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K-Jetronic Fuel Injection System

K-Jetronic is a mechanically and hydraulically operated fuel injection pump,
Introduced by BOSCH GmbH in the year 1973.
The K-Jetronic pump requires no form of drive and
One of its features is that it can meter the fuel as a function of the intake air quantity. The letter ‘K’ stands for continuous in German. Therefore, K-Jetronic pumps
Continuously inject the fuel in the intake ports of the engine.

It can optimize the air-fuel mixture formation at different operating conditions
Such as starting and driving performance,
Power output and exhaust composition.

The 3 main functional areas of a K-Jetronic are:

  • Air-flow measurement
  • Fuel supply
  • Fuel metering

The air-flow is controlled by a throttle valve and it can be measured with the help of an air-flow sensor.

The fuel supply is controlled with the help of an electric pump. The pump delivers the fuel to the fuel distributor via an accumulator and a filter.

Fuel metering is dependent on the position of the throttle valve.
The amount of air drawn is measured by the air-flow sensor,
Which in turn controls the fuel quantity to be supplied to the fuel distributor.

Fuel from the fuel distributor is supplied to the injection valves,
Which injects the fuel over the intake valve. The air-fuel mixture is formed over the intake valve.
The air-fuel mixture has to be varied according to the various operating
Conditions such as start, warm-up, idle and full load.

The K-Jetronic system consists of injection valves which inject the fuel
Continuously into the intake ports where it is mixed with the air.
When the intake valves open,
The air-fuel mixture is drawn inside the combustion chamber.


The fuel supply system consists of the following parts:

  • Electric fuel pump
  • Fuel accumulator
  • Fuel filter
  • Pressure regulator
  • Fuel distributor
  • Injection Valves

Electric Fuel Pump:

The electric pump is a roller cell pump which delivers fuel from the tank to the
Fuel rail at a pressure of approximately 5 bar. The roller cell pump is driven by a permanent magnet electric motor.

It consists of a roller race plate which is eccentric. A rotor plate with notches (4 to 6) around its circumference is placed eccentrically inside the roller race plate. Each notch is provided with a roller. The roller race plate has an inlet port and an exit port.

When the engine is switched ON, the electric motor drives the pump. The motor drives the rotor plate inside the roller race plate. Due to the eccentric shape of the race plate,
The rollers in the rotor move outwards pressing against the roller race plate due to centrifugal force. The fuel is trapped between the roller and the notch in the inlet port
Side and as the rotor rotates towards the exit port side,
The fuel is pressurized and sent out through the exit port.

A check valve before the pump ensures that the fuel doesn’t flow back to the tank.

Fuel Accumulator:

Fuel accumulator is provided to maintain the pressure in the fuel system for a certain;
The amount of time after the engine is switched OFF. This is done to help in easy restarting of the engine, especially when the engine is hot.

The accumulator is divided into 2 chambers with the help of a diaphragm. One chamber acts as the fuel accumulator and the other chamber is connected to the atmosphere. When the engine is running, the fuel enters the accumulator volume and pushes the
Diaphragm against the spring force. The diaphragm moves until the springs halt in the spring chamber. Thus, the fuel collected at this point is the maximum accumulator volume.

Fuel Filter:

Fuel filter is often a combination of a paper filter, followed by a strainer. This ensures a higher degree of filtration. The paper filter has an average pore size of 10 µm.

Pressure Regulator:

A pressure regulator is fitted to one end of the fuel distributor. It is used to maintain the pressure in the fuel system constant at about 5 bar. It consists of a plunger that slides in the regulator against a spring.
When the fuel supplied by the fuel pump exceeds the limit,
The plunger moves against the spring to open the exit port. This allows the excess fuel to return to the fuel tank and thus maintaining the pressure.

When the fuel delivery quantity is lower,
The plunger shifts back closing the exit port to allow less fuel to escape to the tank. The constant shifting of the plunger maintains the pressure in the rail.

Fuel Injection Valve:

Fuel injection valve opens at a given pressure and atomize the fuel and inject onto the intake valves. They have a valve needle which sits on a valve seat. When the pressure is high enough, e.g. more than 3.5 bar, the valve needle is raised from the valve seat, thus allowing the fuel to escape. The valve needle oscillates at a high frequency when operated. This results in excellent atomization of the fuel even if it is of small quantity.


The airflow sensor here works on a suspended body principle. As we are aware that the airflow quantity will decide the fuel injection quantity, accurate measurements of the airflow is required. The airflow sensor is located upstream of the throttle valve. It consists of an air funnel over which a sensor plate is free to pivot.

The air flowing through the air funnel deflects the sensor plate from its zero position to a certain amount. This movement of the sensor plate is transmitted to a control plunger of the fuel distributor via a lever. The movement of the control plunger
Decides the quantity of fuel to be injected.


Depending on the position of the sensor plate in an airflow sensor,
The fuel distributor meters the sufficient quantity of fuel to be
Distributed to individual cylinders. The movement of the sensor plate is transmitted to a
Control plunger of the fuel distributor via a lever. The control plunger moves in a barrel. The barrel is provided with metering slits.

Based on the position of the control plunger in the barrel,
The control plunger opens or closes the metering slits to a larger or smaller extent.
For instance, if the air flow rate is high, then the control plunger will move a larger
Distance against the spring to open the metering slit to a greater extent. As a result, more fuel will be delivered to the injection valve.

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