widely used in the research of spray and combustion processes for engine [41]. The spray process of gasoline in this paper is numerically simulated using a CVCC with an injection pulse of 4.25 ms. The injector with 6 holes is used in the test. The hole diameter is 0.205 mm, and the cone angle of the holes is 9°. The nozzle model established in the simulation process and the observation directions are shown in Figure 4. The comparison between the simulation results and experimental data of spray morphology is shown in Figure 5. It could be observed from Figure 5 that in the whole fuel injection process, the spray structures and liquid penetration length (LPL) in two observation directions between simulation and experiment are very close, which confirmed the adaptability and accuracy of the spray model in this study. In this paper, the LPL is defined as the maximum of 95% liquid fuel along the injection direction.Figure 4
Diagram of fuel spray distribution.Figure 5
Verification of spray model [40].
(a)
Comparison of LPL(b)
Comparison of spray structureThe fuel used in this WRE is RON92 gasoline, with 92% (v/v) isooctane and 8% (v/v) n-heptane as the surrogate fuel for the commercial gasoline (#92 Sinopec, China-V). Meanwhile, the reduced primary reference fuel (PRF) skeletal mechanism of 47 species and 142 reactions was selected to achieve the combustion of the surrogate fuel, and the adaptability of this mechanism has been widely verified [34, 35, 38]. To verify the adaptability of the turbulence model, the combustion model, the initial, and the boundary conditions, a three-dimensional model of the WRE was established according to the literatures [9, 39]. The engine model in refs [9, 39] is a gasoline rotary engine with a displacement of 404 cm3 and a compression ratio of 9.6. The comparison of average in-cylinder pressure between the simulated results and the experimental data is shown in Figure 6. It can be seen that the simulation results and the experimental data show a highly consistent trend. In the whole evolution process of stress, most errors of the two pressure curves are within the range of 2-3%, and the maximum error is less than 10%. The larger deviations mainly occur in the early stages of the combustion process. The reason is that the quenching phenomenon is not fully considered in the simulation model due to the reduced reaction mechanism adopted in the simulation. For numerical calculation, a simulation model with such computational accuracy is usually acceptable [39, 42].Figure 6
Model validation for experimental results [8] and modeling data.


## 2.1. Engine Basic Parameters

In this study, an air-cooled single-rotor small-scaled gasoline WRE with peripheral-ported was used. The engine parameters are given in Table1, where BTDC is the acronym of before top dead center, ATDC is the acronym of after top dead center, ABDC is the acronym of after bottom dead center, and BBDC is the acronym of before bottom dead center. The simulation analysis was carried out at a speed of 5000 r/min, with the throttle widely open and the equivalent ratio of 1.0. In the fuel supply system, a double-hole nozzle with a hole diameter of 0.22 mm was used, and the cone angle of the holes is 12°. An injection pressure of 0.3 MPa was employed. The engine is equipped with two spark plugs, i.e., the leading spark plug (LSP) and the trailing spark plug (TSP). The two spark plugs are set as the same ignition timing, which is 30°EA BTDC. The schematic diagram of the studied engine is shown in Figure 1.Table 1
Specifications of the studied gasoline WRE.
Engine parametersValueGenerating radius/R (mm)47.25Eccentricity/e (mm)6.75Chamber width/B (mm)28Number of rotorsOneDisplacement/Vh (L)0.05Compression ratio8.5Intake and exhaust methodPeripheral-portedIgnition sourceTwo spark plugsIgnition timing30°EA BTDCIntake timing80°EA ATDC/40°EA ABDCExhaust timing60°EA BBDC/70°EA ATDCFigure 1
The schematic of the studied gasoline WRE.


## 2.2. Mesh Generation

A small-scaled WRE model including three combustion chambers, intake, and exhaust ports was established using the software SolidWorks. After the model was established, meshing was performed. Considering the complex eccentric motion of the rotor and the variation of the combustion chamber geometry, the flow field in the cylinder of WRE is very specific. In order to accurately reflect the phenomenon in the cylinder of WRE, the commercial CFD software CONVERGE with adaptive mesh refinement (AMR) and fixed embedding (FE) was used in this paper. Four different meshing schemes, 1 mm, 2 mm, 2.5 mm+AMR, and 3 mm + AMR, were used to eliminate the influence of mesh size on the simulation results. A comparative analysis of the simulation results was performed to determine the appropriate meshing method. The variation of average in-cylinder pressure for different grid sizes is illustrated in Figure2.Figure 2
Comparison of in-cylinder pressure under different grid sizes.As can be seen from Figure2, the simulation results of in-cylinder pressure with 2.5 mm+AMR are very close to those with the basic grid size of 1 mm. This indicates that these two grid size schemes have less influence on the simulation results. The AMR technique significantly refines the grid only for regions where the computational field variables are large enough. Compared with the meshing mode with the base grid size of 1 mm, the grid number in the 2.5 mm+AMR meshing mode is much less. In addition, under the meshing mode of AMR, the FE function was also used at the rotor walls, injector, and spark plugs to ensure that the mesh accuracy can meet the information capture. Thus, the 2.5 mm+AMR meshing mode provides a balance between computational accuracy and computational cost. In this study, the meshing scheme for all subsequent computational models is 2.5 mm+AMR. With this meshing mode, the minimum mesh size is less than 0.625 mm. Studies have shown that the flow field and flame could be accurately captured using this grid size for engines of the same size [4, 29]. The mesh model generated during the calculation is shown in Figure 3. The meshes number for a single working chamber is about 10,000 to 120,000. Due to the special centrosymmetric triangular rotor structure, the three chambers of WRE have similar operating processes. Hence, simulation of any one of these three working chambers can generally represent the overall engine performance [30]. The working chamber III was chosen as the computing object in this study.Figure 3
The grid model of studied WRE in the simulation process.


## 2.3. Boundary Conditions and Simulation Models

In this paper, the studied WRE is naturally aspirated. The boundary condition is classified into two categories. The first category is the inlet and outlet boundary conditions gotten from reference [9]. In the simulation work, the boundary of inlet and outlet ports was defined as the pressure boundary. The pressure at the inlet port was set to 0.1 MPa and the pressure at the exhaust port was set as 1 atm. The rotor was set as a motion boundary by providing a WRE motion file. The second category is the wall boundary conditions of the simulation model. The wall boundary conditions adopted in this study are set according to the one-dimensional simulation results and comprehensive consideration of references [4, 8, 31]. The conditions of the calculation area except for the intake port, exhaust port, and the rotor were fixed wall boundary. The temperature of the fuel and air was decided to be 300 K, which is in accordance with the normal atmospheric temperature. The wall temperature was set to 400 K.The main models involved in this study are listed in Table2. It needs to be mentioned that for the downsize rotary engine at high speed, the combustion tends to be in the distributed reaction zone regime. The SAGE-detailed chemical kinetics solver provides a versatile modeling of combustion, which is more suitable for rotary engine. All the models selected in this paper have been widely used in the simulation analysis of WREs, and its effectiveness has been widely verified. Validation of the model in this paper will be carried out in the next subsection.Table 2
Mathematical models and chemical mechanisms.
ObjectSelection of model and mechanismSpray modelKH-RT [32, 33]Reaction kineticsPRF skeletal mechanism [34, 35]TurbulenceRNG k-ε model [4, 36]IgnitionSpherical selection module [37, 38]CombustionSAGE model [8, 39]


## 2.4. Model Verification

To verify the reliability of the three-dimensional simulation model in this study, the adaptability of models, initial, and boundary condition were verified. Firstly, in order to complete the validation of the spray model, the numerical simulations of gasoline spray process were carried out under the same experimental conditions as in reference [40]. In the experiment, the ambient temperature and pressure were 298 K and 0.14 MPa, respectively. In recent years, the constant volume combustion chamber (CVCC) has been widely used in the research of spray and combustion processes for engine [41]. The spray process of gasoline in this paper is numerically simulated using a CVCC with an injection pulse of 4.25 ms. The injector with 6 holes is used in the test. The hole diameter is 0.205 mm, and the cone angle of the holes is 9°. The nozzle model established in the simulation process and the observation directions are shown in Figure 4. The comparison between the simulation results and experimental data of spray morphology is shown in Figure 5. It could be observed from Figure 5 that in the whole fuel injection process, the spray structures and liquid penetration length (LPL) in two observation directions between simulation and experiment are very close, which confirmed the adaptability and accuracy of the spray model in this study. In this paper, the LPL is defined as the maximum of 95% liquid fuel along the injection direction.Figure 4
Diagram of fuel spray distribution.Figure 5
Verification of spray model [40].
(a)
Comparison of LPL(b)
Comparison of spray structureThe fuel used in this WRE is RON92 gasoline, with 92% (v/v) isooctane and 8% (v/v) n-heptane as the surrogate fuel for the commercial gasoline (#92 Sinopec, China-V). Meanwhile, the reduced primary reference fuel (PRF) skeletal mechanism of 47 species and 142 reactions was selected to achieve the combustion of the surrogate fuel, and the adaptability of this mechanism has been widely verified [34, 35, 38]. To verify the adaptability of the turbulence model, the combustion model, the initial, and the boundary conditions, a three-dimensional model of the WRE was established according to the literatures [9, 39]. The engine model in refs [9, 39] is a gasoline rotary engine with a displacement of 404 cm3 and a compression ratio of 9.6. The comparison of average in-cylinder pressure between the simulated results and the experimental data is shown in Figure 6. It can be seen that the simulation results and the experimental data show a highly consistent trend. In the whole evolution process of stress, most errors of the two pressure curves are within the range of 2-3%, and the maximum error is less than 10%. The larger deviations mainly occur in the early stages of the combustion process. The reason is that the quenching phenomenon is not fully considered in the simulation model due to the reduced reaction mechanism adopted in the simulation. For numerical calculation, a simulation model with such computational accuracy is usually acceptable [39, 42].Figure 6
Model validation for experimental results [8] and modeling data.


## 3. Injection Timing

Proper injection timing results in better interaction and cooperation between the fuel bundle and the transient turbulence during the intake stroke, which is the key factor in the formation of a good combustible mixture [43, 44]. The in-cylinder environmental conditions vary with the eccentric angle. Therefore, for different injection timing, the in-cylinder temperature and pressure of WRE are different. According to the order of delayed injection timing, the in-cylinder conditions are divided into five cases, and the corresponding temperatures and pressures are listed in Table 3.Table 3
The in-cylinder condition under different eccentric angle.
CaseEccentric angle (°EA BTDC)Temperature (K)Pressure (MPa)12503280.10522263370.1131773670.1541483970.2051294210.25The numerical simulations of gasoline spray and combustion processes under different injection timing were carried out. The total injected mass is 4.28 mg. In order to visually observe the atomization process of gasoline, the fuel spray distribution under different cases is presented in Figure7. It can be seen from Figure 7 that in generally, the gasoline LPL and the cross-sectional area of spray top decrease gradually with the delay of the injection timing. When the injection timing is earlier, the above phenomenon is less obvious. When the injection timing is delayed from the intermediate time, the cross-sectional area of spray top and the LPL decreases significantly. This is mainly because with the delay of injection timing, the in-cylinder temperature and pressure gradually increase, raising the interference of the surrounding gas to the fuel bundles [45, 46]. The LPL and the cross-sectional area of spray top affect the contact area between the fuel bundle and the air, which in turn influences the fragmentation and evaporation processes of fuel particles. The number of droplets, the Sauter mean diameter (SMD), and the total fuel evaporation at the end of injection process are given in Table 4 to further analyze the atomization characters of gasoline. SMD is the ratio of the total volume of the droplet to the total surface area of the droplet, and it is used to represent the mean diameter of the droplet. The smaller the SMD, the more favorable for the engine fuel atomization.Figure 7
Fuel spray distribution with different conditions.Table 4
Characteristics of gasoline evaporation and atomization.
CaseDropletsSMD (μm)Fuel evaporation (g)15095231923.3×10−425119121833.2×10−435126051802.6×10−445144011712.3×10−455177611642.1×10−4It can be found that with the delay of injection timing, the number of droplets gradually increases and the SMD gradually decreases at the end of the injection process. This is mainly because the higher in-cylinder temperature and pressure are more conducive to fuel fragmentation, making the droplets finer into smaller diameter particles. The increase of droplets number and the decrease of SMD are favorable to the improvement of atomization quality. However, detrimental to the improvement in atomization quality is that as the injection timing is delayed, the total fuel evaporation decreases. Research has shown that in the engine, the earlier the fuel injection, the longer the mixing time of fuel and gas, which is more conducive to the fuel evaporation [47]. This could provide a good support for the above phenomenon. Furthermore, this is because the higher the pressure of the ambient medium in the cylinder, the greater the resistance of the fuel bundle as it progresses outward. The higher resistance reduces the contact area between the fuel bundle and the air, which is damaging to fuel evaporation. In addition, in order to evaluate the combustion quality, Figure 8 displays the integrated heat release for different injection timing. The integrated heat release is considered as an important indicator of the completeness of fuel combustion. As can be seen from Figure 8, the integrated heat release changes significantly with the variation of injection timing. From the general trend, with the delay of injection timing, the integrated heat release increases first and then decrease. The largest integrated heat release is obtained at the earlier injection timing (226°EA BTDC). This indicates that at this injection timing, the number of particles, the SMD, and total fuel evaporation at the end of injection process are at the optimal level.Figure 8
Integrated heat release under different injection timing.


## 4. Results and Discussion

### 4.1. Injection Position and Direction

In addition to the injection timing, the injection position and direction in the cylinder of WRE are also investigated. Based on the above analysis, the injection timing is determined to be 226°EA BTDC. The injection pulse was 96°EA in all the injection strategies studied. According to the injection timing, the injection pulse as well as the engine speed, the rotor position during the injection process could be determined. The solid and dashed lines in Figure9(a) indicate the rotor positions at the beginning and the end of injection process, respectively. Following this, the zone available for installing the nozzle could be located. Installing nozzle in this zone ensures that the fuel is injected into the current working chamber during the entire injection process. Within this zone, three injection positions (A, B, and C) are placed, as shown in Figure 9(a).Figure 9
Summary of injection position and direction.
(a)
Injection position and angle(b)
Installation directionAdditionally, since a dual-hole nozzle is used in this paper, the injection direction involves two factors, which are the injection angle and the installation direction. Firstly, the nozzle is regarded as a unit, and the angle between the nozzle center axis and the horizontal line X is defined as the injection angle. Two injection angles are set for each injection position, indicated by angle 1 and angle 2, as shown in Figure9(a). Angle 1 is injecting towards the combustion chamber pocket, and angle 2 is injecting along the direction of rotor rotation. The installation direction is used to describe the relationship between the line connecting two holes and the cylinder thickness direction. In this paper, for each injection angle, the parallel installation and vertical installation are studied. Parallel installation is the line between the two holes parallel to the cylinder thickness direction, and the vertical installation is the line between the two holes vertical to the cylinder thickness direction, as illustrated in Figure 9(b). In summary, there are 12 different injection strategies in this study, and the specific injection parameters are listed in Table 5. In the simulation process, except for the parameters listed in Table 5, other parameters are invariants.Table 5
Injection parameters of different injection strategies.
CaseInjection positionInstallation directionInjection angle (°)AP-1AParallel10AP-2AParallel40AV-1AVertical10AV-2AVertical40BP-1BParallel0BP-2BParallel30BV-1BVertical0BV-2BVertical30CP-1CParallel-40CP-2CParallel-10CV-1CVertical-40CV-2CVertical-10


### 4.2. Spray Characteristics in Cylinder of WRE during Injection Process

Under the action of injection pressure, in-cylinder flow field and combustion chamber pocket, the fuel injected from the nozzle will show different spray characteristics [48, 49]. For analyzing the atomization characteristics of gasoline, Figure 10 presented the LPL and SMD for different injection strategies at the end of injection process. In general, the LPL and SMD vary greatly with the change of injection position and direction. The maximal LPL (72.10 mm) is 4.1 times higher than the minimum of 17.66 mm, and the minimum SMD (12.98 μm) is only 57% of the maximum 22.59 μm. At the same injection position, the LPL and SMD under different injection directions are also different. For the injection position A, when the nozzle is parallel-installed, the LPL for injecting along the direction of rotor rotation is significantly larger than that of injecting towards the combustion chamber pocket. When the nozzle is vertical-installed, compared with injecting along the direction of rotor rotation, the LPL obtained by injecting towards the combustion chamber pocket is significantly smaller.Figure 10
The LPL and SMD at the end of injection process.Similar rule is also shown at injection positions B and C; that is, under the same installation direction, the LPL for injecting towards the combustion chamber pocket is smaller than the LPL for injecting along the direction of rotor rotation. In particular, when injecting along the direction of rotor rotation wall at injection position B, the LPL of parallel-installed is larger than that of vertical-installed. This is different from injection positions A and C. At injection positions A and C, the LPL is largest when the nozzle is vertical-installed and injecting along the direction of rotor rotation wall. In summary, the largest LPL is observed for injection strategy AV-2. The SMD also follows the same rule; i.e., under the same installation direction, the SMD for injecting along the direction of rotor rotation wall is larger than the SMD for injecting towards the combustion chamber pocket. Interestingly, the SMD at injection positions A and B is significantly larger than that at injection position C for injecting along the direction of rotor rotation wall. The largest SMD is found in injection strategy BP-2.To explore the reasons for the above phenomenon, Figure11 shows the fuel movement for different injection strategies during the injection process. Because for different injection positions, the fuel development is similar. Therefore, injection position A is used as an example to describe the fuel movement in the cylinder of small-scaled WRE. As can be seen from Figure 11(a), during the whole injection process, the fuel bundles of the parallel-installed are mainly injected into the two sides of the combustion chamber pocket. For vertical-installed, the fuel bundles are mainly concentrated in the center of the combustion chamber pocket. It can be concluded that the installation direction mainly determines the fuel distribution in the transversal direction of combustion chamber. When injecting towards the combustion chamber pocket, the fuel bundles are mainly concentrated at the rear of combustion chamber at the early stage of injection process. At the end of injection process, the fuel bundles are mainly distributed in the middle and rear areas of the combustion chamber. When injecting along the direction of rotor rotation wall, at the early stage of injection process, the fuel bundles are mainly distributed in the middle of combustion chamber. At the end of the injection process, the fuel bundles basically fill the entire combustion chamber pocket. This reveals that the injection angle mainly determines the fuel development along the longitudinal direction of the combustion chamber.Figure 11
Fuel bundles movement under different injection strategies.
(a)
Injection position A(b)
Injection positions B and CMoreover, when injecting into the combustion chamber pocket, the impingement between the fuel bundles and the combustion chamber pocket could be obviously observed at the beginning of the injection process. On the one hand, the impingement accelerates the fragmentation of the particles. On the other hand, the impingement hinders the development of the fuel bundles in the longitudinal direction. These could well explain why the LPL and SMD obtained by injecting towards the combustion chamber pocket are smaller compared with injecting along the direction of rotor rotation wall. In addition, it can be clearly seen from Figure11(a) that the fuel bundle of injection strategy AV-2 penetrates the entire combustion chamber, indicating that the LPL reaches the maximum within the allowable space of the combustion chamber. This is a good demonstration for the fact that injection strategy AV-2 shows the largest LPL among all the injection strategies.In addition, to illustrate the reason for the variation in LPL and SMD at different positions, the partial fuel bundle movement of injection positions B and C is demonstrated in Figure11(b). As can be seen in Figure 11(b), the LPL of injection strategy BP-2 develops to the maximum with the space limitation of the combustion chamber. This is not only a good explanation for the fact that at injection position B, when injecting along the direction of rotor rotation wall, the LPL of vertical-installed is larger than that of parallel-installed. Furthermore, it also gives the reason why the SMD of injection strategy BV-2 is the largest among all injection strategies. Obviously, this is mainly due to that for injection strategy BV-2, there is no hindrance to the fuel bundle during its development and no impingement with the combustion chamber pocket. In addition, it is found that at injection position C, the fuel bundles noticeably impinged on the combustion chamber pocket both for injecting towards the combustion chamber pocket and injecting along the direction of rotor rotation wall. In the impingement zone, fine atomized particles could be clearly observed. This is a good explanation for the phenomenon that the SMD at position C is significantly smaller than that at positions A and B for injecting along the direction of rotor rotation wall.


### 4.3. Fuel-Air Mixture after the Injection Process

The fuel-air mixture after the injection process also plays an important role in the combustion quality [50]. The injection strategies at injection position A are used as an example to describe the fuel-air mixing process in the small-scaled gasoline WRE. The in-cylinder airflow streamlines and fuel mass distribution under different eccentric angle of injection position A are illustrated in Figure 12(a). With the action of injection pressure, the fuel is present in the form of fuel bundles at the end of the injection process. However, at ignition timing, the shape and position of the fuel bundles for all injection strategies vary significantly. During this process, the variation of fuel shape and position is mainly controlled by the interaction of the in-cylinder airflow with the fuel. In general, the airflow of all injection strategies follows the direction of rotor rotation, i.e., from the rear to the front of the combustion chamber. At the end of the injection process, the airflow movement is very complicated. For parallel-installed, the airflow moves forward as a unidirectional state at the rear of the combustion chamber. When approaching the fuel bundles, the airflows on both sides of the combustion chamber converge to the middle, resulting in a crossover. Subsequently, the crossed airflow gradually returns to the unidirectional state. For vertical-installed, the airflow is also unidirectional at the rear of the combustion chamber. The difference is that the airflow then repeatedly crosses to form multiple eddies. The eddy promotes the mixing process of fuel and air, which is beneficial to the formation of a homogeneous mixture gas. Moreover importantly, under the action of such eddy, the fuel moves more easily towards the front of combustion chamber.Figure 12
Contours of airflow streamlines with fuel mass.
(a)
Fuel-air mixture process of injection position A(b)
Airflow streamlines with fuel mass at ignition timingFor observing the fuel distribution at the ignition timing of all injection strategies, the contours of airflow streamline with fuel mass for injection position is B or C are shown in Figure12(b). A comparative analysis shows that at injection position A, the fuel distribution area is the largest, which is basically distributed in the entire combustion chamber. At injection position B, the fuel injecting towards the combustion chamber pocket is mainly distributed in the middle of the combustion chamber. When injecting along the direction of rotor rotation, the fuel is concentrated in the front of the combustion chamber. At injection position C, the fuel is mainly distributed in the rear of the combustion chamber when injecting towards the combustion chamber pocket. When injecting along the direction of rotor rotation, the fuel is concentrated in the front of the combustion chamber. It can be concluded that the injection direction has an important affection on the fuel distribution. At the same injection position, the fuel injecting along the direction of rotor rotation wall is more likely to move towards the front of the combustion chamber. In addition, it could be found that due to the limitation of the small combustion chamber volume of small-scaled WRE, the airflow of all injection strategies presents a unidirectional state at the ignition timing.


### 4.4. Combustion Process

The combustion process is the most important part of the working process for WRE and affects the engine performance [51]. Peak in-cylinder pressure is an important indicator to characterize engine performance. Hence, to evaluate the performance of the small-scaled gasoline WRE, Figure 13 shows the peak in-cylinder pressure for injection strategies with different injection positions and directions. The A, B, and C in Figure 13 correspond to injection positions A, B, and C in Figure 9, respectively. The peak in-cylinder pressure under different injection positions and directions is significantly different. When the nozzle is parallel-installed at injection position C and injecting along the direction of rotor rotation, the peak in-cylinder pressure is the largest. The maximal peak in-cylinder pressure is 2.2 MPa, which is 25% higher than the minimum 1.75 MPa. Referring to Figure 12, it can be seen that for the injection strategy CP-2, a denser mixture is distributed near the spark plug at the ignition timing. Moreover, the continuous mixture is distributed between the two spark plugs. The mixture composition near the spark plug has a significant effect on the combustion process [51]. It was shown in reference [12] that the more continuous the mixture distribution from spark plug to spark plug and the higher the fuel concentration near the spark plug, the more conducive to the improvement of combustion efficiency. This could provide a good explanation for the higher peak in-cylinder pressures of injection strategies CP-2. In other words, because of the concentrated and continuous distribution of mixture, the ignited fuel could spread rapidly in the combustion chamber, allowing the fuel to release heat more fully, thus achieving a higher peak in-cylinder pressure. In summary, the higher peak in-cylinder pressure is mainly attributable to the good mixture distribution at the ignition timing, which is consistent with the conclusion in literature [52].Figure 13
Peak in-cylinder pressure for different injection strategies.Furthermore, it should be mentioned that the peak in-cylinder pressure of original port injection mode is 1.82 MPa. According to the above results, it could be found that not all DI strategies could achieve the effect of increasing the peak in-cylinder pressure. However, proper fuel injection position and direction do result in good fuel-air mixture distribution and thus improved the engine performance. All injection strategies whose injection position is A, as well as injection strategies BV-2 and CP-2, could achieve the purpose of improving engine performance. The peak in-cylinder pressure of injection strategy CP-2 is 21% higher than that of the original port injection.In addition, it can be seen from Figure12(b) that at ignition timing, the airflow of all injection strategies presents a unidirectional state. The unidirectional flow hindered the advancement of fuel at the rear of combustion chamber, aggravating the fuel incomplete combustion phenomenon [53]. For small-scaled WRE, the unidirectional flow is more severe under the limitation of small combustion chamber space. Therefore, to evaluate whether the low-pressure DI strategy could solve the incomplete combustion problem of studied small-scaled gasoline WRE, the fuel distribution at the end of combustion process of the above six injection strategies that can improve engine performance is illustrated in Figure 14. It can be found that although after the combustion process, there is still unburned fuel in the combustion chamber of small-scaled WRE. However, the intensity of this phenomenon varies under different low-pressure DI strategies. Among them, only a small amount of unburned fuel could be observed in the rear of the combustion chamber for the injection strategies AV-1, AV-2, and CP-2, indicating that the problem of incomplete combustion is basically solved. In particular, when the nozzle is parallel-installed at injection position C and injected along the direction of rotor rotation, the incomplete combustion problem at the rear of combustion chamber for the small-scaled gasoline WRE could be almost completely solved.Figure 14
Unburned fuel at the end of combustion process.


## 4.1. Injection Position and Direction

In addition to the injection timing, the injection position and direction in the cylinder of WRE are also investigated. Based on the above analysis, the injection timing is determined to be 226°EA BTDC. The injection pulse was 96°EA in all the injection strategies studied. According to the injection timing, the injection pulse as well as the engine speed, the rotor position during the injection process could be determined. The solid and dashed lines in Figure9(a) indicate the rotor positions at the beginning and the end of injection process, respectively. Following this, the zone available for installing the nozzle could be located. Installing nozzle in this zone ensures that the fuel is injected into the current working chamber during the entire injection process. Within this zone, three injection positions (A, B, and C) are placed, as shown in Figure 9(a).Figure 9
Summary of injection position and direction.
(a)
Injection position and angle(b)
Installation directionAdditionally, since a dual-hole nozzle is used in this paper, the injection direction involves two factors, which are the injection angle and the installation direction. Firstly, the nozzle is regarded as a unit, and the angle between the nozzle center axis and the horizontal line X is defined as the injection angle. Two injection angles are set for each injection position, indicated by angle 1 and angle 2, as shown in Figure9(a). Angle 1 is injecting towards the combustion chamber pocket, and angle 2 is injecting along the direction of rotor rotation. The installation direction is used to describe the relationship between the line connecting two holes and the cylinder thickness direction. In this paper, for each injection angle, the parallel installation and vertical installation are studied. Parallel installation is the line between the two holes parallel to the cylinder thickness direction, and the vertical installation is the line between the two holes vertical to the cylinder thickness direction, as illustrated in Figure 9(b). In summary, there are 12 different injection strategies in this study, and the specific injection parameters are listed in Table 5. In the simulation process, except for the parameters listed in Table 5, other parameters are invariants.Table 5
Injection parameters of different injection strategies.
CaseInjection positionInstallation directionInjection angle (°)AP-1AParallel10AP-2AParallel40AV-1AVertical10AV-2AVertical40BP-1BParallel0BP-2BParallel30BV-1BVertical0BV-2BVertical30CP-1CParallel-40CP-2CParallel-10CV-1CVertical-40CV-2CVertical-10


## 4.2. Spray Characteristics in Cylinder of WRE during Injection Process

Under the action of injection pressure, in-cylinder flow field and combustion chamber pocket, the fuel injected from the nozzle will show different spray characteristics [48, 49]. For analyzing the atomization characteristics of gasoline, Figure 10 presented the LPL and SMD for different injection strategies at the end of injection process. In general, the LPL and SMD vary greatly with the change of injection position and direction. The maximal LPL (72.10 mm) is 4.1 times higher than the minimum of 17.66 mm, and the minimum SMD (12.98 μm) is only 57% of the maximum 22.59 μm. At the same injection position, the LPL and SMD under different injection directions are also different. For the injection position A, when the nozzle is parallel-installed, the LPL for injecting along the direction of rotor rotation is significantly larger than that of injecting towards the combustion chamber pocket. When the nozzle is vertical-installed, compared with injecting along the direction of rotor rotation, the LPL obtained by injecting towards the combustion chamber pocket is significantly smaller.Figure 10
The LPL and SMD at the end of injection process.Similar rule is also shown at injection positions B and C; that is, under the same installation direction, the LPL for injecting towards the combustion chamber pocket is smaller than the LPL for injecting along the direction of rotor rotation. In particular, when injecting along the direction of rotor rotation wall at injection position B, the LPL of parallel-installed is larger than that of vertical-installed. This is different from injection positions A and C. At injection positions A and C, the LPL is largest when the nozzle is vertical-installed and injecting along the direction of rotor rotation wall. In summary, the largest LPL is observed for injection strategy AV-2. The SMD also follows the same rule; i.e., under the same installation direction, the SMD for injecting along the direction of rotor rotation wall is larger than the SMD for injecting towards the combustion chamber pocket. Interestingly, the SMD at injection positions A and B is significantly larger than that at injection position C for injecting along the direction of rotor rotation wall. The largest SMD is found in injection strategy BP-2.To explore the reasons for the above phenomenon, Figure11 shows the fuel movement for different injection strategies during the injection process. Because for different injection positions, the fuel development is similar. Therefore, injection position A is used as an example to describe the fuel movement in the cylinder of small-scaled WRE. As can be seen from Figure 11(a), during the whole injection process, the fuel bundles of the parallel-installed are mainly injected into the two sides of the combustion chamber pocket. For vertical-installed, the fuel bundles are mainly concentrated in the center of the combustion chamber pocket. It can be concluded that the installation direction mainly determines the fuel distribution in the transversal direction of combustion chamber. When injecting towards the combustion chamber pocket, the fuel bundles are mainly concentrated at the rear of combustion chamber at the early stage of injection process. At the end of injection process, the fuel bundles are mainly distributed in the middle and rear areas of the combustion chamber. When injecting along the direction of rotor rotation wall, at the early stage of injection process, the fuel bundles are mainly distributed in the middle of combustion chamber. At the end of the injection process, the fuel bundles basically fill the entire combustion chamber pocket. This reveals that the injection angle mainly determines the fuel development along the longitudinal direction of the combustion chamber.Figure 11
Fuel bundles movement under different injection strategies.
(a)
Injection position A(b)
Injection positions B and CMoreover, when injecting into the combustion chamber pocket, the impingement between the fuel bundles and the combustion chamber pocket could be obviously observed at the beginning of the injection process. On the one hand, the impingement accelerates the fragmentation of the particles. On the other hand, the impingement hinders the development of the fuel bundles in the longitudinal direction. These could well explain why the LPL and SMD obtained by injecting towards the combustion chamber pocket are smaller compared with injecting along the direction of rotor rotation wall. In addition, it can be clearly seen from Figure11(a) that the fuel bundle of injection strategy AV-2 penetrates the entire combustion chamber, indicating that the LPL reaches the maximum within the allowable space of the combustion chamber. This is a good demonstration for the fact that injection strategy AV-2 shows the largest LPL among all the injection strategies.In addition, to illustrate the reason for the variation in LPL and SMD at different positions, the partial fuel bundle movement of injection positions B and C is demonstrated in Figure11(b). As can be seen in Figure 11(b), the LPL of injection strategy BP-2 develops to the maximum with the space limitation of the combustion chamber. This is not only a good explanation for the fact that at injection position B, when injecting along the direction of rotor rotation wall, the LPL of vertical-installed is larger than that of parallel-installed. Furthermore, it also gives the reason why the SMD of injection strategy BV-2 is the largest among all injection strategies. Obviously, this is mainly due to that for injection strategy BV-2, there is no hindrance to the fuel bundle during its development and no impingement with the combustion chamber pocket. In addition, it is found that at injection position C, the fuel bundles noticeably impinged on the combustion chamber pocket both for injecting towards the combustion chamber pocket and injecting along the direction of rotor rotation wall. In the impingement zone, fine atomized particles could be clearly observed. This is a good explanation for the phenomenon that the SMD at position C is significantly smaller than that at positions A and B for injecting along the direction of rotor rotation wall.


## 4.3. Fuel-Air Mixture after the Injection Process

The fuel-air mixture after the injection process also plays an important role in the combustion quality [50]. The injection strategies at injection position A are used as an example to describe the fuel-air mixing process in the small-scaled gasoline WRE. The in-cylinder airflow streamlines and fuel mass distribution under different eccentric angle of injection position A are illustrated in Figure 12(a). With the action of injection pressure, the fuel is present in the form of fuel bundles at the end of the injection process. However, at ignition timing, the shape and position of the fuel bundles for all injection strategies vary significantly. During this process, the variation of fuel shape and position is mainly controlled by the interaction of the in-cylinder airflow with the fuel. In general, the airflow of all injection strategies follows the direction of rotor rotation, i.e., from the rear to the front of the combustion chamber. At the end of the injection process, the airflow movement is very complicated. For parallel-installed, the airflow moves forward as a unidirectional state at the rear of the combustion chamber. When approaching the fuel bundles, the airflows on both sides of the combustion chamber converge to the middle, resulting in a crossover. Subsequently, the crossed airflow gradually returns to the unidirectional state. For vertical-installed, the airflow is also unidirectional at the rear of the combustion chamber. The difference is that the airflow then repeatedly crosses to form multiple eddies. The eddy promotes the mixing process of fuel and air, which is beneficial to the formation of a homogeneous mixture gas. Moreover importantly, under the action of such eddy, the fuel moves more easily towards the front of combustion chamber.Figure 12
Contours of airflow streamlines with fuel mass.
(a)
Fuel-air mixture process of injection position A(b)
Airflow streamlines with fuel mass at ignition timingFor observing the fuel distribution at the ignition timing of all injection strategies, the contours of airflow streamline with fuel mass for injection position is B or C are shown in Figure12(b). A comparative analysis shows that at injection position A, the fuel distribution area is the largest, which is basically distributed in the entire combustion chamber. At injection position B, the fuel injecting towards the combustion chamber pocket is mainly distributed in the middle of the combustion chamber. When injecting along the direction of rotor rotation, the fuel is concentrated in the front of the combustion chamber. At injection position C, the fuel is mainly distributed in the rear of the combustion chamber when injecting towards the combustion chamber pocket. When injecting along the direction of rotor rotation, the fuel is concentrated in the front of the combustion chamber. It can be concluded that the injection direction has an important affection on the fuel distribution. At the same injection position, the fuel injecting along the direction of rotor rotation wall is more likely to move towards the front of the combustion chamber. In addition, it could be found that due to the limitation of the small combustion chamber volume of small-scaled WRE, the airflow of all injection strategies presents a unidirectional state at the ignition timing.


## 4.4. Combustion Process

The combustion process is the most important part of the working process for WRE and affects the engine performance [51]. Peak in-cylinder pressure is an important indicator to characterize engine performance. Hence, to evaluate the performance of the small-scaled gasoline WRE, Figure 13 shows the peak in-cylinder pressure for injection strategies with different injection positions and directions. The A, B, and C in Figure 13 correspond to injection positions A, B, and C in Figure 9, respectively. The peak in-cylinder pressure under different injection positions and directions is significantly different. When the nozzle is parallel-installed at injection position C and injecting along the direction of rotor rotation, the peak in-cylinder pressure is the largest. The maximal peak in-cylinder pressure is 2.2 MPa, which is 25% higher than the minimum 1.75 MPa. Referring to Figure 12, it can be seen that for the injection strategy CP-2, a denser mixture is distributed near the spark plug at the ignition timing. Moreover, the continuous mixture is distributed between the two spark plugs. The mixture composition near the spark plug has a significant effect on the combustion process [51]. It was shown in reference [12] that the more continuous the mixture distribution from spark plug to spark plug and the higher the fuel concentration near the spark plug, the more conducive to the improvement of combustion efficiency. This could provide a good explanation for the higher peak in-cylinder pressures of injection strategies CP-2. In other words, because of the concentrated and continuous distribution of mixture, the ignited fuel could spread rapidly in the combustion chamber, allowing the fuel to release heat more fully, thus achieving a higher peak in-cylinder pressure. In summary, the higher peak in-cylinder pressure is mainly attributable to the good mixture distribution at the ignition timing, which is consistent with the conclusion in literature [52].Figure 13
Peak in-cylinder pressure for different injection strategies.Furthermore, it should be mentioned that the peak in-cylinder pressure of original port injection mode is 1.82 MPa. According to the above results, it could be found that not all DI strategies could achieve the effect of increasing the peak in-cylinder pressure. However, proper fuel injection position and direction do result in good fuel-air mixture distribution and thus improved the engine performance. All injection strategies whose injection position is A, as well as injection strategies BV-2 and CP-2, could achieve the purpose of improving engine performance. The peak in-cylinder pressure of injection strategy CP-2 is 21% higher than that of the original port injection.In addition, it can be seen from Figure12(b) that at ignition timing, the airflow of all injection strategies presents a unidirectional state. The unidirectional flow hindered the advancement of fuel at the rear of combustion chamber, aggravating the fuel incomplete combustion phenomenon [53]. For small-scaled WRE, the unidirectional flow is more severe under the limitation of small combustion chamber space. Therefore, to evaluate whether the low-pressure DI strategy could solve the incomplete combustion problem of studied small-scaled gasoline WRE, the fuel distribution at the end of combustion process of the above six injection strategies that can improve engine performance is illustrated in Figure 14. It can be found that although after the combustion process, there is still unburned fuel in the combustion chamber of small-scaled WRE. However, the intensity of this phenomenon varies under different low-pressure DI strategies. Among them, only a small amount of unburned fuel could be observed in the rear of the combustion chamber for the injection strategies AV-1, AV-2, and CP-2, indicating that the problem of incomplete combustion is basically solved. In particular, when the nozzle is parallel-installed at injection position C and injected along the direction of rotor rotation, the incomplete combustion problem at the rear of combustion chamber for the small-scaled gasoline WRE could be almost completely solved.Figure 14
Unburned fuel at the end of combustion process.


## 5. Conclusion

In this paper, the spray, fuel-air mixture, and combustion processes of a small-scaled gasoline WRE under different low-pressure DI strategies were numerically analyzed, and the optimal injection strategy was determined. The main conclusions can be summarized as follows:(1)
For low-pressure injection, with the delay of injection timing, the integrated heat release increases first and then decreases. In this study, the maximal value is reached at the middle injection timing. This indicates that at this injection timing, the number of particles, the SMD, and total fuel evaporation are in the best combination. Based on this, the optimal injection timing could be determined(2)
Injection angle is a key factor in determining the atomization characteristics of gasoline. At the same injection position, the LPL and SMD for injecting towards the combustion chamber pocket are significantly smaller than those for injecting along the direction of rotor rotation. This is mainly due to the impingement between fuel bundles and combustion chamber pocket caused by injecting along the direction of rotor rotation wall(3)
The installation direction of nozzle plays an important role in the airflow movement. For vertical-installed, the airflow repeatedly crosses to form multiple eddies. Because of this, the fuel moves more easily towards the front of combustion chamber, helping to alleviate the problem of incomplete combustion at the rear of combustion chamber to some extent(4)
Injection position and injection angle have a significant effect on the peak in-cylinder pressure. This is mainly attributed to the different fuel development and mixture distribution for different injection strategies. The peak in-cylinder pressure is the largest when the nozzle is parallel-installed at the lower edge of the installing zone and injecting along the direction of rotor rotation. By this injection strategy, the incomplete combustion problem of small-scaled gasoline WRE is almost completely solved



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*Source: 3716468-2023-02-06.xml*