L-Jetronic Fuel Injection System

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.