 Common Rail 2
The most important distinguishing feature from all other systems is the decoupling of pressure generation and injection. Because of the time in between, such a system is also called a 'storage injection system'. It came at
the right time insofar, as certain manufacturers nowadays consider very late post-injections to be necessary for the regeneration of the particle filters, which is not possible with the other systems.
Of course, the question remains whether, for example, possible oil dilution should not be avoided and injected directly into the exhaust system. Nevertheless, the freedom of choice when programming the engine control
with essentially constant pressure is not the most fascinating thing about Common Rail. In addition, there is of course the possibility of up to seven (!) injections and the associated decisive influence on the engine's
running.
The best-known example of the advantages is provided by VW with its spectacular switch from pump-injector to CR, and the entire truck elite followed suit. And it's not just about the constant guaranteed injection pressure,
but also about its freely selectable adjustment to the various operating ranges, which not least also improves efficiency.
kfz-tech.de/PDM52
Roughly speaking, an electrical switching element has been added to the CR system between the pump and the injector, which by the way still bears a great deal of resemblance to the perforated dies of the past. The
pump is also not so much different from before in that it is self-sufficient again, i.e. not adjusted to the position of the engine's camshaft or crankshaft. Nevertheless, it is driven by one of the two, as it was before.
There is no need for a precise assignment, the pump only has to generate pressure. This can then be regulated on the high or low pressure side, depending on the system. Incidentally, some high-pressure pumps look
awfully similar to those in radial piston pumps. This is different with piston pumps, because they used to be arranged in a row above a camshaft, but today they are arranged around such a shaft in the middle. A cam is
now responsible for several pump elements instead of just one.
The only thing that is really new are the switching elements combined with the perforated dies, which turn them into 'injectors'. It's crazy that a relatively simple pipe gave the system its name and that this has now even
been passed on to direct petrol injection. Its dimensions are already mentioned in Common Rail 1.
It is true that, depending on the manufacturer, diesel engines are almost as smooth as petrol engines, or if they are not immediately as good, that they are no longer audible at all when idling, for example. Was that the
main reason for switching from VW? The pump-injector has now all but disappeared from the vehicle sector (including trucks).
Perhaps another important feature of the CR system, in addition to the division of the main injection, contributed to this. We have already mentioned the noises that such a system can produce. But with CR, these violent
pressure fluctuations are a thing of the past once and for all. The high-pressure pump is smaller and generates a uniform pressure, and the control by injection has little to no effect on it.
However, another component is now under much more stress, the electrical switching unit. In the course of development, the most important ones have been with a solenoid valve, then piezo-controlled and finally again
with a solenoid valve, the latter probably more cost-effective and now also competitively fast. Piezo crystals have the physically more advantageous property of being able to elongate at lightning speed when voltage is
applied, but by such a small amount that you need hundreds of them to be able to open an injector properly.
As drivers, we just need to know that we must not unplug the plug of even one injector while the engine is running, otherwise it will continue to inject. It's best not to do this with any of the systems anyway, so please turn off
the engine every time you make any changes to the injection system. Sure, these days you have to press the clutch or brake every time you start, but you should still make that much effort.
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