The low-pressure system has the task of supplying the high-pressure pump (distributor pump, in-line fuel-injection pump, single-plunger pump, unit Injector or unit pump) with enough fuel to compress and distribute to the individual injection lines. The low-pressure system includes:
• electric fuel pump, or gear pump
• fuel filter
• fuel cooler
• low-pressure lines
The electric fuel pump is installed upstream of the high-pressure pump. Various systems are used depending on the type of injection pump employed.
In-line fuel-injection pumps are generally combined with plunger pumps. These are usually flanged on the side of the injection-pump casing. Their plunger is driven by the camshaft of the injection pump. By contrast with more modern systems, plunger pumps can only supply the fuel in pulses. Depending on their construction, they supply the largest amount at every second plunger stroke (single-acting pump) or supply the same amount at every stroke (double-acting pump).
In modern Common Rail, UPS and UIS engines, gear pumps are used as presupply systems. By contrast with plunger pumps, they provide a continuous supply of fuel to the injection system and are often integrated in the casing of the injection pump.
Gear pumps normally consist of a casing with inlet and outlet lines together with two gear wheels of which one is driven. The most frequent type is the so-called outside-gear pump, where the fuel is conveyed in the spaces between teeth and casing. Their simple construction makes them sturdy and inexpensive.