Turbocharger: Description and Operation
GF09.40-P-3000MOS Boost Pressure Control, Function
- ENGINE 275.953 in MODEL 216.376, 221.176 as of Model Year 09 /AEJ 08
- ENGINE 275.982 in MODEL 216.379, 221.179 as of Model Year 09 /AEJ 08
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 is about 0.9 on engine 275.953 and about 1.5 bar on engine 275.982 (AMG).
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 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), motor speed
- ETC control unit (N15/3), gear range over the drive train CAN (CAN C)
- Atmospheric air pressure sensor in the ME-SFI 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))
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 control unit.
Depending on the performance map and load, the ME-SFI control unit actuates the pressure transducer with a PWM (pulse width modulated) signal with a duty cycle of 5 to 95 %. Depending on the duty cycle, the boost pressure from the charge air cooler of the RH cylinder bank acts totally or partially on the vacuum cells (boost pressure control flaps). Through a linkage, these open the boost pressure control flaps, which close off the bypasses . The boost pressure control flaps thus allow the exhaust flow to bypass the turbine wheels, and in this way control the boost pressure and limit the turbine speed.
To monitor the current boost pressure, the pressure sensor upstream of the throttle valve actuator sends the corresponding voltage signal to the ME-SFI control unit.
Die 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 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 control unit as 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 when the "boost pressure control adapted" message is displayed with the Diagnosis Assistance System (DAS). If the ME-SFI 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 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 control unit.
Fast load requests below the basic boost pressure are controlled by the throttle valve actuator.
Boost pressure control shown with a duty cycle of (ti) less than 5%
5/2 Charge air cooler for right cylinder bank
110/3 Vacuum cell (boost pressure control flap)
110/3a Boost pressure control flap
110c Turbine wheel
110d 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.
Boost pressure control shown with a duty cycle of (ti) greater than 5%
5/2 Charge air cooler for right cylinder bank
110/3 Vacuum cell
110/3a Boost pressure control flap
110c Turbine wheel
110d 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.
