 In-line pump
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The purely mechanical injection pumps until the development of the electronically actuated ones have actually always functioned in the same way since the A-pump of 1927. An injection pressure should be generated that far
exceeds the nozzle opening pressure of say 130 bar. The piston pump is predestined for high pressures, especially as here there is no uniform delivery, but a powerful impact at the time of the so-called start of delivery. The
challenge, however, is sealing the piston against its cylinder. We are talking about a thousandth of a millimeter tolerance and sometimes even more.
It is said to have been a secret process at Bosch, machining based on the principles of fluid friction, also known as 'lapping'. The first pump is an in-line pump, i.e. as many pump elements as the engine has cylinders. The
relatively small pistons are driven by a camshaft, which in the diesel engine is also driven at the speed of the engine camshaft via roller tappets, i.e. half the crankshaft speed.
So all of them nicely in a row and their cams positioned in such a way that the respective piston begins to rise shortly before the start of the working stroke of its engine cylinder. The first way is in vain because the inlet
opening(s) must first be closed. Then there is only one possible path for the fuel, up through some kind of non-return valve to the nozzle of the engine cylinder and injection.
| Far left | Start of filling |
| Left | Start of delivery |
| Right | End of delivery |
| Far right | Top dead center |
Before the pump piston reaches TDC, its control edge releases the inlet bore again. Since the pressure in the pump chamber is significantly higher than in the inlet, the fuel now flows back into the inlet instead of to the
engine cylinder. The action has ended. The actual delivery stroke only made up part of the total stroke of the pump piston. The path from BDC to the start of delivery is always the same, that from the end of delivery to TDC
varies depending on the quantity to be injected.
As a first approximation, one would assume there are two edges on a pump piston, a natural one at the upper end and one at the bottom, which is formed by a circumferential groove. However, this would only ever achieve
the same delivery stroke. However, the torque and speed behavior of the engine is regulated by the injection quantity controlled by the gas pedal. The lower control edge or groove must therefore be arranged at such an
angle that the piston is brought to different delivery rates by rotation.
So once again: The stroke movement results in the pressure build-up or start of delivery, the rotation changes the injection quantity (delivery quantity). It must be clear that the latter must be changed equally on all cylinders.
This is ensured by a kind of toothed rack (picture above), which engages in small clamping sleeves with external teeth, which enclose the pump pistons but at the same time do not impede their stroke movement.. For this
reason, you can think of the accelerator pedal as connected to this toothed rack.
Now it may also become clear why an older diesel engine sometimes soots. The engine is perhaps at its load limit at medium speed and the driver is forcing so much fuel on it when depressing the accelerator pedal that it
cannot process it and it also lacks the corresponding amount of air. A diesel soots when there is a lack of air. A petrol engine would quit with this 'rich' mixture, but not the diesel.
There's a lot more to come. The petrol engine runs out of air above a certain speed, and then the performance drops by itself. Because the diesel engine works with excess air, there are no limits as long as enough fuel is
injected. But it tolerates higher speeds much less well than the petrol engine. It must therefore be regulated, as is also the case with idling. Injection systems without electronics therefore have a classic centrifugal governor.
There are certain designs of injection pumps where a defect can invalidate this regulation. Truck fitters report credibly about (expensive) engines in which this effect sets in and which turn from a standstill in previously
unknown speed regions until they end up with a massive defect. Since the performance also increases beyond the normal level, the engine often cannot be tamed by shifting gears and release the clutch. If you start the
rescue operation a little too late, the latter simply slips through.
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