Research into Rail Pressure Control Strategy of Multi-stage Closed Loop based on Injector Pressure Relief for Diesel

Transcription

1 Research into Rail Pressure Control Strategy of Multi-stage Closed Loop based on Injector Pressure Relief for Diesel 1 WEI Xiong, 2 MAO Xiaojian*, 3 ZHU Keqing, 4 Feng Jing, 5 JIANG Zuhua, 6 WANG Junxi 1,First Author School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 224, China. weixiong @126.coml *2,Corresponding Author School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 224, China. mxj_sjtu@163.com Abstract The development of electric control Common Rail injection (CRI) diesel system lags behind in China. Rail pressure control of diesel engines is very important and requires high control accuracy, and increasingly stringent emission regulations exacerbate this requirement. Taken the fact that domestic existing rail pressure control strategies cannot meet the higher precision of diesel engines as the background, the paper proposes a brand new rail pressure control strategy-cri multi stage closed loop control strategy based on injector pressure relief and improves its control precision from various aspects. In the control of fuel into common rail pipes, it studies and designs multi stage closed loop control algorithm for IMU (Inlet Mass Unit) based on actual rail pressure and drive current. In such a way, it solves current accuracy problem and improves rail pressure control precision. On the other hand, in the control of fuel flowing out of common rail pipes, it designs an injector pressure relief control algorithm based on self adaptation. Without adding any auxiliary equipment, the software solves the control lagging and further improves rail pressure response. Test results show that, the control strategy has a very high precision under full working conditions, and steady-state deviation is within 5bar while transient deviation is within 2bar, which can meet the higher control accuracy requirement of diesel engines. Keywords: Common Rail Injection (CRI), Electric Control Diesel Engine, Control Strategy, Multistage Closed Loop, Injector Pressure relief 1. Introduction The injector pressure is closely related with diesel emissions, and flexibly control of it is one of the effective measures to improve diesel emission [1]. Under a high load, Particulate Matter (PM) and NOx emissions can be compromised through improving injection pressure and timing delay; in small loads, to reduce the premixed burning ratio, NOx emission can be reduced by injection pressure reduction[2-3]. Therefore, the optimal injection pressure is different under various working conditions, and electric control CRI shall control injection pressure by different working conditions. On the other hand, CRI uses time-pressure fuel measurement, and only by controlling the fuel pressure inside the common rail pipes accurately and reducing pressure fluctuation, diesel cyclic injection could be precisely controlled to provide a solid basis for improving driving performance[4][5][6]. Hence, to ensure a high precision of rail pressure control under full working conditions is very important to diesel performance and emission. Local CRI diesel car start late and so far there is no substantive progress yet. Compared to commercial vehicles, diesel cars usually requires low injection and are more sensitive to fuel consumptions, and even for small fuel of commercial vehicles, they can have a dramatic change on car performance; but diesel cars most of the time are applied in bustling city streets with varied working conditions and higher driving and comfort requirements which makes rail pressure control of diesel cars more complicated; in addition, National IV the above law poses a more restrict emission, injection precision and pressure requirements, which aggravate the rail pressure control difficulty[7][8]. Under such background, the paper proposes a brand new rail pressure control strategy, and from multiple aspects it improves the control accuracy to meet higher precision control requirement of diesel cars. 2. GD CRI system International Journal of Digital Content Technology and its Applications(JDCTA) Volume7, Number14, October

2 Research into Rail Pressure Control Strategy of Multi-stage Closed Loop based on Injector Pressure Relief for Diesel GD (Green Diesel) electric control CRI system is illustrated in figure 1 and it mainly consists of ECU, high pressure pump, IMU, common rail pipe, injector and pressure sensor High pressure pump IMU Common rail High pressuer fuel Pressure sensor injectors High pressure pump Rail pressure feedback IMU Driving Injecting driveing Other sensors input Figure 1. The sketch of GD high pressure common rail injection system By adjusting the solenoid valve (IMU), the high pressure pump accurately measure low pressure fuel consumption, and compresses the fuel into the high pressure common rail to get rail pressure and injection by ECU calculation of working conditions. The internal structure and hydraulic pressure cycle are as shown in figure 2. The pump integrates a vane furl delivery pump to produce low pressure fuel, a high pressure fuel pump with one inner cam ring and one pair of diametrically opposed pistons. The cam is driven by camshaft rotation and two opposed pistons sit inside the pump head. The cam ring has 4 convex, and as the camshaft rotates 1 circle, the high pressure oil pump can realize the 4 oil pumps. The quantity of high pressure fuel pumped by the high pressure pump is provided by the fuel delivery pump and it is controlled by a low pressure IMU. According to the difference between the actual and target rail pressure, IMU automatically adjusts the valve opening to allow proper fuel compressed into common rail pipes. IMU Delivery Pump High Pressure Pump Figure 2. The hydraulic sketch of fuel pipeline for common rail system IMU is an open proportional control valve, and under a certain input pressure, the flow-current is as shown in figure 3: the less current, the bigger flow. IMU is controlled by ECU with Pulse Width Modulation (PWM) driving manner. ECU adjusts IMU current by timely changing driving duty ratio and frequency, so that it accurately controls pumping fuel quantity and rail pressure. 85

3 Fuel flow(l/h) Current(mA) Figure 3. The flow-current characteristic of IMU 2.2. Common rail pipe The common rail pipe is used to store and distribute high pressure fuel. Attached to it include one high pressure fuel pump adaptor, rail pressure sensor and 4 connectors with restrictors to provide fuel to injectors as shown in figure 4. Rail pressure sensor Connect to injectors Figure 4. The outline of common rail The rail pressure sensor is used to monitor rail pressure. It transmits signals to ECU which will control rail pressure and fuel injection accordingly Injectors Fuel inlet Control Valve needle (a) Valve and needle closed, no injecting (b)valve (c) Valve and (d) Valve closed needle needle opened, closed needle opened, no injecting opened, stop injecting injecting (e) Valve and needle closed, no injecting Figure 5. The working process for electronic injector The working principle of injectors is as shown in figure 5. When the electromagnet is off, the control valve connected with it is closed, and by inlet throttling orifices high pressure fuel is located in the top face of the hydraulic piston which disables needle hoist. When the electromagnet is on, it drives the control to a quick open which reduces pressure inside the control chamber, and then the needle opens for injection. After power off and under spring forces, the balance control valve closes and 86

4 pressure on the piston top face increases, and the needle falls onto the seat, so injection stops. In conclusion, the controlling valve controls the pressure increase or discharge of the control chamber, injectors can indirectly control fuel injection. When the needle hoist is required (injection starts), the control valve is opened in order to transmit the control pressure to fuel discharge route. When the needle is closed (injection closes), the control valve closes and pressure inside the control chamber returns to normal. According to the working principle of injectors, in some special working conditions of diesel engine, in order to improve the stability and response of rail pressure control, it uses fuel return characteristics of the injector and gives assistant control of rail pressure. During pressure relief, injectors are imposed to a drive pulse width which is small enough which can open the control valve of the injector (>15us) but not enough to hoist the needle valve (<2us). This can return partially high pressure fuel from injectors to fuel tank without injection to relieve the pressure ECU Motorola PowerPC561 as Micro Control Unit(MCU); Dual voltage solenoid valve driving circuit with; Self-protection and diagnostic circuit; Software designed by the MATLAB/Simulink, then auto-code-generated by the Targetlink, finally compiled and downloaded by Cygwin and BDI2. 3. Rail pressure control strategy The basic principle of rail pressure control is according to the difference between the actual and target rail pressure, fuel coming and flowing out of the common rail pipes reaches a dynamic balance to stabilize the rail pressure. Therefore, the trick is to balance two fuels coming and flowing out of the common rail pipe IMU multi stage closed loop control Fuel quantity pumped from the high pressure fuel pump each time is controlled by IMU. The basic principle is to adjust valve opening through IMU drive and compresses proper fuel into the common rail pipe to stabilize rail pressure according to the difference between the actual and target rail pressure. Target pressure Command Target First current current PID transient Pre-control Second PID PWM Rail system IMU current feedback Real pressure feedback Figure 6. The control of IMU driving based multi stage close loop control The traditional rail pressure control method is based on the deviation of actual and target rail pressure and produces a direct IMU drive duty ratio, while the variation of IMU characteristics is ignored. In the process of practical control, the characteristics of IMU will change by drives and environments which will lead to a current accuracy control failure thus it affects the accuracy of rail pressure control. In order to eliminate the influence of IMU parameters, the paper designs a multi stage closed loop control algorithm as shown in figure 6. The control flow is briefly illustrated as follows: at first 1st level closed loop calculates IMU command current according to the deviation of target and the actual rail pressure, and then get IMU target drive current by a transient compensation to improve the 87

5 response of the transient working condition of rail pressure, and at last 2nd level closed loop calculates final IMU drive duty ratio according to the deviation of target and the actual current to achieve the multi stage closed loop rail pressure control First current closed loop control Based on IMU flow-current characteristics, rail pressure control is essentially the control of IMU drive current. To improve rail pressure control precision, the paper designs a current closed loop control model with feed forward control as shown in figure 7: closed loop trim plus feed forward currents produce the final output target current. Figure 7. The close-loop control of dictating driving current The feed forward current is a function of diesel speed and command injection quantity showing the effective pumped fuel demand under diesel working conditions. It has the following advantages: In various operating conditions it provides a closed-loop drive current for the closed-loop controller to improve the control precision and response; It provides reference data for common rail system fault diagnosis Transient pre control Speed(r/min)/Fuel (mg)/pressure (bar) Rail pressure overshot Engine speed Real pressure Target pressure Fuel injection Time(s) Figure 8. Rail pressure control at urgent acceleration and deceleration condition In the process of driving, the rapid deceleration and acceleration may appear frequently. In the diesel engine acceleration conditions, in order to improve the speed response to the throttle and reduce the emission of soot and particles, target injection pressure and injection quantity usually increase significantly in a short period of time. In the diesel deceleration, throttle suddenly closes and ECU will completely stop the fuel injection, and meanwhile the target injection pressure will also decrease significantly in a short time. Therefore, during the acceleration and deceleration there is also a significant change of demand rail pressure and injection. On the other hand, because of the hysteresis of rail pressure control loop, rail pressure will overshoot as shown in Figure 8. According to the 88

6 characteristics of acceleration and deceleration, the paper designs the rail pressure transient pre control algorithm to improve its transient response. Figure 9. Pre-control of transient condition The transient pre control is through inspecting the command injection change to determine the diesel engine enters rapid acceleration or deceleration, and the control logic is as shown in figure 9. The correction current will be calculated by command fuel change ratio, and the greater the rate of change, the more correction. A correction current is equal to two times injection differences multiplied by the current compensation coefficient (a function of diesel speed and the command injection F (n, q)). In addition, in order to improve the smoothness of control transitions, when being out of control, the correction current will transit to by low pass filtering Second PWM closed loop control After receiving IMU target current, it enters PWM closed-loop control to obtain IMU drive signals. Because the ECU current control is by adjusting IMU drive frequency and duty ratio, the drive voltage and frequency on IMU ends in the control process are in a dynamic stabilizing process. In addition, the electrical characteristics of IMU is a nonlinear inductance whose inductance L is not only related to its physical properties, such as the displacement of armature coils, temperature change and the aging degree of coils but also changes with the drive current and frequency. Therefore, in the control process of rail pressure, IMU has a complex impedance characteristic. So considering the complex impedance characteristics of IMU the paper obtains IMU drive impedance by closed loop algorithm based on actual and target current bias, and according to drive voltage, it calculates IMU drive duty ratio to achieve closed loop control of IMU drive current. IMU impedance calculation procedure is shown as figure 1: the coil drive impedance consists of coil basic impedance and impedance trim, in which the former is a basic nominal constant determined by IMU physical property; the latter is calculated by target drive current and actual feedback current. Figure 1. Close-loop control of IMU driving 89

7 The detailed calculation logic of IMU drive frequency and duty ratio is shown as figure 1. In order to avoid IMU working in the resonance region which may affect the control precision of the rail pressure, the driving frequency is designed for one-dimensional pulse spectrum function of diesel speeds. In addition, in order to improve the control precision of driving current, the duty ratio compensates the voltage drop caused by ECU circuit impedance during calculation Closed loop controller Computer control is a kind of sampling control, and only according to the deviation sampling time value it can calculate the control quantity. It is required to discretize integral and derivative items, and the expression PID is as follows: n j x( n) K e( n) K e( j) K [ e( n) e( n 1)] p i In which: n stands for sampling serial number, n=, 1, 2... ; x(n) is the computer output of the n sample time; e(n) is the input deviation value of the n sampling time; e(n-1) is the input deviation value of n-1 sampling time; Kp is a proportional constant; Ki is a integral constant; Kd is a differential constant Injector pressure discharge control IMU is based on the actual rail pressure feedback control signals, and only the rail pressure changes, IMU will act correspondingly to eliminate the variation. Therefore, it has a certain lag. According to the working principle of injector and the injection characteristics, the paper analyzes that rail pressure controlled is assisted by common rail flowing out fuel, and it eliminates IMU control delay and improves rail pressure control precision. The injector pressure relief control is applying a certain frequency drive signal to the injector under specific working condition and its strength is not enough to open the needle (<2us) but just right to open the electromagnet valve to achieve an fuel return (>15us), without affecting the normal fuel injection, it assists the rail pressure control by increasing the amount of high pressure fuel returned to fuel tank. The actual effect of the pressure relief drive signal is as shown in Figure 11, and the high frequency pulse signals between injections are the pressure relief drive signals. d Current(A) Driving current injection driving Cam speed signal relief driving Time(s) Figure 11. Relief driving signal of injector voltage(v) Pressure relief enable control Whether use injector pressure relief assistant control depends on rail pressure status and diesel working conditions. Regarding car diesel working characteristics, the paper divides pressure relief control into 4 types: joint, relief, off and transient. Each feature and function is illustrated as follows: 9

8 Joint status: enable in small amount of fuel and injection stops condition. Such as the max speed empty load and deceleration condition, and pressure relief of the injector enables IMU work in its ideal range which is helpful to improve control precision and stability. Relief status: the target rail pressure decreases sharply. Because of the lag of IMU closed-loop control, the actual rail pressure will lag behind the target rail pressure changes. At this time, the fuel injector pressure control is introduced and improve actual rail pressure companion by increasing fuel quantity. Off status: apply to over pressure or engine stop conditions. The system stops providing fuel for common rail pipes meanwhile control injection pressure relief to decrease the rail pressure rapidly. Transient status: under a sharp decrease of fuel injection, rail pressure relief control can offset the decreased injection fuel and avoid rail pressure overshoot to improve transient control precision Pressure relief drive frequency To achieve an ideal pressure relief, the best injector pressure relief driving frequency shall be specified according to different relief requirements. Pressure relief frequency calculation of each status is shown as in figure 12, and the final relief frequency is basic plus correction frequencies. The detailed calculation is as follows: Off status: basic frequency is nominal constant, and correction frequency = deviation between max rail pressure value and actual rail pressure * overpressure weighing coefficient K3. Relief status: basic frequency is nominal constant, and correction frequency = deviation between target and actual rail pressure * rail pressure difference weighing coefficient K1. Joint and transient status: the basic frequency is based on a function of diesel speed and command injection, reflecting pressure relief requirements of each working condition, and correction frequency is deviation of target and actual rail pressure * weighing coefficient K2. Figure 12. Calculation of discharge driving Self adaptation of pressure relief pulse width According to the principle of pressure relief, the drive pulse width shall be proper, not too big, and not too small. Being too much will open the injector needle to produce injection and if too little it will not open injector control valve for fuel return. Therefore, the relief drive pulse width is important and the control will not be closed to the actual effect of the rail pressure, but also affects the engine performance and emissions. However, with the extension of fuel injector use, not only the solenoid valve will be aging, and injector precision couplers will have a certain degree of wear which leads to the injector actual effect change and affects the minimum pulse width. Therefore, the relief pulse width shall be self adapted, that is, vary by injector status. Therefore, the paper designs pressure pulse width adaptive control based on working conditions, and the control flow is shown as figure 13. Test minimum pulse width under deceleration conditions 91

9 gets relief pulse width. When diesel is in deceleration, the self adaptive drive pulse width starts drive, and it gives a certain initial driving output, waiting getting an increasing step of the pulse in delay and tests speed change rate; if the rate is in the range of normal deceleration change, the drive pulse width repeats the above testing after a step increase; when test shows the speed change rate is near to, the system believes a injection is produced under such pulse width and keeps the previous one as relief pulse width, and the self adaptation is over, drive width is reset. Self adaptation of drive pulse width can effectively improve pressure relief precision and reliability. Enter release driving self-adaptation N Judge if deceleration state without fuel Y Driving pulse initiate Last diving pulse + step Delay for calculating driving step N Monitoring engine speed: Speed variation < certain value? Storage last diving pulse and clear driving Y 4. Tests and verifications Figure 13. Self-adapted control flow of discharge driving The paper conducts series of tests and verifications of control strategies on YC4W11-4 diesel matching GD high pressure common rail injection system. Figure 14 is the rail pressure transient control process under rapid deceleration conditions. In the accelerator release, the system detects injection change rate lower than the enable threshold, and when pre control enable starts, it immediately compensates drive current and avoids rail pressure overshoot; when exit the control system, a compensation current transits to by a certain filter coefficient. Transient control partially offsets the lag of closed loop control and it improves the transient response of rail pressure. End speed(r/min)/pres sure(bar)/ current(ma) Engine_speed Rail pressure Trim_current Fuel_injection Enable_flg Time(s) Figure 14. Control process of pre-control in deceleration Fuel(mg) 92

10 Figure 15 shows pressure relief control process under each condition. Initially the diesel is operating stably in idle speed of 75 ± 5rpm, and when in the late acceleration for speed adjusting, the injection falls rapidly and pressure relief can work to avoid the rail pressure overshoot ( Transient demand). Subsequently, the diesel engine work stably in 28rpm and fuel injection is less than 1mg/str. In order to improve the stability of rail pressure control, fuel injector pressure relief enables ( Joint demand). In the diesel deceleration conditions when the system detects actual rail pressure lagging behind target value to a certain amount, injector pressure relief enables and it can improve its rail pressure companion ( Relief demand). Speed(r/min)/Pressure (bar) Engine_speed Fuel_injection Rail_pressrue Enable_flg Transient Relief demand demand Joint demand Time(s) Figure 15. Release control in different working condition As shown in Figure 16 is the adaptive control process of pressure relief drive pulse width. In third second diesel engine decelerates, and self adaptation of pressure relief drive pulse width starts by an initial 1us width. The rate of change of speed detection in delay waits to acquire the pulse width increase step, if the rate of change is in the deceleration range; the drive pulse width is automatically incremented after a step. When the pressure relief incremental pulse width is 2us, the speed change ratio tends to be equal to. The system believes an injection is made under this pulse width and keeps the previous 19us as the pressure relief pulse width. The adaptive control is over and driving pulse width is reset Fuel(mg) Speed(r/min)/pressure (bar) Engine_speed Fuel_injection Rail_pressure Release_pulse Fuel(mg)/Pulse(us) Time(s) Figure 16. Control process of self-adapted control for discharge driving pulse wide Table 1. Each strategy for improving rail pressure control in deceleration condition Control strategy Pressure overshot-bar Spend time for stable-s Returned fuelmg/str Single close loop Multi close loop Multi close loop + pre-control Multi close loop + pre-control + Release Rail pressure control result in Bosch system Rail pressure control result in Delphi system / / 93

11 Table 1 is the test result of each rail pressure control strategy improving pressure control effects under rapid decelerations. Results show that after these measures, the rail pressure overshoot is reduced from 1bar to 2bar and the stable time is shortened from 6.2s to 2.3s. Among the measures to reduce the rail pressure overshoot, the most obvious improvement is pre control and pressure relief control. In improving rail pressure stability, multi stage closed loop and pressure relief control are the most effective. By using these measures, the rail pressure control result of GD system is better than Bosch and Delphi system. Figure 17 are the test results of rail pressure control under different loads and speed conditions. Test results show that, in the whole diesel engine operating range, the deviation between rails pressure actual and target values is very small, and the steady state deviation is within 5bar while transient deviation is within 2bar, which meets higher rail pressure control precision requirement of diesel cars. Pressure(bar) Real_pressure Target_pressure Engine_speed Summary and conclusion Time(s) Figure 17. Experimental result of rail pressure control 1. The paper proposes a multi stage closed loop control strategy based on pressure relief, improves rail pressure control accuracy from various aspects. In the fuel control into the common rail pipes, it designs a multi stage closed loop control algorithm based on rail pressure and current, achieves precise control of IMU drive current and enhances rail pressure control precision; it also builds transient pre control models of rail pressure and improves its transient response. 2. In the fuel control flowing out of common rail pipes, it designs injector pressure relief control algorithm, avoids IMU control lagging and further improves transient responses. Self adaptive control of the drive pulse width ensures the accuracy and high reliability of rail pressure control. 3. Through the research work of the paper, the rail pressure transient deviation of target diesels reduces from 1bar to 2bar while the steady-state is within 5bar which provides a possibility of higher injection pressure and fuel precision of diesel cars in China and is very helpful for low emission machines and vehicles manufacture. 4. Under existing hardware as well as price constraints of market demand, the paper carries out an optimal design of control strategy software to compensate actuators so that existing systems can have the potential to perform to the extreme. This method saves the development and manufacturing cost of the entire system and adapts to the scientific research and market demand of our current electric control high pressure common rail system. 6. References [1] Zhang Mei-juan, You Li-hua.Study on the Control Method of Injection Pressure of Electronically Controlled High-Pressure Common Rail Fuel Injection System. Journal of JiangNan university(natural science edition), 27,6(4):15~19. [2] HAO Yong, SUN Jian, JI Xue-zhi, WANG Qi-feng. Current Status of Light-duty Vehicle Diesel Engine Technology in China, INTERNAL COMBUSTION ENGINE & POWER PLANT, 29(4): 24~28. Speed(r/min) 94

12 [3] Sun Zhong-qiang. The condition and development of the diesel engine for car. DIESEL ENGINE, 25(5):3~34 [4] ZHU Ke-qing. Experiment-based software optimization for multiple injection control in a common-rail system, Proceedings of the Institution Mechanical Engineers, Part D [J]: Journal of Automobile Engineering. 28, 222(1):1717~173. [5] YOU Li-hua, AN Wei, ZHANG Mei-juan. Control method Based on Neural-Network for rail Pressure control of Common Rail System in Diesel Engine. CHINESE AGRICULTURAL MECHANIZATION, 29, (6). [6] [JIN Jiangshan, PING Tao, LING Lixun. Technical Research on Common Rail Pressure-Control of High-Pressure Common Rail Fuel Injection System for Diesel Engine. DIESEL ENGINE, 26, 28(3):59~63. [7] LIU Zhen, ZHANG Xiao-feng, ZHAO Yin-wei. Experimental study of common rail pressure control based on spilling valve, JOURNAL OF NAVAL UNIVERSITY OF ENGINEERING, 21, 22(4): 41~43 [8] SONG Wen-fu, LI Jie-hui,ZHAO Zhe, LI Zhong-hai. Simulating Study of Injection Characteristics of Common Rail Injector, TRACTOR & FARM TRANSPORTER, 29, 36(3):16~2 7. Notation CRI ECU GD IMU mg/str MCU PM PWM r/min s us Common Rail injection Electronic Control Unit Green Diesel Inlet Mass Unit milligram per stroke Micro Control Unit; Particulate Matter Pulse Width Modulation rotation per minute second microsecond 95