Turbocharging, function

General function requirements

 Circuit 61 ON

Charging, general

Charging serves to make an appropriate boost pressure available to the engine in every operating phase and thus to increase engine performance and engine torque. For exhaust-gas turbocharging the flow energy of the exhaust gases are used to drive the two turbochargers. The turbocharger draws in fresh air at the compressor intake, compresses it and directs it through the compressor outlet, the charge air pipe, the charge air cooler and the air intake manifold to the engine's individual cylinders.

Turbocharging is regulated by the boost pressure control.

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The boost pressure is regulated by the CDI control unit (N3/9) depending on the following values and associated components:

- Boost pressure, boost pressure sensor (B5/1)

- Coolant temperature, coolant temperature sensor (B11/4)

- Charge air temperature, charge air temperature sensor (B17/8)

- Atmospheric pressure, pressure sensor downstream of air filter (B28/5)

- Exhaust temperature, left catalytic converter exhaust gas temperature sensor (B16/4) and right catalytic converter exhaust gas temperature sensor (B16/5)

- Exhaust temperature, temperature sensor upstream of left turbocharger (B16/9) and temperature sensor upstream of right turbocharger (B16/10)

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- The injection quantity which is determined via the injection time of fuel injectors (Y76) and the fuel pressure in the rail. The fuel pressure in the rail is detected by the rail pressure sensor (B4/6).

- Exhaust back pressure in exhaust system, left exhaust back pressure sensor (B60/1) and right exhaust back pressure sensor (B60/2),

- Engine speed, crankshaft Hall sensor (B70)

Function sequence

After evaluating the input signals, the CDI actuates the left boost pressure positioner (Y100) and the right boost pressure positioner (Y100/1) with a pulse-width modulated (PWM) signal according to the boost pressure performance map. The position of the guide vanes in the turbocharger is altered via the control linkage on the boost pressure actuator. The adjustable guide vanes modify the flow cross-section through which the exhaust gas flows to the turbine. The guide vanes thus match the gas pressure present at the turbine to the required boost pressure. The turbocharger speed is used to determine the quantity of compress air and thus the boost pressure.

The boost pressure is constantly monitored by the charge pressure sensor and passed on to the CDI control unit.

As a protection against thermal and mechanical overloading of the turbocharger, the exhaust temperature is monitored by the CDI control unit through the left and right catalytic converter exhaust temperature sensors, left and right temperature sensors upstream of the turbochargers, as well as the left and right exhaust back pressure sensors.

The boost pressure is reduced if there is a risk of thermal or the mechanical overload.

 The charge air heated by the charging is cooled in the charge air cooler.