Charging, function

Function requirements for charging - general

 Circuit 87M (engine control ON)

 Engine running

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. For forced induction, the flow energy of the exhaust gases is used to drive both turbochargers. Forced induction is performed by 2 inline exhaust gas turbochargers of different size which function together in different ways depending on the operating range.

The turbocharger suctions in fresh air through the air filter on the compressor 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 volume flow compresses the air in the charge air pipe. Compression heats the charge air, which now flows through the charge air pipe to the charge air cooler. This cools off the charge air and feeds it through the charge air pipe to the charge air manifold.

Turbocharging is regulated by the boost pressure control. The boost pressure is regulated by the CDI control unit (N3/9) depending on the following values and associated components:

- Atmospheric pressure sensor, for measuring the atmospheric pressure

– charge air temperature sensor (B17/8)

– temperature sensor upstream of turbocharger (B19/11)

- the injection quantity which is determined via the injection time of fuel injectors (Y76) and the fuel pressure in the rail. The fuel pressure in the rail is detected by the rail pressure sensor (B4/6).

– boost pressure control flap pressure transducer (Y93)

- boost pressure control pressure transducer (Y31/5)

– coolant temperature, over the coolant temperature sensor (B11/4)

- atmospheric pressure, via the atmospheric pressure sensor in the CDI control unit

– intake air pressure, via the pressure sensor downstream of the air filter (B28/5)

– engine speed, via the crankshaft Hall sensor (B70)

– boost pressure, over the boost pressure sensor (B5/1)

– boost pressure, over the low-pressure turbocharger boost pressure sensor (B5/4)

– exhaust pressure, via the exhaust pressure sensor (B60)

– intake air mass, via the hot film mass air flow sensor (B2/5)

– intake air temperature, over the intake air temperature sensor (B2/5b1)

– DPF differential pressure sensor (B28/8), soot content of the diesel particulate filter (DPF)

Function sequence for charging

The function sequence is described in the following steps:

 Function sequence for boost pressure with wide open throttle operation up to about 1200 rpm

 Function sequence for boost pressure with wide open throttle operation as of about 1200 rpm

 Function sequence for boost pressure with wide open throttle operation as of about 2800 rpm

Function sequence for boost pressure control with wide open throttle operation up to 1200 rpm

Up to an engine speed of 1200 rpm in wide open throttle operation, the exhaust flap is closed and the entire flow of exhaust gas flows via the turbine wheel of the high-pressure turbocharger to the turbine wheel of the low-pressure turbocharger and then to the exhaust system.

The largest part of the exhaust gas energy acts on the turbine wheel of the high-pressure turbocharger which thereby generates the main part of the necessary boost pressure.

The remaining exhaust gas energy acts on the turbine wheel of the low-pressure turbocharger which drives the compressor wheel via the supercharger shaft. The low-pressure turbocharger does not therefore act as a hydrodynamic retarder. The wastegate actuated by the boost pressure control pressure transducer and the compressor bypass are closed.

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Function sequence for boost pressure control with wide open throttle operation as of about 1200 rpm

As of an engine speed of 1200 rpm in wide open throttle operation, the exhaust flap is opened depending on the required boost pressure.

The larger part of the exhaust flow continues to flow via the turbine wheel of the high-pressure turbocharger to the turbine wheel of the low-pressure turbocharger and then to the exhaust system.

The largest part of the generated boost pressure is provided by the high-pressure turbocharger.

The low-pressure turbocharger is continuously activated and precompresses the drawn-in clean air.

The wastegate actuated by the boost pressure control pressure transducer and the compressor bypass continue to be closed.

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Function sequence for boost pressure control with wide open throttle operation as of about 2800 rpm

As of an engine speed of 2800 rpm, the compressor bypass and the exhaust flap are opened. The largest part of the charge air generated by the low-pressure turbocharger flows in front of the compressor housing of the high-pressure turbocharger via the charge air pipes to the charge air cooler and charge air manifold. The low-pressure turbocharger thus generates the necessary boost pressure. Part of the exhaust flow drives the turbine wheel of the high-pressure turbocharger. The boost pressure generated is regulated by the wastegate actuated by the boost pressure control pressure transducer. The CDI control unit uses the boost pressure sensor to monitor the boost pressure actually generated.

The low-pressure turbocharger boost pressure sensor detects the boost pressure downstream of the low-pressure turbocharger. By comparing the boost pressure actually generated with the boost pressure stored in the performance map and the boost pressure downstream of the low-pressure turbocharger, it is possible to detect faults at the high-pressure and low-pressure turbocharger.

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The compressor bypass is self regulating dependent on the pressure, whereby the spring-loaded check valve takes the high-pressure turbocharger out of the delivery stream at high loads.

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