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 914 - Part 2
This section describes the fuel injection system of the 914/4. First of all, there's the D-Jetronic. Incidentally, the 'D' prefix was added later to distinguish the system from, for example, the L, LE, and LH Jetronic
systems.
Although there were also the K and KE Jetronic systems, the only thing they actually have in common is that fuel is injected rather than, as with a carburetor, usually drawn into the airflow by vacuum.
Let's set aside the benefits of D-Jetronic, which are usually far too general, for now, because, for example, the fuel savings on the 914/4 are fairly modest. Yes, the innovation in this system lay in the electronic
control system, in this case, even analog electronics. This means that nothing is stored; all data collected by the sensors immediately affects the quantities and injection timings.
So what really sets this electronics apart is its incredible speed. Thus, even at very high RPMs, it finds a window of time during which it can inject a highly variable amount of fuel into the intake manifold without
interference
from the closing intake valve. If cost considerations weren't also a factor, it could use only the opening period of each valve for this purpose.
The D-Jetronic was already available in the 1967 VW Type 3, that is, before the 914 was even introduced. It's also available, for example, in Mercedes' eight-cylinder engines. One can already see an advantage
here, namely that, unlike with carburetor systems, for example, it doesn't really matter how many cylinders need to be powered. The costs increase less than with multi-carburetor systems.
On the other hand, we'll also see that it's not enough to simply add more fuel injectors and just change the software. And even though BMW and others will, of course, soon join the exclusive club, let's stick
with the 914's four-cylinder engine for now. Above, you can see how this principle is applied in systems built even today. As close as possible to, or better yet, inside, the tank pressure is generated that can
be released from each cylinder via an electromagnetically controlled valve.
| Inline electric pump of the 914 . . . |
Of course, this fuel must be coarsely pre-filtered before entering the electric pump (pictured above) and then post-treated accordingly. In addition, it should be maintained for a good while even after the engine
has been shut off, so that no steam bubbles form that would ruin a potential hot start. At least with the D-Jetronic, it's still pretty simple: the pressure is set to about 2 bar using an adjustable preload on a
diaphragm regulator, and that's it.
Anything above 2 bar is then returned to the tank. It is also important to note that the pump runs for no more than two seconds longer than the engine, so that in the event of an accident, such as a fire in the
engine compartment and a broken fuel line, no additional gasoline is pumped into the fire. The engine speed is monitored electronically, so to speak. A typical example of this is a brief, relay-controlled start-up
of the pump after turning on the ignition without starting the engine.
So, to recap briefly. An injection valve is located in the intake manifold of each intake valve; depending on the group to which it belongs, this valve injects fuel either into the combustion chamber through the
open intake valve or in front of the closed intake valve. The second method is called 'pre-positioning'; according to Bosch, it has no adverse effect on the engine's performance or other characteristics.
To precisely determine the start of fuel injection, a signal is taken directly from the lowest level of the ignition distributor, where small, cam-controlled switches transmit their signals at very specific positions of
the
camshaft. The control unit determines the duration of the injection signal based on engine speed, temperature, and pressure in the intake tract. Oh, and there's also a potentiometer and a switch for idle
and full throttle on the gas pedal-operated throttle valve.
Apparently, there were some pre-production models without throttle cutoff, but an injection system without throttle cutoff is actually unthinkable. In the D-Jetronic system, the control module detects when
the idle switch closes at an engine speed of, for example, over 1800 rpm and shuts off all fuel injectors. And if the RPM drops below, say, 1300/min, fuel injection is reactivated. By the way, that difference is
called 'hysteresis'.
Neither the cold-start valve nor the auxiliary air damper (pictured above) has anything to do with the electronics in the 914. While the 12V cold-start valve connected to the fuel circuit injects fuel into all cylinders
at a central point in parallel with the starter motor, there is a so-called thermo-timer switch that reacts to the prevailing temperature via a bimetal strip and allows the appropriate amount of idle air to flow through
the idle air damper to raise the idle speed.
Unfortunately, because the 914 is air-cooled, the bimetal cannot be controlled by coolant temperature. So, just like the automatic starter on the Beetle back in the day, it’s electrically heated, and works just as
poorly. At some point, it closes and restores the engine to its original idle speed.
The intake manifold pressure sensor is much more interesting and also more important for the operation of the D-Jetronic. By the way, it's not just it who can really throw the system into disarray. This is even
more likely to happen due to a
defective temperature sensor, which is why it's also called the 'General'. The good thing about this error is its low price and the very simple way to test its functionality.
The intake manifold pressure sensor (pictured above) has only one connection to the intake manifold and, of course, electrical wires to the control unit. Inside, it has a primary and secondary winding with an
armature that can be moved by two
barometer cans. They are made of copper-colored metal and are designed to expand as internal pressure increases or external pressure decreases. This way, one of them can have a fixed
filling, while the other can have a connection to the outside world.
So if the rest of the can is subjected to the respective intake manifold pressure, the inductance, and thus the signal to the control unit, changes either due to a different intake manifold pressure or outside
pressure (altitude correction) or both. In the end, the pressure sensor, as the system's most important component, gave the system its name.
| The individual components of the system . . . |
kfz-tech.de/PPo1
| 1 | Control Unit |
| 2 | Injection valve |
| 3 | Intake Manifold Pressure Sensor |
| 4 | Distributor |
| 5 | Injection Start Switch |
| 6 | Throttle Valve Potentiometer |
| 7 | Supplementary Air Damper |
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