When the engine is very cold, the air compression temperature may not be sufficient to ignite the fuel. In this case, engine preheating is required. For this purpose, an electric glow plug is installed in each engine cylinder. The duration of engine preheating depends on the outside air temperature and is regulated by a special relay of the DDE control unit.
Fuel is sucked in by the fuel pump located in the fuel tank and fed to the high-pressure fuel pump (HPFP). In the HPFP, the fuel is compressed to the pressure required for injection. Injection occurs in a manner similar to the ignition sequence in a gasoline engine.
Fuel injection into the cylinders can be carried out in three ways: through a pre-chamber, a swirl chamber or directly into the combustion chamber.
With swirl-chamber and pre-chamber injection, diesel fuel is injected into the pre-chamber of the corresponding cylinder. The hot fuel mixture immediately ignites. However, the volume of oxygen contained in the pre-chamber is sufficient to burn only part of the injected fuel. The remaining unburned fuel, under the action of combustion pressure, is thrown into the main combustion chamber, where complete afterburning of the fuel occurs.
With direct injection, as used in the BMW 3 Series engine, the high-pressure fuel pump directly injects fuel into the combustion chamber, namely, into the cavity in the piston. In this case, the high-pressure fuel pump creates a pressure of about 1,300 bar and injects fuel in two stages. Thanks to the multi-nozzle injectors, a small amount of fuel is first pre-injected, which, when burned, creates favorable conditions for the injection of the main amount of fuel. This injection process ensures soft and silent combustion, similar to swirl-chamber combustion. The amount of injected fuel is regulated by an electronic control unit.
Before the fuel enters the injection pump, it is cleaned in the fuel filter from dirt and water. Therefore, it is very important to regularly clean the fuel filter as part of maintenance.
The high-pressure fuel pump does not require maintenance. All moving parts are lubricated with diesel fuel. The high-pressure fuel pump is driven by a chain.