Generally, when we talk about EGR system, we understand that it is the exhaust gas recirculation system. By introducing exhaust gas into the combustion chamber, we can reduce the peak combustion of the engine and achieve the purpose of reducing NOx emission.
The diesel vehicle adopts an external system, that is, a pipeline is used between the exhaust pipe and the intake pipe, and a valve is installed in the pipeline to control the amount of exhaust gas. It can be seen that EGR valve is the core component of the whole system, and how to accurately control and monitor it is the development direction of EGR system.
EGR: the full name is exhaust gas recirculation. After combustion, the internal combustion engine separates a part of the exhaust gas and introduces it to the intake side for re combustion. The main purpose is to reduce the nitrogen oxides (NOx) in the exhaust gas and improve the fuel consumption rate when sharing part of the load.
Exhaust gas recirculation system (EGR) is to return a small part of the exhaust gas produced by diesel engine or gasoline engine to the cylinder of recirculation system.
Because of its inertia, the recycled exhaust gas will delay the combustion process, that is, the combustion speed will slow down, resulting in the slow pressure formation process in the combustion chamber, which is the main reason for the reduction of nitrogen oxides.
In addition, increasing the exhaust gas recirculation rate will reduce the total exhaust gas flow, so the total pollutant output in exhaust emission will be relatively reduced of recirculation system.
The task of EGR system is to make the recirculation amount of exhaust gas reach the best condition at each working point, so as to make the combustion process always in the best condition, and finally ensure the lowest pollution component in emissions.
Because the change of exhaust gas recirculation may have opposite effects on different pollution components, the so-called best situation is often a compromise to achieve the best scheme for the total emission of relevant pollutants of recirculation system.
For example, although improving the exhaust gas recirculation rate has a positive impact on reducing the emission of nitrogen oxides (NOx), it will also have a negative impact on the increase of particulate matter and other pollution components.
Different engine operating conditions have different requirements for EGR. In order to make the EGR system work more effectively, the amount of exhaust gas participating in EGR must be limited.
With the increase of load, the amount of EGR increases accordingly and can reach the optimal value;
At idle speed and low load, the NOx emission concentration is low. In order to ensure normal combustion, EGR is not carried out;
During warm-up, the engine temperature is low and the NOx emission concentration is low. In order to prevent EGR from deteriorating the combustion process, EGR is not carried out;
EGR shall not be carried out under heavy load, high speed or full throttle to ensure the power performance of the engine;
During acceleration, EGR plays a role in the transition process in order to ensure the acceleration and necessary purification effect of the vehicle.
According to the above EGR design principles, EGR must be controlled and adjusted to make the application of EGR in the engine achieve the expected effect. There are many methods of EGR control and regulation, which can be classified from different angles according to its main characteristics of recirculation system
1、 Mechanical EGR system (figure below)
By controlling the negative pressure of intake pipe and exhaust pressure, the gas control mode is suitable for EGR rate of 5% ~ 15% of recirculation system.
Advantages: simple structure, low cost and easy implementation.
Disadvantages: the system lacks flexibility and low degree of freedom.
With the implementation of diesel engine electronic control system, EGR system has also entered electronic control, and the working process is controlled by ECU. ECU controls the opening of EGR valve indirectly (EGR solenoid valve) or directly (motor) according to the coolant temperature, intake air volume, speed, starting and other information of the engine, and makes fine adjustment to the EGR valve by detecting the duty cycle signal, EGR valve opening and other feedback signals in combination with the engine working conditions, so as to form a high-precision closed-loop control. This time, we mainly introduce the electronically controlled eg with EGR solenoid valve of recirculation system.
2、 Electronic control solenoid valve EGR system
Component composition: EGR solenoid valve, EGR valve, EGR valve opening sensor and ECU.
1. EGR solenoid valve
Function: the ECU controls the EGR solenoid valve through the duty cycle signal to reduce the back pressure of the EGR valve, so as to open the EGR valve.
Structure (as shown in the figure below): the EGR solenoid valve has three vent ports. When the EGR solenoid valve is not energized, the spring compresses the valve body upward, and the vent port to the atmosphere is closed.
At this time, the EGR solenoid valve connects the intake manifold with the EGR valve vacuum chamber; When the EGR solenoid valve coil is energized, the generated electromagnetic force moves the valve body downward, the lower end of the valve body closes the vacuum channel to the intake manifold, and the atmospheric valve port at the upper end is opened, so the vacuum chamber of the EGR valve is connected with the atmosphere.
2. EGR valve
Function: control the valve of exhaust gas entering the air inlet pipeline.
Structure (as shown in the figure below): one side (lower part) of EGR valve diaphragm is connected to the atmosphere, and the other side equipped with spring is a vacuum chamber, and its vacuum degree is controlled by EGR solenoid valve. Increase the vacuum degree of the vacuum chamber to make the diaphragm arch up against the spring force, the opening of the valve will increase and the exhaust gas recirculation flow will increase. When the upper part loses vacuum, the diaphragm arches downward under the action of spring force to close the valve and block exhaust gas recirculation.
3. EGR valve opening sensor
Function: the EGR position sensor monitors the EGR opening position and transmits it to the ECU for EGR exhaust gas recirculation control.
Pin definition: A22: power supply 5V; A40: ground wire; A12: signal output
2. Inspection of EGR valve
Apply 15kpa vacuum to the EGR valve, and the EGR valve shall be able to open; EGR valve shall be able to close completely when no vacuum is applied.
What is EGR exhaust gas recirculation system? How does it work
3. Inspection of EGR solenoid valve
The measured resistance value shall be 33 ~ 39 Ω. In case of no power supply, the air blown from the connector on the side of the air inlet pipe shall be unblocked, and the air blown from the filter screen connected to the atmosphere shall be blocked. When powered on, it is just the opposite of the above.