Audi A4

since 1994 release

Manual of the car

In order that it is better to understand functioning of system of injection in general, in the beginning it is important to learn about problems of its separate knots.

1 — the engine speed Sensor
2 — the ignition moment Sensor
3 — Hall's Sensor
4 — the Sensor of temperature of cooling liquid
5 — the Sensor of temperature of the soaked-up air
6 — the Potentiometer of a butterfly valve and the switch of the mode of idling
7 — the Sensor of a detonation of I
8 — the Sensor of a detonation of II
9 — the Lambda probe of I
10 — the Lambda probe of II
11 — injection Valves
12 — the Output cascade of power
13 — Coils of double ignition
14 — the idling stabilization Valve
15 — the Electromagnetic valve of system of the coal filter
16 — the Control unit of heating a lambda probes
17 — the Relay of the fuel pump

This scheme illustrates the electric party of system of ignition/injection, in this case on the example of MPFI with the air pressure sensor in an inlet collector. On the left side sensors and sensors which influence behavior of the control unit are placed. On the right those knots of ignition and injection to which the control unit sends the signals are represented.

Fuel party of system of ignition/injection of MPI/MPFI

Sleva: valves of injection (1, 2 and 3) in the six-cylinder engine are located by groups on three on the right and to the left of the central inlet collector.

On the right: behind on the right in a motive compartment there is a measuring instrument of a stream of fresh air (1), in the engine with a working volume of 2,8 l priflantsovanny to the case of the air filter. Figure "2" designated the connection socket.

Control unit

Between the entering information (from various sensors) and valves of injection there is an electronic control unit. Depending on the existing conditions of loading and temperature it sends absolutely certain amount of fuel to the engine. For this purpose the control unit varies duration of opening of the injection valves operated in the electromagnetic way. The amount of injectable fuel can change only at the expense of injection duration. Information for determination of duration of injection comes to the control unit from various knots, it:

• Only in the engine of 2,8 l with MPI: measuring instrument of a stream of fresh air; from it information on amount of the soaked-up air arrives.
• Only in the engine 2,6 with MPFI: the sensor of temperature of the soaked-up air; in combination with the air pressure sensor in an inlet collector (in the control unit) it gives the comparative size of the soaked-up amount/weight of air.
• Sensor of temperature of cooling liquid; he delivers the comparative size of temperature of the engine.
• Potentiometer of a butterfly valve; from it information on load of the engine arrives.
• Speed sensor; it transmits a signal of crankshaft speed.
• The signal of start of the engine arrives from the plug 50 of the lock of ignition (starter).
• Lambda probes send a signal of correctness of composition of fuel-air mix.
• Other sizes exerting impact arrive from: sensors of a detonation, the transmission, a speedometer and if is, from climatic installation.

Injection valves

In the channel of absorption of each cylinder of the engine about one valve of injection is placed. It gives amount of fuel necessary at present to each cylinder and at the same time provides thin dispersion of gasoline.

Valves are put in action in the electromagnetic way. At the same time the dosing needle rises in the saddle approximately on 0,1 mm – fuel can flow.

Distributive tube of fuel

It serves for uniform supply of fuel to all valves of injection. Besides, the distributive tube functions as the collector and thanks to it does not allow fluctuations of pressure. The ring-shaped form of a tube allowing to give fuel to all six valves is interesting.

Fuel pressure regulator

It settles down behind on the right on a distributive tube of fuel and has to keep the constant pressure of fuel in a distributive tube. It provides it by increase or reduction of amount of the fuel flowing on a recirculation tube. If more fuel comes back, pressure goes down; at small return of fuel it rises.

Due to connection of a vacuum hose pressure regulator at the same time obtains information on a condition of load of the engine. At full loading it lifts pressure a little more. As a result of it more fuel which the engine needs for achievement of full capacity is injected.

Fuel pump and relay

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

Measuring instrument of pressure of fresh air

Only MPI

The wire which heats up in the electric way is placed in a stream of the soaked-up air. Depending on the mass of the soaked-up air the stream of air which cools a wire stronger or more weakly changes. Change of temperature causes change of resistance of a wire, and it is measured by the control unit.

The air pressure sensor in an inlet collector

Only MPFI

The air pressure sensor in an inlet collector is in the MPFI control unit. The inlet collector and the sensor unite among themselves a thin hose. For the control unit air pressure in an inlet collector is the major information for calculation of load of the engine. It exerts impact on duration of injection and at the time of ignition.

Sensor of temperature of a stream of fresh air

Only MPFI

The sensor of temperature of the soaked-up air is screwed in to the canal of absorption of the third cylinder (behind on the right). In addition to the air pressure sensor it serves the control unit for calculation of load of the engine. At high temperature of the soaked-up air (it is equivalent the low density of air) injection time, for example, has to be reduced and the moment of ignition is slightly more shifted in the direction "later".

Branch pipe of a butterfly valve

Where the stream of fresh air enters an inlet collector of the engine, in the union there are two butterfly valves. Smaller of them it is connected by a cable of a butterfly valve to a pedal in salon. It doses a stream of the soaked-up air in the engine to the provision of semi-gas.

In process of increase in pressing the accelerator pedal the system of drafts opens the second, big gate until at full gas both butterfly valves completely are not open.

Potentiometer of a butterfly valve

The potentiometer of a butterfly valve is put in action by its shaft. The potentiometer defines the provision of a butterfly valve at present and in the form of voltage sends a signal to the control unit. This information on loading, besides other signals, is used by the control unit for adjustment of idling, the choice of the characteristic of ignition and calculation of duration of injection.

Idling stabilization valve

As it is clear according to the name, this valve provides the constant speed of idling – not important, the engine is heated-up or not, consumers of power (climatic installation) are connected or not.

The valve at the same time is only an executive element. A brain of adjustment is the MPI or MPFI control unit. It compares speed to nominal at present and provides thinly adjusted opening and the closing of the adjusting valve adjusting speed. At the same time the cross section of the auxiliary air channel which bypasses a butterfly valve changes. If the auxiliary channel is open, then more air and therefore the measuring instrument of a stream of fresh air "considers" that the butterfly valve is open is soaked up. It, in turn induces system of injection to direct the necessary amount of fuel that leads to increase in speed of the engine.

It is necessary to notice still that various valves of stabilization of idling are used: stepless MPI adjusting valve; the valve with the drive from the so-called step electric motor at MPFI. The last changes an opening of a baypas the small, precisely adjusted steps.