Charging, Function - GF09.00-P-2000MRS
Engine 264.9 in model 205, 253
Function requirements, general
- Engine runs
The ME-SFI [ME] control unit (N3/10) detects the engine running via the evaluation of the crankshaft Hall sensor signals (B70). The ME-SFI [ME] control unit directly reads in the signals of the crankshaft Hall sensor.
Design of charging system
Charging
The cylinder charging efficiency is improved as a result of forced induction. The engine torque and the engine performance are therefore increased. The fuel quantity corresponding to the increased air mass is metered by the ME control unit.
During forced induction, the flow energy of the exhaust gases is used to drive the ATL. The exhaust gas turbocharger intakes fresh air via the air filter at the compressor inlet and supplies this via the compressor outlet to the charge air pipe. The high rotational speed of the compressor wheel and the resulting high flow volume compresses and heats the air in the charge air pipe. The max. boost pressure is, in this connection, approx. 0.7 to 1.5 bar depending on the engine output variant.
The charge air flows via the charge air pipe to the charge air cooler where it is cooled down to temperatures of approx. 30 °C. The charge air is then routed to the charge-air distributor where it is supplied to the cylinders for combustion.
Charging is subdivided into the following points:
- Charge pressure control
- Overrun mode bypass air
Charge pressure control
The boost pressure control is realized via the boost pressure control flap actuator. The boost pressure control flap actuator is actuated in a characteristics map- and load-dependent manner by the ME-SFI [ME] control unit for boost pressure control. To do this the ME-SFI control unit evaluates the following sensor signals and functions of the engine management:
- Boost pressure and charge air temperature
Pressure and temperature sensor upstream of throttle valve (B28/26)
Pressure and temperature sensor downstream of throttle valve
- Intake pressure
Pressure sensor downstream of air filter
- Load request from driver (accelerator pedal position)
The powertrain control unit (N127) reads in the accelerator pedal sensor (B37) signals directly, and transmits the accelerator pedal position via the drive CAN (CAN C1) to the ME-SFI [ME] control unit.
- Engine speed Crankshaft Hall sensor
- Knock control, transmission overload protection, overheating protection
In full-load operation, the max. boost pressure is built up in a mechanically dependent manner. To reduce the boost pressure, the exhaust flow for the drive of the turbine wheel is diverted via a bypass by opening the boost pressure control flap.
The boost pressure control flap actuator actuates - via a linkage - the boost pressure control flap that closes the bypass. Part of the exhaust flow is directed through the bypass past the turbine wheel, whereby the boost pressure is regulated and the turbine speed limited. In this way the boost pressure can be adapted to the current load demand on the engine.
To monitor the current pressure and temperature conditions in the charge air duct from the exhaust gas turbocharger to the charge-air distributor, the ME-SFI [ME] control unit evaluates the signals from the pressure and temperature sensors and adjusts the boost pressure to the engine-related requirements.
Overrun mode bypass air
The exhaust gas turbocharger continues turning in overrun mode due to the inertia of the shaft, compressor and turbine wheels. In the case of rapid closing of the throttle valve, a charge pressure wave therefore runs back to the compressor impeller. 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 the bypass air switchover valve prevents this through rapid depressurization through a bypass in the intake side of the ATL.
Schematic display of ATL with divert air switchover valve
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 the closing of the throttle valve and thus overrun mode via actual value potentiometer 1 (M16/6r1) and actual value potentiometer 2 (M16/6r2), it actuates the overrun mode bypass air switchover valve. 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.
| Electrical function schematic for charging | PE09.00-P-2050-97FBI | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-P-9998MRS |