With a very cold engine, the compression temperature of the air 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 cylinder of the engine. The duration of engine preheating depends on the outside temperature and is controlled by a special relay on the DDE control unit.
Fuel is sucked in by the fuel priming pump located in the fuel tank and supplied to the high pressure fuel pump (injection pump). In the injection pump, the fuel is compressed to the pressure required for injection. Injection is carried out in the same order as the ignition order in a gasoline engine.
Fuel injection into the cylinders can be carried out in three ways: through the prechamber, 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 ignites immediately. However, the volume of oxygen contained in the prechamber is sufficient to burn only a portion of the injected fuel. The remaining unburned part of the fuel under the action of combustion pressure is thrown into the main combustion chamber, where the complete afterburning of the fuel occurs.
With direct injection, used in the BMW 3 Series engine, the injection pump directly injects fuel into the combustion chamber, namely into the cavity in the piston. At the same time, the injection pump creates a pressure of about 1300 bar and injects fuel in two stages. Thanks to multi-nozzle injectors, a small amount of fuel is first injected, which, when burned, creates favorable conditions for the injection of the main amount of fuel. This injection process provides soft and silent combustion similar to swirl chamber combustion. The amount of injected fuel is controlled by an electronic control unit.
Before fuel enters the injection pump, it is cleaned in the fuel filter from contaminants and water. It is therefore very important to clean the fuel filter regularly as part of maintenance.
The injection pump does not require maintenance. All moving parts are lubricated with diesel fuel. The injection pump is driven through a chain.