Injection control strategy of diesel engine based on speed segment PID and operating parameter adaptation

Abstract

Aiming at the problems of low precision, poor anti-interference and poor follow-up in the control parameters for the diesel engine fuel injection system, this paper studies the control method of the high-pressure common rail electronic control fuel injection system of the diesel engine, constructs the high-pressure common rail fuel injection control system based on the ECU, and establishes the speed segment PID control model of fuel injection quantity, common rail pressure, fuel injection timing and fuel injection rate by using MATLAB/Simulink. The fuel injection quantity and timing are simulated. In order to realize all-round and flexible control of the diesel engine under different working conditions, and to achieve the desired optimal performance in all aspects, the optimization control method of the injection law for the diesel engine is studied. The diesel engine fuel injection control strategy based on speed segment PID and operating parameter adaptation is proposed to realize precise control of the common rail pressure, injection quantity, injection timing and injection rate under different working conditions. The simulation calculation and bench test show that the maximum fluctuation of rail pressure at idle speed is only 5 MPa, and the time to reach stability is only 1.25 s, which greatly improves the control accuracy, anti-interference and follow-up ability of the injection parameters.

Keywords

Diesel engine electronic control injection system high pressure common rail optimization control injection parameter speed segment PID

1. Introduction

A lot of researches have been carried out on engine fuel injection system abroad. Ranz established the relationship between the atomization characteristics and the flow inside the nozzle [1]. A dual controller was designed to control common rail pressure, which can ensure that the current value of fuel metering unit (FMU) and pressure control valve (PCV) can be stabilized near the expected value when the voltage and coil impedance change [2]. A model reference adaptive control (MRAC) method was proposed [3]. When the parameters of diesel engine change and the rail pressure fluctuation is caused by non-periodic disturbance, the parameters can be identified in time, but the control process is somewhat complicated. The fuel injection control by means of a large number of experimental calibration and mapping methods has also been studied deeply. Domestic studies have put forward some new methods, such as: Wu Qinglin of Chongqing University of Technology measures fuel injection through high-speed flowmeter, and designs a fuel injection fuzzy controller, which uses genetic algorithm to optimize control parameters and improve control accuracy [4]. Finally, the hardware test plateform on the ring shows that the control effect has reached the expected level, and it is necessary to conduct in-depth research. In 2016, Zhang Yue of Zhenjiang Shipping College established the control model of main fuel injection volume and carried out bench test with its control algorithm [5]. The control algorithm was calibrated by calibration software to verify the correctness of the control algorithm model. The results show that the fluctuation of fuel volume is small and the fuel supply is stable during the idling process. Li Degang of Jilin University put forward the control method of fuel injection quantity by analyzing the traction demand of high pressure common rail diesel engine [6]. According to the expected torque required by diesel engine traction during starting acceleration, the fuel injection quantity is calibrated. The data show that the deviation between the calibrated torque and the expected value is small.

It can be seen from this that most of the fuel injection control is achieved by the open-loop control method which monitors the status signal of diesel engine, and maps the fitting figures according to a large number of bench calibration experimental data [7]. There are also some studies on the fuel injection control algorithm, but in practice, the closed-loop control algorithm based on the speed is used to speed control, so as to avoid the large deviation between the actual speed and the expected speed under different working conditions, which will lead to too much or too little fuel injection after querying the mapping figures, and make the performance of diesel engine worse. However, the above control strategies do not take into account the actual intake pressure, cooling water temperature and intake temperature and other environmental factors, which reduce the control accuracy [8].

The design of fuel injection system and fuel injection law of diesel engine are closely related to combustion and emission performance. In the paper [9], Hydsim simulation platform is used to establish the simulation model of the fuel injection system. Based on the simulation model, the test platform of the fuel injection system is built, and the test analysis under the rail pressure of 150 MPa is carried out. The validity and accuracy of the simulation model are verified by comparing the simulation value with the test value and the standard value. The influence of structural parameters such as inlet throttle hole, outlet throttle hole and control piston on injection characteristics, and the selection principle of each parameter in system design are mastered to provide the basis for the system structure optimisation. In the paper [10], to deal with the load disturbances and model parameter perturbation of the diesel engine, a model for the relationship between engine speed and fuel injection is established on the basis of the mean value engine model, and an active disturbance rejection control (ADRC) approach is designed to achieve finite-time stability.

In conclusion, the fuel injection law plays an important role in power performance, economy and emission characteristics of diesel engine. At present, the injection control parameters of diesel engine, such as injection quantity, common rail pressure, injection timing and injection rate under different working conditions, often have the problems of low control accuracy, slow response speed and poor anti-interference. Using high-pressure common rail and speed segment PID control method, an injection control system based on ECU model reference and error compensation is developed. The control precision of injection parameters, anti-interference ability and following ability are improved. Aiming at the problems of poor economy and serious emission caused by insufficient combustion of diesel engine, the multiple injection method composed of guided injection, pre-injection, main injection and post-injection is proposed, which optimizes the injection law, further improves the control performance of the system, and achieves the goal of good economy and low emission.

In this paper, the speed segment PID control and operating parameter adaptation method are proposed for the first time in diesel engine injection control strategy. The effectiveness of the method is verified by simulation analysis and experimental research.

2. Overall model of diesel engine

In order not to affect the research and analysis of diesel engine performance, this paper divides it into three parts: diesel engine body, fuel injection system and control unit [11]. The diesel engine body is composed of four basic modules: ventilation system, torque calculation, cooling system and dynamic calculation, which can realize the information transmission with the control unit. The overall model structure is shown in Fig. 1.

The fuel injection system studied in this paper is independently developed and improved according to our product requirements. The main parameters used in diesel engine modeling are shown in Table 1.

Table 1
Related parameters of diesel engine modeling

Type	4 cylinders
Cylinder diameter $\times$ Stroke	85 mm $\times$ 83 mm
Displacement	19L
Compression ratio	18.5:1
Common rail pipe	Volume: 10 cm ${}^{3}$ , Maximum rail pressure: 180 MPa
High pressure oil pump plunger	Lift: 5 mm, Area: 32.1 mm ${}^{2}$ , Number: 3
Low pressure pipe volume	125 cm ${}^{3}$
Rated speed	3200 r/min
Maximum torque	285 Nm
Speed ratio of oil pump and diesel engine	1:2

Figure 1.

Overall model structure.

3. Injection control system

The structure of the high pressure common rail electronic control injection system has been innovated and improved on the previous injection system. It has the following characteristics: it can inject high pressure fuel. Compared with the ordinary in-line pump, the injection pressure has doubled and the maximum pressure exceeds 200 MPa; the injection pressure is independent of the speed of the diesel engine, which can provide the injection pressure and the injection rate under different operating conditions, and has the obvious performance improvement for starting and low speed of the diesel engine; not limited to its structure, it can complete the precise adjustment of injection volume and injection advance angle; it can also complete the control of injection rate, achieve a process of first-slow and then-urgency, and get the ideal injection effect [12]; it can also complete multiple pre-injection, which can greatly reduce combustion noise, fuel consumption and emissions.

The fuel injection system is mainly composed of a series of auxiliary monitoring devices such as fuel tank, low pressure oil pump, filter, high pressure oil pump, common rail pipe, injector and electronic control unit (ECU) and sensor [13], as shown in Fig. 2.

Figure 2.

Structure and composition of fuel injection system.

Firstly, the system delivers the fuel in the tank to the high-pressure oil pump through the low-pressure oil pump. The high-pressure oil pump compresses the low-pressure oil into the high-pressure state and then transfers it to the common rail pipe. It flows into the injector through the common rail pipe and then begins to split into two paths, one way into the oilstorage chamber and the other way into the control chamber [14]. Driven by the ECU signal, the solenoid valve of the injector changes the pressure in the control chamber to make the needle valve act to inject oil. When the power is turned on, the pressure decreases rapidly, the needle valve rises, and the fuel in the oilstorage chamber is injected into the cylinder. When the power is turned off, the pressure in the control chamber recovers quickly because of the common rail pressure, and the needle valve falls back and the injection is completed. Therefore, the oil volume injected into the cylinder is mainly determined by the time when the solenoid valve is electrified and the common rail pressure. The oil pressure in the common rail pipe is determined by the oil volume flowing into it. The proportional throttle valve opening at the inlet of the high-pressure oil pump is adjusted by the output state parameter signal to realize the regulation of the displacement of the high-pressure oil pump. Once the common rail pipe pressure is too large, proportional control valve can be used to reduce the pressure rapidly and substantially. In addition, the rail pressure sensor on the common rail pipe monitors the pressure in the pipe in real time and feeds back to ECU for the closed-loop control to ensure the desired common rail pressure under various working conditions.

In order to make diesel engine achieve all-round flexible control under different working conditions and achieve the desired optimal performance in all aspects, it is far from enough to guarantee good mechanical performance and hardware conditions of the ECU module alone. The most important thing is to rely on the control strategy and control algorithm of diesel engine electronic control injection system. In the research and development of high-pressure common rail electronic control fuel injection system, the focus is the research and design of control strategy. The ideal control strategy can ensure the stability and reliability of the system. The overall structure of the fuel injection control system is shown in Fig. 3. The state parameters of the diesel engine are monitored in real time by sensors such as water temperature, crankshaft, throttle position, ambient temperature and cam position installed on the diesel engine, which are conveyed to the ECU module to distinguish the working conditions accurately, and then the precise control of common rail pressure, fuel injection volume, injection timing and fuel injection rate under different working conditions is completed.

Figure 3.

Overall structure of fuel injection control system.

4. Fuel injection volume control

4.1 Fuel injection volume control under starting condition

The starting of diesel engine can be divided into three stages according to the change of speed: towing period, starting period and heat engine period, as shown in Fig. 4. When starting, the influence of two factors, the speed of diesel engine and the temperature of cooling water, is mainly considered, as shown in Fig. 4. When the speed is low, the demand for fuel is larger, but when the speed is increased, the fuel injection decreases slowly to [15] the value at idle speed. The temperature of cooling water can be used to distinguish the state of cold engine from that of hot engine, while the state of cold engine needs to increase the fuel quantity and to accelerate fuel evaporation and atomization, so as to improve the starting performance. When the heat engine, the phenomenon of black smoke should be avoided and the fuel injection quantity should be moderately reduced. Thus, according to the different conditions of starting, the flexible control of diesel fuel injection can be realized, and the optimal starting effect can be obtained. The process of fuel injection control under starting condition is shown in Fig. 5.

Figure 4.

The starting process and the relationship between injection quantity, speed and water temperature.

Figure 5.

Injection control process under starting condition.

4.2 Control of fuel injection volume after starting

After starting, the fuel injection quantity is mainly obtained by the method of checking the MAP figure. At the same time, the maximum fuel injection quantity must be obtained by the intake pressure and the speed of the diesel engine at this time, so as to limit its target value. In addition, the influence of in-cylinder pressure should also be taken into account. Finally, the injection pulse width can be obtained from the maximum limit value and the pressure difference between common rail pressure and in-cylinder pressure, as shown in Fig. 6.

Figure 6.

Fuel injection quantity control after starting.

In order to ensure that the speed of diesel engine can run steadily according to the expected speed, the closed-loop control of the speed is used to correct the fuel injection volume. The error between the expected value and the measured value of the sensor is transmitted to the controller to correct the fuel injection volume on-line, so as to change the fuel injection volume and maintain the stability of the system. At present, because the structure of the PID controller is easy to understand and the parameters are easy to adjust, the application of the PID controller is the most common. Moreover, the PID controller can also be adjusted on-line according to engineering experience for the control object with complex mathematical model and variable system parameters, which achieves ideal control effect.

Because the fuel injection system itself has the characteristics of non-linearity, parameter-variability and time-delay, the best PID parameter is limited to a specific working condition and the best control effect cannot be satisfied under the whole working condition. Therefore, a sectional PID controller is adopted here, as shown in Eq. (1):

$\displaystyle y(t)=K_{c}\left[X\cdot e(t)+Y\cdot\frac{1}{T_{t}}\int e(t)dt+Z% \cdot T_{d}\frac{de(t)}{dt}\right]$ (1)

In the formula, $y(t)$ is the output signal of the controller, $e(t)$ is the deviation of the controller, $K_{c}$ is the proportional constant, $T_{t}$ is the integral time constant and $T_{d}$ is the differential time constant.

The sectional PID control only multiplies one coefficient before the traditional PID control’s proportional, differential and integral terms, and its magnitude varies with the error range. Equation (2) is the distribution-range of $X$ , and $Y$ or $Z$ is similar. The final value of $X$ , $Y$ and $Z$ should be adjusted on-line.

$\displaystyle X=\left\{\begin{array}[]{ll}A&|{e(t)}|<p\\ B&p\leqslant|{e(t)}|\leqslant q\\ C&|{e(t)}|>q\\ \end{array}\right.$ (2)

According to the speed variation characteristics of diesel engine under actual operating conditions and Eq. (2), the sectional PI controller is selected and the D value is fixed. When the deviation is large, the response can be quick. The $X$ value needs to be large, so as to avoid the phenomenon that the integral is limited when the integral saturation occurs, that is $Y=$ 0. When the error is near the stable region, the small $X$ and $Y$ are selected. After reaching the stability, to prevent a large fluctuation in the speed, the $X$ value should be smaller, and the $Y$ value larger.

Figure 7.

PID control model of speed segment.

Figure 8.

Simulation model of fuel injection control.

The sectional PID control model of rotational speed is built using MATLAB/Simulink, as shown in Fig. 7. The simulation model is built using MATLAB/Simulink, as shown in Fig. 8. The fuel injection mode is selected according to the judgment of the starting state of the system input. The output mainly includes the measured and target values of the rotational speed, the ambient pressure, the intake pressure, the current rail pressure and the applied load.

5. Common rail pressure control

The greatest advantage of high-pressure common rail system is that it does not depend on the speed and load. The compressibility and pressure fluctuation of the fuel itself can cause the residual change in the high-pressure fuel pipe s after each injection cycle. But the high-pressure common rail system can avoid that the final injection state is not consistent with the established law of plunger motion fuel supply [16], and the unstable injection phenomena occur. Therefore, to highlight the advantages of the injection system and improve the accuracy of common rail pressure control is the basis for optimizing other control variables.

According to the working principle of high-pressure fuel pump, it can be seen that the main way to control the intake of high-pressure fuel pump is to change the opening of proportional throttle valve. Therefore, how to adjust the proportional throttle valve in time and accurately is the key to realize the precise control of rail pressure. When starting, low speed and signal detection may cause time delay, so open-loop control must be used to quickly establish a certain common rail pressure by setting the opening of a proportional throttle valve as large as possible through a fixed frequency. When the rail pressure rises to the desired value, the control of the proportional throttle valve is switched to the closed-loop mode. Figure 9 shows that the target rail pressure can be obtained by looking up the target rail pressure chart according to the speed of diesel engine and the current fuel injection volume, and then the target value can be corrected by combining the state information of diesel engine, and the final target rail pressure can be obtained by limiting the change rate of the target rail pressure. In order to prevent rail pressure fluctuation and maintain the stability of common rail pressure under different working conditions, the actual value of rail pressure monitored by sensors is compared with the final target value, and the real-time closed-loop control of common rail pressure is completed by using adaptive neuro-fuzzy control algorithm. The simulation model of common rail pressure control is built using MATLAB/Simulink, as shown in Fig. 10.

Figure 9.

Common rail pressure control.

Figure 10.

Simulation model of common rail pressure control strategy.

Figure 11.

Injection timing control.

6. Injection timing control

The control accuracy of injection timing determines the combustion performance, and also greatly affects the fuel economy, power and emissions of diesel engine [17]. In the process of injection, the injection time should neither be too early nor too late. If the injection time is too early, the combustion burst pressure will be too high, which may prolong the ignition, increase fuel consumption, raise the negative power of diesel engine compression, and affect its life. If the injection time is too late, the fuel atomization effect will become worse and the after-burning phenomenon will occur, resulting in insufficient combustion and black smoke. At the same time, too high exhaust temperature will have a negative impact on the turbine. Therefore, the selection of injection timing must be accurate and appropriate.

In practical engineering applications, the effective method to complete accurate control of injection timing is the injection timing chart, which uses the method of adding the basic value $\theta_{b}$ of injection timing and the correction value $\theta_{c}$ . Starting with the monitoring speed and current injection volume, the fuel injection timing chart is checked to determine its basic value $\theta_{b}$ . Then the corrected value $\theta_{c}=\theta_{w}+\theta_{in}+\theta_{P}$ of the relevant environmental parameters is obtained from the relationship between the real-time state parameters such as cooling water temperature $T_{w}$ , intake temperature $T_{in}$ , intake pressure $P_{in}$ and the injection timing. The corrected target injection timing $\theta_{g}=\theta_{b}+\theta_{c}$ is shown in Fig. 11. The injection timing $\theta_{g}$ is expressed by the front angle of the top dead-end of the crankshaft rotation angle, and the angle is converted to time according to the rotational speed. The time interval $t_{c}$ is calculated by taking the first 30 degrees of the above dead-end as the standard [18]. Finally, the starting point of the operation of the solenoid valve of the injector is obtained. Finally, the simulation model of fuel injection timing control strategy is established using MATLAB/Simulink, as shown in Fig. 12.

Table 2
Function and purpose of multiple injection

Mode	The achieved effect	Fuel injection quantity	Fuel injection timing (CA)
The guided fuel injection	Reducing particle emission through premixed combustion	$<$ 5%	$-$ 70 $\sim$ $-$ 40
Pre-injection	Alleviating ignition delay of main injection, reducing NOx amount of harmful gas and combustion noise	$<$ 5%	$-$ 10 $\sim$ $-$ 40
Main injection	Main process of combustion, power output	$>$ 90%	$-$ 10 $\sim$ $-$ 2
Rear injection	Promoting diffusion combustion, reducing PM emission, improving emission temperature and promoting post-treatment by post combustion	$<$ 2%	10 $\sim$ 50

Figure 12.

Simulation model of injection timing control strategy.

Figure 13.

Ideal injection rate curve.

7. Fuel injection rate control

Fuel injection rate control can not only improve the power and fuel economy of diesel engine, but also reduce emissions and noise. During the whole injection process, high-pressure fuel is transported to the common rail system through the high-pressure oil pump and stored in the common rail pipe. Common rail pressure is not affected by the speed of diesel engine, and can maintain high pressure continuously. The lift of needle valve is controlled by solenoid valve to realize fuel injection. Therefore, under different working conditions, the injection rate and injection law of diesel engine can be completely independent and freely controlled.

According to the influence of injection process on combustion quality, the ideal injection rate control requires that the initial injection rate is expected to be very low, while the middle injection rate is relatively high and stable, while the later injection period can be cut off immediately without fuel leakage, that is to say, to achieve the “first slow and then urgency” injection effect [19], as shown in Fig. 13.

When injecting oil, it does not affect the formation of injection pressure. The injector is controlled by a high frequency solenoid valve. It can accurately control the opening and closing of the injector according to the starting point, pulse width and pulse interval of the solenoid valve. Four injection rate modes in a cycle are realized, namely, guided injection, pre-injection, main injection and post-injection. Table 2 shows the role and purpose of multiple injection.

8. Simulation analysis

8.1 Simulation of fuel injection control strategy

The injection quantity mainly depends on the injection pulse width and common rail pressure, which can be adjusted at any time. So the injection process is more flexible and changeable. The rail pressure does not change during start-up, and the injection volume is determined only by the injection pulse width. Therefore, when the rail pressure is 70 MPa during normal starting, the simulation results under 1.0 ms, 0.8 ms and 0.6 ms fuel injection pulse width are obtained, as shown in Fig. 14. It can be seen from the figure that when the speed is lower than 250 r/min, the speeds are equal under different pulse widths. In this process, no fuel injection is carried out and the engine is driven by the motor. When the injection pulse width is 1 ms, the diesel engine starts the fastest, but the overshoot of the simulation curve is the largest. When the injection pulse width is 0.6 ms, the starting speed is the slowest, and the speed after stabilization is slightly lower than the expected value. Therefore, the choice of starting fuel injection must be appropriate, not too large or too small. When too large, overshoot is also too large. When too small, starting is slow.

Figure 14.

Simulation results of starting under different fuel injection quantity.

Figure 15.

Change of injection pulse width.

Figure 15 shows that the starting effect is the best when the injection pulse width is 0.8 ms. At this time, the steady-state time is less than 2 s, and the fluctuation is very small. Therefore, keeping the rail pressure of 70 MPa unchanged, the change of pulse width during the transition to idle speed is observed in the starting pulse width of 0.8 MS. As shown in Fig. 5.6, during the period from 0 to 0.2 s, the pulse width is 0; when the speed increases to 250 r/min, the fuel injection begins with a fixed pulse width of 0.8 ms, and at this time the starting rail pressure is 20 MPa; when the starting time passes through 0.8 s, the speed is about 500 r/min, and the pulse width starts to decrease slowly and finally stabilizes at 0.5 ms, and the speed is also stable around 800 r/min, indicating that the engine is in idle condition at this time. The pulse width under this condition is selected to inject oil. From Fig. 15, it can be observed that the fluctuation of speed during starting process is small, about $\pm$ 8 r/min, and the time for reaching the steady condition is 1.2 s. During the transition period from starting to idling, the speed response is rapid and the fluctuation is close to 0 when the speed is stable. In summary, the results show that under the suitable injection pulse width, the control strategy of starting and idling is satisfactory.

8.2 Simulation of fuel injection timing control strategy

The simulation charts of diesel injection timing are shown in Fig. 16. As can be seen from the figure, with the increase of rotational speed, the injection timing begins to move backwards. The idle condition is reached in about 2 s. Because the speed and injection pulse width are fixed, and the injection timing (i.e. injection advance angle) is obtained from the MAP chart according to the speed and injection volume, so the injection timing does not change. When starting, the cooling water temperature and intake pressure are very low. The diesel engine needs to go through the cold start process, and the injection timing should be advanced, so the starting effect is better. After starting, the temperature and intake pressure of the diesel engine start to rise, and the injection timing moves backward under the idle condition after 2 s.

9. Bench test [20]

The diesel engine test bed and test equipment are shown in Fig. 17. The equipment used in the test process is shown in Table 3.

Table 3
Test equipment

Name	Type	Function
Control and measuring cabinet	EST2008	Measuring and controlling diesel engines
Hydraulic dynamometer	W series	Loading the diesel engine and absorbing its power
Fuel consumption meter	STYH-125	Measuring fuel consumption of diesel engines
Exhaust analyzer	DiGas4000Light	Measuring the diesel exhaust emissions of CO, CO ${}_{2}$ , O ${}_{2}$ , HC, and NO ${}_{\text{x}}$
Electronic control unit (ECU)	Self-developed ECU	Writing program, making calibration experiment and controlling the diesel engine
Debugging tool	DAP MinWiggle	Communicating, debugging and downloading the program between ECU and calibration software
Calibration software	INCA	Monitoring the software parameters in ECU and modifying the parameters online to realize the calibration of diesel engine

Figure 16.

Simulation characteristic chart of injection timing.

Figure 17.

Diesel engine test bed and test equipment.

The starting and idle test mainly verify the control of common rail pressure of diesel engine in open and closed-loop control under the starting and idle conditions. The common rail pressure control process under starting and idle conditions is shown in Fig. 18.

Figure 18.

Common rail pressure control process under starting and idling conditions.

When starting, the temperature of diesel engine is 20 ${{}^{\circ}}$ C, and the injection quantity is 30 mg/cyc. It can be seen from Fig. 18 that at the beginning of starting, the diesel engine is driven by the starter, and the rail pressure begins to increase after the speed reaches 250 r/min, and stops nearby for a while. This is due to the fact that it takes time for the high-pressure oil pump to deliver oil to the common rail pipe and to establish a certain pressure. When the rail pressure is equal to 20 MPa, the fuel is injected formally. Because of the combustion work, the diesel engine is separated from the starter and starts to rotate, and the speed increases rapidly. It takes only 0.3 s for the diesel engine to reach the starting pressure 20 MPa rapidly from the beginning. When the rail pressure is greater than 45 MPa and the speed is equal to the threshold speed 600 r/min of the open and closed-loop, the rail pressure control transits from the open-loop mode to the closed-loop mode, and the injection quantity also begins to decrease. When the diesel engine speed reaches 750 r/min, the diesel engine begins to transit to the idle condition. The idle speed is 800 r/min, fuel quantity is 10 mg/cyc and target rail pressure is 30 MPa. At this time, the rail pressure drops rapidly, and the idle speed value reaches 800 r/min after 0.4 s, and the rail pressure fluctuates slightly. It indicates that the closed-loop PID control mode begins to enter. The deviation of rail pressure fluctuation is into $\pm$ 5 MPa, the time taken for the rail pressure to stabilize from 0 to the target rail pressure at idle speed is about 1.25 s.

The transient condition test is mainly to verify the rationality of the corresponding control strategy by observing the changes of each control parameter (injection quantity, throttle, rail pressure, speed) under the acceleration and deceleration of diesel engine. The specific operation process is as follows. When the diesel engine is at idle speed 800 r/min, quickly press down the accelerator pedal and keep the opening of the accelerator unchanged. When the speed increases and stabilizes, release the accelerator and fall back to the idle condition again. The parameters of diesel engine under transient condition are shown in Fig. 19.

Figure 19.

Variation of various parameters of diesel engine under transient condition.

It can be seen from Fig. 19 that when accelerating, the throttle takes about 0.4 s from 0% to 23%, and then does not change. When the throttle is increased, the rail pressure increases from the idle rail pressure 30 MPa to 65 MPa in only 0.6 s, and the injection quantity also increases from 10 mg/cyc to 20 mg/cyc fast. The instantaneous increase of the throttle causes the injection quantity to increase sharply in a short time, and then decrease slowly. At this stage, the speed is still increasing. According to the rail pressure control strategy, the rail pressure will continue to increase. When the fuel injection quantity and speed are changing, the rail pressure control is complex, and the rail pressure change is complex. After the completion of acceleration, the speed is stable at about 2000 r/min, the fuel injection quantity is stable, the diesel engine is in a steady state, and the rail pressure value is stable at about 55 MPa. Compared with the simulation results [20], the response time is also less than 5S. But due to the sudden change of working conditions, the diesel engine itself vibrates greatly, the overshoot is greater than the simulation results, and the time to reach stability is longer.

When decelerating, the throttle will change quickly to 0%, the fuel injection quantity will change to 0 mg/cyc, and the speed will decrease. When decelerating to 800 r/min, the fuel injector will inject the fuel quantity 10 mg/cyc into the cylinder at idle speed, and the diesel engine speed will no longer decrease and will be stable at idle speed. At this time, because of the sudden drop of fuel injection to 0 mg/cyc, the speed decreases slowly, and the rail pressure also decreases. When the diesel engine reenters the idle condition, the fuel injection quantity is stable nearby 10 mg/cyc, the rail pressure decreases sharply, and a certain amount of fluctuation occurs. Finally, the rail pressure and speed remain at the idle value, and the maximum deviation of rail pressure is 10 MPa.

To sum up, it can be seen that the rail pressure responds rapidly to the changes of speed and fuel injection quantity, the control strategy of the system adapts to the sudden change of diesel engine working conditions, and the actual system control has good following performance.

10. Conclusion

(1)

The relationship among rail pressure, injection quantity, rotation speed, injection timing and the injection control strategy for the high pressure common rail electronic control injection system are studied and analyzed. Considering the change of working conditions, different control schemes are designed, the overall structure of the control strategy of the injection system is determined, and the simulation model of the control unit of the injection system is established in the MATLAB/Simulink software environment.

(2)

The simulation analysis and experimental verification of the diesel engine control strategy are completed, and the rail pressure control algorithm is embedded into the whole control unit simulation model. The simulation results show that the injection quantity and injection timing basically meet the system requirements under the starting and idle conditions, and the common rail pressure fluctuates slightly from the starting to idle and transient conditions, But the simulation results are consistent with the expected trend of rail pressure, and the overshoot of the control algorithm is less than that of the conventional PID algorithm. So it is more stable, faster in response.

(3)

The test results show that the time for fuel injection quantity, speed and rail pressure to reach the predetermined value is short during the starting process of diesel engine. When the starting pressure is reached, the opened and closed-loop control states for the rail pressure are switched in time, the maximum fluctuation at idle speed is only 5 MPa, and the time to reach stability is only 1.25 s. Under the transient condition, the rail pressure has a good response to the change of speed and fuel injection quantity, the control strategy of the system adapts to the sudden change of diesel engine conditions, and the actual system control has a good following performance. In the steady condition, the rail pressure has a great influence on the injection parameters. In the process of increasing the rail pressure, the injection advance angle and injection pulse width decrease nonlinearly, and the change trend of each parameter is normal. Thus it ensures good injection and combustion conditions.

(4)

Because of the simplification of the actual fuel injection system modeling and the measurement accuracy of the instruments used in the bench test, the error of the results is caused, but the trend of change is right. In this paper, the injection control strategy based on injection quantity, injection timing and injection rate is analyzed and studied. The corresponding control algorithm and multiple injection control technology need to be further studied.

Footnotes

Acknowledgments

This research was financially supported by the National Natural Science Foundation of China (NO. 51675148).

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