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 inject 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.
FUEL SUPPLY SYSTEM:
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 in shape. 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 amount of time after the
engine is switched OFF. This is done in order 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 strainer. This ensures 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 which 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 open at a given pressure and atomize
the fuel and inject onto the intake valves. They have a valve needle which sit
on a valve seat. When the pressure is high enough, for 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.
AIR-FLOW SENSOR:
The air flow sensor here works on suspended body principle. As
we are aware that the air flow quantity will decide the fuel injection
quantity, accurate measurements of the air flow is required. The air flow
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.
FUEL DISTRIBUTOR:
Depending on the position of the sensor plate in air flow
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.
Related Topics:
- Gasoline Fuel Injection
- L-Jetronic (Gasoline)
- Chokes in cars (Gasoline)
- Multi point fuel injection (MPFI- Gasoline)
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