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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 to air mass/combustion (mg/c). The value is sent to box 3.

2. Cruise control request

When 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 of the two values. The value is sent to box 5.

4. Engine torque limitation

Maximum permissible air mass/combustion varies depending on the engine variant. During operation, the maximum permissible air mass/combustion must also be limited to protect the engine, gearbox, brakes or turbo.

Even external units such as the automatic transmission or ESP can request maximum permissible air mass/combustion. Limitations can also be made in case of a system fault. The control module selects the lowest value, which is sent to box 5.

5. Select lowest value

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

6. Compensation request

When the A/C compressor is on, and when the electrically heated rear window or radiator fan is on, the air mass/combustion required to compensate for the increased 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 values

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

9.
10. Total system air request

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

11. Throttle control

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

Note

The system regulates the current air mass/combustion so that it corresponds with the requested value. It is completely normal for the throttle not to be fully open despite the accelerator pedal being fully depressed.


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 for throttle sensor 1 is finely adjusted.

13. Turbo control

If the requested air mass/combustion is too high to be handled by the throttle control alone, the turbo control will take over. The excess is converted to a PWM, which controls the charge air control valve. The absolute atmospheric pressure is used to correct this conversion.

14. Current air mass/combustion

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


Diagnosis

In principle, the air mass/combustion is the same as engine torque. Air mass control is therefore of great interest when diagnosing bad performance.

There is a special menu in the diagnostic tool from which values of interest can be read. The most important is the deviation between requested and current air mass/combustion.

The current value must always follow the requested value, occasionally after a short delay. If this does not take place, there may be a fault in the turbocharger.

Note

A fault in the throttle control can be eliminated because deviations between the present throttle position value and its desired value place the throttle control in limp-home.