Turbocharger: Description and Operation
GF09.00-P-2000MMC Charging, Function
- ENGINES 157.9 in MODEL 216.3, 221.0 /1
- ENGINES 278.9 in MODEL 216.3, 221.0 /1
Function requirements for charging - general
- Circuit 87 M ON (engine timing ON)
- Engine running
Forced induction, general
Cylinder filling efficiency is improved by forced induction. This raises the engine torque and engine power.
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 turbocharger.
The turbochargers 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 volume flow rates, the intake air becomes compressed in the charge air pipes.
Compression heats the charge air, which now flows through the charge air pipes to the charge air cooler. This finally cools the air which was heated by the compression and leads it further over the charge air pipe to the intake manifold.
Function sequence for forced induction
The function sequence is divided into the following subfunctions:
- Function sequence for boost pressure control
- Function sequence for charge air cooling
Function sequence for boost pressure control
The boost pressure is controlled electro-pneumatically over the boost pressure control pressure transducer (Y77/1) (boost pressure control pressure transducer). The vacuum is generated by the mechanical vacuum pump attached to the engine. The boost pressure regulator is actuated dependent on the characteristics map and the load by the ME-SFI [ME] control unit for the purposes of boost pressure control. Do this the ME-SFI [ME] control unit evaluates the following sensors and functions of the engine timing:
- Intake air temperature sensor (B17)
- Pressure sensor downstream of air filter, LH cylinder bank (B28/4)
- Pressure sensor downstream of air filter, right cylinder bank (B28/5)
- Pressure sensor upstream of throttle valve actuator (B28/6), boost pressure
- Pressure sensor downstream of throttle valve actuator (B28/7), charge air distributor air pressure
- Accelerator pedal sensor (B37), load request from driver
- Crankshaft Hall sensor (B70), engine speed
- Knock control, transmission overload protection, overheating protection
In full-load range, the maximum boost pressure is built up.
To reduce the boost pressure, the exhaust flows that drive the turbine wheels are each redirected through bypasses by opening the boost pressure control flaps.
To do this the boost pressure regulator actuates the boost pressure control flap vacuum cell with vacuum from the vacuum pump. The vacuum cells react by closing the boost pressure control flaps 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 actuator 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 distributor by the intake air temperature sensor and transmitted to the ME-SFI [ME] control unit in the form of a voltage signal.
Diagnosis of boost pressure control
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 [ME] 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 [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 of the RH cylinder bank, a "boost pressure too high" fault is stored in the ME-SFI [ME] control unit. Rapid load requests below the basic charge pressure are regulated over the throttle valve actuator (M16/6).
Shown: the flow pattern of the intake air
1 Left air filter housing
3 Left turbocharger
4 Right turbocharger
7 Right air filter housing
8 Charge air distributor
9 Charge air cooler
10 Exhaust manifold on the left
11 Exhaust manifold on the right
A Intake air (downstream of air filter)
Shown: flow pattern of the intake air/ charge air
2 Vacuum cell (boost pressure control flap)
3 Left turbocharger
9 Charge air cooler
10 Exhaust manifold on the left
M16/6 Throttle valve actuator
A Intake air (downstream of air filter)
B Charge air (uncooled)
Shown: flow pattern of the charge air
2 Vacuum cell (boost pressure control flap)
3 Left turbocharger
7 Right air filter housing
8 Charge air distributor
9 Charge air cooler
10 Exhaust manifold on the left
M16/6 Throttle valve actuator
C Charge air (cooled)
Boost pressure control shown with a duty cycle of (ti) less than 5%
2 Vacuum cell (boost pressure control flap)
2a Boost pressure control flap
3 Left turbocharger
3a Turbine wheel
3b Compressor impeller
9 Charge air cooler
12 Mechanical vacuum pump
Y77/1 Charge pressure positioner
A Intake air (downstream of air filter)
B Charge air (uncooled)
C Charge air (cooled)
D Exhaust
E Atmospheric pressure (atmospheres)
t Time
ti Duty cycle
Boost pressure control shown with a duty cycle of (ti) greater than 5%
2 Vacuum cell (boost pressure control flap)
2a Boost pressure control flap
3 Left turbocharger
3a Turbine wheel
3b Compressor impeller
9 Charge air cooler
12 Mechanical vacuum pump
Y77/1 Charge pressure positioner
A Intake air (downstream of air filter)
B Charge air (uncooled)
C Charge air (cooled)
D Exhaust
E Atmospheric pressure (atmospheres)
F Vacuum
t Time
ti Duty cycle
Function sequence for charge air cooling
Through charge air cooling the charge air temperature is kept less than 60°C (for engine 278) or less than 65°C (for engine 157) for a 20°C ambient temperature.
The cooled air downstream of the charge air coolers 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 water/charge air cooler. The water/charge air cooler is attached to the low temperature cooling circuit with the low-temperature cooler and the charge air cooler circulation pump (M44).
If the charge air temperature is above 35°C, the ME -SFI [ME] control unit (N3/10) actuates the charge air cooler circulation pump through the circulation pump relay (N10/2kQ).
If the charge air temperature fall s below 25°C, the charge air cooler circulation pump is switched off again.
The charge air temperature is detected in the charge air distributor by the intake air temperature sensor and transmitted to the ME-SFI [ME] control unit by means of a voltage signal.
Only open the cap on the low-temperature coolant circuit when the charge air temperature is increased (lack of power) and the engine is cold. The coolant must reach up to the cap.
Shown: charge air cooler coolant circuit
9 Charge air cooler
13 Vent line
14 Low-pressure cooler
15 Expansion reservoir
M44 Charge air cooler circulation pump
A Coolant feed
B Coolant return flow
