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 2.8 bar (absolute).

The compressed charge air flows via the charge air pipe to the charge air cooler. This then cools the charge air which was heated by the compression and routes it via the charge air pipe to the charge air distributor.

Forced induction function sequence

The function sequence is divided into the following subfunctions:

 Function sequence for boost pressure control

 Function sequence for twin-scroll system

 Function sequence for charge air cooling

Function sequence for boost pressure control

The boost pressure control occurs electropneumatically over the boost pressure control pressure transducer (Y77/1).

The pressure transducer boost pressure control is actuated on a characteristics map and load-dependent basis by the ME-SFI [ME] control unit for 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 downstream of throttle valve (B17/9)

 Pressure sensor downstream of air filter (B28/5), intake pressure

 Pressure sensor upstream of throttle valve (B28/6), boost pressure

 Pressure sensor downstream of throttle valve (B28/7), boost 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 gas stream 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 to the current load demand on 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 air intake pipe 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.

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Function sequence for twin-scroll system

A so-called twin-scroll ATL is used for engine 133.

Separation of the exhaust flows downstream of the cylinder outlet allows more energy (pulsed energy and kinetic energy) to be transported to the turbine wheel. In order to maximize utilization of this energy, the manifold runner diameters and the manifold runner lengths are adapted to the twin-scroll ATL.

For this reason, a fan-type exhaust pipe is used for engine 133 and manifold runners 1 + 4 and 2 + 3 are combined (flow separation). As a result, it was possible to achieve a good response time and the desired maximum performance.

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Function sequence for charge air cooling

The charge air cooling maintains a charge air temperature of < 60 °C at 20 °C ambient temperature. The cooled air downstream of the charge air cooler has higher density. This increases the cylinder charge, and therefore engine performance. The tendency to knock is reduced due to lower exhaust temperatures and there is reduced output of nitrogen oxides (NOx). The coolant cooled charge air cooler is connected to the low-temperature circuit.

If the charge air temperature is > 35°C, the ME-SFI control unit actuates the charge air cooler circulation pump via the circuit M4487 relay (F58kS).

If the charge air temperature falls below 25°C, the charge air cooler circulation pump is switched off again.

The charge air temperature is detected in the charge air distributor by the charge air temperature sensor and sent to the ME-SFI [ME] control unit in the form of a voltage signal.

 Only open the cap in the low-temperature circuit when the charge air temperature is increased (lack of power) and the engine is cold. The coolant must reach up to the cap.

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