To keep the engine running «purely», the ratio of air and fuel in the mixture must be correct. The starting point for calculating the air/fuel ratio in the MPI injection system is the amount of air that has been introduced. This information is provided by the intake air mass flow meter. For the MPFI system, the source information is intake manifold pressure and intake air temperature.
To start the engine, a rich air-fuel mixture is needed, because a lot of fuel droplets on the way to the combustion chambers settle on the walls of the intake port and do not burn out. In this case, the electronic control unit uses the cold engine start program stored in its memory. At the time of the start-up, the duration of the opening of the nozzles increases. Factors such as coolant temperature and engine speed also affect injector opening times. After the engine has reached a certain number of revolutions, the amount of injected fuel is gradually reduced to the nominal volume.
At the same time, even after starting, the engine needs a rich air-fuel mixture for some time, because some of the fuel still settles in the form of condensate on the walls of the intake cable. Therefore, the control unit performs a so-called post-launch program for enriching 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 desired idle speed.
When the accelerator pedal is released, the throttle valve closes and a small amount of air bypasses it. This air is taken into account by the mass air mass meter or respectively by the intake manifold pressure sensor; this air is mixed with the required amount of fuel. The amount of intake air is determined by the idle speed stabilization valve.
When the accelerator pedal is depressed strongly, the acceleration air-fuel mixture is enriched if the increase in the mass of intake air per second exceeds a certain value. The throttle angle potentiometer also sends a signal to the control unit to accelerate.
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 influence of its own mass and the engine does not need fuel supply. The control unit recognizes this situation by a high engine speed and a closed throttle (from damper angle potentiometer) and switches to fuel saving mode.
The electronic control unit constantly monitors the number of engine revolutions by comparing their number at the current moment with the inherent maximum permissible value of 6500 rpm. If this value is exceeded, the unit closes the fuel cock.
The adjustable catalyst can only work correctly when the value ((lambda) is equal to 1, which corresponds to the ratio of fuel and air as 14.7:1. To ensure this, the MPI/MPFI system has a mechanism for adjusting the lambda value.
For this purpose, the lambda probe measures the oxygen content in the exhaust gases, thus generating an initial value that is taken into account when forming the air-fuel mixture. If the value obtained by the lambda probe does not correspond to the nominal value, then the control unit requires the air-fuel mixture corrector to make the appropriate corrections. Thus, the lambda regulation mechanism works flexibly, because only in the range of values (= 0.8-1.2 the catalyst is able to ensure the combustion of harmful substances.
The lambda probe is able to give the appropriate signal to the electronic control unit only when heated to a temperature of 350°C. For this reason, it has electric heating.
Until the moment 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, AUDI 80 is equipped with two catalysts.
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