Talking about Diesel High Pressure Common Rail Technology

First, high pressure common rail technology

Let us first understand the limitations of traditional diesel engine fuel injection systems:

The working process of the traditional diesel engine fuel injection system is: After the diesel engine raises the oil pressure through the high-pressure oil pump, it then passes through the injector according to a certain timing of fuel supply and the amount of fuel to be injected into the cylinder combustion chamber. During the fuel injection process, there is a secondary fuel injection due to pressure fluctuations. Due to the fact that the secondary fuel injection cannot be completely burned, smoke and hydrocarbon emissions are increased, and fuel consumption is also increased. In addition, the residual pressure in the high pressure fuel line changes after each injection cycle, resulting in unstable injection, especially in the low rotation speed range. In severe cases, not only the injection is uneven, but also intermittent non-ejection occurs. phenomenon.

With the development and progress of engine automatic control technology, in order to solve the fuel injection combustion defects caused by changes in the diesel fuel pressure, modern diesel engines adopt a high pressure common rail electronic fuel injection technology, so that the performance of the diesel engine has been fully improved.

After the diesel engine has been injected by mechanical injection, supercharged injection and ordinary electric injection, common rail high pressure injection has appeared in recent years. Common Rail EFI technology refers to the pressure build-up, injection pressure control, and injection in closed-loop systems consisting of high-pressure oil pumps, pressure sensors, and electronic control units (ECUs), as compared to common injection systems. Oil processes are independent of each other and can be flexibly controlled. It is a high-pressure oil pump that delivers high-pressure fuel to a common rail (Rail). The oil pressure in the public supply pipe can be precisely controlled so that the pressure of the high-pressure pipe is independent of the engine speed, which can greatly reduce the diesel engine's pressure. Supply pressure varies with engine speed.

In addition, the high-precision components of the common rail fuel injection system require high surface machining quality, high geometric accuracy, and special requirements. The processing of the components is micron, sub-nanometer, and represents the highest level of machining in the machinery manufacturing industry. 
Second, the composition and working principle of high pressure common rail system
2.1, high pressure common rail injection system composition 
 

The high pressure common rail injection system is mainly composed of high pressure oil pump, common rail tube, electronically controlled fuel injector, various sensors and electronic control unit ECU, as shown in Figure 1. When the engine is working, the gear pump on the high-pressure pump absorbs oil from the tank through negative pressure, and sends the filtered fuel to the high-pressure pump with a certain pressure (about 5-7 bar). After the fuel enters the high pressure plunger chamber, it is compressed and enters the common rail tube through the high pressure oil pipe to form a high pressure. Each cylinder injector is connected to the common rail tube through a high pressure oil pipe to achieve high pressure injection.

2.1.1 High pressure pump

The high pressure oil pump is one of the key components in the high pressure common rail system. Its main function is to pressurize the low pressure fuel into high pressure fuel, store it in the fuel rail and wait for the ECU's injection instructions. The high pressure oil pump consists of a gear pump, an oil metering unit, an overflow valve, an inlet and outlet valve, and a high pressure plunger. As an example, CPN2.2BL, which has been widely used in China's commercial vehicle market by Bosch and has already begun local production, is shown in Figure 2 [12].
 

The design criteria for the supply of high-pressure oil pumps are:

The high pressure oil pump must be guaranteed over the entire life span and under any operating conditions

The oil supply can meet the fuel injection requirements of the engine at a certain rail pressure, ie the oil balance.

CPN2.2BL uses two in-line plunger design, driven by the engine camshaft, transmission ratio of 1:2, similar to the traditional mechanical pump, in order to facilitate the Euro II engine upgrade. The lubrication method is oil lubrication, and the lubrication oil path is directly connected to the engine lubrication oil path. The task of the gear pump is to supply enough high-pressure fuel to the high-pressure oil pump, installed at the rear end of the pump body of the high-pressure pump, and driven by the gear at the end of the camshaft of the high-pressure pump. Its rotational speed is 2.85 times that of the high-pressure pump. When the fuel enters the high pressure section, it enters the high pressure plunger chamber all the way through the fuel metering unit, enters the fuel rail after being compressed, and excess fuel passes through the spill valve and returns to the fuel tank. The main function of the oil metering unit is to regulate the amount of oil entering the high pressure plunger chamber to control the size of the fuel pressure in the common rail tube.

  2.1.2 Common Rail

The common rail is a special component of the electronically controlled high pressure common rail system, which mainly includes high pressure joints, orifices, rail pressure sensors and pressure limiting valves, as shown in Figure 3. The main role of the common rail is to accumulate and distribute fuel, damp fuel pressure fluctuations, and also limit the maximum fuel pressure so that it does not exceed safety limits. The rail pressure sensor provides the ECU with a real-time pressure signal in the common rail as an input to the rail pressure closed-loop control. The orifice design at the inlet and outlet of the rail reduces the pressure fluctuations in the common rail and high pressure tubing.

The pressure limit valve is a mechanical valve that opens when the pressure exceeds a certain limit value to ensure that the pressure of the common rail tube is quickly released to ensure system safety. When the pressure limit valve is opened, it can still maintain the rail pressure in a normal range (such as 700 ~ 800Bar), so that the vehicle can continue to run to the maintenance site in the event of a fault, that is, limp home.

2.1.3 Injector

The injector is the most critical and complex part of the electronically controlled high pressure common rail system. Its role is to control the opening and closing of the solenoid valve according to the electrical signals sent by the ECU, and to optimize the injection of fuel in the high pressure fuel rail. The timing, fuel injection rate and fuel injection rate are injected into the combustion chamber of the diesel engine. The injector is mainly composed of the injector body, solenoid valve, nozzle, needle valve assembly and spring. Figure 4 shows Bosch's second-generation commercial vehicle injector CRIN2. When the solenoid valve is not energized, the armature presses the ball valve tightly on the valve seat. At this time, the pressure in the control chamber and the pressure chamber is balanced. The nozzle needle valve is pressed against the surface of the nozzle seat by the spring pre-tightening force and is not raised. The fuel injector is not fueled; when the solenoid valve is energized, the solenoid valve lifts the armature by suction. At this time, the fuel in the control room leaks through the orifice of the ball valve, the pressure in the control room drops rapidly, and the pressure in the pressure chamber does not change, so the needle The valve is lifted, ie, the injector starts to inject oil; when the solenoid valve is closed, the pressure in the control room rises, the pressure at the two ends of the nozzle needle is again balanced, and the nozzle needle valve is seated under the action of the spring pre-tightening force to close the spray. The oil injector completes the injection process.

2.2 How Common Rail System Works

Accumulation of high-pressure fuel from the oil pump through the larger-capacity common-rail lumen eliminates pressure fluctuations in the fuel, forms a constant high-pressure fuel, and distributes it to each injector, with the aid of integration in each injection. The high-speed electromagnetic on-off valve on the device is opened and closed, and the amount of oil injected by the injector into the diesel engine combustion chamber is controlled periodically and quantitatively. This ensures that the diesel engine achieves good fuel atomization, optimum combustion ratio, and optimum ignition time, sufficient ignition energy, and minimal pollution emissions.

The electronic control unit (ECU) controls the fuel injection quantity of the injector (the fuel injection quantity depends on the fuel rail, the common oil supply pipe), the pressure and the opening time of the solenoid valve. The basic control function of the ECU is to inject a certain amount of diesel fuel into the combustion chamber at a set pressure at an appropriate moment to ensure high power, low fuel consumption, low emissions, and low noise of the diesel engine.

Third, the development of diesel engine electronically controlled fuel injection

The electronic fuel injection management of diesel engines began in the 1970s. The development of electronically controlled fuel injection technology for diesel engines has undergone roughly the following three generations:

The first generation: Positional electronic control system. Electronic servo mechanism (such as linear solenoid, linear DC motor) is used instead of mechanical governor to control the position of the oil supply rod (in-line pump) or to control the position of the oil spill ring (distribution pump) to control the fuel injection quantity. The electro-hydraulic actuator controlled by the (electronic control unit) changes the phase between the engine drive shaft and the camshaft of the injection pump or controls the movement of the advancer piston and controls the injection time. The first-generation common-rail high-pressure pump always maintained at the highest pressure, resulting in wasted fuel and high fuel temperature.

The second generation: time-based electronic control system. Its structural feature is that the oil supply still maintains the traditional pulse type plunger pump oil supply mode, that is, the fuel boost pressure is achieved through the injection pump or the cam of the engine, but the fuel injection timing (the time of the start of pressure increase) and the fuel injection amount (The amount of fuel injected from the start of boosting to the end of boosting) is determined by the ECU adjusting the timing of opening and closing of the high-speed solenoid valve. Due to the adoption of high-speed solenoid valves, the control accuracy has been greatly improved over the first generation. Pre-injection reduces engine noise: A small fraction of a millionth of a second before the main injection is injected into the cylinder to ignite and preheat the combustion chamber. The preheated cylinder makes the compression after the main injection easier, the pressure and temperature inside the cylinder no longer increase abruptly, and it is beneficial to reduce the combustion noise.

The third generation: time-pressure control system, also known as high pressure common rail electronically controlled fuel injection system, with piezoelectric in-line injectors. It is a brand new electronically controlled fuel injection system developed in the mid-1990s. It completely eliminates the conventional fuel injection pump and sub-cylinder fuel supply system, and it has an electronically controlled injector through common rail pressure storage and high-speed solenoid valve actuators. The comprehensive control of fuel injection pressure, time, fuel injection volume, and various complex injection characteristics is more obvious than the first and second generation electronically controlled fuel injection systems.

The fourth-generation common-rail diesel electronic control system has gradually been produced. The fourth-generation common rail electronic control system for diesel engines is based on the technology of the third generation common rail system, which allows free control of injection parameters such as injection pressure, fuel injection quantity, fuel injection time, and fuel injection rate. The work is optimally matched to the engine's operating conditions, and comprehensive control of various additional functions such as exhaust gas recirculation (EGR), boost pressure control, idle speed control, cruise control, etc., has expanded the electronic control system that has not yet been used by others in diesel engines. It adopts the “change emission” energy-saving technology and the cutting-edge technology of “urgent brakes stepped on the throttle safety precautions”, and integrates three major energy-saving and emission reduction high-end emission reductions, “HCCI” and “low-temperature plasma exhaust gas purification”. technology. Theoretically, it is estimated that the fuel saving rate can be increased by nearly 30%, and the emission of exhaust pollutants can meet or exceed the Euro IV standard.

Fourth, the advantages and disadvantages of high pressure common rail system

4.1 Advantages of Common Rail System

The common-rail diesel injection system is different from the diesel injection system that was previously driven by a cam. It completely separates the injection pressure generation and the injection process from each other, making the common-rail high-pressure common rail system an unattainable function in conventional injection systems. It can be summarized as the following four points:

4.1.1 The fuel injection pressure in the common rail system can be adjusted flexibly. The optimum injection pressure can be determined for different working conditions to optimize the overall performance of the diesel engine.

4.1.2 The injection timing can be independently controlled flexibly, and with the high injection pressure (120~200MPa), the NOx and particulates, PM2.5 can be controlled at the same time to meet the emission requirements.

4.1.3 flexible control of the injection rate changes, to achieve the ideal fuel injection law, easy to achieve pre-injection and multiple injection, can reduce diesel engine NOx, but also to ensure excellent dynamic and economical.

V. Research on Common Rail Technology at Home and Abroad

1. Research status of domestic high pressure common rail technology

Domestic research on electronically controlled high pressure common rail fuel injection systems started late, and most of them are limited to research on electronic control unit hardware and other aspects. Only some universities, enterprises, and research institutes participate in the development of common rail system components. At present, no mature diesel electronically controlled fuel injection system has been developed yet, and there is still a long way to go before productization. The following are the domestic research results in the diesel engine electronically controlled fuel injection system: 1) The electric-controlled inline pump-pipe-valve-mouth (PPVI) system developed and researched by Tsinghua University has developed a corresponding high-performance solenoid valve. 2) PAIRCUI, a new common-rail accumulating type electronically controlled fuel system developed by Tianjin University’s State Key Laboratory of Internal Combustion Engine Combustion, is in the hardware-in-the-loop simulation and real machine testing phase. 3) Zhejiang University conducted a high pressure common rail system matching test on the Xichai CA6110 engine. 4) Shanghai Jiaotong University is based on the self-developed GD-1 high pressure common rail system in preparation for the Yuchai YC6110 diesel engine. 5) Guangxi Yulin Diesel Engine Factory cooperated with Tsinghua University to develop a GDI diesel engine high pressure common rail injection system. 6) Wuxi Oil Pump Yuzu Research Institute cooperated with Wuxi Diesel Engine Factory and successfully tested the common rail injection system of the CA6110 turbocharged and intercooled diesel engine. 7) Wuxi Weifu Group and Bosch have jointly established Wuxi Bosch Automotive Diesel Engine System Co., Ltd. and started production of high pressure common rail system.

2. Development Status of Foreign High Pressure Common Rail Technology

So far, various countries have developed and produced various electronic control systems for diesel engines, which have effectively eased the current global energy crisis and car pollution. The electronic control technology of diesel engines in some developed countries in the automotive industry is currently quite advanced.

    At present, in European and American countries, diesel engines are used in 100% of heavy vehicles and 90% of light vehicles. The proportion of diesel cars in Europe is more than 40%, and the proportion of new cars is more than 50%. Bosch's conservative forecast for the Chinese market is that by 2015, the proportion of diesel vehicles will reach 25%, and by 2015, according to some articles, the market share of diesel vehicles will increase to approximately 15%.

3. Research direction and problems to be solved in high pressure common rail technology

The research and development hot spots of common rail technology are as follows: 1) Numerical simulation technology of fuel injection system. The numerical model of the electronically controlled high pressure common rail fuel system is established by simulation software, and the influence of the fuel injection process and system parameters on the fuel injection characteristics is analyzed, providing a theoretical basis for the optimal design and failure analysis of the fuel system. Reduce product development costs and shorten development cycles. 2) Solve the problem of non-uniform injection volume due to minor fluctuations in common rail pressure. The dynamic pressure stability of the high pressure common rail system directly affects the realization of the ideal fuel injection system law. 3) Research on a new type of electromagnetic valve. Future solenoid valves require faster response, accuracy, repeatability, reliability, and good flow capabilities. Such as the use of piezoelectric ceramic driver to develop a high response solenoid valve. 4) Sensor technology. With the constant increase of injection pressure, new smart sensors with higher accuracy and response speed are required. 5) Research on the best control strategy. Multiple injection control technology and the regulation of fuel injection by controlling the pressure.

Comprehensive analysis of the history and status quo of diesel engine electronically controlled fuel injection system at home and abroad. Electronically controlled high pressure common rail system has a lot of room for development. Mainly to further tap the flexible persity of electronic control and common rail system pressure. The potential of time control principles. The key to achieving an ideal injection pattern (Figure 5) is to increase the injection pressure and improve the flexibility of the injection rate control. The main technical measures are multi-stage pressure control and multiple injections.