Common Rail Injection

A high-pressure pump keeps fuel in a shared rail at very high pressure, typically well above 1,000 bar on modern engines. Electrically controlled injectors open for precise moments, and the engine can inject several times per combustion cycle: pilot, main and post injections.

This control makes modern diesels quieter, cleaner and more efficient than older pump-injection systems. It also makes them sensitive to fuel quality: water or dirt in the fuel can damage the pump and injectors. Common rail made modern diesels much quieter and cleaner because the injection pressure no longer depends on engine speed. It is used in almost all passenger car diesels built in recent decades.

A rail pressure sensor reports the actual pressure, and a regulator or metering valve adjusts it to the target. The control unit drives each injector with timing and duration calculated for load and speed. A pressure sensor on the rail reports the actual pressure, and a regulating valve adjusts it. Injectors are opened by solenoids or piezo crystals and can fire several times per combustion cycle.

Rail pressure faults (for example P0087) cause hard starting, loss of power and limp mode. A clean fuel filter and correct fuel are the main protection for the system. The system works at very high pressure, so you must never open fuel lines with the engine running. Clean fuel and a regularly changed fuel filter protect the high-pressure pump and the injectors.