Boost pressure control, function

P07.61-2726-09

5/2 Charge air cooler, right cylinder bank

12 Intake manifold

110/3 Vacuum cell

110/3a Charge pressure control valve

110/3b Control rod

110a Left turbocharger

110b Right turbocharger

M16/6 Throttle valve actuator

N3/10 ME-SFI [ME] control unit (pressure sensor for ambient pressure integral)

Y31/5 Boost pressure control pressure transducer

CAN Data bus

a Ambient pressure

E Charge air

M Modulated boost pressure

B17/8 Charge air temperature sensor

B28/4 Pressure sensor downstream of air filter for left cylinder bank

B28/5 Pressure sensor downstream of air filter for right cylinder bank

B28/6 Pressure sensor upstream of throttle valve actuator

B28/7 Pressure sensor downstream of throttle valve actuator

B37 Accelerator pedal sensor

L5 Crankshaft position sensor

The ME-SFI [ME] control unit actuates the boost pressure control pressure transducer for the boost pressure control by means of pulse width modulation with a duty cycle of 5 to 95% and with a frequency of 30 Hz. According to the duty cycle, the boost pressure operates fully or partially in the vacuum cells of the turbocharger.

The boost pressure control valves (wastegate valves) are actuated via vacuum cells and control rods. For limitation of the boost pressure, the boost pressure control valves open and part of the exhaust is guided past the turbine wheel.

The boost pressure is calculated based on the pressure conditions downstream and upstream of the turbochargers (based on the pressure values of the pressure sensor downstream of air filter and pressure sensor upstream of throttle valve actuator).

The boost pressure control is performed in line with the performance map.

Identification of the load (dependent on throttle valve angle) is performed through the signal of the pressure sensor downstream of throttle valve actuator .

P07.61-2741-11

Boost pressure control shown on a turbocharger

Duty cycle (ti) on boost pressure control pressure
transducer <5 %

5/2 Charge air cooler, right cylinder bank

110/3 Vacuum cell

110/3a Charge pressure control valve

110 C Turbine wheel

110 D Compressor impeller

Y31/5 Boost pressure control pressure transducer

a Ambient pressure

B Exhaust

D Clean air (downstream of air filter)

E Charge air

Without actuation, the entire boost pressure in the vacuum cell is used. The boost pressure control valve opens at approx.
300 mbar boost pressure (mechanical basic boost pressure). The boost pressure does not increase further and power shortage is the consequence.

P07.61-2742-11

Boost pressure control shown on a turbocharger

Duty cycle (ti) on boost pressure control pressure transducer >5%

5/2 Charge air cooler, right cylinder bank

110/3 Vacuum cell

110/3a Charge pressure control valve

110 C Turbine wheel

110 D Compressor impeller

Y31/5 Boost pressure control pressure transducer

a Ambient pressure

B Exhaust

D Clean air (downstream of air filter)

E Charge air

The vacuum cell is vented to the atmosphere so that no boost pressure can act anymore in the vacuum cell. In unpressurized condition, a spring holds the boost pressure control valve closed in the vacuum cell. The whole exhaust gas drives the turbine wheel and the maximum boost pressure is generated.

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If the hose lines between the vacuum cells, pressure transducer and charge air cooler on the right cylinder bank are leaky the fault "Boost pressure too high" is stored in the ME-SFI [ME] control unit.

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The input parameters for the boost pressure control are:

- Pressure values of pressure sensor downstream of air filter

- Pressure values from pressure sensor upstream of throttle valve actuator

- Charge air temperature, signal from charge air temperature sensor

- Load recognition from pressure sensor downstream of throttle valve actuator

- Ambient pressure (altitude correction), signal from pressure sensor in ME-SFI [ME] control unit

- Engine speed

- Coolant temperature

- Engaged drive mode

- Engine oil temperature

- Pressure drop in air filter, detection by pressure sensor downstream of air filter

- Exhaust gas temperature (from model calculation/performance map)

- Knock control active (maximum boost pressure only possible with RON 98)

- Catalytic converter damaging misfire recognized/not recognized

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The maximum boost pressure is only released via the signal from the charge air temperature sensor, if the charge air temperature is less than < 70°C.

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If the charge air temperature becomes too high, the reason for this could be air bubbles in the low temperature cooling circuit.

In order to protect the exhaust gas turbochargers from overspeed when in altitude mode and in case of soiled air filters, the maximum boost pressure is limited pressure conditions upstream/downstream of the turbochargers.

Further to the boost pressure control it is possible under certain conditions (e.g. for an excessively high boost pressure due to leaky lines to the vacuum cell) for load limiting to take place through closing of the throttle valve.

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The function of the boost pressure control can only be evaluated if the message "Boost pressure control adapted" is read out using the DAS. After replacing the ME-SFI [ME] control unit or a turbocharger is a long drive under certain operating conditions is required to perform the adaptation.