High pressure common rail in the end is what?

The high-pressure fuel pump delivers fuel to the public fuel supply pipe, and directly injects fuel into the cylinder by controlling the fuel injector. The high-pressure common rail completely separates the injection process from the oil pressure generation, so that the fuel supply pressure will not be affected by the engine speed.

Advantages:

1. The injection pressure in the high-pressure common rail system is flexibly adjustable, and the optimal injection pressure can be determined for different working conditions, thereby optimizing the overall performance of the diesel engine.

2. It can independently and flexibly control the fuel injection timing, and with high injection pressure (120MPa ~ 200MPa), it can simultaneously control NOx and particulate matter (PM) in a smaller value to meet emission requirements.

3. Flexible control of fuel injection rate changes, to achieve the ideal fuel injection law, easy to achieve pre-injection and multiple injection, not only can reduce diesel engine NOx, but also ensure excellent power and economy.

4. The fuel injection is controlled by the solenoid valve, and the control accuracy is high. There will be no bubbles and zero residual pressure in the high-pressure oil circuit. Therefore, within the operating range of the diesel engine, the circulating fuel injection volume changes little, and the fuel supply of each cylinder is uneven. Can be improved, thereby reducing the vibration of the diesel engine and reducing emissions.

Representative model:

At the end of 1999, a Smart equipped with a 3-cylinder common rail diesel engine was born. It has a displacement of only 799mL, a maximum power of 30kW, and a maximum torque of 100Nm at 1800~2800rpm.

The E320 introduced by Mercedes-Benz is equipped with a second-generation common rail engine, with a maximum power of 150kW/1000rpm and an output torque of 250Nm, 85% of the peak torque can be obtained at 1400rpm, and a peak torque of 500Nm can be achieved in a wide area of 1800~2600rpm.

The acceleration time of 0~100 km/h is only 7.7 seconds, and the maximum speed is 243 km/h. The comprehensive fuel consumption is 6.9L/100km, and the 80L fuel tank enables the endurance to reach 1000km. The combined fuel consumption of the E320 equipped with a gasoline engine is 9.9L/100km.

Technical summary:

In a diesel engine, high-speed operation makes the diesel injection process only a few thousandths of a second. Experiments have proved that the pressure of the high-pressure oil pipe varies with time and position during the injection process.

Due to the compressibility of diesel and the pressure fluctuation of diesel in the high-pressure fuel pipe, the actual fuel injection state is quite different from the plunger fuel supply rule specified by the fuel injection pump.

The pressure fluctuation in the oil pipe sometimes causes the pressure in the high-pressure oil pipe to rise again after the main injection, reaching the pressure that opens the needle valve of the injector, and reopens the closed needle valve to produce secondary fuel injection. Because the secondary fuel injection cannot be completely burned, the smoke and hydrocarbon (HC) emissions are increased, and fuel consumption is increased.

In addition, the residual pressure in the high-pressure oil pipe will change after each injection cycle, which will cause unstable injection, especially in the low-speed area. In severe cases, not only the fuel injection is uneven, but also intermittent non-injection will occur. phenomenon. In order to solve the defect of diesel engine fuel pressure change, modern diesel engine adopts a technology called common rail.

High-pressure common rail technology refers to a fuel supply method that completely separates the generation of injection pressure and the injection process in a closed-loop system composed of a high-pressure fuel pump, pressure sensor and ECU. The high-pressure fuel pump delivers high-pressure fuel to the public fuel supply pipe.-Realize precise control of the oil pressure in the public fuel supply pipe, so that the pressure of the high-pressure oil pipe has nothing to do with the engine speed, it can greatly reduce the change of diesel engine fuel supply pressure with engine speed, so it also reduces the defects of traditional diesel engines. The ECU controls the fuel injection volume of the fuel injector. The fuel injection volume depends on the pressure of the fuel rail (public fuel supply pipe) and the length of the solenoid valve opening time.

Technical principle:

The high-pressure common rail system is mainly composed of an electronic control unit, a high-pressure oil pump, a pressure accumulator (common rail pipe), an electronic control injector and various sensors. The low-pressure fuel pump inputs the fuel into the high-pressure fuel pump, and the high-pressure fuel pump pressurizes the fuel into the high-pressure fuel rail (accumulator). The pressure in the Shenzhen high-pressure fuel rail is measured by the electronic control unit according to the fuel rail pressure measured by the fuel rail pressure sensor And the need to adjust, the fuel in the high-pressure fuel rail passes through the high-pressure fuel pipe, according to the operating state of the machine, the electronic control unit determines the appropriate fuel injection timing and fuel injection duration. The electronic fuel injector controlled by electro-hydraulic injects fuel into the cylinder.

1. High pressure oil pump

The design criterion for the fuel supply of the high-pressure fuel pump is to ensure that the demand for the sum of the fuel injection volume and the control fuel volume of the diesel engine under any circumstances, as well as the demand for changes in fuel volume during startup and acceleration. Since the fuel injection pressure in the common rail system has nothing to do with the fuel injection process, and the fuel injection timing is not guaranteed by the cam of the high-pressure fuel pump, the pressure cam of the high-pressure fuel pump can follow the lowest peak torque, the lowest contact stress and the most wear-resistant The design principle to design the cam.

Most companies use a three-cylinder radial piston pump driven by a diesel engine to generate pressures up to 135MPa. The high-pressure oil pump uses multiple oil pressure cams in each oil pressure unit, so that its peak torque is reduced to 1/9 of that of the traditional high-pressure oil pump, the load is also relatively uniform, and the operating noise is reduced.

The control of the pressure in the high-pressure common rail cavity in this system is realized by discharging the fuel in the common rail cavity. In order to reduce the power loss, when the fuel injection volume is small, the three-cylinder radial piston will be closed. An oil pressure unit in the pump reduces the oil supply.

2. High pressure fuel rail (common rail pipe)

The common rail pipe distributes the high-pressure fuel provided by the fuel supply pump to each injector and acts as a pressure accumulator. Its volume should reduce the fuel supply pressure fluctuation of the high-pressure fuel pump and the pressure shock caused by the fuel injection process of each injector, so that the pressure fluctuation in the high-pressure fuel rail is controlled below 5MPa. However, its volume cannot be too large to ensure that the common rail has sufficient pressure response speed to quickly track changes in diesel engine operating conditions.

The high-pressure common rail pipe is also equipped with a pressure sensor, a flow buffer (restrictor) and a pressure limiter. The pressure sensor provides the pressure signal of the high-pressure fuel rail to the ECU; the liquid flow buffer (current limiter) WeChat Plus Car Connect Shenzhen ensures that the fuel supply to the fuel injector is cut off when the fuel leakage fault occurs in the fuel injector, and can reduce Pressure fluctuations in the common rail and high-pressure oil pipe; the pressure limiter ensures that the pressure in the high-pressure fuel rail is quickly released when the pressure is abnormal.

3. Electronic control injector

The electronically controlled fuel injector is the most critical and complex component in the common rail fuel system. Its function is to control the opening and closing of the solenoid valve according to the control signal sent by the ECU to optimize the fuel in the high-pressure fuel rail. The fuel injection timing, fuel injection volume and fuel injection rate are injected into the combustion chamber of the diesel engine.

In order to achieve a predetermined fuel injection shape, a reasonable optimization design 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, and the needle valve cannot quickly cut off the fuel at the end of fuel injection, which makes the later fuel atomization poor; the volume of the control room is too small, It cannot provide enough effective stroke for the needle valve to increase the flow resistance of the injection process. Therefore, the volume of the control room should also be reasonably selected according to the maximum fuel injection volume of the model.

In addition, the minimum injection pressure of the injection nozzle depends on the flow rate of the oil return hole and the oil inlet hole and the end surface area of the control piston. In this way, after determining the structural dimensions of the fuel inlet hole, the fuel return hole and the control room, the stable and shortest fuel injection process with the needle valve of the fuel injector fully opened is determined, and the stable minimum fuel injection of the fuel injector is determined at the same time. Quantity. 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 fuel injection volume of the nozzle.

However, the volume of the control room cannot be reduced indefinitely, and it should be able to ensure the lift of the needle valve of the fuel injector to fully open the needle valve. The two control holes determine the dynamic pressure in the control room, which determines the movement law of the needle valve. By carefully adjusting the flow coefficient of these two holes, the ideal fuel 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 the nozzle is very small. For example, the nozzle diameter of BOSCH's nozzle is 0.169mm × 6. Under such a small nozzle diameter and high injection pressure, the fuel flow is in an extremely unstable state, the spray cone angle of the fuel beam becomes larger, and the fuel atomization is better, but the penetration distance becomes smaller, therefore, the swirl intensity of the intake air and the shape of the combustion chamber 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, considering 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 oil pipe

The high-pressure fuel pipe is the channel connecting the common rail pipe and the electronically controlled fuel injector. It should have sufficient fuel flow to reduce the pressure drop when the fuel flows, and make the pressure fluctuation in the high-pressure pipeline system smaller, and can withstand high-pressure fuel The impact of the impact, and the pressure in the common rail can be quickly established when starting.

The length of the high-pressure oil pipes of each cylinder should be as equal as possible, so that each injector of the diesel engine has the same injection pressure, thereby reducing the deviation of the fuel injection volume between the cylinders of the engine. The high-pressure oil pipes in Shenzhen should be as short as possible to minimize the pressure loss from the common rail to the fuel injector.

BOSCH's high-pressure oil pipe has an outer diameter of 6mm and an inner diameter of 2.4mm, while Japan's high-pressure oil pipe has an outer diameter of 8mm and an inner diameter of 3mm.

The high pressure in the common rail cavity is directly used for injection, which can eliminate the pressure boosting mechanism in the injector; and the common rail cavity is continuously high pressure, and the driving torque required by the high-pressure oil pump is much smaller than that of the traditional oil pump.

Through the pressure adjustment solenoid valve on the high-pressure oil pump, the oil pressure in the common rail cavity can be flexibly adjusted according to the engine load condition and the requirements of economy and emission, especially the low-speed performance of the engine is optimized.

The solenoid valve on the injector controls the injection timing, injection volume and injection rate, and can also flexibly adjust the injection volume of pre-injection and post-injection under different working conditions and the interval with the main injection.

The high-pressure common rail system consists of five parts, namely high-pressure oil pump, common rail cavity and high-pressure oil pipe, fuel injector, electronic control unit, various sensors and actuators. The fuel supply pump pumps fuel from the fuel tank into the fuel inlet of the high-pressure fuel pump. The high-pressure fuel pump driven by the engine pressurizes the fuel and sends it into the common rail cavity, and then the solenoid valve controls the injectors of each cylinder to inject fuel at the corresponding time.

Pre-injection Before the main injection, a small part of the fuel is injected into the cylinder, and pre-mixing or partial combustion occurs in the cylinder to shorten the ignition delay period of the main injection. In this way, the in-cylinder pressure increase rate and peak pressure will drop, the engine works more gently, and the in-cylinder temperature decreases to reduce NOx emissions. Pre-injection can also reduce the possibility of misfire and improve the cold start performance of the high pressure common rail system.

Reducing the injection rate at the initial stage of the main injection can also reduce the amount of oil injected into the cylinder during the ignition delay period. Increasing the injection rate in the middle of the main injection can shorten the injection time and shorten the slow burn period, so that the combustion is completed within the engine's more effective crankshaft angle range, increase output power, reduce fuel consumption, and reduce soot emissions. Rapid fuel cut-off at the end of the main injection can reduce incomplete combustion of fuel, smoke and hydrocarbon emissions.