Common Rail

A clever idea is turning diesel injection technology (inside out|upside down): Common Rail, a common fuel high pressure conduit for all cylinders. Up to now each cylinder had a conduit of his own.

The injection pump had to build up the injection pressure by the aid of a cam for each cylinder separately and for each processing step anew until now. The time of injection depended on the setting of the cams as well.

A single high pressure pump providing a constant pressure for all cylinders is sufficient for the Common Rail system. The clou is a pipe that can hold up to 30 cubic centimetres in which the fuel - like the air in an air pump - is compressed. Due to his compressibility the fuel functions as a pressure storage. The pump presses the fuel with a pressure of up to 1350 bar into this common conduit (Common Rail). Conduits branch to the injection nozzles from there.

The pressure on the common conduit can be freely adjusted for each load and for each number of revolutions per second. Sensors measure the number of revolutions per second on the cam shaft as well as on the crankshaft in addition to the pressure on the rail and send the measured data to the control device. A pressure regulate valve precisely (sets|adjusts) the pressure in the rail. In exhaustive engine tests the engine developers have scrutinised and programmed the optimal composition of revolutions per second. load and pressure into the controlling device.

The real (heart|central) piece of the whole system is a magnetic valve inside each injection nozzle which opens and closes the nozzle within a fraction of a millisecond. This permits to very precisely control the beginning and the amount of the injection even under high pressures.

The advantages over traditional injection systems: A high injection pressure can be made available even for lower numbers of revolutions per second and is not only - as in traditional cam-driven systems - available with a high number of revolutions per second.

Because of short switching times of the magnetic valve a "pilot injection" is possible. This means: Just before the main injection the nozzle opens for a short moment and presses a spatter of fuel - less than what fits into a pinhead - into the combustion chamber. This makes the start of the subsequent main injection smoother and dampens the noise of the fuel saving direct injector to the level of a prechamber engine. Besides, the temperature rise is not that high during combustion.

This again means fewer nitrogen oxides harmful to the environment. Karl-Heinz Hoffmann, project head "Common Rail" at Daimler-Benz sums up the advantages: "The higher the injection pressure, the smaller the injection spatters and the better the pre-processed fuel mix will burn."