Common rail, as the name suggests, is a shared fuel line. The rail is an accumulator supplied by a pump, which maintains the fuel at a pressure of up to 2000 bar – which is equivalent to the weight of a luxury saloon on the space of one square centimetre.
The piezo injectors used by Audi open and close within fractions of a millisecond and can inject precise and minute quantities weighing less than a thousandth of a gram. The highly dynamic system allows the number of injection processes per power stroke to be widely varied – in the case of the 3.0 TDI for example, there are up to five partial injections. This strategy reduces emissions and, thanks to pre-injection, ensures a smoother combustion process, which in turn has an extremely positive effect on the engine acoustics.