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This is how the Bosch K-Jetronic mechanical injection works

This is how the Bosch K-Jetronic
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D he electronics department at Bosch developed the D-Jetronic and launched it in 1967. As soon as this was on the market in significant numbers, the Bosch hydraulics department began to develop another injection system that worked purely hydraulically - without electronics. This system dispensed with the regulation of the injection quantity by the duration of an injection pulse, but injected continuously, i.e. continuously. Hence the name K-Jetronic. The D- and the later developed L-Jetronic were called intermittent injection. Competition stimulates business - even if it comes from the same group but from a different department.

911, Golf GTI and S-Class with K-Jetronic

Porsche continued the K- Jetronic from 1973 in the G series of the 911. VW then revved up the Golf GTI with this injection, and Mercedes used it in the mighty W116, the S-Class, for example with the 6.9 liter V8 engine. In what was then the fastest series production sedan in the world, the Mercedes 450 SEL 6.9, a mighty eight-cylinder with K-Jetronic ensured impressive performance. The engine with 286 hp and 549 Newton meters of torque helped this 'over-S-Class' to achieve sporty performance and the title of fastest sedan in the world.

The mighty V8 of the Mercedes 450 SEL 6.9 is supplied by a K-Jetronic from Bosch.

Mercedes converted the fuel injection system of the 2.8-liter, 3.5-liter and 4.5-liter injection engines in the winter of 1974/75. With this, the Stuttgart-based automobile manufacturer wanted to better comply with the emission limit values ​​that have now become stricter in most European countries. You change from the electronically controlled Bosch D-Jetronic to the newly developed, mechanically controlled Bosch K-Jetronic.


At the beginning of the 1970s, the Bosch company developed a hydraulic fuel injection system that initially managed without electronics. Due to higher environmental protection requirements, however, the requirements became higher and higher, so that more electronic interventions were necessary. In order to drive more economically, an overrun cut-off was added first. For this purpose, a bypass was switched parallel to the air flow meter, which opened this detour in overrun mode and thus no longer allowed air to flow through the air flow meter. The air flow meter reported: 'No air'. The fuel supply was cut off. The lambda control was added to reduce exhaust gas along with a three-way catalytic converter.

System diagram of the K-Jetronic

Structure of the K-Jetronic: The path of the intake air is marked with the blue arrows. The fuel is marked in yellow.

The fuel is sucked in by an electric fuel pump (2), via a fuel reservoir (3) and a Fuel filter (4) fed to the mixture regulator (9). This consists of the two main parts of the fuel flow divider (9a) and the air flow meter (10). The fuel then reaches the injection valves, which are located in the intake manifold (6).

The air required for combustion is sucked in through the air filter (not shown) and comes through a deflectable baffle plate (10a) of the air flow meter (10) , the throttle valve, the intake manifold and the inlet valves in the combustion chamber.

Injection valve

The injection valves of the K-Jetronic work purely mechanically from an opening pressure of 3.3 bar. The amount supplied by the control piston pushes the valve needle open more or less and leads to a rattling noise. The valve needle vibrates in its seat due to the fuel flowing past. This rattling makes the fuel droplets smaller and finely distributed.

Mixture regulator

The mixture regulator consists of the air flow meter and the fuel flow divider. Mixture regulator: on the left the baffle plate, on the right the flow divider for an engine with four cylinders. The air flow meter is available in a rising current version or - for better adaptation to certain engine designs - as a downdraft version as shown opposite. Depending on the suckedAir volume per time, the flap adopts a certain position. This acts via a lever system on a control piston in the flow divider.

Air flow meter

The air flow meter consists of a baffle plate that is lifted up by the air and a funnel that is shaped by its shape serves to raise the lever and thus the control piston for each volume of air supplied at full load than in the middle area, the so-called partial load area.

Air flow meter principle: the more air is sucked in , the further the baffle plate rises.

The throttle valve adopts three positions: idle black, partial load green and full load red. The ring area around the baffle plate is small when idling, larger at partial load and largest at full load. As a result, the incoming air lifts the baffle plate in the funnel. The baffle plate is attached to a lever that presses on a control piston. This control piston is responsible for supplying fuel to the injection valves. The cone shape of the air funnel has different areas for the different load conditions.

Cone correction: Each engine has its own specially adapted funnel for its displacement and maximum speed. 1 for full load, 2 for part load, 3 for idling.

Fuel flow divider

Like the air flow meter, the fuel flow divider is part of the mixture regulator. It is used to dose the required amount of petrol in the current load condition. It consists of a control piston, the slot carrier and theDifferential pressure valves. A thin metal membrane separates the upper and lower part of the flow divider. It also contains the flow throttle for the control pressure. If it is not used for a long time, it can corrode due to condensation in the fuel. So that the entire fuel injection quantity is evenly distributed to the individual nozzles, each injection nozzle has its own differential pressure valve.

System pressure regulator

A spring-loaded system pressure regulator is located on the flow divider, which keeps the system pressure constant at 4, 5 bar.

Control valve

The control piston moves in the slot carrier, a cylindrical component with a number of 0.2 mm wide openings, the control slots, corresponding to the number of cylinders. The more air is sucked in, the higher the control piston is pressed and the larger the cross-section that is released by the control slots.

Due to the pressure difference between the system pressure and the pressure above the membrane, the fuel flows into the upper part of the flow divider and from there into the control valves. The control piston has a groove so that the system pressure does not move the control piston. The system pressure on the upper and lower ring surface cannot shift it.

Control pressure

The control pressure is located above the control piston. This is influenced by the warm-up regulator. It changes in a range from 0.5 to 3.7 bar. If the control pressure drops, it is pushed further upwards by the same amount of air. The control slots open wider and the mixture becomes richer.

The control pressure is derived from the system pressure. A flow restrictor taps the system pressure of the fuel meter. If you look at a removed membrane and hold it up to the light, you will see the throttle as a tiny hole. However, the flow divider is usually not taken apart without necessity.

Differential pressure valves

These valves cause a constant pressure drop at the control throttles, the differential pressure is constant 0.1 bar. The flat seat valves are located in the flow divider. If the pressure above the diaphragm rises, the drain opening is widened, the control piston moves upwards and more fuel flows in the direction of the differential pressure valves. This means that the fuel supply works proportionally and linearly to the movement of the control piston and the air flow meter - without being influenced by the pressure.

Fuel filter

With so much precision mechanics and hydraulics, tiny slits and just as small Chokes would have a devastating effect on the functionality of the system. Therefore the system needs a fuel filter.

Throttle valve

Function of the throttle valve.

There is a cam on the throttle valve shaft, which in turn changes the control pressure. This component was built into the Porsche 911. When idling (left figure), the opening cross-section of the fuel return is large. This makes the mixture richer.

In the partial load range (middle figure) the opening cross-section is small, the control pressure is high and the mixture is lean. At full load, however (right figure), the opening cross-section is large and therefore the control pressure is low. The mixture is enriched.

Warm-up regulator

The warm-up regulator ensures that the engine runs smoothly while the engine is warming up from a cold start. It is attached directly to the engine so that it 'feels' the engine temperature directly as a control variable. The bimetal spring is heated.

When the engine is cold, the cold bimetal spring pushes the spring plate of the valve spring down. This allows the valve membrane to move downwards and the fuel control pressure to flow off via the return line.

When the engine is warm, the bimetal spring bends upwards. This allows the valve spring to push the diaphragm against the return via the pin and pressure piece. The fuel can no longer escape and the control pressure increases.

The warm-up regulator influences the control pressure of the fuel flow divider. It is lowered when the engine is cold. This means that the same amount of air can push the control piston further up via the lever. The control slots are opened wider and the amount of fuel injected increases. The mixture is enriched. Some warm-up regulators have an additional vacuum connection. This achieves full load enrichment by lowering the control pressure.

At full load, the negative pressure pulls up the full load membrane on which the closing spring of the warm-up regulator is located. At full load, the inner spring therefore presses the full load membrane downwards. The intake manifold vacuum does not pull as strongly on the vacuum port as it does when idling and at part load. This allows the control pressure to escape via the fuel return. He sinks. This in turn means that the membrane is no longer pressed so strongly against the drainage pipe and the control pressure drops. The control piston slides further up in the slot carrier, and this increases the amount of fuel injected. The mixture is enriched.

Principle of overrun cut-off

In overrun mode, no power is required from the engine,and that's why it doesn't need any fuel. Therefore the feed is simply turned off. All cars today have fuel cutoffs. In the city cycle, a few percent fuel is saved.

With the K-Jetronic, the shutdown takes place by means of a bypass valve, in which the supplied air is simply led around the air flow meter. He's being tricked, so to speak. Above the idling speed, the air bypass valve (= overrun cut-off valve) is opened in overrun mode with the throttle valve closed. This causes the air sucked in by the engine to flow around the baffle plate. This falls into the zero position and interrupts the fuel injection. The engine receives no mixture during coasting operation, only air.

If the driver accelerates again, the air bypass valve closes and the air flows through the mixture regulator again, the baffle deflects and fuel injection starts again. The speed information from the speedometer ensures that the overrun cut-off only works from a speed above 35 km /h.

Cold start valve

So that the engine starts better when it is cold, the K- Jetronic an electromagnetic starting valve. It is supplied with power for a short time during the starting process via terminal 50 via a thermal timer. Depending on the temperature, the thermal timer switches off the power supply to the cold start valve after eight to no more than 15 seconds.

Roller cell pump

The electric fuel pump is a roller cell pump through which the fuel flows. It is therefore also referred to as a 'wet' pump. To avoid vapor lock formation, more fuel is pumped than is consumed. The fuel that is not required is returned to the tank without pressure.

Pressure accumulator

When the vehicle is switched off, the engine is still hot. All fuel-carrying components such as warm-up regulators and injection valves absorb this heat. Especially in hot weather, vapor bubbles could form when the fuel pressure decreases. That is why the K-Jetronic has a pressure accumulator. This maintains the pressure in the system for several minutes after the engine has been switched off.

During operation, it dampens the noises and fluctuations in the fuel pressure that the roller cell pump inevitably generates due to its pulsating delivery.

Development to KE-Jetronic

The first gasoline injections with continuous injection K-Jetronic managed without electronics. A further development was the mechanically working gasoline injection with overrun fuel cut-off, the KA-Jetronic, which, in conjunction with the lambda control, opened up an electronic control option.

The further developed KE-Jetronic was used in the Mercedes 190E.

In 1982 the KE-Jetronic was used for the first time in the Mercedes Benz 190 E installed. It was a further development of the K and KA Jetronic. The basic hydraulic-mechanical principle of the K-Jetronic was retained. The emergency running properties correspond to the properties of a normal K-Jetronic. The corrections required in practice are controlled electronically. This eliminates the need for the K-Jetronic's warm-up regulator, for example.

The following typical problems were encountered with the K-Jetronic:

▶ The engine temperature could not be recorded accurately enough
▶ Poor throttle response with rapid load changes during the warm-up phase
▶ Overrun cut-off complex
▶ Lambda control complex.
These problems led to the further developed KE-Jetronic.


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