Author: Site Editor Publish Time: 2020-09-23 Origin: Site
The fuel metering unit is also called the fuel metering proportional valve，suction control valve（SCV）. It is installed at the fuel inlet position of the high-pressure fuel pump, used to adjust the fuel supply and fuel pressure, and is controlled by the ECU. The control process is affected by the value measured by the rail pressure sensor. Simply put, the metering unit can be compared to a throttle valve, or to an adjustable faucet.
The internal oil path of the high pressure pump
Fuel tank → coarse consideration → hand oil pump → gear pump → careful consideration → metering unit → high pressure pump → fuel rail, it can be seen that the metering unit is the only way for the fuel from the tank to enter the high pressure pump, and the quality of the metering unit directly affects the entry The amount of oil in the high-pressure pump will affect the power and economic performance of the vehicle.
The fuel metering unit has two types: normally open and normally closed. It is a proportional solenoid valve. The normally open type is mainly used in transportation vehicles, while the normally closed type is often used in passenger vehicles. For the normally open metering unit, it is normally open when the power is off, the flow is the largest, and it is closed when the power is on, and the flow is small. For the normally closed metering unit, it is normally closed when the power is off, and the flow is the smallest, and it is open when the power is on, and the flow is large.
When the control coil is not energized, the fuel metering unit is turned on and can provide the fuel pump with the maximum flow of fuel. The ECU controls the on and off time of the metering unit through the duty cycle signal to control the amount of oil entering the high-pressure pump to achieve the level of rail pressure. All in all: the shorter the power-on time, the longer the metering unit is on, the higher the rail pressure will be
Common models are:
The normally closed metering unit is the opposite of the normally open type. The normally closed metering unit is closed when it is powered off. No fuel enters the high-pressure pump. The longer the metering unit is energized, the more oil that enters the high-pressure pump, the higher the rail pressure.
Common models are:
The amount of oil entering the high-pressure pump is determined by the opening time of the metering unit, not the opening angle. The longer the opening time, the greater the fuel supply, the higher the rail pressure.
There are two types of fuel metering valves: normally open and normally closed. Pay special attention to them during maintenance. Due to the failure of the fuel system oil circuit itself, such as fuel quality, fuel pump wear and other reasons, the fuel metering valve wear or jam failure indirectly caused, when replacing the new fuel metering valve, the entire fuel system oil circuit must be checked at the same time , Clean and replace fuel that meets standards.
Because the huge pressure generated when the high-pressure fuel overflows may cause serious damage to the skin and eyes, and the leaked high-pressure fuel may be ignited when it encounters the exhaust system, and there is a danger of fire. Therefore, when performing any operation on the high-pressure fuel system including fuel metering valve and its corresponding components, the disassembly and assembly specifications provided by the manufacturer must be followed.
When the fuel metering unit fails, such as stuck in the closed position and cannot be opened, the fuel cannot enter the high-pressure pump, so the engine will stall or fail to start.
When the fuel metering unit fails such as stuck in the normally open position, the fuel is always supplied at the maximum flow rate, the fuel rail pressure is over-high and the pressure limiting valve will be opened, and the ECU enters the failure protection mode. The engine speed is limited to 1500 rpm
When the fuel metering unit fails, such as the valve core is worn, it will cause the actual fuel supply to be inconsistent with the fuel required by the ECU. The common failure phenomenon of the vehicle is that it is not easy to start, insufficient power, and increased fuel consumption
The failure of the metering unit will affect the power reduction of the engine, and even the engine flameout and the increase of fuel consumption. The automobile fuel metering unit is also called the fuel metering proportional valve. It is installed at the fuel feed position of the high-pressure fuel pump and is controlled by the ECU and is used to adjust the fuel supply volume and fuel pressure value.
In the common rail system, the fuel metering unit is a very easily damaged component. When the metering unit fails, the engine fault light will first light up. At the same time, it will be accompanied by vehicle weakness, unstable engine idling, and black smoke from exhaust.