Usually we talk about the EGR system, the understanding is the exhaust gas recirculation system, by introducing exhaust gas into the combustion chamber, thereby reducing the peak of engine combustion, to achieve the purpose of reducing NOx emissions. Diesel vehicles use an external system, that is, a pipeline is connected between the exhaust pipe and the intake pipe, and a valve is installed in the pipeline to control the amount of exhaust gas entering. It can be seen that the EGR valve is the core component of the entire system, and how to accurately control and monitor it is the development direction of the 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 to reburn the technology (method or method). The main purpose is to reduce the nitrogen oxides (NOx) in the exhaust gas and increase the fuel consumption rate when sharing part of the load.
EGR working mechanism
Exhaust GasRecirculation is abbreviated as EGR, which returns a small part of the exhaust gas produced by a diesel engine or a gasoline engine back to the cylinder.
Due to the inertness of the recirculated exhaust gas, the combustion process will be delayed, which means that the combustion speed will slow down and the pressure formation process in the combustion chamber will be slowed down. This 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 (mass flow), so the total pollutant output in exhaust emissions will be relatively reduced. The task of the EGR system is to make the recirculation amount of exhaust gas reach the best condition at each operating point, so that the combustion process is always in the most ideal situation, and ultimately to ensure the lowest pollution components in the emissions. Since changes in the amount of exhaust gas recirculation may have diametrically opposite effects on different pollution components, the so-called optimal situation is often a compromise, so that the total emission of related pollutants can reach the best plan. For example, although increasing the exhaust gas recirculation rate has a positive effect on reducing nitrogen oxide (NOx) emissions, it will also have a negative effect on the increase of particulate matter and other pollutants.
Different engine operating conditions have different requirements for EGR amount. In order for the EGR system to function more effectively, the amount of exhaust gas participating in EGR must be limited.
As the load increases, the amount of EGR increases correspondingly, and can reach the optimal value;
At idling speed and low load, the NOx emission concentration is low. To ensure normal combustion, EGR is not performed;
During the warm-up process, the engine temperature is low, and the NOx emission concentration is also low. In order to prevent EGR from deteriorating the combustion process, EGR is not performed;
When heavy load, high speed or full throttle, EGR is not used to ensure the power of the engine;
When accelerating, in order to ensure the acceleration of the car and the necessary purification effect, EGR plays a role in the transition process.
way to control
According to the above-mentioned design principles of EGR, EGR must be controlled and adjusted so that the application of EGR in the engine can achieve the desired effect. There are many ways to control and adjust EGR, which can be classified from different angles according to its main characteristics
One. Mechanical EGR system
The gas control method by controlling the negative pressure of the intake pipe and the exhaust pressure is suitable for EGR rate of 5% to 15%.
Advantages: simple structure, low cost, easy to implement.
Disadvantages: The system lacks flexibility and low degree of freedom.
With the implementation of diesel electronic control system, EGR system has also entered electronic control, and the working process is controlled by ECU. ECU controls the opening of the EGR valve indirectly (EGR solenoid valve) or directly (motor) according to the engine’s coolant temperature, intake air volume, speed, starting and other information, and detects the duty cycle signal and feedback signals such as EGR valve opening. Make fine adjustments to the EGR valve in combination with the engine operating conditions to form a high-precision closed-loop control. This time we mainly introduce the electronically controlled EG with EGR solenoid valve
two. Electronically controlled solenoid valve EGR system
Components: EGR solenoid valve, EGR valve, EGR valve opening sensor, ECU.
1. EGR solenoid valve
Function: ECU controls the EGR solenoid valve through the duty cycle signal to reduce the back pressure of the EGR valve, thereby opening the EGR valve.
Structure: The EGR solenoid valve has three air ports. When the EGR solenoid valve is not energized, the spring presses the valve body upward, and the atmospheric valve port 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 electromagnetic force generated causes the valve body to move down, and the lower end of the valve body closes the vacuum passage leading to the intake manifold, and The upper air valve port is opened, so that the vacuum chamber of the EGR valve is open to the atmosphere.
2. EGR valve
Function: The valve that controls the exhaust gas to enter the intake pipe.
Structure (as shown in the figure below): One side (lower part) of the EGR valve diaphragm is open to the atmosphere, and the other side with a spring is a vacuum chamber, the vacuum degree of which is controlled by the EGR solenoid valve. Increasing the degree of vacuum in the vacuum chamber causes the diaphragm to arch up against the spring force, the valve opening degree increases, and the exhaust gas recirculation flow rate increases. When the upper part loses vacuum, the diaphragm arches downward under the action of the spring force to close the valve and block the recirculation of exhaust gas.
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
Signal voltage: The larger the EGR valve opening, the higher the signal voltage fed back to the ECU.
Point A: valve closed position (“zero” point position)
Point B: starting position of anti-shock during valve closing
Point C: 6mm position point (100% opening position point)
Point D: Maximum stroke position (fully open dead center)
three, the working principle of the exhaust gas recirculation system
1. EGR on-off control
2. EGR rate control
If the EGR rate is too high above 20%, the combustion speed will be too slow, the combustion will become unstable, the misfire rate will increase, HC will increase, and the power and economy will decrease; if the EGR rate is too small below 10%, NOx emissions will not meet the regulations. Requirements, prone to knocking, engine overheating and other phenomena. Therefore, the EGR rate must be controlled according to the requirements of the engine operating conditions. The EGR rate is usually controlled in the range of 10% to 20%.
Excessive exhaust gas will worsen the combustion of the engine and reduce power and economy. Therefore, the amount of exhaust gas recirculation must be strictly controlled. And in some special working conditions, the exhaust gas recirculation is turned off.
The amount of exhaust gas introduced is called exhaust gas recirculation rate (EGR rate)
3. Working principle
Exhaust gas contains a large amount of CO2 and water vapor, which are close to chemically inert gas. After introducing it into the cylinder, it can dilute the mixed gas in the cylinder, and the oxygen concentration will be reduced accordingly, thereby alleviating the violent combustion reaction. ·CO2 can’t burn but can absorb heat and lower the temperature. Reduce the production of NOX.
4. EGR control strategy
Comprehensive consideration of power, economy, and emission performance.
1) Cold engine, idle speed and light load: low NOx, in order to ensure normal combustion, EGR is not performed.
2) Heavy load and high speed: to ensure power, at the same time, the mixture is rich, NOx is low, EGR is not performed or the EGR rate is reduced.
3) Partial load: As the load increases, the allowable EGR rate also increases.
four. Overhaul of EGR control system
1. General inspection
When idling, remove the vacuum hose on the EGR valve, the engine speed should not change, and there should be no suction when touching the test vacuum tube port with your hands; if the speed reaches 2500r/min or more, if you also remove the vacuum hose, the engine speed should increase significantly (Exhaust gas recirculation is interrupted).
2. Inspection of EGR valve
Apply 15kPa vacuum to the EGR valve, the EGR valve should be able to open; when no vacuum is applied, the EGR valve should be able to close completely.
3. Inspection of EGR solenoid valve
Measure the resistance value, it should be 33～39Ω. When there is no electricity, the air blowing in from the side joint of the air intake pipe should be unblocked, and the air blowing in from the air filter should be blocked. When energized, it is just the opposite of the above.