Operating procedure (Audi 100 C4)

Interaction of constituent elements

The pistons rising and falling while the engine is running draw in air. If you fully depress the accelerator pedal, the engine draws in the greatest possible amount of air, because both throttle valves are then fully open. Accordingly, when the throttle valves are closed or partially open, the air quantity/mass is smaller. For the engine to run cleanly, air must be mixed with the fuel in a precisely defined proportion. To determine the fuel/air ratio, the MPI injection system uses the intake air mass. The air flow meter serves as a source of information for the MPI control unit on the intake air mass. A comparable value for the MPFI injection system is information on the pressure in the exhaust manifold and on the temperature. The control unit can give the correct impulses for opening and closing the injection nozzles. A longer opening is necessary if a large amount of fuel is required - a short opening is sufficient if only a small amount of fuel is needed. The "injection power" of the nozzles thus remains constant. The reduction in quantity occurs by reducing the "injection time".

Starting a cold engine

To start a cold engine, a rich mixture is necessary because many of the fuel droplets condense on the inner walls of the injection system already on the way to the combustion chambers and do not participate in the combustion process. Therefore, it is necessary to enrich the mixture.



The cold start program takes care of this. During cold start regulation, the injection duration of the injectors is increased. The injection duration is influenced by factors such as coolant temperature and engine speed. After a certain number of revolutions, the amount of fuel during a cold start is gradually reduced to normal.

Engine warm-up phase

After starting, the engine also requires a rich mixture for some time, since some fuel still continues to condense in the intake system. For this purpose, there is "post-start enrichment". Over a certain period of time, depending on the temperature, more fuel is supplied. The control unit receives the necessary information about the engine temperature from the coolant temperature sensor. The relatively viscous engine oil causes increased internal friction when the engine is cold. Therefore, more force is required (richer mixture), in order to maintain the required rotation speed. The supply of the enriched mixture is provided by the already mentioned engine shaft rotation speed stabilization valve in idle mode.

Idling

With the throttle valve closed (released accelerator pedal) a small amount of air passes through the channel past the throttle valve. The amount of this air is determined by the air flow meter or pressure sensor and is supplemented with the required amount of fuel to obtain a mixture for idle mode. By the way, the mixture for idle mode is richer than the normal mixture, which is done to ensure that the engine runs smoothly. At what point the enrichment for the idle mixture should be performed, the control unit learns from the throttle potentiometer. How much air will pass through the bypass is determined by the idle speed stabilization valve.

Acceleration

If the accelerator pedal is suddenly depressed, this causes the mixture to be enriched for acceleration if the increase in the quantity/mass of intake air per second exceeds a certain value. The acceleration signal also comes from the throttle potentiometer. If the engine is still cold, even more fuel is needed for acceleration. The control unit therefore recognizes the corresponding pulse from the air flow meter or pressure sensor and the throttle potentiometer as an acceleration signal and increases the fuel supply.

Speed limitation

The injection system compares the current rotation speed with the maximum, which is 6500 rpm, and if it is exceeded, it simply stops supplying fuel (see also chapter "Engines").

[The original article is posted on the resource: AudiManual.ru]