The purpose of the common rail system is to increase the efficiency of diesel engine performance. During operation, an amount of fuel is delivered from the fuel pump into the common rail, which is a kind of accumulator, where it is then sent to the injectors. These electronically controlled injectors control the amount of fuel and timing when this fuel is injected under a certain pressure.
The engine ECU can control injection quantity and timing to a high level of precision, independent of engine speed or load. Even multi-injection (multiple fuel injections in one injection stroke) is possible. This ensures stable injection pressure at all times, even in the low engine speed range, and dramatically decreases the amount of black smoke emitted during start-up and acceleration.
Once the fuel is delivered from the tank to the engine, the common rail pump creates the high pressure ready for it to be put into the combustion chambers via the injectors. This makes the common rail pump a key part of the common rail system.