Fuel Distributor Operation

Fuel Distributor Cut-away:

Fuel Pressure Regulators:

The Fuel Distributor is the part of the mixture control unit that meters the fuel and then distributes it to the individual fuel injectors. The Fuel Distributor uses the position of the air flow sensor plate to measure the amount of air drawn in by the engine. The position of the plate is transmitted to the control plunger by a lever.

Depending upon the position of the control plunger in the plunger cylinder, the control plunger opens or closes metering ports in the plunger cylinder to a greater or lesser degree. The fuel flows through the open section of these ports to the upper and lower chambers of the Fuel Distributor and then to the fuel injectors. If the sensor plate travel is only very small, the control plunger is only lifted slightly and, as a result, only a small section of the metering ports are opened for the passage of fuel. On the other hand, with a greater plunger travel, a larger section of the port is opened and more fuel can flow. There is a linear (direct) relationship between sensor plate travel and the amount of metering port opened for fuel flow.

A hydraulic pressure is applied at the top of the control plunger, and acts in opposition to the movement resulting from the sensor plate. A constant air-fuel pressure drop at the sensor plate is the result, this ensures that the control plunger always follows the movement of the sensor plate lever. A pressure spring (not shown) is used to assist the hydraulic pressure and also prevents the control plunger from being drawn up due to vacuum effects when the system cools down. A damping orifice serves to dampen oscillations that can occur because of sensor plate forces. When the engine is switched off, the control plunger sinks until it comes to rest against an axial sealing ring. This is secured by an adjustable screw and should be set to the proper height to ensure that the metering ports are closed completely by the control plunger when it is in its zero position. This feature serves to prevent pressure loss due to leakage past the control plunger, and prevents the fuel accumulator from emptying through the control plunger gap.

The air flow sensor has a linear characteristic. This means that if double the quantity of air is drawn in, the sensor plate travel is also doubled. If this (linear) travel is to result in a change of the basic injection quantity in the same relationship, then a constant pressure drop must be guaranteed at the cylinder metering ports independent of the amount of fuel flowing through them. The metering valves maintain the drop in pressure between the upper and lower chambers constant, independent of fuel flow-volume. The difference in pressure is 0.2 bar, and ensures a high degree of metering accuracy.

Fuel Distributor Channels:

The metering valves are of the flat-seat type, and are located in the Fuel Distributor. The upper and lower chambers are separated by means of a diaphragm. The lower chambers of all the valves are provided with a helical spring and are connected to one another, as well as to the differential pressure regulator. The metering valve seat is located in the upper chamber, and each upper chamber is connected to a plunger cylinder metering port and its corresponding fuel injector line. The upper chambers are completely sealed off from each other. The pressure differential at the cylinder metering port is determined by the force of the helical spring in the lower chamber, together with the differential pressure regulator.

Differential Pressure Valves:

If a large basic injection quantity flows into the upper chamber, the diaphragm bends downwards and opens the outlet of the metering valve until the set differential pressure is reached again. If the flow quantity drops, the metering valve opening is reduced due to the equilibrium of forces at the diaphragm until a pressure differential of 0.2 bar is again reached. This means that an equilibrium of forces exists at the diaphragm which can be maintained for every basic injection quantity by controlling the metering valve opening size.