Bosch launched the first common rail system in 1997. The system is named after the shared high-pressure reservoir (common rail) that supplies all the cylinders with fuel. With conventional diesel injection systems, the fuel pressure has to be generated individually for each injection. With the common rail system, however, pressure generation and injection are separate, meaning that the fuel is constantly available at the required pressure for injection.
Common rail systems have a modular design. Each system consists of a high-pressure pump, injectors, a rail, and an electronic control unit.
Mode of operation
With conventional diesel injection systems, the fuel pressure has to be generated individually for each injection. With the common rail system, however, pressure generation and injection are separate, meaning that the fuel is constantly available at the required pressure for injection. Pressure generation takes place in the high-pressure pump. The pump compresses the fuel and feeds it via a high-pressure pipe to the inlet of the rail, which acts as a shared high-pressure reservoir for all injectors – hence the name "common rail". From there, the fuel is distributed to the individual injectors, which inject it into the cylinder's combustion chamber.
Broad range of solutions
Bosch offers common rail systems for all vehicle models from microcars to heavy luxury limousines. The high-pressure pumps operate at pressures between 1,100 and 2,200 bar. Systems with individual pumps are also available (plug-in pumps). The injectors use either solenoid valve or Piezo technology.
The high-pressure pump compresses the fuel and supplies it in the required quantity. It constantly feeds fuel to the high-pressure reservoir (rail), thereby maintaining the system pressure. The required pressure is available even at low engine speeds, as pressure generation is not linked to the engine speed. Most common rail systems are equipped with radial piston pumps. Compact cars also use systems with individual pumps which operate at a low system pressure.
The injector in a common rail system consists of the nozzle, an actuator for Piezo injectors or a solenoid valve for solenoid valve injectors, as well as hydraulic and electrical connections for actuation of the nozzle needle.
It is installed in each engine cylinder and connected to the rail via a short high-pressure pipe. The injector is controlled by the Electronic Diesel Control (EDC). This ensures that the nozzle needle is opened or closed by the actuator, be it solenoid valve or Piezo. Injectors with Piezo actuators are somewhat narrower and operate at a particularly low noise level. Both variants demonstrate similarly short switching times and enable pre-injection, main injection and secondary injection to ensure clean and efficient fuel combustion at every operating point.