Common Rail 3

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A lot can be regulated and adjusted with the Common Rail, that's how it is designed. In the first systems there is only the high pressure, which is maintained by a pressure control valve on the high pressure pump or on the
rail. So imagine a valve whose ball is held on their place by an electromagnet.
If you do not want to release any steam bubbles into the return line after switching off the system, a certain residual pressure should be maintained in the rail, because everything that is already there does not have to be
restored when the engine is started again. A spring then initially acts on the ball valve, which is supported by the electromagnets to significantly increase or permanently change the rail pressure during operation.
'Pulse width modulation' is the magic word for building up very different pressures depending on the operating state. This is where the pulse duty factor comes into play, which can switch the voltage supply for the magnet
on and off again so quickly that it can increase the closing force of the spring over a wide range. This then results, if appropriate, in the enormous pressures that are always mentioned in connection with CR. Of course, the
high-pressure pump must also be able to deliver such pressures without complaint.
Which brings us to the key point. Starting with the second generation CR, high-pressure control is no longer the only option, because from an energy point of view it is pointless to first build up pressure and then release it,
even if only partially, into the return line. If you don't want to generate it in the first place, then control on the low-pressure side is required. Incidentally, this also saves on the fuel cooler, which in the first generation had to
deal with the additional heat input caused by the released high pressure.

Glycerine-damped high pressure gauge |
In contrast to high-pressure control, depending on the manufacturer, pure time control is also possible in addition to pulse width modulation. Now, at least the electrics can be dispensed with in the high-pressure control,
unless the speed of the control is particularly important, e.g. when the accelerator is briefly released. In this case, high-pressure control is somewhat superior to that in the low-pressure range.
Now you might think that the safest way to avoid pressure fluctuations during the injection process is to make the rail as large as possible. But it should be remembered that after switching off, there is either a particularly
low or no increased pressure, so the operating pressure must first be built up again during the start-up process. The rail volume must therefore be as small as possible to enable the engine to start quickly.

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Whether the rail is elongated (picture at the top) or spherical (picture above) has no influence on its function. The only important thing is that there are connections for each cylinder, the line from the high-pressure pump,
the rail pressure sensor and the valve just discussed. Surprisingly, as with injection pumps in the past, manufacturers strive to have roughly the same line length between the rail and the individual injectors, although
sometimes there is still a great similarity between the individual lines.
Which brings us to the most important parts of a CR system, apart from the high-pressure pump. The electrical part of the injectors respectively protrudes from the top of the cylinder head. As is always the case with hard-
working electromechanics, it is very sensitive to heat. The protrusion promises a little cooling, but this is immediately hindered by the upper engine cover that is common today. Surprisingly enough, the circuit is one of the
few parts of an injector that cannot be replaced, so a repair that is common today and much cheaper by replacing parts is not possible.

So here the rail pressure is set to the correct pressure for the respective operating state. It only needs to be distributed through the at least five blind holes (picture above) to the actual combustion chambers in the
individual pistons. Important addition: It is not just the quantities that matter, but also the time. In diesel engines, the distribution of mixture formation and ignition known from gasoline engines does not exist. Both have
traditionally been done by the injection system alone. The how much corresponds to the mixture formation and the when to the ignition.
However, it is not quite that simple, because even in a gasoline engine, ignition is often carried out several times. If you look at the partial quantities of an injection process, then the ignition can also be controlled by the
distribution of the quantities, which is called the 'start of delivery' here. The start of delivery is therefore not actually the point of the first injection, but should probably be the injection
which causes the actual burn-through of the resulting mixture.
To describe an injector, we will use the hole nozzle described in the 'Process' chapter. Its needle is now changed significantly so that the nozzle still extends a little into the combustion chamber, but at the same time the
circuit above the cylinder head has access to it at its upper end. Now you might think that this circuit is simply controlled to raise the needle against a spring in the event of an injection being necessary. But that doesn't work
because the currents and the energy required for this would be far too high.
2000 bar or even more cannot be opened and closed so easily with a 12V system. So you resort to a trick. You apply the rail pressure from above to the entire surface of the needle and from below to a partial surface. This
is because the needle is much thinner at the bottom of the space that is always filled with fuel than above it. This creates a circular ring at the top, which is called a pressure shoulder.

The primary task of this pressure shoulder is to try to lift the nozzle needle with the help of the rail pressure. But since its surface is smaller than that at the top of the nozzle, where there is also rail pressure, the nozzle
needle remains closed, especially since it is also pressed downwards by a spring. But the pressure from below has a relieving effect. The force of this no longer has to be applied by the electrical system.

It is even a little easier because the solenoid valve does not act on the needle at all, but only lowers the closing pressure above the needle by opening it towards the return flow. Now the force of the pressure from below is
greater than that of the spring and the nozzle needle is raised. By throttling the rail pressure supply at the top accordingly, the needle is prevented from closing again. This only happens when the electronics close the
access to the return flow again.
The electronically controlled electromechanics therefore control the supply and discharge of fuel pressure and only the difference between two forces acting from above and one from below must be overcome. The system
must be fast. The car diesel engine does not rev at high speeds, but the nominal speed of most is between 3600 rpm and 4400 rpm. If we take 4000 rpm as a good middle, then that is 67/s and 0.067/ms, which is a crank
angle of 24°.
A pre-injection needs a little less time with the voltage rising and falling, a main injection needs a little more. Now you distribute them with just a little time in between, so at nominal speed hardly more than two main
injections make sense. Of course, things look a little different at lower speeds. But even a piezo injector can't do it much faster.
Of course, post-injections can and should only take place towards the end of the working cycle and pre-injections well before TDC. The latter increase the temperature and turbulence. Then the main injection has it easier.
Less time passes before it ignites (ignition delay) and it doesn't happen as violently. So nowadays you can control the acoustic and vibration behavior of a diesel engine quite precisely.
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