Charging, Function - GF09.00-D-2000TSB

Engine 274 in model 907 

Block diagram 

Function requirements for charging - general 

• Circuit 87M ON
• Engine running.

Forced induction, general 

The cylinder charging efficiency is improved as a result of forced induction. This raises the engine torque and engine power output.

The fuel quantity corresponding to the increased air mass is metered by the ME-SFI [ME] control unit (N3/36).

During forced induction, the flow energy of the exhaust gases (B) is used to drive the ATL (50).

The ATL (50) draws in intake air (A) via the air filter at the compressor inlet and leads this via the charge air cooler into the compressor outlet to the charge air pipe (110/2).

The high rotational speed of the compressor impeller and the resultant high volumetric flow rate compacts the air in the charge air pipe to the charge air cooler (110/2). The max. boost pressure in this connection is approx. 0.8 to 1.2 bar depending on the engine variant. The noise damper (50/3) on the compressor outlet dampens the charge pressure fluctuations and thus the associated flow noises which occur for rapid changes in engine speed.

The compacting charge air (C) flows over the charge air pipe to charge air cooler (110/2) into the charge air cooler (110/3). This cools off the charge air (C) heated up by the compression and leads it over the charge air pipe downstream of the charge air cooler (110/4) to throttle valve and subsequently further in the charge air distributor (12).

Design of charging system 

Forced induction function sequence 

The function sequence is divided into the following subfunctions:

• Function sequence for boost pressure control
• Function sequence for bypass air

Function sequence for boost pressure control 

The boost pressure control takes place electropneumatically via the boost pressure control pressure transducer (Y77/10).

The pressure transducer boost pressure control is actuated on a characteristics map and load-dependent basis by the ME-SFI control unit for boost pressure control.

To do this the ME-SFI [ME] control unit evaluates the following sensors and functions of the engine management:

• Pressure sensor downstream of air filter (B18/8)
• Charge air temperature sensor upstream of throttle valve (B17)
• Charge air temperature sensor downstream of throttle valve (B17/16)
• Pressure sensor upstream of throttle valve (B5/13)
• Intake manifold pressure sensor (B28/24)
• Accelerator pedal module (A68), via powertrain control unit (N127) and drive CAN (CAN C)
• Crankshaft Hall sensor (B70/2)
• Knock sensor system
• Transmission overload protection
• Overheating protection

In wide open throttle operation, maximum boost pressure builds up.

In order to reduce the boost pressure, the exhaust flow (B) for driving the charger turbine is diverted via a bypass by opening the boost pressure control flap (50/1).

To do this the boost pressure control pressure transducer (Y77/10) actuates the boost pressure control flap vacuum unit (50/2) using boost pressure from the vacuum reservoir in the charge air distributor (12). The vacuum unit then opens the boost pressure control flap (50/2) over a linkage the boost pressure control flap (50/1), which closes the bypass.

The exhausts (B) pas round the turbine wheel through the boost pressure control flap (50/1), whereby the boost pressure regulates and the turbine speed is finite.

The boost pressure of max. 0.8 to 1.2 bar can thus be adapted, depending on the engine variant, to the current engine load demand. To monitor the current boost pressure, the pressure sensor upstream of the throttle valve (B5/13) transmits a corresponding voltage signal to the ME-SFI [ME] control unit.

The pressure sensor upstream of the throttle valve (B5/13) that is located in the suction line (110/1) upstream of the exhaust gas turbocharger (50) serves the ME-SFI [ME] control unit for monitoring the charging.

The charge air temperature is detected in the charge-air distributor (12) by the charge air temperature sensor downstream of throttle valve (B17/16) and communicated to the ME-SFI [ME] control unit in the form of a voltage signal.

Function sequence for bypass air 

The ATL (50) continues turning for a period of time after the start of deceleration mode due to the inertia of the shaft, compressor and turbine wheel.

In the case of rapid closing of the throttle valve, a charge pressure wave therefore runs back to the ATL (50). This charge pressure wave would create a condition with a low delivery volume and high pressure conditions at the compressor impeller, which causes charger pumping (brief howling and mechanical stress).

The opening of the overrun mode bypass air switchover valve (Y101) prevents this through rapid pressure reduction via a bypass in the intake section of the exhaust gas turbocharger (50).

Schematic display of ATL with divert air switchover valve 

In load operation of the engine, the bypass is kept closed by means of a diaphragm under boost pressure.

If the engine is switched off, the diaphragm is pressed into the seat via a spring integrated in the overrun mode bypass air switchover valve (Y101).

If the ME-SFI [ME] control unit detects the closing of the throttle valve via the actual value potentiometer 1 (M16/50r1) and actual value potentiometer 2 (M16/50r2) and thus overrun mode, the overrun mode bypass air switchover valve (Y101) is actuated.

The diaphragm is pulled open against the spring force and boost pressure and opens the bypass duct to the intake side. The excess boost pressure is thereby relieved.

If the engine changes from overrun mode to load mode, the overrun mode bypass air switchover valve (Y101) is no longer actuated.

The spring presses the diaphragm in the direction of the seat. The diaphragm is then pulled into the seat by the prevailing boost pressure thereby closing the bypass again.

Sectional view of deceleration air switchover valve 

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