Fuel rail:
A fuel rail is mounted on engines that use high-pressure injection, such as common-rail diesel engines and nowadays increasingly on petrol engines. The high-pressure pump supplies a pressure that is maintained in the fuel rail. All injectors are connected to it. Each injector injects at a different moment. As a result, there is always a regular pressure “loss”, which is corrected again by the high-pressure pump. Further on this page, the differences and pressures of the rails on petrol and diesel engines are described in more detail.
The fuel rail not only supplies the injectors with fuel, but also ensures that the pressure fluctuations caused by the pump are damped. This way, the injectors are not affected by them. A fuel rail always has a supply line (from the pump), a return line (which goes back to the tank), a pressure sensor and a pressure control valve.
Rail pressure sensor:
The rail pressure sensor is a MAP sensor (a piezoresistive semiconductor sensor). This sensor transmits the pressure in the fuel rail to the ECU. When the pressure changes, a different voltage is transmitted, which the control unit then recognizes.
If the pressure becomes too high, the pressure control valve will reduce it. More fuel will then go to the return side (and be routed back to the tank). If the pressure is too low, the fuel pump will increase the pressure in the fuel rail.
Fuel rail on a diesel engine:
Below is an image of a V8 common-rail diesel engine from BMW. It clearly shows how the line from the high-pressure pump runs to the valve block and then branches off to both fuel rails. Each cylinder bank has its own rail. In this engine, a valve block has been chosen so that the pressure control valve and the rail pressure sensor are mounted centrally and therefore do not need to be duplicated.
From the fuel rail, metal lines with large unions run to the injectors. This is necessary because the pressure in the rail can rise to as much as 1300 bar.
Fuel rail on a petrol engine:
In the image there are two fuel rails of a V8 petrol engine. These fuel rails differ greatly from those of the diesel engine. The injectors on the petrol version are simply mounted to the fuel rail with clips, in contrast to the diesel engine, where the injectors are mounted to the rail with separate lines and large unions. On the petrol engine, the pressure rises to between 3 and 8 bar, where the clips (with which the injectors are attached to the rail) are sufficiently resistant to these pressures.
