Charging, function

Function requirements for charging - general

 Circuit 87M (engine management ON)

 Engine runs

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 (N3/10).

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

The ATL 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 rotational 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. 0.6 to 1.5 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 distributor.

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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 is controlled electropneumatically via the boost pressure control pressure transducer.

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 sensor signals and functions of the engine management:

 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) (with rotational recognition), engine rpm

 Knock control

In wide open throttle operation, maximum boost pressure builds 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.

The boost pressure control pressure transducer actuates the boost pressure control flap vacuum cell using vacuum from the vacuum reservoir to do this. The vacuum cell then opens the boost pressure control flap over a linkage which closes the bypass.

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

In this way the boost pressure can be adapted from a maximum of 0.6 to 1.5 bar depending on the engine variant to the engine's current load requirement.

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 air intake duct upstream of the ATL, is used by the ME-SFI [ME] control unit to monitor charging.

The charge air temperature is detected in the charge air distributor 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 ATL continues turning for a period of time after the start of deceleration mode due to the inertia of the shaft, compressor and turbine wheel.

Upon rapid closing of the throttle valve through the throttle valve actuator a boost pressure shaft therefore runs back to the ATL.

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 through a bypass in the intake side of the ATL.

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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 by a spring integrated into the deceleration air switchover 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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