In order to ensure safe operation, a pressure limiting valve is set in the common rail system.
The pressure limiting valve can be understood as a safety valve. Its function is to limit the oil pressure in the high pressure rail. When the rail pressure exceeds the preset upper limit, the pressure limiting valve opens, and a part of the fuel flows back to the fuel tank through the oil collecting pipe, so as to achieve pressure relief. the goal of.
In fact, when the pressure limiting valve opens, it means that there is a fault in the common rail system, and the system will limit the engine output torque to ensure that the high-pressure fuel rail maintains a certain pressure so that the driver can drive the car to the nearest repair shop for repairs.
The opening of the pressure limiting valve is divided into active opening and passive opening.
- 01Actively open
When receiving failure information of some related components, the engine electronic control unit (ECU) will implement a protection strategy and command the pressure limiting valve to open (even if the rail pressure is not too high), so as to avoid serious failure of the engine. Which component failure information will trigger the ECU to execute the protection strategy?
(1) Rail pressure sensor signal
When the rail pressure sensor sends a rail pressure overrun signal to the ECU, if it is too high or too low, it will cause the ECU to execute a protection strategy and open the pressure limiting valve. There are many reasons for the rail pressure sensor to report a fault message. The essence is abnormal oil inlet pressure or oil return pressure, such as insufficient oil supply in the low-pressure oil circuit, wear and tear of the high-pressure pump, poor sealing of the pressure limiting valve, excessive oil return from the injector, Oil circuit blockage, etc.
(2) Fuel metering valve fault signal
In order to improve efficiency, the high-pressure pump of the common rail system is equipped with a metering unit for fuel quantity control, namely a fuel metering valve. It controls the oil pressure in the high pressure rail by regulating the amount of oil entering the high pressure pump. The ECU controls the metering unit by changing the duty cycle inside the metering unit through a pulse signal. When the metering valve fails, it means that the ECU cannot complete the precise control of the rail pressure. Therefore, once receiving fault information related to the metering valve, such as short circuit, open circuit fault, or abnormal temperature fault, the ECU will execute the protection strategy to limit the The pressure valve will also open.
(3) Sensor power supply module 3 fault signal
The sensor power supply module 3 provides 5V power supply for multiple sensors, including rail pressure sensors. When the sensor power supply module 3 fails, it means that all the sensors it is responsible for may work abnormally, so once the ECU receives the fault information related to the sensor power supply module 3, it will execute the protection strategy, and the pressure limiting valve will also be opened accordingly.
(4) ECU-related faults
As the commander-in-chief of the common rail system, the consequences of ECU failure can be imagined, so when the system detects ECU-related failure information, it will also implement protection strategies.
- 02 Passive open
In addition to the active opening controlled by the ECU, the pressure limiting valve will also be opened passively due to excessive rail pressure. Common reasons for passive opening include: fuel metering valve or overflow valve failure, high-pressure pump failure, rail pressure loss of control due to rail pressure sensor damage, oil return pipe blockage, etc.
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