Charging, function

Function requirements, general

 Engine runs (circuit 87M On)

Forced induction, general

The cylinder charging efficiency is improved as a result of forced induction. As a result, the engine torque and power output are boosted. The fuel quantity corresponding to the increased air mass is metered by the ME-SFI control unit (N3/10).

With forced induction, the flow energy of the exhaust gases is used to drive the ATL. The ATLs draw in fresh air via the air filters at the compressor inlets and lead it via the compressor outlets in the charge air pipes upstream to the charge air coolers.

Due to the high rotational speed of the compressor impellers and the resulting high volumetric flow rates, the intake air becomes compressed in the charge air pipes. The compressed charge air flows via the charge air pipes to the charge air coolers. These subsequently cool the air which was heated by the compression and lead it via the charge air distributor to the cylinders.

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The charging is divided into the following subsections:

 Charge pressure control

 Overrun mode bypass air

 Charge air cooling

Charge pressure control

The boost pressure control occurs electropneumatically over the boost pressure control pressure transducer (Y77/1). The vacuum is generated by the mechanical vacuum pump attached to the engine. 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.

The ME-SFI [ME] control unit evaluates the signals from the following assembly parts for this:

 Left charge air temperature sensor

 Right charge air temperature sensor

 Left pressure sensor upstream of throttle valve

 Right pressure sensor upstream of throttle valve

 Left pressure sensor downstream of throttle valve

 Right pressure sensor downstream of throttle valve

 Pressure sensor downstream of left air filter

 Pressure sensor downstream of right air filter

 Accelerator pedal sensor (B37)

 Crankshaft Hall sensor (B70)

The ME-SFI [ME] control unit also evaluates the activities that take place within the following functions:

 Knock sensor system

 Transmission overload protection

 Overheating protection

In wide open throttle operation, maximum boost pressure builds up. To reduce the boost pressure, the exhaust flows that drive the ATLs are redirected through a bypass each by opening the boost pressure control flaps. In order to do this, the boost pressure control pressure transducer actuates the boost pressure control flap vacuum cells with vacuum. The boost pressure control flap vacuum cells react by closing the boost pressure control flaps via a linkage, and therefore the bypasses. If there is no vacuum at the boost pressure control flap vacuum cells, the boost pressure control flaps and therefore the bypasses are opened. The boost pressure control flaps lead the exhaust flow past the turbine wheels of the ATL. The boost pressure can be adapted to the respective load requirement in this way. If there is a leakage in the line between the vacuum pump and the vacuum cells, boost pressure build-up is not possible. The pressure sensors upstream of the throttle valve record the boost pressure. The pressure sensors downstream of the air filter record the charging. The charge air temperature is recorded in the charge air coolers by the charge air temperature sensors.

 The boost pressure control can only be evaluated with the diagnostic tester if the "Boost pressure control adapted" message is shown on the display. After replacement of ME-SFI [ME] control unit or an exhaust gas turbocharger, a long driving distance under specific operating conditions is required. Only then can this ME-SFI [ME] control unit adaptation be performed. If hose lines between pressure cells, boost pressure control pressure transducer, and charge air cooler are leaky, "Boost pressure too low" fault is stored in ME-SFI [ME] control unit. Quick load requirements below the basic charge pressure are controlled via the throttle valve actuator.

Overrun mode bypass air

The ATLs continue turning for a period of time after the start of deceleration mode due to the inertia of the shaft, compressor and turbine wheel. Through rapid closing of the throttle valve back pressures and unfavorable vibrations occur in the air column downstream of the ATLs. This vibrations would create a condition with a low delivery volume and high pressure conditions at the compressor impellers, which causes charger pumping (brief howling and mechanical stress).

The ME-SFI [ME] control unit detects the closing of the throttle valves and therefore the onset of deceleration mode by evaluating the signals of the following assembly parts:

 Actual value potentiometer 1, left (M16/60r1)

 Actual value potentiometer 2, left (M16/60r2)

 Actual value potentiometer 1, right (M16/61r1)

 Actual value potentiometer 2, right (M16/61r2)

The ME-SFI [ME] control unit then actuates the bypass air switchover valves. These are located on the charge air coolers and they each open a bypass from the boost pressure side downstream of the ATLs to the intake side downstream of the air filters. The excess boost pressure and the corresponding air volume are quickly reduced, thus preventing deceleration of the ATL compressor impellers. If the engine changes from deceleration mode to load operation, the bypass air switchover valves are no longer actuated. The bypasses are closed.

Charge air cooling

Charge air cooling keeps charge air temperature below 60 °C (at ambient temperature of 20 °C). The cooled air downstream of the charge air coolers has a higher density. This increases the cylinder charge, and therefore engine performance.

The tendency to knock and, due to the lower exhaust gas temperatures, the formation of nitrogen oxides (NOx) are also reduced. Both cylinder banks are fitted with a common coolant-cooled charge air cooler. The charge air coolers are connected to the low-temperature circuit.

The ME-SFI [ME] control unit records the current charge air temperature in the charge air coolers through the charge air temperature sensors.

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If the charge air temperature is over 35 °C, the ME-SFI [ME] control unit transmits the request to switch on the low-temperature circuit circulation pumps via the drive CAN (CAN C1) to the powertrain control unit (N127). The powertrain control unit actuates the low-temperature circuit circulation pumps via the drivetrain LIN (LIN C3). If the charge air temperature drops below 25 °C, the low-temperature circuit circulation pumps are switched off again.