Audi 80/Avant (B4)

since 1991-1995 release

Repair and operation of the car

The scheme in detail shows system of ignition and injection on the example of MPFI system with pressure sensor in an inlet collector. On the left side the sensors and sensors influencing behavior of the control unit are located. On the right components of systems of ignition and injection to which the control unit sends the teams are presented.

System of injection of MPI/MPFI with t.z. fuel supply

Electronics helps to constrain a few "appetites" of the engine even the most powerful Audi 80 version. However, at more than probable fuel consumption over 12 l on 100 km it is not always possible to be quite convinced that this good intention is carried out. But nevertheless follows that high engine capacity justifies this extra charge for high high-speed qualities of the car.

Multipoint injection

The 2,8-liter engine is supplied with system of injection with designation of MPI. The English words Multi Point Injection – in Russian "multipoint injection" are behind this reduction. (The distinctive sign of system of injection of MPI is a flowmeter of air in an air inlet sleeve behind on the right in a motor compartment).

Almost identical system of injection of the 2,6-liter engine is called MPFI actually only for their distinction. This reduction is in turn deciphered as Multi Point Fuel Injection – that is in Russian "multipoint injection of fuel". (A distinctive sign of system – lack of the air-flow meter).

The technical distinctive sign of these both systems is a way of measurement of quantity (or masses) the arriving air:

• MPI uses for this purpose the termoanemometrichesky air-flow meter with the heated thread.
• MPFI uses for this purpose pressure sensor in a final collector (it is built in a monitor and it is connected with a final collector a connecting hose), and also the sensor of temperature of the arriving air (in a final collector).

Additional functions

• MPI/MPFI is very difficult system of injection which control unit in addition includes also electronic system of ignition.
• The control unit of system of injection of MPI/MPFI is completely suitable for diagnostics. It means that the malfunctions arising during operation register in memory of the store of malfunctions which is erased only after shutdown of the battery.
• Feature – storing even short-term malfunctions. From practice it is known that it is those defects which are extremely difficult for finding.
• After 50 starts of the engine the malfunction which arose only once is erased from memory of the store of malfunctions.

Basic elements

For the best understanding of the general functioning of this system of injection it is necessary to get acquainted with separate problems of components at first.

Control unit

Between arriving (from various sensors) the control unit is information and vpryskny nozzles. It allows the engine, depending on the operating conditions of loading and temperature, to receive precisely certain amount of fuel. For achievement of it the control unit varies duration of opening of nozzles with the electromagnetic drive. As pressure in a fuel supply system constantly remains almost invariable, the amount of injectable fuel can be regulated only by means of change of duration of injection. From where the control unit receives information according to which it establishes this duration of injection? Various sensors are responsible for it:

• Only the 2,8-liter engine with system of injection of MPI: air flowmeter; it issues information on amount of the arrived air.
• Only the 2,6-liter engine with system of injection of MPFI: temperature sensor in a final collector; he reports in combination with pressure sensor in an inlet collector (in the control unit) the size of amount/weight of the arrived air.
• Sensor of temperature of cooling liquid; he reports engine temperature size.
• Potentiometer of a butterfly valve; he gives information on load of the engine.
• Sensor of frequency of turns; it transmits a signal of rotation frequency for system of ignition/injection of MPI/MPFI.
• Ignition moment sensor; he reports about the provision of a cranked shaft. So the control unit learns, what cylinder on turn for ignition or injection.
• The signal of start-up arrives from the plug 50 (the lock switch of ignition and a starter).
• Lambda probes report, whether the composition of mix is correct.
• Other indicators arrive from sensors of detonation combustion, from transmission, from a tachometer, even from the conditioner.

Vpryskny nozzles

Is in an inlet collector of each cylinder of the engine on one vpryskny nozzle. They bring to the corresponding cylinder amount of fuel necessary at present and at the same time provide thin dispersion of gasoline.

Nozzles are set in motion by an electromagnet. Thus the needle of a spray rises from the saddle approximately on 0,1 mm – fuel can arrive. That circumstance is interesting that management of each separate vpryskny nozzle happens precisely in an injection step. That means that injectable fuel even has no time to be condensed on walls of an inlet collector (there are no fuel losses).

Fuel distributor

It is intended for uniform supply with fuel of all vpryskny nozzles. Besides, the distributor of fuel operates and as the fuel store, thereby preventing pressure differences. The U-shaped form of a pipe allowing a fuel supply to all six nozzles is very interesting.

Fuel supply pressure regulator

It is located behind on the right on the distributor of fuel and has to – according to the name – to support the constant level of pressure in the fuel distributor. It is carried out by means of more or less strong outflow of fuel back in a fuel tank on the drain highway. If on the drain highway more fuel leaves, pressure decreases; if it is less – pressure increases.

Through connection of the vacuum pipeline to pressure regulator besides information on load of the engine arrives. At full loading the regulator increases pressure even more. Thanks to it more fuel, necessary for the engine for achievement of the maximum power is injected.

Fuel pump and relay

In more detail you learn about the electromagnetic fuel pump, the relay of the fuel pump and other MPI/MPFI relays in the head the Fuel tank and the fuel pump.

Air flowmeter

Only MPI

On the way of a stream of the soaked-up air there is a wire which is warmed up electrically. Depending on the let-in quantity the air stream changes that leads to more or less strong cooling of the mentioned wire. Change of temperature leads to change of electric resistance of a wire which is measured by the control unit.

Pressure sensor in an inlet collector

Only MPFI

Pressure sensor in an inlet collector is in the MPFI control unit. Connection between an inlet collector and the sensor represents a thin hose. Pressure in an inlet collector is for the control unit of information front view for calculation of loading of the engine. It influences duration of injection and the moment of ignition.

Sensor of temperature of inlet air

Only MPFI

The sensor of temperature of inlet air is screwed in in the inlet canal of the third cylinder (behind on the right). In addition to pressure sensor in an inlet collector it is for the control unit a source to an inofrmation for calculation of load of the engine. At high temperature of inlet air (that is equivalent the low density of air) it is necessary to reduce, for example, duration of injection and to shift the ignition moment aside "later" a little.

Case of throttle knot

Where the stream of the soaked-up air gets to an inlet collector of the engine, in one case there are two butterfly valves. Smaller from gates it is connected by means of the hummock drive to an accelerator pedal. It doses a stream of inlet air in the engine to the provision of half loading. By further pressing an accelerator pedal the lever of draft opens the second, big gate until in the provision of full loading there are no completely open both gates.

Potentiometer of a butterfly valve

The potentiometer of a butterfly valve is put in action by the roller of a butterfly valve. The potentiometer defines the provision of a butterfly valve of time at present and transfers to the control unit this information in the form of electric resistance. The control unit needs this information on loading, for example, for regulation of frequency of rotation in the mode of idling, a choice of the characteristic of ignition and calculation of duration of injection.

Valve of stabilization of frequency of idling

As it is already clear according to the name, this valve constantly provides the constant frequency of rotation of a shaft of the engine in the idling mode – all the same, whether the engine is cold or heated-up, are included or not powerful consumers of the electric power like the conditioner.

Thus the valve – the executing body. The center of regulation is the control unit of systems of injection of MPI or MPFI. It compares rotation frequency with standard at present and thus provides the coordinated opening and closing of the adjusting valve for a rotation frequency equilibration is thin. Thus the cross section of the additional air channel laid bypassing butterfly valves varies. When the channel is open, the bigger amount of air is let in, thereby the flowmeter of air or the sensor of pressure in a final collector because of increase in amount of air "thinks" that the butterfly valve is open. That in turn gives the grounds to system of injection to increase amount of the arriving fuel to the necessary.

It is necessary to notice still that in different systems of injection different valves for stabilization of frequency of rotation operate: in system of injection of MPI – smoothly regulating; in system of injection of MPFI – put in action by the so-called step engine. This the last regulates opening of channels in small, is thin the adjusted steps.