With the state-of-the-art common-rail direct fuel injection used in the S 320 CDI and S 400 CDI, Mercedes-Benz has achieved an ideal compromise between economy, torque, ride comfort and long life. Whereas conventional direct-injection diesel engines must repeatedly generate fuel pressure for each injection, in the CDI engines the pressure is built up independently of the injection sequence and remains permanently available in the fuel line.

The common rail upstream of the cylinders acts as an accumulator, distributing the fuel to the injectors at a constant pressure of up to 1600 bar. Here high-speed solenoid valves, regulated by the electronic engine management, separately control the injection timing and the amount of fuel injected for each cylinder as a function of the cylinder's actual need.

In other words, pressure generation and fuel injection are independent of each other. This is an important advantage of common-rail injection over conventional fuel injection systems.