On the M57, the rail pressure sensor is screwed onto the end of the rail and on the M67 it is screwed in the distributor block from the bottom to the top.
Task
The rail pressure sensor must measure the current pressure in the rail
• with sufficient accuracy and
• in correspondingly short time
and supply a voltage signal corresponding to the pressure applied at the control unit.
Structure
The rail pressure sensor consists of the following components:
• An integrated sensor element
• A printed board with an electrical evaluation switch
• A sensor housing with an electrical plug connection
The fuel reaches a sensor diaphragm through the high pressure connection. There is a sensor element located on this
diaphragm (semiconductor component) to convert the changes in shape caused by pressure into an electrical signal. The signal generated is sent to an evaluation switch via connecting lines, which relays the prepared measuring signal to the control unit via the connections.
Function
The rail pressure sensor operates according to the following principle:
The diaphragm’s electrical resistance varies when its shape changes. This change in shape (approx. 1 mm per 500 bar) created by the accumulation of system pressure alters the electrical resistance and changes the voltage of the resistance bridge which has a supply of 5 volts.
This voltage is in the 0 – 70 mV range (depending on the pressure) and is increased by the evaluation switch to a range of 0.5 – 4.5 volts. The exact measurement of the rail pressure is essential if the system is to function correctly. For this reason, the permissible tolerance ranges for the pressure sensor are also kept to a minimum when the pressure is measured. In the main operating range, the measuring precision is at 30 bar, i.e. at approx. ± 2 from the final value. If the rail pressure sensor malfunctions, the pressure control valve is activated by the control unit’s emergency function.
