|Decoupled, quieter and better exhaust gas properties|
Common Rail is characterized by the ability to hold a relatively constant pressure in all parts of the high pressure equipment. The substantially steadier pressure build-up is also easier on the timing belts, in contrast
e.g., to the unit-injector (pump-jet) system. In the past there were pipes with
pressure pulsations, which
in this case, one would prefer to avoid if possible. This determines the size of the common rail (see picture 3), from where the system also gets it's name.
|The rail-volume is sufficient to hold the pressure when injecting|
In the past, the disadvantage of pressure pulsation was, that the full pressure was not available from the word go, because it had to be provided from a central injection pump, through the conduction system. The
storage size must be calculated in such a way that, on the one hand, the pressure when injecting from one injector does not drop too low, and on the other hand, does not last too long when starting the engine.
|Independent parameters, multiple injection is possible|
Injection pressure- and timing can be selected freely by the control device and independent of each other. Through multiple, short pre-injecting of approx 0,4 milliseconds in the partial-load range, the temperature
increase (nitrogen oxide emission) and peak pressure can be kept lower and a certain air movement in the cylinder can be supported. The more the injection is divided, the more the hardness of the combustion is
increased thus, also a reduction of combustion noise.
|Up to seven partial injections are possible|
Nowadays in theory, up to seven partial amounts are quite possible. Post-injections raise the temperature in the exhaust system and create regeneration possibilities for NOX storage catalytic converters
and particle filters. At the same time the injection pressures rise to up to almost 2000 bar.
|Electrically driven low- and mechanically driven high-pressure pump|
The Diesel fuel for the rail comes from the high-pressure pump (see picture 4), which is mechanically driven from the
engine. Nowadays, for the fuel supply, the electric tank-internal pump, with it's 3 - 6 bar supply pressure, is almost always used, in the past there were also mechanical gearwheel pumps. Depending on the outside
temperature a pre-heating element is also active.
|First high- and later only low-pressure regulation|
The high pressure costs engine performance and lowers the efficiency. For certain araes of operation it's not
necessary. Even from the first generation, it could be reduced by the control device. Under driving conditions, e.g., during fuel cut-off, it can be completely switched off. From the second generation onwards the
regulation is done on the low pressure side.
The high pressure rail, which should have the same transport distance to all the cylinders is connected with all the injectors (see picture 5). The pressure valve (on the right of the rail in picture 1) allows excess fuel to
flow, through the cooler (only first generation) back to the tank. The control device gets it's pressure control signals from a sensor on the left of the rail. The rail can also be sperical in shape. 10/11
|All Diesel-motor cars/transporters|
|Occasionlly in heavy utility vehicles|