Common Rail System: Design

The primary pump (1) supplies fuel to a high-pressure reservoir, the fuel rail (2), where the fuel is stored at the optimal pressure for the engine's instantaneous operating conditions in preparation for subsequent injection.

Each of the engine's cylinders is equipped with an injector (4) featuring an integral solenoid valve. This solenoid valve opens and closes to define the starting point and mass of the injection process.

The driver's demand is defined by the accelerator pedal. The ECU (3) registers driver demand and current vehicle operating conditions as the basis for calculating the required fuel pressure, injection duration (= fuel mass) and injection timing according to the parameters defined in the program map


Common Rail System: Types

In addition, Common Rail technology differs from conventional systems by providing multiple injections per working cycle. This cycle is divided into pre-injection (pilot injection) for quiet engine running, main injection for ideal power deployment and secondary injection for reduced emissions. The fuel reaches the injector via short pressure lines and then through the injection nozzles into the combustion chamber.

The higher the injection pressure, the finer the injection system vaporizes the fuel, thus permitting even more efficient combustion. In 2005, Bosch already introduced the third Common Rail generation with injection pressures of 1,800 bar for light commercial vehicles onto the market. A version of this system for medium-sized and heavy commercial vehicles will follow in 2007. The first Bosch generation for commercial vehicles went into series production in 1999 with an injection pressure of 1,400 bar. The second generation with a pressure of 1,600 bar followed in 2001.

Against the background of ever-stricter exhaust-gas pollutant values worldwide, Bosch has continued to further develop its injection systems. Based on the 1,800 bar injector, the company will continue to develop its systems for pressures of 2,000 bar and 2,200 bar.

Common Rail System: Injectors

The injectors are installed in the engine's cylinder head, and have the same function as nozzles and nozzle holders on the previous injection systems. The main injector components are:
Hole-type nozzle, hydraulic servo-system, solenoid valve

Injector functions

The forces required to open and close the nozzle needle cannot be generated by the solenoid valve on its own. The nozzle needle is therefore indirectly triggered via a hydraulic force-amplification system.

With the solenoid valve closed, the complete chamber volume and the rail are at the same pressure. The nozzle needle is forced against its seat by a spring.

When the solenoid valve opens, fuel flows from the valve control cavity and into the fuel return.The feed throttle prevents complete pressure equalization, and the pressure in the cavity drops. The excess pressure in the chamber volume overcomes the spring force and lifts the needle so that injection can start.

The solenoid valve is no longer energized and closes the opening to the fuel return. The force applied to the control plunger increases along with the increasing pressure in the valve control cavity. The needle closes and injection stops.

Information regarding common rail system was provided by Bosch:
http://rb-kwin.bosch.com/us/en/powerconsumptionemissions/dieselsysteme/dieselsystem/commercialvehiclesystems/injectionsystems/commonrailsystem/index.html