The L-Jetronic is an electronically controlled fuel
injection system which has the advantage of direct air flow sensing. It injects
intermittently into the intake ports. The task of a fuel injection system is to
supply precise amount of fuel to the combustion chamber at that particular
moment.
An engine’s operating conditions keep changing rapidly,
hence fuel injection system should be fast enough to adhere to the changes and
vary the fuel supply quantity at that very moment. L-Jetronic, which is an electronically
controlled fuel injection system, is particularly suitable for the above
mentioned conditions.
The control unit processes signals from a variety of sensors
and calculates the exact amount of fuel to be supplied to the combustion
chamber.
Functions of
L-Jetronic:
There are 3 major functions of an L-Jetronic:
·
To
pressurize fuel: L-jetronic system supplies fuel from tank to the intake
valves at a certain pressure required for injection. Maintaining the pressure
throughout the supply is at most important.
·
To
monitor the sensors: The control unit has to register the important signals
from various sensors such as air-flow sensor, throttle valve sensor, engine
speed sensor, engine temperature sensor, etc.
·
To
regulate fuel quantity: The signals from the sensors are processed by the
control unit and pulses are generated to vary fuel injection amount.
DESIGN OF
L-JETRONIC’S FUEL SYSTEM:
The fuel system of an L-Jetronic consists of the following
components:
- Electric pump
- Fuel Filter
- Fuel rail
- Pressure regulator
- Fuel injectors
Electric Pump:
The electric pump is a roller cell pump which delivers fuel
from the tank to the fuel rail at a pressure of approximately 2.5 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 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.
Fuel Rail:
The function of a fuel rail is to maintain the pressure and
to supply equal amount of fuel to each injector.
Pressure Regulator:
Pressure regulator is provided at one end of the fuel rail.
It maintains the pressure difference between the fuel rail pressure and the
manifold air pressure. The injection of fuel by the electronic fuel injectors
depends on the inlet valve opening time. Therefore, the pressure in the fuel
rail depends directly on the inlet manifold pressure.
The pressure regulator is a diaphragm controlled regulator
which regulates fuel pressure at 2.5 bar. If the pressure exceeds the set
pressure, then the fuel from the rail flows back to the tank via a return valve
in the regulator. The diaphragm is pre-loaded by a spring and the diaphragm
chamber is connected to the inlet manifold stream through a tube. Another
chamber is provided for the fuel return valve and line. The fuel flows back to
the tank without any pressure.
Electronic Fuel
Injectors:
The electronic fuel injectors inject precise amount of fuel
over the inlet valves. Each cylinder is provided with its own fuel injector. All
the injectors are solenoid operated valve. The solenoid is controlled by
electric pulses which are generated by the control unit (ECU).
The solenoid valve is provided with a solenoid winding.
There is a needle valve sitting inside the winding. The needle is pressed
against its seat with the help of a helical spring. When electric pulse is
passed, the solenoid winding is magnetized and the needle valve lifts from its
seat to allow the fuel to be injected through the orifice. The front end of the
needle is pintle shaped for better atomization of the fuel. The needle is
lifted approximately 0.08 to 0.1 mm from its seat.
SENSORS:
Sensors are an essential part of the L-Jetronic system as it
detects the operating conditions of an engine. The most important ones are the
engine speed sensor and the air-flow sensor.
Air-flow sensor:
Air-flow sensor measures the force of air on the air-flow
sensor flap. The sensor flap moves against the opposing spring forces. The flap
moves in proportion to the air flow and the compensation flap also moves the
same distance as the sensor flap moves. The compensation flap is connected to a
variable resistance potentiometer.
When the flap moves, a voltage is generated in proportion to
the distance it moves. The closed position of the flap will generate zero
voltage and fully open position will generate approximately 5 V. There is an
idling air passage to allow some amount of air to flow when the engine is
running at idling speed.
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