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Thermal Management, Function - GF07.10-P-1012MNH

Engine 176.9, 177.9 in model 463 

as of model year 2019 

Function requirements for thermal management, general points 

IMPORTANT The circuit relay87M (K40/8 kN) is switched on for circuit 15 ON.

Thermal management, general 

The coolant temperature of the engine is regulated with the thermal management controlled by the ME-SFI control unit (N3/10). The following advantages arise from this:

The ME-SFI control unit reads in the following signals for controlling the thermal management:

Coolant circuit, shown schematically 

G13655860Courtesy of MERCEDES-BENZ USA

Function sequence for thermal management 

The thermal management function encompasses the following subfunctions:

Function sequence for heating the two-slide thermostat 

The temperature of the coolant can be controlled variably by the heatable two-slide thermostat. There is a coolant thermostat heating element in the two-slide thermostat which is actuated as required by the ME-SFI control unit using a ground signal.

The two-slide thermostat can assume five positions:

Stationary coolant 

G13655861Courtesy of MERCEDES-BENZ USA

At a coolant temperature< 80°C and an engine speed< 3000 rpm, both valves on the two-valve thermostat are closed completely.

IMPORTANT Shortening of the engine warm-up phase by stationary coolant leads to fuel saving and therefore reduction of the CO2 output.

Bypass mode 

G13655862Courtesy of MERCEDES-BENZ USA

In partial-load range, the coolant temperature can be raised to approx. 105 °C (heating element de-energized). Therefore the friction power can be improved due to a higher engine oil temperature and the mixture formation improved due to less fuel condensation on the cylinder barrels.

Mixed mode 

G13655863Courtesy of MERCEDES-BENZ USA

Radiator operation 

G13655864Courtesy of MERCEDES-BENZ USA

Through heating the two-slide thermostat (heating element energized) this opens and the coolant is led through the engine radiator. For wide open throttle the two-slide thermostat is very quickly opened. The coolant temperature can be lowered whereby the best possible engine cooling and knock-free combustion are achieved.

Fail-safe position 

G13655865Courtesy of MERCEDES-BENZ USA

To avoid damage the fail-safe position is initiated for stationary coolant and an engine rpm > 3000 rpm. The coolant is passed back over the differential pressure disc to the engine (short circuit).

IMPORTANT Above a coolant temperature of approx. 120 °C, the two-valve thermostat is always fully opened (limp-home function) irrespective of the energization of the heating element).

Low-temperature circuit, shown schematically (for engine 177.9) 

G13655866Courtesy of MERCEDES-BENZ USA

Function sequence for charge air cooling (engine 176.9) 

The charge air cooling keeps the charge air temperature < 60 °C at 20 °C ambient temperature. The cooled air downstream of the charge air coolers has a higher density. This increases the cylinder charge, and therefore engine performance.

The tendency to knock is reduced due to lower exhaust temperatures and there is reduced development of nitrogen oxides (NOx ). Each cylinder bank has a coolant-cooled charge air cooler, which is connected to the low temperature circuit.

The coolant in the low-temperature circuit is constantly circulated to prevent overheating at certain points in the circuit. For this purpose, the low-temperature circuit circulation pump 1 (M43/6) is steadily controlled as required by the powertrain control unit via the drive LIN (LIN C3).

The closed-loop control depends, among other things, on the following variables:

The coolant temperature in the low-temperature circuit is detected via the low-temperature circuit temperature sensor (B10/13), which sends its signal to the powertrain control unit.

The charge air temperature is detected in the charge air cooler by the left and right charge air temperature sensors and sent to the ME-SFI control unit with a voltage signal.

For the appropriate distribution of the coolant flow volume in the low-temperature circuit, the low-temperature circuit switchover valve is controlled by the powertrain control unit.

Low-temperature circuit, shown schematically (for engine 176.9) 

G13655867Courtesy of MERCEDES-BENZ USA

Function sequence for fan control 

The powertrain control unit actuates the fan motor directly. The ME-SFI control unit transmits the fan nominal speed via drive CAN to the powertrain control unit. If the powertrain control unit does not receive a valid fan request, the fan motor is actuated at maximum rpm. In the case of a fault in the signal line (loss of frequency) by the powertrain control unit, the fan motor switches itself to the maximum rpm (fan emergency mode).

The climate control unit sends the air conditioning system status via the interior CAN and suspension FlexRay to the powertrain control unit including a fan requirement. If the powertrain control unit does not receive a valid fan request, the fan motor is actuated at maximum rpm.

Delayed fan switch off:

The fan motor runs on for up to 5 min with "ignition OFF" if the coolant temperature, the temperature of the ME-SFI control unit or a thermal input integral calculated based on an engine load, coolant temperature, vehicle speed and outside temperature (averaged over the last 6 min) has exceeded a prescribed threshold value. If the battery voltage drops down a lot, the delayed fan switch off is suppressed.

IMPORTANT The delayed fan switch off is not broken off by "ignition ON". When starting the engine in delayed fan switch off the fan regulation for normal operation is suppressed until the delayed fan switch off is completed.

Function sequence for overheating protection 

In a case of thermal overload the overheating protection protects the catalytic converters against engine damage and overheating damage. To do this, the ME-SFI control unit reads in the signals of the coolant temperature sensor.

The following measures should be taken to avoid overheating of the engine:

IMPORTANT If the engine oil or coolant temperature is too high, a warning message in instrument cluster is shown. To do this, the ME control unit transmits an appropriate signal via the drive CAN, powertrain control unit, suspension FlexRay, electronic ignition lock control unit, and the user interface CAN to the instrument cluster.

Function sequence for heating system switch-off 

To heat up the engine more quickly, the ME-SFI control unit switches off the heating system coolant circuit by actuating the heating system shutoff valve (Y16/2).

Function sequence for air flap control 

The air flowing through the engine compartment is led over a air duct (passive duct) on the engine hood in a targeted manner into the area between the cylinder rows in order to appropriately flow into the components (exhaust gas turbocharger and catalytic converters) positioned there.

In certain operating conditions the air cross-section of the air duct must be closed off in order, for example, to prevent recirculation of the cooler exhaust air over the air duct for engine fan operation at low ground speeds. This cross-section is released or closed off by the air flap. The air flap is actuated via the left engine cooling air flap actuator motor (M108/1) and the right engine cooling air flap actuator motor (M108/2). The powertrain control unit actuates the engine cooling air flap actuator motor via the Drivetrain LIN.

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