Charging, Function - GF09.00-P-2000MRH
Engine 274.9 in model 205.047/053/054/147/247/253
Engine 274.9 in model 253.354/954
Function requirements for charging - general
- Circuit 87 M (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 draws in fresh air via the air filter at the compressor inlet and leads this via the charge air cooler into the compressor outlet to the charge air pipe.
The high rpm of the compressor impeller and the resulting high volumetric flow rate compresses the air in the charge air pipe.
The maximum boost pressure here is approx. 0.7 to 1.5 bar depending on the engine variant involved. 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 distributor.
Design of charging system
Forced induction function sequence
The function sequence is divided into the following subfunctions:
- Function sequence for boost pressure control
- Function sequence for bypass air
- 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 vacuum is generated by the mechanical vacuum pump attached to the engine. 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 sensor signals and functions of the engine management:
- Charge air temperature sensor upstream of throttle valve, charge air temperature
- Charge air temperature sensor downstream of throttle valve, charge air temperature
- Pressure sensor downstream of air filter, intake air pressure
- Pressure sensor upstream of throttle valve, boost pressure
- Pressure sensor downstream of throttle valve, boost pressure
- Accelerator pedal sensor (B37), load request made by driver
- Crankshaft Hall sensor (B70), engine rpm
- Knock control, transmission overload protection, overheating protection
In wide open throttle operation, maximum boost pressure builds up. In order to reduce the boost pressure, the exhaust flow for driving the turbine wheel is diverted via a bypass by opening the boost pressure control flap.
To do this the boost pressure control pressure transducer actuates the boost pressure control flap vacuum cell using boost pressure from the vacuum reservoir in the charge air distributor. The vacuum cell then opens the boost pressure control flap over a linkage which closes the bypass. The boost pressure control flap allows the exhaust flow to bypass the turbine wheel (bypass), whereby the boost pressure is controlled and the speed of the turbine speed is limited. This means that the boost pressure of a maximum of 0.7 to 1.5 bar, depending on the engine variant involved, can be adapted to the engine's current load requirement.
Boost pressure buildup is not possible if there is a leakage in the line between the vacuum pump and the vacuum cell.
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 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.
Function sequence for bypass air
The ATL continues turning for a period of time after the start of deceleration mode due to the inertia of the shaft, compressor and turbine wheel. In the case of rapid closing of the throttle valve, a charge pressure wave therefore runs back to the ATL. This charge pressure wave would create a condition with a low delivery volume and high pressure conditions at the compressor impeller, which causes charger pumping (brief howling and mechanical stress). Opening of the bypass air switchover valve (Y101) prevents this by rapidly reducing the pressure via a bypass duct in the intake side of the ATL.
Schematic display of ATL with divert air switchover valve (Y101)
In load operation of the engine, the bypass is kept closed by means of a diaphragm under boost pressure.
If the engine is switched off, the diaphragm is pressed into the seat by a spring integrated into the deceleration air switchover valve. If the ME-SFI [ME] control unit detects via actual value potentiometers 1 and 2 (M16/6r1, M16/6r2) that the throttle valve has closed and therefore deceleration mode is active, the bypass air switchover valve is actuated. The diaphragm is pulled open against the spring force and boost pressure and opens the bypass duct to the intake side. The excess boost pressure is thereby relieved.
If the engine changes from deceleration mode to load operation, the bypass air switchover valve is no longer actuated. The spring presses the diaphragm in the direction of the seat. There the diaphragm is pulled into the seat by the prevailing boost pressure and thus closes the bypass duct again.
Sectional view of deceleration air switchover valve
Function sequence for charge air cooling
The liquid-cooled charge air cooler is connected to the low-temperature circuit with the low-temperature cooler and the low-temperature circuit circulation pump 1 (M43/6). If the charge air temperature is > 35°C, the ME-SFI [ME] control unit actuates the low-temperature circuit circulation pump 1 over the engine CAN (CAN C), the powertrain control unit (N127) and the drivetrain-LIN (LIN C3).
If the charge air temperature drops below 25°C, the low-temperature circuit circulation pump 1 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.
The low-temperature circuit temperature sensor (B10/13) also records the temperature of the coolant downstream of circulation pump 1 and sends corresponding signals to the powertrain control unit.
Shown on low-temperature circuit up to model year 2017
Shown on low-temperature circuit as from model year 2017
| Electrical function schematic for charging | PE09.00-P-2050-97FBH | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-P-9998MRH |