L-Jetronic Theory of Operation
INTRODUCTION
The L-Jetronic is an electronically controlled fuel injection system. Fuel is injected intermittently into the intake ports. The amount of fuel injected is controlled by the electronic control unit (E.C.U.).
The E.C.U. bases the injection amount from various inputs that it receives from sensors located on the engine. The system can adapt rapidly to changing driving conditions and offers the benefits of improved performance and economy while reducing the level of exhaust emission.
The L-Jetronic system can be broken down into 3 subsystems:
- The fuel supply system
- The air intake system
- The electronic control system
FUEL SUPPLY SYSTEM
The fuel supply system consists of the following components:
- electric fuel pump
- inline fuel filter
- fuel rail
- pressure regulator
- injection valves
Fuel Pump
The fuel pump is a roller cell type driven by a permanent magnet electric motor. The metal rollers are fitted to an eccentrically mounted rotor plate Centrifugal force causes the rollers to be pressed against the pump housing. Fuel is carried in the cavities formed by the rollers.
The fuel pump delivers more fuel than the maximum requirement of the engine. This is so that the pressure in the fuel system can be maintained under all operating conditions.
A check valve in the pump outlet prevents fuel from flowing back to the tank when the engine is switched off, thus maintaining system pressure.
Fuel Rail
The injection waves are connected to the fuel rail witch supplies an equal quantity of fuel to each injector. The volume of fuel stored in the rail, compared with the amount of fuel injected during each cycle is large enough to prevent variations in pressure
Pressure Regulator
The pressure regulator keeps the pressure differential between the fuel pressure and manifold pressure constant. Thus, the amount of fuel injected depends solely on injector opening time. The diaphragm in the regulator limits the system pressure to 2.5 bar.
The spring chamber of the regulator is connected by a vacuum hose to the manifold. Manifold vacuum adjusts the pressure of the regulator to compensate for changes in the manifold vacuum. This slices the pressure drop across the injector to remain constant for any throttle position.
Injection Valves
The valves are electromagnetic solenoid type valves. The valve body contains the solenoid winding needle valve and return spring. The valves are opened by electric pulses from the control unit. The amount of fuel injected depends on the opening time of the injector. The end of the needle valve is designed to atomize the fuel as it is sprayed into the manifold.
The injectors are pulsed once for each rotation of the crankshaft, or twice per working cycle of each cylinder. All injectors are opened at the same time.
AIR INTAKE SYSTEM
The air intake system consists of the following components:
- air cleaner
- air flow meter
- throttle valve
- idle control valve
Air Flow Meter
The air flow meter consists of a sensor flap compensation flap, idle bypass, potentiometer and intake air temperature sensor. It measures the volume of intake air entering the engine and converts it to a voltage signal. This voltage signal is used as an input signal to the control unit. It becomes one of the main measured variables used in determining the fuel injection amount.
The compensation flap dampens out pulsations of the sensor flap caused by the intake strokes of the pistons.
The meter incorporates an idle air bypass that is used to set or adjust the idle CO.
Throttle Valve
The throttle valve is connected to the accelerator pedal and controls the volume of air entering the engine. It incorporates a throttle switch to detect closed and open throttle position.
Auxiliary Air Valve
In order to achieve a smoother idle during engine warm up, the auxiliary air valve bypasses metered air around the throttle plate. When the engine is started cold, the gate valve is pushed open by the bimetal strip. As the engine is running, the heating coil heats the bimetal causing it to bend and the spring pulls the gate valve closed.
ELECTRONIC CONTROL SYSTEM
The amount of fuel supplied to the engine is controlled by the opening time (duration) of the injectors. The injectors are controlled by ground pulses from the electronic control unit (E.C.U.), which bases the injection time on signals from various switches and sensors on the engine. The switches and sensors detect any change in the engine operating conditions.
The initial (or basic) opening time of the injectors is determined from two main signals:
- air flow volume -- air flow meter
- engine R.P.M. -- ignition system
The input signals from the other switches and sensors are used by the E.C.U. to lengthen or shorten this initial injection time. This allows the E.C.U. to adjust the air/fuel mixture to ensure good driveability for all driving modes.
COLD START ENRICHMENT
When the engine is started, additional fuel is injected with the cold start valve. The cold start valve is energized for a limited time period which varies with engine temperature.
The cold start valve is a solenoid operated valve that is controlled by a relay during cold starts. A thermo-time switch limits the duration of injection of the valve. The thermo-time switch is an electrically heated bimetal switch which opens or closes its contact depending on its temperature.
The thermotime switch is installed in an engine coolant passageway and is heated by both the coolant and a heating coil. The heating coil is primarily responsible for heating the switch during cold starting. While the engine is at operating temperature, engine heat will prevent the valve from operating.
COLD IDLE/WARM UP
In addition to the increased idle speed provided by the auxiliary air valve, the engine requires additional fuel to ensure a stable idle. The fuel required for the cold idle/warm up phase is supplied by increasing the injection time. The E.C.U. receives, as an input, the engine operating temperature from an NTC temperature sensor. As the temperature of the sensor increases the resistance of the sensor decreases. This signal allows the E.C.U. to decrease the injection time as the engine warms up.
SUPPLEMENTARY FUNCTIONS
Adaption to intake Air Temperature
The quantity of fuel injected is adapted to the intake air temperature. Cooler air, being denser than warmer air requires additional fuel to maintain proper combustion. The air temperature sensor in the air flow meter signals the control unit which adjusts the fuel injection time accordingly.
Deceleration Fuel Cut
In order to reduce fuel consumption and achieve lower emissions, the injectors are cut on deceleration. The throttle valve switch is used as an input signal for deceleration. An idle contact signal along with a high R.P.M. signal indicates deceleration.
Maximum Speed Limit
When the maximum engine speed is reached, the control unit shuts off the injectors.
Acceleration Enrichment
During acceleration, additional fuel is required to ensure good transitional response. The E.C.U. lengthens the injection time to achieve this enrichment. The E.C.U. uses the rapid movement of the air flow sensor flap, as the throttle is opened abruptly, as an input signal for acceleration.
Idle/Full Load Correction
The throttle valve switch is used to signal the control unit for both closed and wide open throttle positions. The amount of fuel supplied is controlled by the control unit lengthening the injection time.
Lambda Closed Loop Control
Very low levels of harmful exhaust pollutants can be achieved when the air/fuel ratio is kept within the range of 14.7:1. (so called stoichiometric point or Lambda = 1).
In order to maintain this ratio the exhaust gas has to be monitored and the injection time has to be corrected accordingly.
A sensor is positioned in the exhaust gas flow to measure the oxygen content of the exhaust. The oxygen sensor will generate a voltage signal depending on the amount of oxygen in the exhaust. When the oxygen content is high (lean or Lambda > 1), the voltage signal is low.
When the oxygen content is low (rich or Lambda < 1), the voltage signal is high.
The voltage output of the oxygen sensor is in the range of 100 MV to 1000 MV with the transitional range from lean to rich being 450 to 500 MV. The control unit uses this signal to richen and lean the mixture to maintain Lambda = 1
Exhaust Emissions/Control Systems
Complete combustion of gasoline results in the formation of carbon dioxide (CO2) and water vapor (H20), neither of which is harmful. The engine exhaust gas also contains by-products of combustion. Incomplete combustion produces carbon monoxide (CO) and partially combusted or noncombusted hydrocarbons (HC). In addition, oxides of nitrogen (NOx) are formed by the combustion process from the nitrogen inducted with the air.
Carbon monoxide is a colorless, odorless toxic gas. CO is produced with a deficiency of oxygen, it is a direct result of the air/fuel ratio.
Hydrocarbons can be both an irritant and toxic due to the variety found in the exhaust gas. Hydrocarbons are also produced with a deficiency of air but can also be produced by mechanical or ignition related problems with the engine.
Oxides of nitrogen are colorless toxic gases. Production increases with higher combustion temperatures and pressures. Combined with hydrocarbon, oxides of nitrogen contribute to the formation of smog in the presence of sunlight.
Three-way Catalytic Convertor
Using Lambda controlled fuel injection has significantly reduced the levels of HC, CO & NOx emissions to a point where the only exhaust emission control device needed is a three-way catalytic converter.
The three-way catalytic converter oxidizes HC and CO emissions into H2O and CO2. At the same time, the NOx emissions are reduced to N2 and 02. It is only efficient within the range of Lambda = 1. The graphs represent exhaust emissions with and without convertor operation. The most effective conversion occuring at Lambda = 1.
The ceramic material within the convertor is coated with the noble metals platinum and rhodium. These metals cause the chemical reaction to take place without changing their composition.
The catalytic convertor is subject to damage by overheating, lead coating and engine oil residues.
The ideal operating temperature is within the range of 400° to 800°C (700 to 1400°F).