Charging, Function - GF09.00-P-2000MRT
Engine 256.9 in model 213, 238, 257, 290
up to model year 2021
Engine 256.9 in model 222
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.
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 air in the charge air pipe is compressed and heated due to the high rotational speed of the compressor wheel and the resultingly high flow volume. The maximum boost pressure in this case is approx. 2.2 bar.
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.
Design of charging system, view from below
Charging is subdivided into the following points:
- Charge pressure control
- Electric additional charging
- Overrun mode bypass air
Charge pressure control
Boost pressure control is carried out via the boost pressure control flap actuator (M16/7). 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
Charge air pressure and temperature sensor (B4/32) Pressure and temperature sensor upstream of throttle valve (B28/26)
Pressure and temperature sensor downstream of turbocharger (B28/17)
- Intake pressure
Pressure sensor downstream of air filter (B28/11)
- Load request from driver
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 pressure and temperature sensors' signals. The boost pressure is adapted to the engine's requirements.
Electric additional charging
The boost pressure is directly dependent on the rpm of the turbocharger that is driven by the exhaust flow. The boost pressure that can be generated by the turbocharger in the lower rpm/partial-load range is thus rather low, and only increases as the engine rpm increases. When high power is demanded rapidly by the driver, it takes a certain time until maximum boost pressure can be built up so that the engine's full output is available. This behavior in supercharged engines is called "turbo lag".
To counteract turbo lag and have evenly high boost pressure available across the entire rpm range, part of the boost pressure in the lower rpm/partial-load range is generated by the electric additional compressor. The boost pressure provided in this way is a maximum of 400 mbar.
Depending on load requirement, the engine's operating state, and ambient conditions, the engine management system calculates the nominal boost pressure at each engine rpm. As the exhaust gas turbocharger cannot build up the specified boost pressure in the low rpm range, the pressure difference between the actual and specified boost pressure is adjusted by actuating the electric additional compressor. To do this, the engine management system calculates the additional compressor rpm metered to the required boost pressure. The ME-SFI [ME] control unit requests additional compressor actuation with rotational speeds of up to 3, 000 rpm via the drive train sensor CAN (CAN I).
During additional compressor actuation, the signals of the pressure and temperature sensor upstream of the throttle valve are registered to monitor the boost pressure. When the turbocharger is running on its own, pressure measurement is carried out via the pressure and temperature sensor downstream of the turbocharger.
View of electric additional compressor
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 diagram of exhaust gas turbocharger
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 throttle valve closing and thus overrun mode via actual value potentiometer 1 (M16/6r1) and actual value potentiometer 2 (M16/6r2) in the throttle valve actuator (M16/6), it actuates the 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.
Sectional view of deceleration air switchover valve
| Electrical function schematic for charging | Engine 256.9 in model 213, 238 up to model year 2021 | PE09.00-P-2050-97DBA | |
| Engine 256.9 in model 222 | PE09.00-P-2050-97SEM | ||
| Engine 256.9 in model 257 up to model year 2021 | PE09.00-P-2050-97XBB | ||
| Overview of system components for gasoline injection and ignition system with direct injection | GF07.70-P-9998MRT |