Determination of air mass, function

W07.15-1259-81

Shown on engine 541.9

A6 MR/PLD control unit

B111 Charge air combination sensor (temperature, pressure)

General information

The air mass, along with the engine temperature or fuel temperature, is one of the most important factors which the engine control (MR) needs to control the fuel quantity. It is only when this is known that is then possible to precisely calculate the delivery angle over which the fuel quantity or injected quantity will be determined.

Function

Determination of the air mass occurs over the MR/PLD control unit (A6). It detects the air temperature and the air pressure in the charge air housing over the charge air combination sensor (temperature, pressure) (B111) and uses these to calculate the air mass.

 Detection of the air temperature in the charge air housing
Detection of the air temperature in the charge air housing takes place over a temperature sensor which is located at the tip of the charge air combination sensor (temperature, pressure) (B111).

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The temperature sensor, a temperature-dependent resistance with a negative temperature coefficient (NTC) - that is an electrical resistance which reduces as the temperature increases, is located directly in the air stream of the sucked in or charged combustion air.
The resistance of the temperature sensor decreases or increases according to the air temperature in the charge air housing and the voltage being applied to the MR/PLD control unit (A6) decreases or increases as a result.
As long as the values remain within a permissible range, a certain temperature will be allocated to the combustion air based on the respective voltage level.

 Detection of the air pressure in the charge air housing
Detection of the air pressure in the charge air housing takes place over a pressure sensor which is located inside the housing of the charge air combination sensor (temperature, pressure) (B111).

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This is a so-called semiconductor pressure sensor which detects the air pressure in the intake manifold based on a piezoelectric principle. In the inside of the housing of the charge air combination sensor (temperature, pressure) (B111) there is a silicon membrane with 4 pressure-dependent resistors (stretch measuring resistor) as well as an electronic analysis system, which supplies the bridge circuit supplies these resistors with voltage and additionally for signal amplification or signal correction.
The electronics analysis system itself is provided with a 5 V DC voltage by the MR/PLD control unit (A6).
The suctioned in or supercharged combustion air in the charge air housing passes over a bore in the silicon membrane with the 4 pressure-dependent resistors and distorts it.

The resistors in the silicon membrane are arranged in such a way that on deforming the membrane, 2 resistors are contracted and 2 resistors elongated.
Contraction and elongation changes the electrical resistance of the resistors which in turn has effects upon the measurement voltage, which is being applied to the electronic analysis system.
The electronic analysis system amplifies the measurement voltage, compensating for possible temperature fluctuations or balancing out possible manufacturing tolerances on the resistors and passes this on to the MR/PLD control unit (A6) which uses it to derive the air pressure of the combustion air in the charge air housing.