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Air mass control, basic function

Air mass control, basic function




1. Driver request

The control module reads pedal potentiometer 1 and converts the voltage into air mass/combustion (mg/c). The value is sent to box 3.

2. Cruise control, request

When the cruise control is active, the air mass/combustion required to maintain the set speed is calculated. The value is sent to box 3.

3. Select highest value

The control module selects the highest value. The value is sent to box 5.

4. Torque limitation

The maximum permitted air mass/combustion varies according to the engine variant. When the car is being driven, the maximum permitted air mass/combustion must also be limited to protect the engine, gearbox, brakes and turbo.

External units, such as the automatic transmission or TC/ABS, or a system fault can also limit the maximum permitted air mass/combustion. The control module selects the lowest value. The value is sent to box 5.

5. Select lowest value

The control module selects the lowest value and moves it to box 8.

6. Compensation request

When the A/C compressor is turned on, and when the electrically heated rear window or radiator fan is on, the air mass/combustion required to compensate for the increased engine load is calculated. The value is sent to box 8.

7. Other air request

The control module calculates the air mass/combustion required for idle speed control. The value is sent to box 8.

8. Totalling of values

The control module totals all the values. The total is sent to box 9.

9.
10. Total system air request

This value is the system's total air request. The value is sent to box 10.

11. Throttle control

The requested air mass/combustion is converted to the requested voltage for throttle position sensor 1. The charge air pressure and intake air temperature are used to correct this conversion. The throttle motor turns the throttle valve until the current voltage for throttle position sensor 1 corresponds to the voltage requested.

Note

The system controls the current air mass/combustion so that it corresponds to the requested value. It is perfectly normal for the throttle valve not to be fully open despite the accelerator being pressed fully down.


12. Current air mass/combustion

The requested air mass/combustion is also compared with the current air mass/combustion (current MAF value/combustion). If necessary, the requested voltage is finely adjusted for throttle position sensor 1.

13. Turbo control

If the requested air mass/combustion is too high to be handled by the throttle control only, the turbo control will take over. The exceeding portion is converted to PWM, which governs the charge air control valve. Absolute atmospheric pressure is used to correct the conversion.

14. Current air mass/combustion

The requested air mass/combustion is compared with the current air mass/combustion and the PWM to the charge air control valve is finely adjusted if necessary.


Diagnostics

Air mass/combustion is basically the same as engine torque. When carrying out fault diagnosis to determine the cause of poor performance, air mass control is therefore extremely important.

The diagnostic tool contains a special menu where relevant values can be displayed. The most important of these is the difference between requested and current air mass/combustion.

The current value must always follow the requested value, sometimes after a certain delay. If this does not take place, a fault in the turbo function can be suspected. A fault in the throttle control can be ruled out as deviations between the set value and the desired value of the throttle position would put the throttle control into limp-home mode.