Lift Pump

The lift pump is a low-pressure pump that supplies fuel to the injection pump, or unit injectors. Often called a transfer or supply pump, the lift pump transfers fuel from the tank to the fuel injection system. In modern vehicles, the tank is mounted below the engine, and the fuel has to be lifted to the level of the engine. Three types of lift pumps are common on light-duty diesel cars and trucks: diaphragm, plunger, and vane.

Diaphragm Lift Pump
FIGURE 54-14
A mounted diaphragm-type lift pump.

The diaphragm lift pump can be mounted on the engine or on the injection pump Figure 54-14. It is fitted with inlet and outlet valves, and an eccentric on a camshaft acts on a two-piece rocker arm connected to a diaphragm. Rotating the eccentric causes the rocker arm to pivot on its pin and pull the diaphragm down. This compresses the diaphragm return spring and increases the volume in the pumping chamber above the diaphragm.

Atmospheric pressure at the fuel tank forces fuel along the fuel line to open the inlet valve. Fuel flows into the pumping chamber. The eccentric keeps rotating, and the rocker arm is released. The spring exerts force on the diaphragm to pressurize the fuel in the chamber. This pressure closes the inlet valve and opens the outlet valve, letting fuel be delivered to the injection system.

If the system does not need all of the fuel delivered, the pressure in the outlet fuel line rises to the same level as in the pumping chamber. This pressure holds down the diaphragm and keeps the diaphragm return spring compressed. As a result, the split linkage in the rocker arm allows the lever to maintain contact with the eccentric, without acting on the diaphragm pull rod; therefore, pumping ceases until the pressure drops and pumping action needs to resume.

Plunger Lift Pump
FIGURE 54-15
A plunger-type lift pump.

A second type of lift pump in light vehicle applications is the plunger pump. It is mounted on the in-line injection pump and is driven by a cam inside the in-line injection pump housing. Internally, a spring-loaded cam follower converts the rotary motion of the camshaft into reciprocating motion. The reciprocating motion is transferred to a spring-loaded plunger, fitted with close tolerance in a cylindrical bore. It has two spring-loaded check valves, which are called the inlet valve and the outlet valve Figure 54-15.

As the engine drives the injection pump, the lobe of the camshaft pushes the cam follower into the plunger pump. The cam follower acts directly on the plunger, pushing it toward the end of the cylinder bore. Fuel is displaced from one side of the plunger, through the outlet check valve, to the other side of the plunger. When the cam follower retracts, spring force on the plunger moves the plunger out of the cylindrical bore. Fuel from the fuel tank enters behind the plunger through the inlet check valve. Fuel in front of the plunger is displaced out of the pump to the fuel injection system.

Vane Lift Pump
FIGURE 54-16
A vane lift pump in a distributor-type injection pump.

A vane lift pump is used in distributor-type injection pumps. Also known as a transfer pump, it is mounted on the input shaft and pumps fuel whenever the distributor pump is driven by the engine Figure 54-16. It consists of a rotor, mounted off-center in a pump housing. Slots are machined into the rotor to carry vanes.

As the rotor rotates, the vanes can move into, and out of, the slots. The vanes seal on the edges of the rotor slots and the pump housing. Trapped fuel is carried around by the action of the rotor until the leading vane uncovers the outlet port. Since the rotor is offset, as it continues to turn, the volume between the vanes reduces, and fuel is squeezed out of the pump. A pressure relief valve controls the pump’s operating pressure.