Design and Experimental Research of Electronically Controlled Common Rail System for Diesel Engines – Engineering Research Paper
ABSTRACT – The electronically controlled common rail system presents many advantages, because the injection timing and common rail pressure can be controlled independently. An electronic control unit(ECU) to realize control functions is developed, and serious experiments about the injection system are conducted.
The purpose of this project is to study the factors that affect injection characteristics. The results indicate that the injector has faster response when the common rail pressure rises, increasing the injection rate. One the other hand, the extension of control pulse width adds the valve open time, which increases the injection duration. The flexible injection characteristics can be acquired by controlling common rail pressure and pulse width coordinately.
With the increasing concern of the environment and more stringent government regulation on exhaust emissions, the reduction of engine emissions is a major research objective in engine development. In order to improve the engine’s emission performance, various technologies are adopted such as electronically controlled common rail system, clean alternative fuel, high efficient combustion organization, advanced turbo charging, exhaust gas recirculation(EGR) and advanced after treatment technology. The harmful emissions are generated in combustion process, so realizing ideal injection characteristics is one of the key aspects to improve combustion and reduce emissions.
Electronically controlled common rail fuel injection system has been accepted as one leading technology in diesel engines, and this system has the following advantages:
(1)The injection pressure and injection quantity can be adjusted and acquired flexibly. With high injection pressure independent of engine load and speed, the NOx and PM emissions can be minimized to meet the requirement of emissions regulation and optimize the engine performance.
(2)The common rail installed between oil pump and injectors functions as a reservoir, and the fluctuation of oil from oil pump is stabled in the common rail, so the disturbance between injectors is decreased greatly.
(3)Exact injection timing and injection duration can be realized with electronic control unit (ECU) over the entire engine load and speed range.
(4)It is easy to package this system to a traditional diesel engine without much change.
This paper discusses the configuration of an electronically controlled common rail system based on Caterpilar HEUI (Hydraulic Electronic Unit Injection) injector, and introduces the development of ECU hardware and software. Long tube method is used to investigate the characteristics of the fuel injection system, and the result shows how the common rail pressure and control pulse affect the injection characteristics, which provides the foundation of soft engine control.
2. SYSTEM OPERATION
Figure 1 is a schematic of the electronically controlled common rail system based on HEUI injector. This system is comprised of oil and fuel adjusting structure, HEUI injector, electronic control unit(ECU) and various sensor. Note this system incorporates two fluid circuits, fuel and oil. The fuel circuit provides fuel to the injector. The fuel transfer pump draws fuel from the fuel tank and delivers
it through a filter to the injector. System pressure is 0.2MPa typically. The oil circuit generates high pressure oil needed for HEUI injector. Oil is drawn from the engine oil sump and supplied trough the filter to the high pressure pump. The high pressure oil pump is an axial piston pump, and it raises the system’s oil pressure to the actuation pressure level typically between 4 and 23MPa. This high pressure oil flows through lines into the oil common rail located near the injector. The rail stores the oil at actuation pressure adjusted by the rail pressure control valve (RPCV).
The ECU reads engine sensors to identify the engine’s current operating condition and produces the waveform required to drive the HEUI injector solenoid. When the ECU gives a control pulse, the solenoid is energized moving the poppet valve from the lower to the upper seat. This action admits high pressure oil to the spring cavity and the passage to the intensifier piston. As the piston and plunger move downward, the pressure of the fuel below the plunger rises, and the nozzle needle lifts allowing injection to occur. Injection continues until the solenoid is de-energized and the poppet moves from the upper to lower seat, blocking oil flow. The plunger return spring returns the piston and plunger to their initial position. As the plunger returns, it draws replenishing fuel into the plunger chamber across a ball check valve. Since the ratio of areas between the intensifier piston and plunger is 7, the actual injection pressure can reach 30~150MPa. Pre-Injection Metering (PRIME) is HEUI’s rate shaping device. When the plunger moves downward, pre-injection occurs before main injection through a precious spill hole. Figure 2 shows a representational cross section of the HEUI injector.
3. ECU STRUCTURE
In electronically controlled common rail system, the major operation parameters are injection quantity and injection timing controlled by High-speed Solenoid Valve (HSV) in the injector. The basic injection quantity is determined by common rail pressure, engine speed and pedal position signal, and other supplementary signals such as atmosphere pressure and oil temperature make modification to the basic injection quantity. A closed loop control strategy is used to stabilize the speed. Figure 3 is the block diagram of speed control principle. The ECU consists of microcontroller, input circuit, HSV drive circuit and display module.
The central part of ECU is the microcontroller (MCU). Motorola’s serious MCU has become an industrial standard in engine control. Both GM Inc. and Caterpillar Inc. adopted Motorola’s MCU in their electronically controlled engine products and made remarkable result. In this paper, the high-performance 8-bit MC68HC908 GP32 MCU is employed to accomplish control functions, and this chip owns the following advantages:
(1)32 Kbytes of FLASH memory with in-chip programming capabilities and in-chip programming firmware for use with host personal computer which does not require high voltage to entry
(2)8 MHz internal bus frequency and clock generator module with 32-KHz crystal compatible Phase Lock Loop (PLL)
(3)Two 16-bit, 2-channel timer interface modules with selectable input capture, output compare, and Pulse Width Modulation (PWM) capability on each channel
(4)8-channel, 8-bit successive approximation analog-to-digital converter (ADC) to satisfy multi-input requirement
(5)System protection features such as optional computer operating properly (COP) rest, low-voltage detection with optional rest and illegal address detection with rest
3.2 Input circuit of ECU
The top dead center (TDC) signal of cylinder must be detected first for precious timing control. A signal plate is installed on the rim of flywheel. When the engine piston arrives at the top dead center position, the signal plate moves faced to a magnetoelectric sensor, and then the TDC signal comes into being. A reshaping circuit is used to regular this signal to supply MCU for input capture. A rotary timing wheel is attached to the engine crank shift, and produces 720 standard square waves every cycle. The engine speed can be calculated in MCU using these signals, and coordinating with TDC signal, the exact moment of HSV operation is determined with a tolerance of less than 0.5 crankshaft angle. The analog signals such as oil pressure, oil temperature and pedal signal are induced to ECU’s AD channel through an amplify circuit and a modulate circuit.
3.3 HSV drive circuit
The HSV in injector undertakes the task of injection control. Fast-open is needed to ensure exact injection timing and forming high pressure quickly, and fast-close is required to satisfy shutting off quickly and unloading stably. The dynamic response characteristics of the injector affect the main performance of injection system directly. In order to create strong force to overcome the resistance of return spring, a 60V high-voltage is adopted for the solenoid, and the Pulse Width Modulation (PWM) control method is used to keep the holding current. Figure 4 shows the principle of drive circuit. When the MCU gives an injection control pulse, the injector gets 60V high voltage, and the current in solenoid rises quickly according to the system resistance and inductance. 1ms later, the circuit turns to PWM mode, and the average current of PWM pulse keeps the valve open with low power dissipation. The maximum open current is 4 ampere and the average holding current is 1.5 ampere. This operation mode ensures the injector to run reliably in a long term.
3.4 Display circuit
In order to acquire engine real-time status, DM-162, a character liquid crystal diode (LCD) display chip, is used to monitor the engine speed, injection timing and injection pulse width. The small-size DM-162 expends low power, and stores 160 different array character graphs in its internal character generator. It has convenient interface to MCU, and it is easy to accomplish the display function by giving the corresponding character storing address in software.
3.5 ECU software structure
The control software is edited by Motorola assembly language, and it includes the following modules.
(1)Injection control module: On the condition of the common rail acquiring stable pressure from pump, the MCU calculates the injection quantity on the basis of engine speed and pedal position signal. After modification, the MCU produce a corresponding pulse to the HSV drive circuit. A closed loop control algorithmic is used to adjust the injection quantity to stabilize engine speed. The injection timing in different engine operation mode is acquired by software table-look-up method.
(2)Data acquisition and process module: This module scans the signals from various sensors, and use software filtering to eliminate false signals due to engine disturbance.
(3)Display module: According to the display demand, the engine speed, the crankshaft angle of starting injection and injection pulse width are showed in different position of the LCD screen. In every software cycle, the data refresh to keep the real-time engine status.
(4)Malfunction process module: This module judges whether the signals from sensors exceed its limited value. When a serious error occurs, the MCU turns off for safety.
4. EXPERIMENTAL RESULTS AND DISCUSSION
After finishing the development of electronically controlled common rail fuel system, long tube method is used to measure the injection characteristics of HEUI injector. A long tube mechanism is attached to injector fuel outlet for realizing this method, and a high-precision pressure sensor is used to gather the fuel pressure information in the long tube. The injection rate and injection quantity per-cycle can be calculated according to pressure wave transmission theory. In practical treatment, formula (1) to (4) are applied for calculation.
The injection quantity on a time basis for one cycle under different common rail pressure and control pulse width are shown in figure 5. The relationship between them can be seen from this figure.
(1)Under a fixed common rail oil pressure, the injection quantity increases proportionally with pulse width in a certain rang. Under a fixed pulse width, when the common rail oil pressure rises, the injection quantity increases correspondingly.
(2)The control pulse width and common rail oil pressure determine the opening properties of HSV. When the pulse coming from MCU is less than 0.8ms, the HSV can not get enough response time for action. If the oil pressure in the common rail is fairly low, the intensifier piston accelerates slowly, therefore the fuel can not acquire enough pressure to open the nozzle in very short time. For example, when the common rail oil pressure is 5MPa, the injection does not occur until 1.5ms-width pulse exerts on the solenoid.
(3)The volume of plunger chamber determines that the maximum injection quantity in one cycle is 88mg/cycle (66ml/min). With the increase of common rail oil pressure, the pulse width to achieve the maximum injection quantity decreases correspondingly.
4.2 Effect of pressure and pulse width on pre-injection
Pre-injection can adjust the initial portion of fuel delivery to control the amount of fuel delivered during ignition delay and main injection. It makes the fuel in main injection to burn quickly and stably. This process modifies the heat release characteristics and is beneficial in achieving low emission and noise levels. Figure 6 shows the effect of pressure and pulse width on pre-injection ratio.
(1)Under a fixed pulse width, the pre-injection ratio presents decline tendency with the decrease of the common rail pressure. However, the over-low common rail pressure makes pre-injection ratio rising. This result appears because the flow duration of the spill hole in the PRIME is quite short under a higher common rail pressure, and this makes the pre-injection quantity keep unchanged, so the pre-injection ratio decreases with the total injection quantity increasing. When the common rail pressure changes at a lower level, the spilling time increases apparently, therefore the proportion of pre-injection quantity adds correspondingly.
(2)Under a fixed common rail pressure, the extension of control pulse width causes the increase of total injection quantity before it reaches the maximum value, accordingly the pre-injection ratio decreases.
4.3 Effect of pressure on injection rate
Figure 8 presents the injection rate shape in various control pulse width when the common rail pressure maintains 10MPa. The injection duration and injection quantity increase significantly with the extension of pulse width, but the injection rate keeps invariant mainly on the whole. To conclude from figure 7 and figure 8, the main factor of affecting injection rate is common rail pressure, and the pulse width is the main factor of affecting injection duration. On the basis of the result, the injection characteristic can be controlled flexibly to satisfy the practical engine operation.
The electronically controlled common rail fuel system opens many opportunities for improving engine performance, and the injection control is the most important part of this system. Through the development of control system and experimental research, the following conclusion can be obtained:
(1)The exact injection control is achieved by applying MC68HC908 GP32 microcontroller to the ECU. Long tube method is a convenient and precious means to measure the injection process of fuel system, and the injection characteristics acquired from this method provides the foundation of soft engine operation.
(2)The structure of the injector determines the maximum injection quantity in one cycle and the minimum pulse width to open the valve.
(3)The desired injection characteristics can be realized by controlling the common rail pressure and pulse width coordinately. The common rail pressure makes major effect on the injection rate, while pulse width makes major effect on the injection duration.
(4) The injection rate and injection duration of electronically controlled common rail system are independent on the engine load and speed.
This study was financially supported by the National Natural Science Foundation of China (No.50136040 and No.50276035). The authors wish to express their gratitude to Li Shuze and Lu Xingcai for their contributions to this paper.
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