The fuel high pressure accumulator (rail) is located beside the cylinder head cover underneath the engine bonnet.
Task
In the rail, the fuel is subjected to high pressure and delivered (accumulated) for injection.
This accumulator which is used by all cylinders (where the name common rail comes from) maintains its internal pressure at a near constant value, even when handling larger quantities of fuel. This ensures that the injection pressure remains at a near constant when the injector is opened.
Fluctuations in pressure resulting from the pump delivery and injection are dampened by the accumulator volume.
Structure
The rail is essentially a thick-walled pipe with connections for lines or sensors.
In the M57, the rail pressure sensor is located at one end of the rail.
In the M67, there are 2 rails which are connected to one another by a ring conduit. The rail pressure sensor in this rail system is screwed from bottom to top in the distributor block. The pressure control valve is also mounted on the distributor block instead of on the HPP.
There is no throughput limiter in the M57 and M67 (unlike other common rail systems).
Because of the different conditions for engine installation, the rail can take on a variety of forms. The smaller the rail volume and/or the bore diameter with the same outer contour, the higher the demands that can be placed on the system. A small rail volume also keeps the demands on the high pressure pump’s delivery capacity to a minimum when the vehicle is started up and when the nominal value of the pressure is altered. On the other hand, the rail volume must, however, be sufficiently high to prevent drops in pressure during injection.
The internal diameter of the longitudinal bore in the rail is approx. 9 mm.
The rail is continuously supplied with fuel from the high pressure pump. The fuel is delivered from this intermediate accumulator to the injectors via the injector connection lines. The rail pressure is set via the pressure control valve.
Function
The volume present in the rail is constantly filled with the pressurized fuel. The spring action of the fuel brought about by the high pressure is used to create an accumulator effect.
If fuel is removed from the rail for the purposes of injection, the pressure in the high pressure accumulator remains at a near constant. Fluctuations in pressure created by the cycled supply of the high pressure pump are dampened and/or compensated.
