Forced induction, function

P09.00-2118-76

Design of charging system

12 Charge air distribution pipe

50 Turbocharger

50/1 Boost pressure control flap

50/2 Boost pressure control flap vacuum cell

50/3 Noise damper

110/1 Suction pipe

110/2 Charge air manifold to charge air cooler

110/3 Charge air cooler

110/4 Charge air manifold to throttle valve actuator

B17/7 Charge air temperature sensor upstream of throttle valve

B17/9 Charge air temperature sensor downstream of throttle valve

B28/5 Pressure sensor downstream of air filter

B28/6 Pressure sensor upstream of throttle valve

B28/7 Pressure sensor downstream of throttle valve

M16/6 Throttle valve actuator

Y77/1 Boost pressure control pressure transducer

Y101 Divert air switchover valve

A Exhaust

B Intake air

C Charge air (uncooled)

D Charge air (cooled)

Function requirements for charging - general

 Circuit 87M (engine control ON)

 Engine running

Forced induction, general

Cylinder filling efficiency is improved by 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 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 turbine wheel and the resultant high volume flow compresses the air in the charge air pipe.
The maximum boost pressure is 0.8 bar.

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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.

Function sequence for forced induction

The function sequence is divided into the following subfunctions:

 Function sequence for boost pressure control

 Function sequence for bypass air

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Function sequence for boost pressure control

The boost pressure is controlled electro-pneumatically 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 sensors and functions of the engine control system:

- Charge air temperature sensor upstream of throttle valve

- Charge air temperature sensor downstream of throttle valve

- Pressure sensor upstream of the throttle valve, boost pressure

- Pressure sensor downstream of the 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 direction of rotation recognition), motor rpm

- Knock control, transmission overload protection, overheating protection

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In full-load 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.

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 from a maximum of 0.8 bar to the current load demand on the engine.

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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 exhaust gas turbocharger, is used by the ME-SFI [ME] control unit to monitor the 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 in the form of 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 and compressor and turbine wheel.

Therefore a charge pressure wave runs back to the turbocharger for rapid closing of the throttle valve by the throttle valve actuator.

This charge pressure wave would create a condition with a low delivery volume and high pressure conditions at the compressor wheel, which causes charger pumping (brief howling and mechanical stress). Opening of the bypass air switchover valve prevents this through rapid depressurization over a bypass duct in the exhaust side of the turbocharger.

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Schematic diagram of turbocharger with bypass air switchover valve

1 Compressor

2 Turbine

3 To throttle valve

4 From exhaust manifold

Y101 Divert air switchover valve

A Fresh air

B Exhaust gases

When the engine is under load the bypass duct is kept closed by a membrane which has boost pressure applied to it. If the engine is stopped, the membrane is pressed over an integral spring in the bypass air switchover valve into the seat.

If the ME-SFI [ME] control unit recognizes closing of the throttle valve and therefore decel mode over the actual value potentiometers 1 and 2 (M16/6r1, M16/6r2), the bypass air switchover valve is actuated. The membrane is drawn against the spring force and boost pressure in the process and opens the bypass duct to the suction side. The excess boost pressure will be removed in this way.

If the engine then changes over from decel mode into operating under load, the bypass air switchover valve is no longer actuated. The spring presses the membrane in the direction of the seat. There the membrane is drawn into the seat by the boost pressure present there and thus closes the bypass duct again.

P09.40-2421-76

Sectional view of the bypass air switchover valve

50 Turbocharger

50/2 Boost pressure control flap vacuum unit

50/3 Noise damper

Y101 Divert air switchover valve

A Status: closed

B Status: opened