Boost pressure control, function

Boost pressure control function requirements - general

 Circuit 87M ON (engine control ON)

 Engine running

Boost pressure control - general

Cylinder filling efficiency is improved by forced induction. This raises the engine torque and engine power.

One turbocharger is installed for each cylinder bank (Biturbo). An optimal response characteristic and a resulting high boost pressure are achieved even at low rpm speeds by installing the turbochargers immediately on the exhaust manifolds.

With forced induction, the flow energy of the exhaust gases is used to drive the turbocharger.

The turbochargers draw fresh air through the air filters into the compressor inlets, from where it passes through the compressor outlets to the charge air pipes upstream of the charge air coolers.

Due to the high rotational speed of the compressor impellers and the resulting high volume flow rate, the intake air becomes compressed in the charge air pipes.

As the charge air is compressed, it heats up, and flows through the charge air pipes to the charge air coolers. These cool the charge air down and guide it through the Y-connection pipe and the throttle valve actuator (M16/6) to the intake manifold.

The turbochargers start to produce boost pressure at approx. 1500 rpm. The maximum boost pressure is reached at approx. 2000 rpm.

The maximum boost pressure for combustion engine 275.954 is about 0.9 bar,

for combustion engine 275.981 (AMG) about 1.5 bar and for combustion engine 275.983 with code (P99) Special model "AMG Black Series" about 2.4 bar.

P09.40-2063-06

View of charge air path

5/1 Charge air cooler, LH cylinder bank

5/2 Charge air cooler for right cylinder bank

12 Intake manifold

110/1 Turbocharger intake line

110/6 Charge air pipe

110/7 Hose section with hose clamp

110/8 Y-connection pipe

110 A Left turbocharger

110b Right turbocharger

121/1 Air filter, LH cylinder bank

121/2 Air filter, RH cylinder bank

M16/6 Throttle valve actuator

B Exhaust

D Intake air (downstream of air filter)

E Charge air

For the boost pressure control, the ME-SFI [ME] control unit (N3/10) reads the following sensors and signals:

- Coolant temperature sensor (/)B11/4

- Charge air temperature sensor (B17/8)

- Pressure sensor downstream of air filter, LH and RH cylinder bank (B28/4, B28/5), intake air pressure

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

- Pressure sensor downstream of throttle valve actuator (B28/7), engine load

- Accelerator pedal sensor (B37), load request by driver

- Oil sensor (oil level, temperature and grade) (B40), engine oil temperature

- Crankshaft position sensor (L5), engine speed

- ETC control unit (N15/3), gear range over the engine compartment CAN (CAN C)

- Atmospheric air pressure sensor in the ME-SFI [ME] control unit, altitude adaptation (maximum boost pressure is limited at high altitudes)

- Exhaust gas temperature (from model calculation/performance map)

- Fuel grade (maximum boost pressure only for RON 98 (Research - OctaneNumber))

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The boost pressure control also depends on the safety functions of the engine control (for example, knock control and overheating/knock protection).

Boost pressure control function sequence

The boost pressure is controlled electropneumatically by the boost pressure control pressure transducer (Y31/5), which is actuated by the ME-SFI [ME] control unit.

Depending on the performance map and load, the ME-SFI [ME] control unit actuates the pressure transducer using a PWM signal with a duty cycle of 5 to 95 %.

Depending on the duty cycle, the boost pressure from the charge air cooler of the right cylinder bank acts totally or partially on the vacuum cells. Through a linkage, these open the boost pressure control flaps, which close off the bypasses .

The boost pressure control flaps therefore allow the exhaust flow to bypass the turbine wheels, thus controlling the boost pressure and limiting the turbine speed.

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To monitor the current boost pressure, the pressure sensor upstream of the throttle valve actuator sends the corresponding voltage signal to the ME-SFI [ME] control unit.

The pressure sensors downstream of the air filter of the LH and RH cylinder banks, which are located in the intake lines upstream of the turbocharger, are used by the ME-SFI [ME] control unit to monitor the charging process (pressure drop through the air filter).

The charge air temperature is detected in the intake manifold by the charge air temperature sensor and sent to the ME-SFI [ME] control unit in the form of a voltage signal. The maximum boost pressure is enabled only at charge air temperatures less than 70°C.

Engine load is detected (according to the throttle valve angle) using the voltage signal from the pressure sensor downstream of the throttle valve actuator.

Diagnosis

The boost pressure control function can only be assessed if the "boost pressure control adapted" message is displayed with the DAS. If the ME-SFI [ME] control unit or one of the turbochargers is replaced, a longer driving distance is required in certain operating conditions, in order to allow the ME-SFI [ME] control unit to perform the adaptation.

Diagnosis

If the hose lines are leaky between the vacuum cells, boost pressure control pressure transducer and charge air cooler of the RH cylinder bank, a "boost pressure too high" fault is stored in the ME-SFI [ME] control unit.



Fast load requests below the basic boost pressure are controlled by the throttle valve actuator.

P07.61-2741-11

Boost pressure control shown with a duty cycle of (ti) <5 %

5/2 Charge air cooler for right cylinder bank

110/3 Vacuum cell

110/3a Boost pressure control flap

110 C Turbine wheel

110 D Compressor impeller

Y31/5 Boost pressure control pressure transducer

A Atmospheric pressure (atmospheres)

B Exhaust

D Intake air (downstream of air filter)

E Charge air

t Time

ti Duty cycle

Without actuation, the entire boost pressure in the vacuum cell is used. The vacuum cell opens boost pressure control flap at about 300 mbar boost pressure (mechanical basic boost pressure). The boost pressure no longer rises, and a condition restriction results.

P07.61-2742-11

Boost pressure control shown with a duty cycle of (ti) >5 %

5/2 Charge air cooler for right cylinder bank

110/3 Vacuum cell

110/3a Boost pressure control flap

110 C Turbine wheel

110 D Compressor impeller

Y31/5 Boost pressure control pressure transducer

A Atmospheric pressure (atmospheres)

B Exhaust

D Intake air (downstream of air filter)

E Charge air

t Time

ti Duty cycle

The vacuum cell is ventilated to atmospheric pressure, so that no boost pressure is then acting on the vacuum cell. In the unpressurized state, the spring in the vacuum cell holds the boost pressure control flap closed. All of the exhaust gas drives the turbine wheel, thus generating the maximum boost pressure.