In order for the engine to run "cleanly", the air-fuel ratio in the mixture must be correct. The starting point for calculating the air-fuel ratio in the MPI injection system is the volume of incoming air. This information is supplied by the mass air flow meter. In the MPFI system, the starting point is the information on the pressure in the intake manifold and the temperature of the intake air.
To start the engine, an enriched air-fuel mixture is required because many drops of fuel on the way to the combustion chambers settle on the walls of the inlet opening and do not burn. In this case, the electronic control unit uses the program stored in its memory for starting a cold engine. At the moment of starting, the duration of opening of the injectors increases. Factors such as coolant temperature and engine speed also affect the duration of opening of the injectors. After the engine has made a certain number of revolutions, the amount of injected fuel gradually decreases to the nominal volume.
At the same time, even after starting the engine, the enriched air-fuel mixture is needed for some time, because some of the fuel still settles as condensate on the walls of the intake manifold. Therefore, the control unit performs the so-called post-start enrichment program of the air-fuel mixture, the duration of which depends on the temperature of the coolant.
In addition, when starting a cold engine, lubrication of its parts is difficult due to the increased viscosity of the oil. Therefore, the engine also requires more fuel to maintain the required idle speed.
When the accelerator pedal is released, the throttle valve closes and a small amount of air bypasses it. This air is measured by the air mass meter or, respectively, the pressure sensor in the intake manifold; this air is mixed with the required amount of fuel. The amount of air admitted is determined by the idle speed stabilization valve.
When the accelerator pedal is pressed hard, the air-fuel mixture is enriched during acceleration if the increase in the mass of the incoming air per second exceeds a certain value. The throttle angle potentiometer also sends a signal to the control unit about the need for acceleration.
At full engine load, the throttle angle potentiometer tells the control unit that the accelerator pedal is fully depressed. In response, the control unit performs actions to enrich the air-fuel mixture.
When driving downhill with the accelerator pedal released, the car moves by inertia under the action of its own weight and the engine does not need fuel supply. The control unit recognizes this situation by a high number of engine revolutions and a closed throttle valve (from the valve angular displacement potentiometer) and switches to fuel saving mode.
The electronic control unit constantly monitors the engine speed by comparing its current value with the set maximum permissible value of 6500 rpm. If this value is exceeded, the unit closes the fuel valve.
The regulated catalyst can only operate correctly when the value of ((lambda) is 1, which corresponds to a fuel-to-air ratio of 14.7:1.To ensure this, the MPI/MPFI system has a lambda value adjustment mechanism.
For this purpose, the lambda probe measures the oxygen content in the exhaust gases, thus generating an initial value taken into account when forming the air-fuel mixture. If the value obtained by the lambda probe does not correspond to the nominal value, the control unit requires the air-fuel mixture corrector to make the appropriate adjustments. Thus, the lambda regulation mechanism works flexibly, because only in the range of λ=0.8-1.2 is the catalyst able to ensure the combustion of harmful substances.
The lambda probe is able to send the corresponding signal to the electronic control unit only when heated to a temperature of 350°C. For this reason, it has electrical heating.
Until the lambda probe warms up, the operation of the catalyst is not regulated, and the electronic control unit uses the average lambda value in its calculations.
As already mentioned, the AUDI 80 is equipped with two catalytic converters.
(A link to the original source is available on the website: Audimanual.ru)