Combustion and Emission Characteristics of a Diesel Engine with a Variable Injection Rate

Abstract

Diesel engine combustion is dependent mainly on the fuel injection characteristics, particularly the injection pressure and rate, which directly affect the engine efficiency and emissions. Herein, an electrically controlled supercharger is added to a traditional high-pressure common rail system to form an ultrahigh-pressure common rail system. Then, the variations in the spray, combustion, and emission characteristics of a diesel engine with a variable fuel injection rate are analyzed. Moreover, a simulation model for a diesel engine combustion chamber is built and verified by experimental results for numerical analysis. The results reveal that the injection rate can be flexibly adjusted via regulation when the solenoid valves are opened on the electrically controlled supercharger. Specifically, (1) the boot-shaped injection rate has greater potential than the traditional rectangular injection rate in terms of combustion and emission; (2) the main injection advance angle at the boot-shaped injection rate can be properly increased to improve combustion; and (3) the pilot injection quantity and advance angle are strongly coupled with the boot-shaped injection rate, potentially enhancing the mixing efficiency of fuel and air in the cylinder to achieve favorable emission results. This paper provides good guidance for the reliable design and optimization of noble-metal-based diesel engines.

1. Introduction

Diesel engines, as the core components of power machines, have been widely applied in various fields. Worldwide, the need for a high-efficiency, low-emission diesel engine has increased [1]. The increasing depletion of petroleum resources and the worsening of environmental pollution have increased the significance of energy conservation and environmental protection in the development of modern high-power marine diesel engines [2]. New diesel engine technology must be developed to fulfill future energy needs and tightening regulations concerning emissions, such as new combustion strategies, ultrahigh-pressure injection, posttreatment systems, and residual heat recovery systems. However, posttreatment and residual heat recovery systems are structurally complex and very expensive to construct; thus, improvements in fuel–air mixing and combustion in cylinders have been adopted as effective strategies for controlling exhaust emissions [3,4].

The injection pressure significantly affects the combustion of a diesel engine. Increased pressure can help to improve the quality of atomization while effectively decreasing nitrous oxide (NO_x) and particulate matter (PM) emissions [5]. Moreover, injection rate control is regarded as the most effective technology for reducing NO_x emissions, PM emissions, and combustion noise [6]. Therefore, efforts have been made to achieve an ideal injection rate as the requirements for injection pressure continuously increase.

Nevertheless, the current common high-pressure rail system suffers from various technical defects. The components of this type of system always require high pressures, which leads to major mechanical stress and fuel leakage. The shape of the injected fuel is approximately rectangular and difficult to adjust [7,8]. To obtain an ideal injection rate, the Denso company designed a throttling orifice and specific boot-shaped valves for the injector of the ECD-U2 high-pressure common rail system. In this manner, the injection rate can be adjusted by changing the movement speed in a needle valve while maintaining the same injection pressure [9]. In addition, the Bosch company flexibly adjusted the injection rate by changing the injection pressure in each injection process in the CRSN4 high-pressure common rail system [10]. Tsinghua University proposed regulations of the pilot injection rate for plunger discharge and solenoid valve control in an electrically controlled pump–valve–pipe–nozzle system and carried out research on the injection rate characteristics [11]. Huang et al. [12] devised a double-staggered nozzle; then, the scholars calculated and evaluated the combustion and emission characteristics of the diesel engine on which the nozzle was installed. The researchers found that a double-staggered nozzle could achieve a greater flow area than a common nozzle, thereby increasing the injection rates [12].

In general, the injection rate in a high-pressure common rail system can be adjusted in different ways, e.g., altering the pressure difference on both sides of an orifice in the structural design, changing the injection pressure during the injection process, or using different orifice flow areas. If a high-pressure common rail system is fixed, the injection rate can be adjusted in a limited manner through structural changes. However, managing injection pressure control can serve as a potentially enhanced strategy for significantly controlling the injection rate.

In recent years, the effects of injection pressure and rate on the performance of diesel engines have been studied worldwide. Cheng et al. [13] explored the influence of injection pressure on the combustion performance of a diesel engine. Their results revealed that an increase in injection pressure leads to an increase in cylinder pressure peak and a decrease in ignition delay time, but the effect diminishes after the injection pressure exceeds 140 MPa. Wang et al. [14] used high-speed cameras to study the effects of ultrahigh injection pressure on the flame structure and soot generation characteristics of an impact diesel engine and employed a two-color flame measurement method to measure the soot discharge temperature and concentration. According to their findings, an ultrahigh injection pressure can increase the flame structure size and decrease the soot generation level. Yuan et al. [5] studied the specific influence of injection pressure on flash boiling spray breakup and its underlying mechanism and compared the impacts of injection pressure on spray shape and spray breakup. This study revealed that, for cold spray, the injection penetration distance can be reduced, and the spray width is not significantly increased if a relatively high injection pressure is used; for overheated spray, increased injection pressure dramatically decreases the spray width and slightly increases the injection penetration distance. Zhang et al. [15] analyzed the combustion characteristics of a high-speed diesel engine with different injection rates. A rectangular injection rate leads to the shortest ignition delay time, while a corresponding premixed combustion generates the highest heat release and the largest heat efficiency. According to their findings, a decrease in the natural gas injection rate significantly decreases the in-cylinder pressure, heat release rate, and average peak temperature. Nevertheless, most of the above studies have addressed the effects of injection pressure on the combustion and emission characteristics of a diesel engine but seldom focused on the mechanisms by which a varying injection rate can cause these influences. Some studies might have involved analyses of the influences of the injection rate, but only non-qualitatively. Therefore, there is an urgent need to explore the combustion and emission characteristics of a diesel engine with various injection rates and discover the regularity of variation to provide a basis for improving the overall performance.

In this paper, a diesel engine was used to reveal the mechanisms by which its combustion and emission characteristics changed at different injection rates. An ultrahigh-pressure common rail system was, therefore, proposed, and a simulation model was developed to explore the effects of controlling the injection rate of this engine. Subsequently, fire software was employed to build a simulation model of the diesel engine combustion chamber. The accuracy of the model was then tested and verified. On this basis, the simulation model was used to analyze the mechanisms by which the combustion and emission characteristics of the diesel engine were affected by various injection rates and boot-shaped injection rates. Besides the related effect coupled with different main injection advance angles, pilot injection quantities and pilot injection advance angles are revealed.

2. Realization of a Variable Injection Rate

A variable injection rate was achieved with a common ultrahigh-pressure rail system. The system consisted of a fuel tank, a high-pressure fuel pump, a common rail pipe, an electrically controlled supercharger, and an injector (shown in Figure 1). By adding a single-inlet and single-outlet electrically controlled supercharger between the common rail pipe and the injector, the system could supply fuel to the injector under different pressure and injection rate conditions based on the changing operating conditions of the diesel engine.

An electrically controlled supercharger is a crucial component of the ultrahigh-pressure common rail system because its performance plays a decisive role in injection with ultrahigh pressures and varying injection rates. The structure of the supercharger is shown in Figure 2. This component contained charging pistons, solenoid valves, check valves, orifices, and other components, and it functioned according to the following principles: at the time of partial load, the fuel (basic pressure) was supplied through a common rail pipe to the charging and control chambers, thus ensuring an equivalent pressure at both ends of the charging piston. The charging piston remained in an equilibrium state at this time, and the fuel was supplied at a basic pressure to the injector. Under high loading conditions, the solenoid valve of the electrically controlled supercharger was opened, and the spool moved toward the coil to disengage the spool head from the valve seat while closing the inlet orifice. Then, high-pressure fuel flowed back from the outlet orifice into the fuel tank, causing the charging piston to move toward the charging chamber. Subsequently, the pressure in the charging chamber increased to supply high-pressure fuel to the injector. After the solenoid valve was closed, the high-pressure fuel supplied from the common rail pipe flowed through the inlet orifice into the control chamber. Under the effect of the reset spring, the charging piston moved toward the basic pressure chamber. Afterward, a state of equilibrium was regained.

For descriptive purposes, a general model was built for the common ultrahigh-pressure rail system using Amesim v14.0 software (Figure 3). The model consisted of an electrically controlled supercharger model, an injector model, and a high-pressure source model in place of a high-pressure pump and a common rail pipe. In this model, components 8 and 9 simulated the piston with a ring groove and the cone valve in the electrical controller supercharger, respectively. These components were considered crucial to the two-position three-way valve. Components 11, 13, and 14 simulated the basic pressure, control, and charging chambers of the electrically controlled supercharger, respectively. Components 15–20 formed the charging piston assembly. Components 23 and 27 simulated the control and pressure chambers of the injector, respectively. Injection control was studied by regulating the opening of the solenoid valves of the electrically controlled supercharger and the injector in the model.

3. Modeling of Combustion and Emission Characteristics

In a real-life military ship system situation, the varied MIAA, PIQ, and PIAA could be caused by many reasons. For instance, when a ship turns, accelerates to start, decelerates to stop, or when the ship is damaged, the performance expectation of a diesel engine is different. In other words, the variation of MIAA, PIQ, and PIAA is unavoidable. Therefore, the article discusses the effects of varying MIAA, PIQ, and PIAA on combustion and emission characteristics. Fire software (v2018.1) was used to construct a combustion simulation model. The calculation covered the period from the closing of the inlet valve to the opening of the exhaust valve. The crank angle was 360–838° in the calculation. For the calculation, the step length was 0.4° at the spray stage but increased to 1° at the combustion stage. The calculations were conducted with the k−ε−f turbulence model in Fire software. The Dukowicz model was implemented for evaporation, the flame three-region model was implemented for combustion, the advanced NO_x model was implemented for NO_x generation, and the advanced soot model was implemented for soot generation. The main parameters of the diesel engine used in the test are given in

Table 1. Main parameters of the diesel engine.

Parameter	Value
Rotational speed/r·min−1	1500
Bore stroke/mm	128/140
Rod length/mm	255
Compression ratio	15
Orifice diameter/mm	0.2
Number of orifices	6
Angle between orifices/°	144

.

The initial pressure in the cylinder was 0.3 MPa. The temperature was 348 K. The exhaust mass fraction was 0. The temperatures of the cylinder head, cylinder liner, and piston were 543 K, 413 K, and 583 K, respectively. The injector was placed at the center of the cylinder head along the vertical direction of the axis and was not inclined. The combustion chamber had no bias. The diesel engine had six orifices; thus, a one-sixth fan-shaped area was selected for simplified calculation, and its corresponding angle was 72°. The calculation grid for the combustion chamber of the diesel engine is presented in Figure 4.

To verify the accuracy of the model, the simulation results were compared with the results of combustion and emission tests using a traditional high-pressure common rail system with a rectangular injection rate. A comparison of the diesel engine in-cylinder pressure between the simulation and test results is presented in Figure 5. The results were clearly shown to be almost identical and satisfactorily consistent, indicating that the simulation model was acceptable and could be used to simulate real-world conditions.

4. Simulation Results Analysis

The distributions of the NO_x and soot mass fractions (i.e., the amount of NO_x or soot mass and the total mass of air in the cylinder) at 726 °CA with various injection rates are presented in Figure 6 and Figure 7, respectively. More specifically, from Figure 6, there are distributions of NO_x mass fractions at 726 °CA; the maximum high mass fractions of NO_x of rectangular is 3.7 × 10⁻⁵, higher than that of inclined and boot-shaped injection conditions, which are 3.04 × 10⁻⁵ and 2.74 × 10⁻⁵, respectively. From Figure 7, the distributions of soot mass fractions are at 726 °CA, the maximum high mass fractions of soot emissions of rectangular is 1.81 × 10⁻³, higher than that of inclined and boot-shaped injection condition, which is 1.29 × 10⁻³ and 0.79 × 10⁻³, respectively. Related contents are added to the revised manuscript. Overall, these figures show that the NO_x and soot emissions had relatively high mass fractions when the injection was rectangular. Moreover, the emissions occurred near the wall of the combustion chamber in most cases. Conversely, the boot-shaped injection rate led to a correspondingly reduced emission mass fraction.

According to an analysis of the above combustion and emission results, the rectangular injection rate was always accompanied by an increased injection pressure, which peaked shortly after the start of the injection. At the early stage of injection, a large amount of fuel was injected into the cylinder. Nevertheless, the temperature and pressure in the cylinder were not sufficiently high for peak mixing; thus, most of the fuel could not be completely mixed, and a portion of the fuel was bumped against the wall, increasing the concentration of emissions near the wall. The boot-shaped injection rate could increase the injection rate with increasing in-cylinder pressure and conditions. Therefore, different injection pressures caused different and possible improvements in combustion. Additionally, the cylinder had a small distribution range and a low mass fraction of emissions [16,17,18]. A more parametric study would be carried out to analyze the possibility of flexibly controlling the peak and level of in-cylinder pressure and temperature.

The distributions of NO_x and soot mass fractions at 726 °CA with different main injection advance angles (MIAAs) are shown in Figure 8 and Figure 9, respectively. The NO_x and soot emission mass fractions were obviously the lowest, and the area of high concentration was small when the MIAA was −15 °CA. This area was followed by that at an MIAA of −14 °CA, while at an MIAA of −13 °CA, the largest NO_x and soot emission mass fractions occurred, causing a slight distribution adjacent to the wall. Specifically, for the boot-shaped injection rate, a properly increased MIAA could increase the amount of fuel involved in the ignition delay to sufficiently mix the fuel with the air in the cylinder. This process would facilitate adequate combustion and result in relatively low and acceptable emission levels [19,20,21].

The distributions of NO_x and soot mass fractions at 726 °CA with different pilot injection quantities (PIQs) are presented in Figure 10 and Figure 11, respectively. Evidently, the NO_x and soot emissions were distributed uniformly at a PIQ of 1.5 mg, and their average mass fractions were relatively low. When the PIQ was 1.0 mg, the NO_x and soot emissions were concentrated near the nozzles, creating an obvious high-concentration area. When the total injection quantity was fixed, a high PIQ caused additional fuel to mix with the air in the ignition delay, facilitating sufficient combustion because a highly uniform mixture led to enhanced combustion and emission. If the PIQ was relatively low, the fuel injected into the cylinder accumulated near the nozzle, undermining atomization and increasing the mass fraction of subsequent emissions.

The distributions of NO_x and soot mass fractions at 726 °CA with different pilot injection advance angles (PIAAs) are presented in Figure 12 and Figure 13, respectively. The average emission mass fraction was relatively low and uniformly distributed when the PIAA was −20 °CA. In contrast, additional areas with high emission mass fractions were observed when the PIAA was −16 °CA. Nevertheless, the overall emission performance did not vary dramatically. When the PIAA was high, fuel was injected into the cylinder in advance and mixed sufficiently with the air, which improved the combustion and emission characteristics. Nevertheless, the pilot injection quality remained unchanged at the three PIAAs; thus, the PIAA did not significantly affect the combustion and emission. It should be noted that, by increasing the injection in Figure 10 and Figure 11, the total emissions could increase as well while the fraction may not increase, which proves that a more homogeneous and complete combustion occurs in the cylinder, but the total emissions increase.

5. Conclusions

In this paper, a variable injection rate is achieved, and the injector in an ultrahigh-pressure common rail system is analyzed. Compared with the traditional rectangular injection rate, the boot-shaped injection rate shows greater potential and superiority regarding the combustion and emission characteristics. The main conclusions are as follows:

1.

The main injection advance angle at the boot-shaped injection rate can be increased to improve the fuel–air mixing in the cylinder and the combustion and emission performance characteristics.

2.

Enhanced atomization can be achieved if the boot-shaped injection rate is coupled with a relatively large pilot injection quantity to improve the combustion and emission performance.

3.

Enhanced atomization can be achieved if the boot-shaped injection rate is coupled with a high pilot injection advance angle to improve the combustion and emission performance.

In summary, the reported simulation method provides a novel tool for analyzing the combustion and emission characteristics of a diesel engine with a variable injection rate. In addition, this study provides good guidance for the optimized design of noble-metal-based diesel engines.