L-Jetronic

from Wikipedia, the free encyclopedia

The L-Jetronic is a driveless injection system for gasoline engines with electronic control, in which the fuel is injected intermittently into the intake manifold. The amount of fuel metered depends on the amount of air drawn in, which is measured electronically. The system was introduced in 1973.

L-Jetronic in a Peugeot 405 SRi

construction

Fuel path

The fuel is pumped from the fuel pump via a filter to the fuel rail, which supplies the injection valves via a fuel pressure regulator . The fuel pressure regulator regulates the system pressure in the fuel rail up to 3.5  bar . The intake manifold pressure is transmitted to the pressure regulator via a hose attached behind the throttle valve , which keeps the pressure difference between the intake manifold and the distributor pipe constant. This means that the volume of fuel injected per millisecond is independent of the absolute pressure of the intake manifold and the position of the throttle valve.

The allocated amount of fuel is therefore determined exclusively via the opening time of the electromagnetic injection valves, which are activated by a control unit . This switches to ground, since the positive supply is applied to the valves. As with the K-Jetronic , the valves in the intake tract are about 70–100 mm in front of the intake valve (manifold injection, as opposed to direct injection). The spray cone has an angle of 25 °. The opening time is around 2.5 ms with the engine at operating temperature and idling speed.

In older vehicles, a cold start valve is connected to the fuel rail , which serves to enrich the mixture during a cold start and is controlled by a thermal timer . In newer vehicles, this cold start enrichment is taken over by the control unit, which keeps the injection valves open for a specified period of time or depending on the cooling water temperature.

Air damper air flow meter

The air flow meter is similar in principle to the air flow meter of a K or KE Jetronic . A spring-loaded flap is located in the air flow and must be moved by it. This controls a potentiometer mounted on the valve shaft . It shows a resistance value corresponding to the flap position, which is evaluated by the control unit. To compensate for the pressure fluctuations generated by the intake strokes, a compensation flap is installed, which is also attached to the shaft described above . Both flaps form one component. A temperature-dependent resistor is installed in the intake area of ​​the air flow meter , which measures the temperature of the air that is drawn in. On some, mostly older vehicles, a screw is attached to the underside of the air flow meter. This tapers a duct in the air flow meter through which unmeasured air enters the intake manifold. By turning this screw, the amount of air changes and the control unit will meter more or less fuel accordingly.

Creation of the injection signal

The control unit receives speed information from terminal 1 of the ignition distributor . There are four injection signals for every 720 ° crankshaft rotation angle (4-cylinder engine). In the control unit's pulse shaper, these four signals are converted into square-wave signals that can be used by the control unit. The downstream frequency divider halves the number of pulses. The resulting signals are now converted into the basic injection time in the division control multivibrator (DSMV). At the beginning of a signal, the DSMV lets a voltage increase linearly, at the end of the signal it is reduced again. The rate of voltage drop is controlled by the resistance of the air flow meter potentiometer. Depending on the amount of air drawn in, the breakdown phase will be longer or shorter. This basic injection time is now processed in the multiplier stage. Two correction times are added or subtracted here. These correction values ​​are based on the one hand on the intake air temperature, the engine temperature and the throttle valve position and on the other hand on the current battery voltage. The resulting injection signal is amplified in an output stage and passed on to the injection valves.

In summary this means:

  • The cycle frequency of the injection valves is determined from the engine speed.
  • The speed and the amount of air drawn in determine the basic injection time.
  • According to information from the measuring sensors and the battery voltage , the multiplier stage extends the basic injection time to the actual opening time of the injection valve.

Mixture correction

Cold start

  • The first versions have an electrically heated thermal timer (bimetal) and an additional cold start valve for mixture enrichment
  • in newer vehicles over longer opening times of the injection valves regulated by the control unit

Warm-up phase

Load changes

  • The throttle valve switch sends information (idle or full load) to the control unit, which changes the injection time depending on the throttle valve angle, which is recorded by the throttle valve potentiometer.

Change in intake air temperature

  • Detection via the intake air temperature resistance. The control unit reacts by extending the injection time.

Speed ​​limitation

  • The control unit receives the speed information from the distributor and reduces the injection time.

Fuel cut-off

  • Detection via speed and throttle valve switch; the control device stops the injection at the accelerator pedal released and above a certain rotational speed (usually 1500 min -1 ).

Lambda control

  • A lambda probe detects a rich or lean mixture, and the injection time is changed accordingly.

L-Jetronic with DME and L-Motronic

A further development of the L-Jetronic in the mid-1980s was the combination of ignition and injection control in one control unit. This made it possible to better adapt the controls to one another, thereby reducing fuel consumption and achieving better emission values. All vehicles equipped with this injection technology already had a lambda probe and catalyst preparation. Upon request, they were also equipped with a catalytic converter in Europe. Furthermore, an activated carbon filter is built into the tank ventilation, which reduces the hydrocarbon emissions from the tank. This means that the L-Motronic can meet the Euro 2 emissions standard. The fuel pump can be integrated in the tank.

Activated carbon filter with regeneration valve

From a certain temperature, volatile hydrocarbons can enter the earth's atmosphere through the tank ventilation . In the Euro 2 standard, this emission was set to a certain limit value. In order to comply with this, an activated carbon filter was installed in the tank ventilation. This absorbs the escaped hydrocarbons and only allows pure air to escape into the atmosphere. A regeneration valve is placed between the activated carbon filter and the intake pipe of the engine. It opens under certain conditions and enables the activated carbon filter to be regenerated. The fuel vapors are sucked into the intake pipe via the regeneration valve, then reach the combustion chamber of the cylinders and are burned there. Since the control unit cannot recognize the composition of the air sucked in via the regeneration valve, it is possible that the mixture becomes too rich or too lean. This is recognized by the lambda probe and compensated for by the control unit. In OBD vehicles, the function of the tank ventilation is monitored by the control probe. If the activated carbon filter is regenerated, the control probe makes a voltage jump in the "rich" direction. If the voltage jump does not occur, the control unit activates the OBD lamp. When the engine is switched off, the regeneration valve remains closed for a few seconds to prevent the engine from refilling.

LH-Motronic

The basic structure of an LH-Motronic is the same as an L-Jetronic . In contrast to this, it is not the amount of air that is measured, but the air mass.

Air mass meter

In the LH-Motronic, either a hot wire air mass meter or a hot film air mass meter can be installed.

Basic principle

1000 liters of air (one cubic meter) have a mass of 1.29 kg at 0 ° C and an air pressure of 1013 hPa - the density is 1.29 kg / m 3 . Changes in temperature, but also changes in air pressure, influence the density of the air. This is because gases completely fill the available space. The individual molecules and atoms have the greatest possible distance from one another. If the temperature rises, the self-movements of the air particles increase and they continue to repel each other. The density of the air decreases. If the temperature drops, the particles move less and the distance between them is reduced. The density of the air increases. If the pressure increases, the particles are compressed into a smaller space and the density increases. If the air pressure drops, the density of the air also drops. If air is allowed to flow past a heated wire, the wire is cooled to varying degrees depending on the air mass over time. If the temperature of the heating wire is kept constant by an electronic control, the current flowing through the heating wire is a measure of the air mass flowing past.

This principle is used in the hot wire air mass meter and in hot wire anemometers .

Function of the hot wire air mass meter

A pipe section with a measuring pipe is attached behind the air filter box. A platinum wire 0.07 mm thick is attached to this measuring tube in such a way that the air flowing through it flows around it. The temperature of the hot wire and a reference wire with less flow around it is kept the same (approx. 100 ° C.) via the current in a Wheatstone bridge circuit . The intake air temperature is determined at the same time. The actual measured variable is the voltage drop across the circuit's measuring resistors. These have a high long-term stability.

The bridge circuit is controlled or evaluated with an operational amplifier . The measured values ​​are obtained 1000 times per second. The control unit then calculates the exact injection time taking into account the other sensors.

Function of the hot film air mass meter

In the case of the hot film air mass meter (see also thermoelectric anemometer ), the platinum wire is replaced by a sensor plate. This consists of three different resistance layers with different functions. These three layers consist of different materials that were vapor-deposited onto a ceramic plate using hybrid technology . It refers to

  • the balancing resistor. It is used to record the air temperature of the air drawn in and shows PTC behavior.
  • the heating resistor. It only serves as a heater and has no sensor function.
  • the sensor resistance. It is used to record the air mass sucked in and is cooled by it. Depending on the air mass, it is heated more or less by the heating resistor so that its temperature-dependent resistance ( NTC behavior) is kept constant.

The hot film air mass meter offers an advantage over the hot wire air mass meter. The ceramic plate is arranged in the measuring tube in such a way that the air does not hit it but can flow past it. The smallest particles are no longer picked up by the sensor unit and can no longer stick there, as was the case with hot wire. The service life was thereby increased. However, the measurement accuracy is no longer as high as with the hot wire air mass meter. Nevertheless, it is still high enough to ensure problem-free functioning of the system.

The same problem occurs when the engine is idling both with the hot wire and hot film air mass meters and with the damper air flow meter. Due to the pressure pulsations of the air column that arise in this operating state, it is possible that one and the same air mass is measured several times. This is z. B. prevented by the fact that the measuring unit is integrated in a bypass channel. Air mass meters with reverse flow detection were also installed.

literature

  • Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition. Friedr. Vieweg & Sohn Verlag, Wiesbaden 2003, ISBN 3-528-23876-3 .
  • Peter Gerigk, Detlef Bruhn u. a .: automotive engineering. 7th edition. Westermann Schulbuchverlag, Braunschweig 2009, ISBN 978-3-14-231800-4 .
  • Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition. Verlag Europa-Lehrmittel, Haan-Gruiten 2001, ISBN 3-8085-2067-1 .