General description
In terms of the accuracy of supplying the air-fuel mixture of fuel, the fuel injection system through one nozzle under the throttle valve is inferior to the injection system directly into the combustion chamber of the cylinder. This function is performed by the corresponding injector on each engine cylinder. For this reason, the 85 kW four-cylinder engine is equipped with a multipoint injection system. We are talking about an electronically controlled system with parameters stored in its memory obtained during engine tests. These parameters relate to the ignition timing, knock-free combustion of the air-fuel mixture, idle stabilization and forced idling.
This system in the sense of automation and computerization is so powerful that the AUDI and VW engineers who created this system gave it the designation Digifant, which is a word formation from parts of two words, which can be translated from German as «software elephant» (see illustration 6.0).
The electronic control unit of the Digifant system receives information from the following sources via a multi-pin plug:
- A) from terminal 50 of the starter, data are received on the beginning and end of the engine start,
- b) the throttle angle potentiometer tells the control unit its current position,
- V) Hall sensor on the ignition distributor informs about the engine speed,
- G) the lambda probe on the catalyst provides information on the oxygen content in the exhaust gases,
- d) the potentiometer of the air volume flow meter informs about the position of the valve shutter, which corresponds to the volume of incoming air,
- and) the temperature sensor on the air volume flow meter supplies data on the temperature of the incoming air,
- h) the coolant temperature sensor in the return pipe reports its temperature,
- e) The knock sensor on the cylinder block informs about the knocking combustion of the fuel.
Based on the data on the number of revolutions and engine load, the electronic control unit determines the duration of the opening of the nozzles and, thus, the amount of fuel injected. To do this, its memory contains information about the needs of the engine in all kinds of situations, obtained during the tests, as well as parameters relating to the ignition timing. The electronic control unit can independently correct the set parameters, but this is possible only after the receipt of the so-called clarifying signals (e.g. intake air and coolant temperature data).
In the intake manifold, there are two throttle valves on one pipe. The smaller of these shutters is connected by a cable «gas» with accelerator pedal (see illustration 6.0a).
Until the accelerator pedal reaches the position «Full throttle», this damper regulates the intake air. And only when the accelerator pedal is sufficiently depressed, the corresponding rod opens the second flap of a larger diameter.
The throttle valve angle potentiometer recognizes its current position. Based on its information, idling stabilization, forced idling or maximum enrichment of the mixture is performed (see illustration 6.0b).
A pressure reducing valve is located in front of the fuel distributor, which regulates the pressure of the fuel supplied to the injectors. For this purpose, the valve is informed of the degree of vacuum in the intake manifold. When idling at closed throttle and high vacuum, the valve reduces fuel pressure. As the vacuum decreases as the engine load increases, the pressure reducing valve increases the fuel pressure. The fuel pump generates a sufficiently high operating pressure of the fuel, however, the pressure reducing valve can increase or decrease the return of fuel to the fuel tank accordingly.
The injectors, with each revolution of the crankshaft, inject fuel into the intake port in front of the intake valve of the corresponding cylinder (see illustration 6.0c).
The duration of the injection is determined by the control unit. Thus, the corresponding cylinder receives one portion of fuel «in reserve», and the second portion enters directly into the combustion chamber at the moment the intake valve opens.
The intake air temperature sensor, located on the air mass meter, detects the temperature of the incoming air and transmits it in the form of a resistance value to the electronic control unit. Air temperature is needed to optimize fuel injection.
The coolant temperature is needed to control several functions performed during fuel injection. These are rich air-fuel mixture at cold start, rich mixture after engine start due to corresponding engine load, rich mixture during acceleration, and forced idle operation. Information about the temperature of the coolant is also transmitted to the electronic unit in the form of resistance values. Based on the data received, the electronic control unit calculates the required injection duration, which, when the engine is warmed up to operating temperature, ranges from 2 to 8 milliseconds. This value can be increased by the electronic control unit if the coolant temperature is -25°C.
The air coming from the air filter presses on the valve shutter of the air volume flow meter. The degree of pressure depends on the volume of incoming air. At the same time, the cover of the vibration damper connected to the valve shutter prevents vibrations of the valve shutter. The corresponding potentiometer reports the position of the valve shutter to the electronic control unit of the system, giving a signal by changing the voltage. High voltage is supplied when the valve shutter is fully open and a large amount of air has entered, and low voltage is supplied when the shutter is slightly opened.
The starting fuel solenoid valve is a nozzle that, when starting the engine, depending on its temperature, injects fuel into the intake manifold for 1-4 seconds, while spraying it. The duration of injection is set by the control unit of the system.
When the engine warms up, when the power steering hydraulic drive is activated, the air conditioner is running, or when the gear is engaged on cars with an automatic transmission, the idle air stabilization valve is turned on, which opens an additional air supply channel bypassing the throttle. An increase in the volume of incoming air simultaneously causes an increase in the fuel supply. This compensates for increased friction of engine parts when starting it in a cold state or an increased load on the engine due to the operation of the power steering pump, air conditioning compressor or automatic transmission. The idle speed stabilization valve is also used when the forced idle mode is turned on.
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