In a very cold engine, it may happen that the temperature required for the fuel to ignite spontaneously is not reached by compression alone. In this case, the engine must be preheated. For this purpose, a glow plug is installed in each combustion chamber, which heats the air in the chamber volume. The duration of the preheating depends on the ambient temperature and is regulated by the DDE control unit using the preheating relay.
Fuel injection in a diesel engine is carried out in three different ways: pre-chamber injection (pre-chamber mixture formation) or a vortex chamber (vortex chamber mixture formation), and also injection directly into the combustion chamber (direct mixture formation).
In the case of swirl-chamber and pre-chamber mixture formation, an electric booster pump located in the fuel tank supplies fuel to the distributor-type fuel injection pump. The required high pressure is created in the fuel injection pump and diesel fuel is injected into the swirl-chamber or pre-chambers of the corresponding cylinders in accordance with their operating order. The resulting mixture immediately self-ignites. The amount of oxygen in the pre-chamber is sufficient to burn only part of the injected fuel. The unburned portion of the fuel, under the pressure formed as a result of the combustion that has begun, is ejected into the main combustion chamber, where it burns completely. In direct mixture formation, the fuel is sucked in by the booster pump from the tank and supplied to the high-pressure pump (HP). In the HP, even at a low crankshaft speed, a constant high pressure of approximately 1350 bar is created.
From the high-pressure fuel pump via the common fuel line (Common Rail) the fuel is supplied to the injectors of the individual cylinders. The common fuel line serves as a pressure accumulator and distributes the fuel under constant pressure to the injectors. The required cyclic fuel supply is precisely dosed by the engine control unit using electromagnetic valves of the injectors. If the microcomputer of the engine control unit gives the command to close the electromagnetic valve, the fuel injection is immediately stopped. In other words: the creation of high pressure and the injection of fuel are carried out independently of each other. This offers the advantage of optimizing the injection process in terms of cyclic supply and exhaust gas parameters, regardless of the crankshaft speed.
The fuel is injected through a multi-hole nozzle in two stages. First, a small amount of fuel is pre-injected, which improves the conditions for ignition of the main part of the fuel. This results in "soft" and low-noise combustion, similar to combustion in a swirl-chamber engine. Unlike other direct injection systems with camshaft control, the BMW injection system changes the parameters of the pre-injection depending on the operating conditions.
Before the fuel reaches the high-pressure fuel pump/injection pump, it flows through the fuel filter. This is where impurities and water are trapped. Therefore, it is extremely important to change the filter regularly in accordance with the operating manual.
The HP/TNVD pump does not require maintenance. All moving parts of the pump are lubricated with diesel fuel. The HP/TNVD pump is driven by a chain from the crankshaft.