## Fuel and Emissions Systems

#### System Descriptions (cont'd)

11-29

Construction and Operation of Supply Pump Outline The function of the supply pump is to regulate the fuel discharge volume, thus generating the internal fuel pressure in the common rail. Construction The supply pump consists of a feed pump, which is similar to that of the conventional inline pump and the fuel metering unit (pump control valves), which are provided at each cylinder, to regulate the fuel discharge volume.

Operation

The fuel metering unit remains open during the plunger's downward stroke, allowing the low-pressure fuel to be drawn into the plunger chamber by way of the fuel metering unit.

Even after the plunger begins its upward stroke, if the valve remains open because current is not applied to the fuel metering unit, the fuel that was drawn in returns via the fuel metering unit, without becoming pressurised.

When current is applied to the fuel metering unit in order to close the valve at the timing that accommodates the required discharge volume, the return passage closes, causing the pressure in the plunger chamber to rise.

Accordingly, the fuel passes through the delivery valve (check valve) to the common rail. As a result, the amount of fuel that corresponds with the lifting of the plunger after the fuel metering unit closes becomes the discharge volume and varying the timing of the closing of the fuel metering unit (plunger pre-stroke) causes the discharge volume to vary, thus regulating the common rail pressure.

After surpassing the cam' s maximum lift, the plunger begins its downward stroke, causing the plunger chamber pressure to decrease. At this time, the delivery valve closes, thus stopping the pumping of the fuel. In addition, because the current to the fuel metering unit valve is stopped, the fuel metering unit opens, allowing the low-pressure fuel to be drawn into the plunger chamber.

a a a