Jetronic fuel injection system - K-Jetronic, KE-Jetronic


Jetronic fuel injection system

Since their introduction, JETRONIC fuel injection systems have been proven numerous times under the worst circumstances of daily driving. The reasons why this is so lies in one single word-BOSCH batteries and motors, synonymous with quality.

Rarely does any mechanical assembly on cars boast such longevity in terms of production as is the case with the most famous fuel-air mixing system - the carburetor. This device has been manufactured for decades and during that time it has been installed in who knows how many tens of millions of passenger cars. During all that time, the principle of its work, but also its main components, remained, so to speak, the same.


However, new requirements regarding engine construction, performance and especially the quality of exhaust gases have recently led to the consideration of more advanced construction solutions that would meet not only the current wishes and needs for better driving and life preservation. environment in terms of cleaner exhaust through lower fuel consumption, but also the desire of designers to build a quality base for further research in the field of fuel and air engine power. As a result of this work and thinking, Bosch first introduced the brand new system to the public in 1973 mechanical gasoline injection with lambda closed loop control called K-Jetronic.

Air to fuel ratio

To operate, a gasoline engine requires a specific air-fuel ratio. The ideal ratio of these two substances is 14,7: 1 and it is called the stoichiometric ratio. In other words, 1 kg of air is required for complete combustion of 14,7kg of gasoline. However, certain operating conditions (eg idling or full load) make it necessary to adjust this ratio to certain limits. And indeed, the relationship between the real and theoretical need for air, which is called λ(Greek letter: lambda), varies from 0,9 to 1,1. The values ​​of the coefficient λ less than the unit correspond to a richer mixture (the ratio of air to fuel is smaller stoichiometric) which results in an increase in engine power, that is, values ​​of coefficient λ that are larger than the unit correspond to a poorer mixture (the ratio of air to fuel is bigger from stoichiometric) which results in a decrease in output power. These details will be discussed later.

Advantages and disadvantages

One of the biggest advantages of the fuel injection system over carburettors is the precision with which it operates, that is, the fuel injection. By omitting the carburettor and introducing the injectors, the path that the fuel needs to go before entering it is shortened cylinder. This means that its deposition along the walls of the intake manifold is largely avoided, which again means that a much higher percentage of the fuel remains in the air in the intake duct than was the case with the carburetor. Another great advantage is the size of the energy droplet, which is now several times smaller, which results in its much easier evaporation in the suction duct, hence the transition to a gaseous state, and thus improved mixing with air. It is clear that the new system now requires much less storage space under the hood than the old one, which designers, of course, use to optimize the intake ducts in order to obtain a more favorable torque curve, to shape the front of the vehicle even more aerodynamically.


The biggest disadvantages, ie shortcomings, of this system are certainly the price, which is in any case higher than the price of the carburetor, ie its complex construction, which, logically, requires a certain professional training of service technicians in a certain car service.


K-Jetronic is a BOSCH mechanical continuous-fuel gas injection system that requires no control. It is divided into three main functional areas: air flow measurement, fuel supply and fuel intake. Measurement of air flow or amount of intake air into the engine was controlled by a gas damper and measured by an air flowmeter. The amount of intake air serves as the main operating variable for determining the basic amount of injection fuel. The fuel is supplied with the help of an electric gas station which supplies the manifold via a fuel accumulator and filters. The fuel manifold distributes that fuel to the injectors in the intake ducts of the cylinders. The intake of fuel is caused by the amount of intake air, which serves as the criterion for fuel dosing for individual cylinders, and it is determined based on the position of the measuring plate in the intake line. This gauge actually represents an air flowmeter that simultaneously controls the fuel rail.

The air flowmeter and the fuel rail are assemblies that are part of the control unit of the mixture. The injection takes place continuously, regardless of the position of the intake valves. At times when these valves are closed, the fuel "piles up" in the suction throat.

Description of the system and its parts

The fuel is extracted from the tank by an electric fuel pump. It is then directed, under pressure, through the fuel accumulator and the fine filter into the fuel manifold housed in the control device of the mixture. The pressure is kept constant by a pressure regulator in the control unit of the mixture from which the fuel flows to the injectors. Injectors Constant (without interruption) inject fuel into the inlets of the engine cylinders.

Electric fuel pump is an armored cylindrical pump with an electric motor permanently submerged in fuel. There is no risk of explosion because there is never a flammable mixture inside the pump body. It always provides more fuel than the engine's maximum needs, so the pressure in the fuel system can always be maintained regardless of operating conditions.

Fuel accumulator maintains fuel pressure in the system for a period of time after the engine has stopped. While the engine is running, it is used to suppress pressure oscillations generated by the operation of an electric fuel pump. After the engine has stopped, the fuel accumulator maintains the fuel pressure in the system to facilitate the restart of the engine, especially when the engine is warm.

Fuel filter is a very important element of the fuel distribution, because of the low tolerances of the various components in the system, a very high quality purification of the propellant is necessary in order to guarantee proper operation K-Jetronic-a. In the order of placement, the filter is located immediately after the fuel accumulator.

Fuel manifold measures (distributes) the correct amount of fuel for individual cylinders in accordance with the position of the air flow meter. The position of the plate is transferred by levers to a piston that controls the amount of fuel to be injected. Depending on its position in the cylinder with the metering openings, the piston opens or closes (by its vertical movement) the openings to a greater or lesser degree of dosing. The fuel flows through the open space of these openings to the differential pressure valves and afterwards to the injectors. If the displacement of the dipstick is very small, then the piston is only slightly raised and as a result only a small portion of the fuel flow orifice is opened. With a larger piston displacement, it opens more of the openings so more fuel can flow. Therefore, there is a linear relationship between the displacement of the dipstick and the openness of the fuel flow orifice in the metering tube.

System pressure regulator maintains a constant pressure in the fuel injection system (about 5 bar). Due to the fact that the fuel pump always provides more fuel (and thus creates more pressure) than the engine needs, in the pressure regulator there is a movement of the piston and the opening of the passage through which excess fuel returns to the fuel tank. When the engine stops working and the pressure in the system starts to drop, the spring pushes (returns) the piston all the way to the position where it completely closes the fuel flow to the tank. In this way, the same pressure is always maintained in the system.

Injector it opens at a certain pressure and injects fuel into the suction throat, just in front of the cylinder suction valve. The fuel is sprayed by the vertical oscillations of the needle valve needle located at the very top of the syringe. In this system, the injectors do not have the ability to dispense fuel independently. They open freely when the opening pressure reaches 3,3 bar.

The mixture control device consists of an air flowmeter and a fuel manifold (called the K-head). This system has the task of dosing, or determining, the required amount of fuel corresponding to the amount of air that is sucked into the engine.

Air flow meter works on the principle of changing the position of the measuring board, thus measuring the amount of air that is sucked in by the engine. Air sucked through the air funnel moves the plate by a certain value from its initial position. Movement of the measuring plate is transferred to the piston by the lever system. This piston determines the amount of fuel injected.

Differential pressure valves in the fuel manifold serve to keep the pressure drop at the metering openings (between piston and diff. valve) constant, regardless of the amount of fuel injected. Each cylinder has its own differential pressure valve.

Cold start

During the start-up process, and depending on the engine temperature, the cold start injector injects an additional amount of fuel into the intake port for a certain time. During a cold start, some of the fuel in the intake mixture is lost due to condensation on the cold walls of the cylinder. In order to compensate for this lack of fuel and to allow the cold engine to start, an additional amount of fuel must be injected at the initial start-up time ie. startups. This extra amount of fuel is injected by the cold start injector into the intake manifold. The injection period of the cold start injector is limited by a thermo-time switch depending on the engine temperature reached. It is of electromagnetic type and for all its time the excess air ratio (χ) is less than 1.

Of the other elements K-Jetronic We will also mention a thermo-time switch whose role is to "switch on" or "switch off" an additional syringe, a temperature controller that regulates the value of control pressure, which ranges from 0,5 bar for a cold engine to 3,7 bar- and for a heated engine, and it rules in the space above the piston in the fuel manifold, the supplementary air assembly or the auto-blower, and so on.


Although it could not infinitely perform its function, the K-Jetronic served as an excellent base for the continued development of the fuel injection system in order to further reduce the amount of particulate matter from the exhaust system, while improving vehicle performance. KE-Jetronic is a combined mechanical-electronic gas injection control system with a closed-loop lambda control. Its main function is to supply fuel to the engine depending on the amount of air that is sucked into the engine (the basic control variable). Unlike the system mentioned above, KE-Jetronic uses in the calculation numerous data about the operation of the engine obtained by the encoder. The output signals from these encoders are processed in a control device controlled by an electro-hydraulic pressure regulator, which adjusts the injected amount of fuel to different operating conditions.

As can be inferred, the essential difference between K-Jetronic i KE-Jetronic The system has an encoder and a control unit that processes the data received from the encoder. In the latter case, the fuel manifold also has the aforementioned electro-hydraulic pressure regulator.

The encoders attached to the system, in this case, are used to read the engine temperature, to determine the position of the throttle (load signal) and the deflection of the air flow sensor (which roughly corresponds to the change in engine power over time). With these encoders, the control unit "commands" the hydraulic pressure regulator to appropriately "deplete" or "enrich" the mixture. KE-Jetronic responds quickly to changes in engine operating condition and improves torque characteristics as well as engine flexibility. Switching off the engine supply with the vehicle speed limiting corresponds to the speed (RPM) and engine temperature, and also switches off the fuel supply during engine braking. When the fuel supply is switched on again (during acceleration), there are no more unpleasant jerks. Effectively designed air intake system KE-Jetronica, allows for increased engine power due to improved cylinder filling. Similar to all other Jetronic systems, KE-Jetronic achieves the already mentioned increase in engine power for the same piston displacement, but not at the expense of increasing fuel consumption, which is very important.

Basic pressure regulator  has, in this case, replaced the temperature controller. Its function is to remind that it maintains the control pressure value constant, since any change in this value directly affects the air to fuel ratio in the cylinders.

Electronic control device it processes the data obtained from various encoders related to the operating state of the engine, and from that data generates a control signal for the electro-hydraulic pressure regulator.

Encoders for registering work data they serve, as their name implies, to register certain phenomena that will serve as additional criteria for determining the optimal amount of fuel needed by the engine and forwards them to the electronic control unit. Encoders are described in the following table with reference to a specific customization function.

Table 1 - Encoder description with respect to the adjustment function

Characteristic operating value Encoder
Full load and idle Throttle switch
Engine speed Ignition System (Int. Div. Ignition)
Running Ignition and start switch
Engine temperature Engine temperature encoder
Air pressure Aneroid box
A mixture of air and fuel Lambda probe

Electro-hydraulic pressure regulator is in the system KE-Jetronic replaced the pressure regulator with a spring that was an integral part K-Jetronic system. Depending on the operating conditions of the engine and the resultant current received from the control, the electro-hydraulic pressure regulator changes the pressure in the lower chambers of the valve for a definite (final) pressure difference. This changes the amount of fuel supplied to the injectors. It is located on the body of the fuel rail and operates on the principle of a plate that closes the opening of the pipe, and the change (drop) of pressure is controlled by the supply of electricity from the control device.

Prepared by: Dušan Ković
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