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The role of common rail pressure sensor

Views:49     Author:Site Editor     Publish Time: 2020-11-20      Origin:Site

Common rail pressure sensor is one of the important components of the diesel engine fuel system. A sensor is used to measure the pressure in the fuel rail and provide electrical signals to the electronic control unit.


diesel engine


Used to feedback the high-pressure diesel pressure signal in the common rail cavity to the ECU. Due to the high-pressure injection used in the common rail diesel control system, the injection pressure is more than 10 times higher than that of a general direct injection engine. Therefore, the ECU will monitor the diesel pressure in the common rail cavity in real-time, make judgments based on the feedback pressure signal and other feedback signals, and send command signals to the injector solenoid valve, EGR solenoid valve, SCV valve, and other control units.


Principle of the common rail pressure sensor


When the pressure in the common rail causes the shape of the silicon diaphragm to change (approximately 1mm at 150MPa), the resistance of the resistive layer connected to the diaphragm will also change, and the changed resistance will cause the passage of a 5V bridge (Wheatstone Bridge) output terminal voltage changes, through the processing of the amplifying circuit, the signal terminal output voltage changes between 0-5V, ECU will calculate the current common rail pressure based on this voltage.


The common rail stores high-pressure fuel, and the high-pressure oscillation generated by the fuel supply of the high-pressure pump and the fuel injection is attenuated in the common rail volume. This ensures that the injection pressure remains instantaneously stable when the injector is opened, and the common rail also acts as a fuel distributor. A common rail pressure sensor used to measure fuel supply pressure is installed on the common rail to perform closed-loop control of fuel pressure (closed-loop control: ECU calculates the theoretical rail pressure based on the signal from the sensor under the current engine working condition to control the fuel intake metering The opening of the proportional valve is used to control the rail pressure, and the rail pressure sensor is used to detect the current actual rail pressure, which is compared and corrected with the theoretical rail pressure to achieve closed-loop control).


PRESSURE-LIMITING-VALVE


The real-time pressure of the fuel in the common rail is measured with sufficient accuracy in a short time, and the pressure signal is converted into a voltage signal and transmitted to the ECU at the same time to realize the control of the injection accuracy and the injection timing.


The common rail pressure sensor is generally installed on the common rail pipe, and the measurement range is generally 0~200MPa. Generally speaking, the pressure sensor cannot be removed from the common rail pipe. If it is damaged, the common rail pipe must be replaced at the same time.


The fuel flows to the common rail pressure sensor through a hole in the common rail pipe, and the end of the hole is sealed by the diaphragm of the pressure sensor. The fuel flows to the diaphragm of the sensor under the action of pressure. The diaphragm converts the pressure signal into an electric signal through the sensing element, and this electric signal is transmitted to the evaluation circuit through the wire.


The working principle of the fuel pressure sensor: When the fuel pressure causes the deformation of the diaphragm, the resistance value in it will change accordingly, thereby generating a voltage change in the resistance bridge. The voltage change caused by the pressure change is generally within 70mV, and the function of the evaluation circuit is to amplify this voltage change, generally to 0.5~4.5V.


nozzle for diesel fuel


Measuring the fuel pressure in the common rail pipe is a safety guarantee for the normal operation of the fuel system. Therefore, the measurement error of the common rail pressure regulator must be small and the measurement accuracy must be relatively high.


Failure mode and cause of failure


Failure phenomenon and mechanism after failure: The engine cannot be started

Reason: At startup, the system uses the pressure of the common rail as a parameter to control the movement of the injector. Under the premise that the pressure of the common rail is known, the system controls the amount of fuel entering the cylinder by controlling the opening and closing time of the injector. If the common rail pressure signal is lost, the system loses important parameters for fuel injection control. At this time, the system controls the engine to be unable to start. In the same way, if the common rail pressure signal is suddenly lost when the engine is running, the engine will stop immediately.


Accelerated weakness or black smoke 

Reason: When the pressure value detected by the sensor differs greatly from the actual pressure value, the system controls the fuel injection according to the pressure feedback from the sensor, which will make the mixture too rich or too lean.


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