Lesson 2/Learning Event 1
The fuel pump is made of three main units: a gear pump, a pressure regulator, and a governor and throttle.
- Gear pump - draws fuel from the supply tank and delivers it under pressure through the pump and
supply lines to the individual injector.
- Pressure regulator - limits the pressure of fuel to the injector.
- Governor and throttle - act independently of the pressure regulator to control fuel pressure to the
Fuel circulates through the injector at all times, except during a short period following injection into the
combustion chamber. From the inlet connection, fuel flows down the inlet passage of the injector, around
the injector plunger, between the body end and cup, up the drain passage to the drain connections and
manifold, and back to the supply tank.
As the plunger comes up, the injector feed passage is opened and fuel flows through the metering orifice
into the cup. At the same time, fuel flows past the cup and out the drain orifice. The amount of fuel
entering the cup is controlled by the fuel pressure against the metering orifice, and fuel pressure is controlled
by the fuel pump. During injection, the plunger comes down until the orifice is closed, and the fuel in the
cup is injected into the cylinder. While the plunger is seated in the cup, all fuel flow in the injector is
Fuel injection pumps must be supplied with fuel oil under pressure, because they have insufficient suction
ability. Therefore, all injection systems require supply pumps to transfer fuel from the supply tank to the
injection pump. Pumps for this purpose have a positive suction lift, and their performance is independent of
any normal variations in viscosity, pressure, or temperature of the fuel. The pumps in use today are gear,
plunger, and/or vane types.