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Common Rail 4

All other injection systems are also available in purely mechanical-hydraulic form without electronics. This is only impossible with common rail. Bosch still wrote in 1999 that the 'injection quantity is specified by the driver',
but that was already history by then. It would be better to say that a driver's request for a certain torque is addressed to the engine control unit, but the implementation is the responsibility of the digital electronics alone. You
only have to operate the accelerator pedal in a strongly alternating manner to be able to feel the 'jerking dampening', for example.
The best way to explain this is to briefly look at the gasoline engine. Whether the desire for more torque is implemented by opening the throttle valve, changing the ignition timing or other control times is something the
driver doesn't even notice. If, for example, the turbocharger is equipped with even more electrical control or even an electric drive, the matter becomes even more unclear.
In the picture above you can see a small selection of incoming and outgoing signals. The vital sensor on the camshaft is still missing, without which the control system cannot distinguish between the first and fourth
cylinders, for example. After the engine has started, however, you could pull the plug on many vehicles. It's a good thing that the error lamp warns you of a defect early on, because the engine would then no longer start.
With the turbocharging that is now always present, the lambda sensor and knock sensor have been added in addition to the boost pressure sensor and exhaust gas recirculation. The calculation of the basic quantity for
injection, which was once still somewhat comprehensible, has long since become an inscrutable mess due to additional loading and deductions. 'Characteristic maps' are the signposts to the correct injection time and
quantity. Since they are stored one after the other in the electronic memory, the number of dimensions or parameters actually no longer plays a role.
Common Rail therefore fulfils functions that would be impossible to fulfil with mechanical-hydraulic control, and we have not even mentioned the various messages and regulations in the exhaust system and the legal
requirements for non-company-specific error messages. Today, it is indicated even if just one pressureless line in the fuel return is open, not to mention larger leaks. Noise comfort has been increased to such an extent
that, for example, a V6 three-litre Audi is not significantly louder after a cold start than at operating temperature.
Which brings us to the management of the glow plug system, which is usually outsourced, and has evolved from pre-heating to after-heating, because a modern CR diesel engine no longer needs the former down to -5°C.
Or the immobilizer, which of course had to be integrated a little deeper into the engine control system. It's not enough to just block the main current. Cruise control was actually included in the system for free, even if there is
still an extra charge for it today. It is simply integrated between the accelerator pedal and the system, but has now become a lot more complicated due to distance and GPS control.
Let's turn to the main thing, the injection process. The division between pre-injection and main injection was already an innovation, if you ignore the pump-injector and cautious developments in the distributor pump. In
principle, with Common Rail, not only the injection quantity and timing are independent of each other, but also the injection pressure and quantity. There is also no increase in injection pressure as with conventional
systems, but the pressure remains constant during an injection, no (unnecessary) peak pressure puts strain on components.
The injection quantity is particularly interesting when it is also divided. Let's assume a trip on the motorway in a diesel sedan or SUV at an average speed of 120 km/h and assume a consumption of 6 litres/100km. That
would be 5 litres/h or 83 cm3 or 83,000 mm3 per minute. Guess an engine speed of 2,000 rpm, four cylinders have 4,000 injections per minute, which works out to just over 20 mm3
for one injection.
Mind you, this is a cube with an edge length of 4.5 mm. And of course you expect the engine control to seamlessly cover all speeds from, say, 900 rpm to 2000 rpm. Unfortunately, we cannot assume an idle consumption
when driving with the clutch disengaged, as we might have done in the past. If we still assume one liter per hour, then the control unit has to increase the injection quantity from 9 to 20 mm3 a total of 22 times
from 900 to 2000 rpm, i.e. half a cubic millimeter every 50 rpm, a cube with an edge length of 0.7 mm.
You have to take up the cudgels for the old systems, because they could do that too. Nobody noticed any erratic behavior when accelerating particularly sensitively. At the same time, however, this simple calculation
example shows why it took as long to develop an injection system as the diesel engine itself. But that is no longer the problem in modern injection systems. Time control has now become almost more important.
To avoid having to do the calculations again, we take the 24° crank angle already determined in Common Rail 3 in one millisecond at 4000 rpm. That is then 12° at 2000 rpm. According to Bosch, there are now two limit
values, namely that pre-injection up to 90° crank angle is possible, but pre-injection only makes sense up to 40° crank angle because otherwise there is a risk of oil dilution. That was before the time of post-injections. On
the other hand, main injection up to a maximum of 40° after TDC is still useful.
If we assume one millisecond for an injection, as in the Common Rail 3 chapter, then at 80°, 6 injections would be possible, but we also have to take the pauses in between into account. So you can see that the
determination of the injections is not entirely free, if you also consider that at idle the total quantities shrink to 9 mm3 and the time is halved at higher speeds up to 4000 rpm. The distribution of the injection
quantities into pre-injection and main injection will therefore certainly vary depending on the speed.
An injection time of less than a millisecond would help. This is probably possible with pre-injection of only 1 to 4 mm3. It serves to increase the compression pressure, which in turn makes the main
combustion less sudden, which is technically known as a shorter ignition delay. And anything that puts less strain on the hearing also has a largely positive effect on the load on the components and possibly also on
consumption. In the end, the diesel engine finds a little more of the equal pressure combustion that has always been required of it.
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