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K-Jetronic In Seven Points

The K-Jetronic in Seven Points
What you really need to know about it, if you’re not supposed to repair it.

The Bosch K-Jetronic, in the Seventies and Eighties was used in countless, especially German cars, including the beloved 911, Golf GTi and many BMWs and Mercs. The web offers a great variety of extensive reports and instructions and help with all kinds of issues. Even the Wikipedia article about the K-Jetronic puts together all relevant information in a very precise way.
However, there’s something wrong with all of these publications: they are getting lost in far too many details and readers can’t see the wood for the trees.
If you have ten minutes left and you’re fine with investing them in increasing your knowledge about the K-Jetronic by reading the rather poor English of a German engineer, here is what you really need to know.

First things first: to make it bang at all, a certain amount of petrol needs to be mixed with a certain amount of air. Almost any combustion engine, let’s take the Diesel engine as an example, intakes a certain amount of air and adds a higher or lower amount of fuel depending on the driver’s wish for torque. That won’t work with a petrol engine. With a petrol engine, basically every molecule of the petrol needs to find a molecule of Oxygen, yielding in 1 kilogram of petrol needing 14,7 kg of air to burn it. If there’s not enough air or fuel, you won’t see the petrol engine running nicely. The main task hence we’re expecting from all kinds of carburetors or injection systems is to add the proper amount of fuel to the air which is taken into the combustion chamber.

The second thing you need to know:
a fine spray of petrol is a relatively stable thing. You can leave it alone for a while without being concerned that it’s going its own way. That’s why Bosch was able to design a continuous injection that keeps on injecting even while the intake valve is closed. The gas stays where it is till next time the valve opens and it’s inhaled into the cylinder. The „K“ in K-Jetronic by the way represents the German word „kontinuierlich“, which means continuous.

Third thing you need to know. The pressure used for injecting the gas is provided by the fuel pump. While with carburettor engines this is a shaky and rather primitive thing that does the job to transport the fuel from the tank to the carburettors, with injection systems it’s where all the great technology starts. If the fuel pump is not doing the right thing, injection won’t work either. Let’s keep this in mind when dealing with the state of our fuel tanks (rust particles), fuel lines and filters.
Less important, but worth having heard about: there’s a number of secondary systems built around the fuel pump that make it less noisy, shut it down in case of accidents, help building up pressure and avoid vapour bubbles.

Fourth. As the job is to mix a certain amount of fuel to a given amount of air, as said above, there’s a flap in the intake channel that is deflected the more, the more air is inhaled. The flap is connected to a lever which acts on a valve to let fuel pass and the whole thing therefore appropriately is named air flow meter. Another component named fuel flow divider adds two more functions: first it distributes equal amounts if fuel to the cylinders and secondly, using a rather complex mechanism consisting of a membrane and little springs, it makes sure that the air flow meter’s input is used to quantify a proper amount of fuel. You may wish to study the exact function of this valve, but actually it’s engineer’s stuff.
The unit described here in chapter four is colored in blue in the picture attached.

Number five. The K-Jetronic uses a system pressure of roughly 5 bar. This is the pressure which is created by the fuel pump and maintained throughout the whole system till it’s finally used by the injection valves to inject fuel. More precisely, the pump is being operated on an even higher pressure, but the an additional pressure controller is used to adjust it to a constant level, as obviously we can’t expect the pump to deliver a constant pressure throughout the entire life time of the car.

Sixth. With the system described so far, the engine will run fine as long as it’s warm. Every other subsystem that we’re going to talk about from now on, has the sole purpose to manage a cold start. Cold engines need a greater amount of fuel per air, as a part of the fuel will condense at the intake channels and therefore won’t even reach the cylinders to get burned. To ensure this, a subsystem named warm-up controller is used and that’s where the so-called control pressure comes in. A part of the fuel is branched off and used to create a closing force on the fuel control valve, i.e a force in opposite direction of the force created by the air flow meter. As long as the engine is cold, only little control pressure is created by the warm-up controller and the fuel mixture is rich. With the engine warming up the control pressure increases and the mixture is getting lean. The controller element by the way is a good old bimetallic strip that acts against a spring. Funnily, it’s not the engine heat itself that is responsible for the strip’s bending, but electrical power with a relay control. Find the warm-up controller again colored in blue in our picture.

Seventh, finally. There are a couple of more parts to support the cold start of the engine: the auxiliary air valve (increases the idling speed), the cold start valve (adds an extra amount of fuel) and the thermo time switch (switches the cold start valve off after a short wile). Given that confusing amount of electric, mechanical and hydraulic control systems it is becoming very clear what blessing the introduction of an electronic control device was. Bosch introduced it already in 1973 with a system named L-Jetronic, but Porsche sticked with the mechanical controllers till in 1983 the 3.2 liter engine appeared.

2 additional images. Click to enlarge.


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