Charging, Function - GF09.00-P-2000MNC
ENGINE 276.8 in model 172.4
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).
With forced induction, the flow energy of the exhaust gases is used to drive the ATL.
The ATLs draw fresh air through the air filters into the compressor inlets, from where it passes through the compressor outlets in the charge air pipes to the charge air cooler.
Due to the high rotational speed of the compressor impellers and the resulting high volumetric flow rate, the intake air becomes compressed in the charge air pipes.
The compressed charge air flows via the charge air pipes upstream of the charge air cooler to the charge air cooler. This then cools the charge air heated up by the compression and routes it to the charge air distributor.
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 via the boost pressure control pressure transducer (Y31/5). 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 (B17/8), charge air temperature
- Pressure sensor downstream of air filter, left cylinder bank (B28/4), intake air pressure
- Pressure sensor upstream of air filter, right cylinder bank (B28/5), intake air pressure
- Pressure sensor upstream of throttle valve (B28/6), boost pressure upstream of throttle valve
- Pressure sensor downstream of throttle valve (B28/7), boost pressure downstream of throttle valve
- 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. To reduce the boost pressure, the exhaust flows that drive the ATLs are each redirected through bypasses by opening the boost pressure control flaps.
To do this the boost pressure control pressure transducer actuates the boost pressure control flap vacuum cell with vacuum from the vacuum pump. The vacuum cells react by closing the boost pressure control flap linkages over a rod, which close the bypasses. If there is no vacuum at the vacuum cells then the boost pressure control flaps and thus also the bypasses are opened. The boost pressure control flaps therefore allow the exhaust flow to bypass the turbine wheels (bypass), thus controlling the boost pressure and limiting the turbine speed. In this way the boost pressure can be adapted to the current load demand on the engine.
If there is leakage in the line between the vacuum pump and the vacuum cells then no build up of boost pressure is possible. 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 sensors downstream of the air filter serve to allow the ME-SFI [ME] control unit to monitor the charging. The charge air temperature is detected in the charge air cooler by the charge air temperature sensor and sent to the ME-SFI [ME] control unit in the form of a voltage signal.
The boost pressure control function can only be assessed if the "boost pressure control adapted" message is displayed with the Xentry Diagnostics.
If the ME-SFI [ME] control unit or one of the ATLs 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. If the hose lines are leaky between the vacuum cells, boost pressure control pressure transducer and charge air cooler, a "boost pressure too high" fault is stored in the ME-SFI [ME] control unit. Quick load requirements below the basic charge pressure, are controlled through the throttle valve actuator (M16/6).
Shown: flow pattern for charging
Function sequence for bypass air
The respective 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 the bypass air switchover valve prevents this through rapid depressurization over a bypass in the intake side of the ATL.
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 switchover valve.
Schematic display of ATL with left divert 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.
View of right ATL with sectional view of right divert air switchover valve
Function sequence for charge air cooling
The charge air cooling maintains a charge air temperature of <t 60°C at 20°C ambient temperature The cooled air downstream of the charge air cooler has higher density This increases the cylinder charge and therefore engine performance The tendency to knock is also reduced and also the tendency to generate nitrogen oxide (NOx) is reduced by low exhaust temperatures
Both cylinder banks are fitted with a common coolant-cooled charge air cooler The charge air cooler is attached to the low-temperature circuit with the low-temperature cooler and the charge air cooler circulation pump (M44)
If the charge air temperature is > 35°C the ME-SFI [ME] control unit actuates the charge air cooler circulation pump over the charge air cooler circulation pump relay (K61) If the charge air temperature falls below 25°C the charge air cooler circulation pump is switched off again.
The charge air temperature is detected in the charge air cooler by the charge air temperature sensor and sent to the ME-SFI [ME] control unit using a voltage signal.
Only open the cap in the low-temperature circuit when the charge air temperature is increased (lack of power) and the engine is cold The coolant must reach up to the cap.
View of low-temperature circuit
| Electrical function schematic for charging | PE09.00-P-2050-97TAH | ||
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