Boost Pressure Control, Basic Function - GF09.40-P-1000A

Engine All (4xWD) 

Engine All (CAR) 

Overview 

This document contains information on:

• General 
• Function requirements 
• Function 
  • Exhaust gas turbocharger with bypass valve 
  • Exhaust gas turbocharger with variable turbine geometry 
  • Exhaust gas turbocharger with "overboost" function 

General 

The objective of the exhaust gas turbocharger is to achieve a charging effect in the medium rpm range, at low exhaust flows. The more the rpm range increases, the more exhaust flows are generated and the quicker these pass through the compressor wheel of the exhaust gas turbocharger.

To prevent a too high boost pressure in the upper rpm range and a possible Engine damage, the boost pressure must therefore be regulated. This boost pressure control can be realized in different ways.

Combinations of different boost pressure controls are also possible.

Function requirements 

• Engine management ON (circuit 87M)
• Engine in operation

Function 

Exhaust gas turbocharger with bypass valve 

The boost pressure regulator changes the position of the bypass valve (2) and thus the boost pressure. The boost pressure regulator is actuated via a vacuum cell (1).

Sample illustration of exhaust gas turbocharger with closed bypass valve 

Courtesy of MERCEDES-BENZ USA

In the low rpm range and thus at low exhaust flows, the bypass valve remains closed to increase the boost pressure (2).

Sample illustration of exhaust gas turbocharger with open bypass valve 

Courtesy of MERCEDES-BENZ USA

Once the boost pressure in the exhaust gas turbocharger has reached a set pressure, the bypass valve (2) is opened by a sensor on the compressor side.

Sample illustration of airflow and exhaust flow 

Courtesy of MERCEDES-BENZ USA

The bypass valve (2) opening results in the exhaust flow (5) that is routed to the turbine wheel (4) being redirected via a bypass (3) and the boost pressure being reduced.

Via the bypass (3), part of the exhaust flow (5) is routed past the turbine wheel (4) directly to the exhaust pipe, resulting in the boost pressure being regulated and the turbine speed being limited.

The boost pressure can be adapted to the current load demand on the Engine.

Exhaust gas turbocharger with variable turbine geometry 

The boost pressure regulator changes the position of the guide vanes (9) in the exhaust gas turbocharger with variable turbine geometry and thus the boost pressure. The boost pressure regulator is actuated via an electric actuator.

Sample illustration of exhaust gas turbocharger airflow and exhaust flow 

Courtesy of MERCEDES-BENZ USA

Sample illustration of exhaust gas turbocharger with closed guide vanes 

Courtesy of MERCEDES-BENZ USA

To increase the boost pressure before the guide vanes (9), at low Engine speeds, the vanes close to form small air gaps and thus reduce the flow cross-section.

The guide vanes (9) remain in this position until the desired boost pressure is achieved. The guide vanes (9) are mounted through the adjustment ring (12), which is actuated via an electric actuator.

Sample illustration of exhaust gas turbocharger with open guide vanes 

Courtesy of MERCEDES-BENZ USA

Once the Engine speed increases, the exhaust flow (5) also further increases. To regulate the boost pressure, the opening is enlarged means of the position of the guide vanes (9) being changed.

The specified value for the boost pressure depends on the following variables:

• Rpm
• Quantity injected
• Coolant temperature
• Intake air temperature
• Atmospheric pressure

As protection against an overloading of the exhaust gas turbocharger, the exhaust gas temperature is monitored by the temperature sensor upstream of exhaust gas turbocharger by the combustion Engine control unit. The exhaust gas pressure, however, is monitored by the exhaust pressure sensor. The boost pressure is reduced if there is any risk of thermal or mechanical overload.

If faults occur, e.g. a faulty pulse width modulated signal, the boost pressure regulator moves to the limp-home position.

In the limp-home position, the boost pressure regulator opens the largest inlet cross-section. Under these conditions, only a limited Engine output is available.

Exhaust gas turbocharger with "overboost" function 

Vehicles with a combustion Engine M254/M256 in combination with a 48 V on-then for a temperature-dependent period. By increasing the boost pressure and adjusting the injection of fuel, the Engine management control unit briefly increases the combustion Engine output.

As long as the exhaust gas temperature remains under a certain threshold value, the "overboost" function can be repeated as often as necessary.

A message on the driver display tells the driver that the function is available.

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