High Pressure Injection System:
A BMW gasoline direct injection system is used for the first time in the N73 engine series. Each fuel rail is supplied with fuel by a high pressure pump (HDP), which is driven via a bucket tappet by a triple lobe cam on the exhaust camshaft.
The two high pressure pumps are supplied with fuel by the electric fuel pump (EKP), which is located in the fuel tank. The high pressure fuel injectors are connected to a pressure rail (accumulator) for each cylinder bank. The two rails are not interconnected.
High Pressure Pumps: A single cylinder high pressure pump is used for each cylinder bank. The pumps are mounted on the cylinder heads and driven via bucket tappets by triple cam lobes on the exhaust camshafts. Each pump has three connecting lines: feed line, high pressure line and leakage line.
Fuel is delivered to each high pressure pump through the feed line at a pressure of 6 bar from the electric in tank fuel pump via a T-branch. In the pump, the fuel passes through the inlet valve (2) into the high pressure area (3).
As the pump plunger is forced up by the camshaft, the fuel is pressurized (up to 120 bar) in this area. The pressurized fuel is then forced out through the high pressure line to the fuel rail. The outlet valve (4) prevents back flow from the rail into the high pressure pump.
Due to the extreme pressure on and around the plunger shaft, a small amount of fuel (max. 1 liter per hour) flows past the plunger shaft against the sealing ring (9). This also serves as lubrication for the plunger shaft. The sealing ring provides a seal between the fuel side of the pump and the engine oil at the pump drive.
To relieve the pump pressure (up to 120 bar) at the sealing ring, the pressure is reduced in two stages, at which point the fuel returns through the leakage line to the tank. The pump pressure is reduced down to 6 bar at the annular groove (7) because it is connected by a channel (8) to the feed area of the pump. The fuel flow from this channel is regulated by the fuel quantity control valve (MSV).
Below the annular groove, some fuel flows past the pump plunger against the sealing ring. At this point the fuel pressure is virtually reduced to atmospheric pressure, which is sufficient to return the fuel through the leakage line to the tank.
Fuel Quantity Control Valves (MSV): The fuel quantity control valve (10) is installed in the high pressure pump to regulate the fuel delivery rate as required based on load and engine RPM. This valve opens a channel from the high pressure chamber (3) to the feed area allowing excess fuel to return to the feed area.
When the pump plunger is at it's lowest position, the valve is energized closed by the ECM. The valve is de-energized as soon as the injection pressure calculated by the ECM is reached during the upwards travel of the pump plunger. The valve is now opened to allow excess fuel to return to the feed area. This switching is repeated three times per camshaft revolution because the drive cam for the pump has three lobes.
The pulsations generated in the pump during the process are absorbed by the pressure attenuator spring (12). The pressure attenuator is sealed by a diaphragm (11) from the pump feed area.