Charging, function

Function requirements for charging - general

 Circuit 87M (engine timing 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.

The fuel quantity corresponding to the increased air mass is metered by the ME-SFI [ME] control unit (N3/10).

During forced induction, the flow energy of the exhaust gases is used to drive the turbocharger.

The turbocharger suctions in fresh air through the air filter on the compactor inlet and feeds it through the compressor outlet into the charge air pipe upstream of the charge air cooler.

The high speed of the compressor impeller and the resultant high volumetric flow rate compacts the air in the charge air pipe. The maximum boost pressure here is approx. 0.8 to 1.2 bar depending on the engine variant.

The noise damper on the compressor outlet dampens the charge pressure fluctuations and thus the associated flow noises which occur for rapid changes in engine speed .

The compressed charge air flows via the charge air pipe to the charge air cooler. This finally cools the air which was heated by the compression and leads it over the charge air pipe to the charge air manifold.

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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 occurs electropneumatically via the boost pressure control pressure transducer (Y31/5).

The pressure transducer is actuated dependent on the characteristics map and the load by the ME-SFI [ME] control unit for the purposes of boost pressure control.

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

 Charge air temperature sensor upstream of throttle valve

 Charge air temperature sensor downstream of throttle valve

 Pressure sensor upstream of throttle valve, boost pressure

 Pressure sensor downstream of throttle valve, boost pressure

 Pressure sensor downstream of air filter, intake pressure

 Accelerator pedal sensor (B37), load request made by driver

 Crankshaft Hall sensor (B70), engine speed

 Knock control, transmission overload protection, overheating protection

In the wide open throttle range, the maximum boost pressure is built up.

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

To do this, the boost pressure control pressure transducer actuates the boost pressure control flap vacuum unit using boost pressure from the vacuum reservoir in the charge air manifold. The vacuum unit then opens the boost pressure control flap over a linkage which closes the bypass.

The boost pressure control flap allows the exhaust flow to bypass the turbine wheel (bypass), whereby the boost pressure is controlled and the speed of the turbine wheel is limited.

This means that the boost pressure of a maximum of 0.8 to 1.2 bar dependent on the engine variant can be adapted to the current load requirement of the engine.

To monitor the current boost pressure, the pressure sensor upstream of the throttle valve sends the corresponding voltage signal to the ME-SFI [ME] control unit.

The pressure sensor downstream of the air filter, which is located in the suction line upstream of the turbocharger, is used by the ME-SFI [ME] control unit to monitor charging.

The charge air temperature is detected in the charge air manifold by the charge air temperature sensor downstream of the throttle valve and sent to the ME-SFI [ME] control unit as a voltage signal.

Function sequence for bypass air

The exhaust gas turbocharger 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 turbocharger. 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).

Opening the bypass air switchover valve prevents this through rapid depressurization via a bypass duct in the intake side of the turbocharger.

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

If the engine is shut off, the diaphragm is pressed into the seat by an integrated spring in the blow-off valve.

If the ME-SFI [ME] control unit detects via actual value potentiometers 1 and 2 (M16/6r1, M16/6r2) that the throttle valve has closed and therefore deceleration mode is active, the bypass air switchover valve 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 deceleration mode to load operation, the bypass air switchover valve is no longer actuated.

The spring presses the diaphragm in the direction of the seat. There the diaphragm is pulled into the seat by the prevailing boost pressure and thus closes the bypass duct again.

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