First, the basic knowledge of diesel engine
Diesel engines and gasoline engines have basically the same structure, with cylinder blocks, cylinder heads, pistons, valves, cranks, crankshafts, camshafts, and flywheels. However, the former uses compression-ignition diesel for work, and the latter uses ignited gasoline for work. One ignition and one ignition are the fundamental differences between the two. Gasoline engine fuel is mixed with air in the intake stroke and then enters the cylinder, and then is ignited by the spark plug. The fuel of the diesel engine is directly injected into the cylinder when the compression stroke is near the end, and is compressed and burned in the compressed air. This difference has caused diesel engines to have their own characteristics in the structure of the fuel supply system.
The fuel injection system of a diesel engine consists of a fuel injection pump, an injector, a high-pressure fuel pipe, and some auxiliary accessories. The simple process of diesel fuel delivery is: the fuel pump sends the diesel fuel to the filter and enters the fuel injection pump after filtration. (In order to ensure adequate fuel and maintain a certain pressure, the fuel supply of the pump is required to be larger than the fuel pump needs. Far more, the excess diesel is returned to the tank via the low-pressure pipe, and the other part of the diesel is compressed by the injection pump to high pressure. The high-pressure fuel pipe enters the injector and is directly injected into the combustion chamber of the cylinder. (The schematic diagram is the diesel fuel supply system, 4 is the high pressure pipeline, 1, 2, 3 is the low pressure pipeline, 5, 6, 7, 8 is the return pipeline).
Second, high pressure common rail electronic control diesel injection system
Modern advanced diesel engines generally adopt technologies such as electronically controlled injection, common rail, and turbo-charged intercooling. They have achieved major breakthroughs in terms of weight, noise, smoke, etc., reaching the level of gasoline engines, and are more environmentally friendly than gasoline engines. At present, foreign light cars and advanced agricultural tractors all use diesel engines.
The main difference between the diesel engine and the gasoline engine in electronically controlled injection is that the electronically controlled injection system of the gasoline engine only controls the air-fuel ratio. The electronically controlled injection system of the diesel engine regulates the output size by controlling the injection time. The diesel injection control is Engine speed and accelerator pedal position (throttle lever position) to determine. Therefore, the basic working principle is that the computer first calculates the basic fuel injection quantity based on the input signals of the speed sensor and the throttle position sensor, and then corrects the signal according to the sensors such as the water temperature, the intake air temperature, the intake pressure, and then from the control sleeve position. The sensor signal is feedback-corrected to determine the optimum fuel injection.
The electronically controlled diesel injection system consists of sensors, ECUs (computers) and actuators. Its task is to electronically control the fuel injection system, realizing the real-time control of the fuel injection quantity and timing of the fuel injection with operating conditions. Sensors such as rotation speed, temperature and pressure are used to simultaneously input real-time detection parameters into the computer and compare them with the stored parameter values. After processing and calculation, the fuel injection pump, exhaust gas recirculation valve and glow plug actuators are executed according to the optimal values. Control, drive the fuel injection system, so that the best operating state of the diesel engine. This type of electronic control system can be divided into: Accumulator type electronically controlled fuel injection system, hydraulic booster type electronically controlled fuel injection system and high pressure common rail type electronically controlled fuel injection system. The following describes the high pressure common rail electronically controlled diesel injection system:
(A) Common Rail Technology
In diesel engines, high-speed operation makes the diesel injection process time only a few thousandths of a second. Experiments have shown that the pressure in the high-pressure fuel pipe varies with time and location during the injection process. Due to the compressibility of diesel fuel and the pressure fluctuation of diesel oil in the high-pressure tubing, the actual injection state is different from the plunger supply regulation provided by the injection pump. Pressure fluctuations in the tubing can sometimes cause the pressure in the high pressure tubing to rise again after the main injection, reaching the pressure at which the needle valve of the injector opens, and reopening the closed needle valve to produce a secondary injection phenomenon. Since the secondary fuel injection cannot be completely burned, smoke and hydrocarbon (HC) emissions are increased, and fuel consumption is 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. In order to solve the defect of diesel fuel pressure change, modern diesel engines use a technology called common rail.
Common-rail technology refers to a type of oil supply system in which a high-pressure oil pump, a pressure sensor, and an ECU comprise a closed-loop system consisting of a spray pressure and a spray process that are completely separated from each other. The high-pressure fuel pump conveys the high-pressure fuel to the public oil supply pipe. The accurate control of the oil pressure in the public oil supply pipe makes the pressure of the high pressure oil pipe irrelevant to the engine speed, which can greatly reduce the variation of the fuel supply pressure of the diesel engine with the engine speed, thus reducing the defects of the conventional diesel engine. The ECU controls the injection quantity of the injector. The injection quantity depends on the pressure of the fuel rail (public supply pipe) and the length of the solenoid valve opening time.
The common rail fuel injection system was only officially put into practical use in the mid to late 1990s. The high pressure common rail system can achieve functions that cannot be achieved in conventional fuel injection systems. The advantages are:
a. The flexibility of the injection pressure in the common rail system can be adjusted to determine the optimal injection pressure required for different operating conditions to optimize the overall performance of the diesel engine.
b. The fuel injection timing can be independently controlled flexibly, and with high injection pressure (120Mpa~200MPa), the NOx and particulates (PM) can be controlled at the same time to meet the emission requirements.
c. Flexible control of fuel injection rate changes, achieving ideal injection rules, easy to achieve pre-injection and multi-injection, can reduce diesel engine NOx, but also ensure excellent power and economy.
d. The fuel injection is controlled by the solenoid valve. The control accuracy is high. There will be no bubbles and residual pressure in the high-pressure oil circuit. Therefore, in the diesel engine operating range, the cyclic fuel injection volume change is small, and the cylinder oil supply is not Uniformity can be improved, thereby reducing diesel engine vibration and reducing emissions.
As the high pressure common rail system has the above advantages, research institutes for diesel engines both at home and abroad have devoted great efforts to research. The more mature systems are CR Systems of ROBERT BOSCH, Germany, ECD-U2 of Denso Corporation, unijet of FIAT Group of Italy, LDCR of DELPHI DIESEL SYSTEMS of the United Kingdom, and Common Rail System of Deere Wait.
(B) High pressure common rail electronically controlled fuel injection system and basic unit
Figure 1 shows the basic composition of a high pressure common rail electronically controlled fuel injection system. It is mainly composed of electronic control unit, high pressure oil pump, pressure accumulator (common rail), electronically controlled injector and various sensors. The low-pressure fuel pump enters the fuel into the high-pressure pump, and the high-pressure pump pressurizes the fuel into the high-pressure fuel rail (pressure accumulator). The pressure in the high-pressure fuel rail is adjusted by the electronic control unit according to the rail pressure measured by the rail pressure sensor and needs to be adjusted. The fuel in the high pressure fuel rail passes through the high pressure fuel pipe. According to the operating status of the machine, the electronic control unit determines the proper fuel injection timing from the preset map, and the injection duration is controlled by the electronic fuel injector. Spray into the cylinder.
1, high pressure oil pump
The design criteria for the fuel supply of high-pressure pumps is the need to ensure the sum of the fuel injection quantity and the control oil quantity of the diesel engine under any circumstances, as well as the change in the quantity of oil at the time of starting and accelerating. Since the injection pressure in the common rail system is independent of the fuel injection process and the injection timing is not ensured by the cam of the high pressure oil pump, the pressure oil cam of the high pressure pump can have the lowest peak torque, the lowest contact stress and the most wear resistance. The design principle is to design the cam.
Most companies use a three-cylinder radial piston pump driven by a diesel engine to produce pressures up to 135 MPa. The high-pressure oil pump uses a plurality of pressure oil cams in each pressure oil unit to reduce its peak torque to 1/9 of the conventional high-pressure oil pump, and the load is also relatively uniform, which reduces operating noise. The control of the pressure in the high pressure common rail chamber of this system is achieved through the deflation of fuel in the common rail cavity. To reduce the power loss, the three cylinder radial piston will be closed under a small injection volume. One oil pressure unit in the pump reduces the amount of oil supply.
2, high pressure fuel rail (common rail)
The common rail tube distributes the high-pressure fuel provided by the fuel supply pump to each injector and functions as a pressure accumulator. The common rail of the ECD-U2 system is shown in Figure 4. Its volume should reduce the supply pressure fluctuation of the high pressure pump and the pressure oscillation of each injector caused by the injection process, so that the pressure fluctuation in the high pressure rail is controlled below 5 MPa. However, its volume must not be too large to ensure that the common rail has enough pressure response speed to quickly track changes in diesel engine operating conditions. The maximum circulating oil supply for the high pressure pump of the ECD-U2 system is 600 milliliters and the common rail volume is 94,000 milliliters.
Pressure sensors, flow buffers (restrictors) and pressure limiters are also installed on the high pressure common rail. The pressure sensor provides the ECU with the pressure signal of the high pressure rail; the flow buffer (restrictor) ensures that the fuel supply to the injector is cut off when the fuel leakage fault of the injector occurs, and the common rail and the high pressure tubing can be reduced. The pressure fluctuations; pressure limiter ensures that the pressure in the high-pressure rail will quickly release the pressure in the high pressure rail.
As can be seen from the above analysis, it is critical to accurately design the volume and shape of the high-pressure common rail tube suitable for the determined diesel engine.
3, electronically controlled injector
Electronically controlled fuel injectors are the most critical and complex components of the common rail fuel system. Their role is based on the control signals sent by the ECU. By controlling the opening and closing of the solenoid valves, the fuel in the high pressure fuel rails is optimized. Fuel injection timing, fuel injection rate and fuel injection rate are injected into the combustion chamber of the diesel engine.
In order to achieve a predetermined injection shape, a reasonable optimization of the injector is required. The volume of the control room determines the sensitivity of the needle valve when it is opened, the volume of the control room is too large, the needle valve can not achieve rapid oil cut at the end of the injection, so that the late fuel atomization is poor; the control room volume is too small, The needle valve can not provide enough effective stroke, so that the flow resistance of the injection process increases, so the volume of the control room should also be a reasonable choice based on the model's maximum fuel injection.
The size of the control orifices A and Z has a decisive influence on the opening and closing speed of the injector and the injection process. The three key structures of the double-volume valve body are the oil inlet hole, the oil return hole and the control room, and their structural size has a great influence on the fuel injection performance of the injector. The difference between the flow rate of the oil return hole and the oil inlet hole and the volume of the control chamber determine the opening speed of the injector needle valve, and the closing speed of the injector needle valve is determined by the flow rate of the inlet orifice and the control room. The volume of the decision. The design of the oil inlet orifice should allow the injection nozzle needle valve to have a sufficient closing speed to reduce the poorly atomized part of the injector nozzle during the late stage of injection.
In addition, the minimum injection pressure of the fuel injector depends on the flow rate of the oil return hole and the oil inlet hole and the area of ​​the end face of the control piston. After confirming the structural dimensions of the oil inlet hole, the oil return hole and the control room, the stable and shortest injection process of the injector needle valve is fully opened, and at the same time the stable minimum injection of the injector is determined. The amount of oil. The reduction of the volume of the control chamber can make the response speed of the needle valve faster, so that the fuel temperature has less influence on the nozzle injection volume.
However, the volume of the control room can not be reduced without limit, it should be able to ensure the lift of the nozzle needle to fully open the needle valve. The two control orifices determine the dynamic pressure in the control room, which determines the motion of the needle valve. By carefully adjusting the flow coefficients of the two orifices, an ideal injection law can be produced.
Because the injection pressure of the high pressure common rail injection system is very high, the cross-sectional area of ​​its injection nozzle is very small. For example, the diameter of the injection hole of the BOSCH nozzle is 0.169mm 6 , and the diameter of the nozzle is so small. Under high injection pressure, the fuel flow is in an extremely unstable state, the spray cone angle of the oil jet becomes larger, the atomization of the fuel is better, but the penetration distance becomes smaller, so the swirl intensity of the intake of the original diesel engine and the shape of the combustion chamber structure should be changed. To ensure the best combustion process.
For the injector solenoid valve, because the common rail system requires it to have sufficient opening speed, taking into account that the pre-injection is an important injection method to improve the performance of the diesel engine, the response time of the control solenoid valve should be shortened.
4, high pressure tubing
The high-pressure fuel pipe is a passage connecting the common rail tube and the electronically-controlled fuel injector. It should have enough fuel flow to reduce the pressure drop when the fuel flows, and make the pressure fluctuation in the high-pressure piping system smaller to withstand the high-pressure fuel. Impact, and the pressure in the common rail can be established quickly when starting. The length of each cylinder's high-pressure fuel pipe should be equal as far as possible, so that each injector of the diesel engine has the same injection pressure, thus reducing the deviation of the fuel injection amount between the cylinders of the engine. Each high pressure fuel line should be as short as possible to minimize the pressure loss from the common rail to the nozzle. The outer diameter of the BOSCH company's high pressure tubing is 6mm and the inner diameter is 2.4mm. The outer diameter of the high pressure tubing of Nippon Denso is 8mm and the inner diameter is 3mm.
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