ME-SFI thermal management, function

Function requirements for thermal management, general points

 Circuit 87M ON (engine control ON)

 Engine running

Thermal management, general points

The coolant temperature of the engine is regulated by the controlled thermal management from ME-SFI [ME] control unit (N3/10). This has the following advantages:

- Operating temperature is reached faster

- Reduction of the exhaust gas emissions

- Fuel savings (up to around 4%)

- Improved heat comfort.

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Thermal management is performed according to the following sensors and signals:

- Hot film MAF sensor (B2/5), engine load

- Intake air temperature sensor (B2/5b1)

- Coolant temperature sensor (B11/4)

- Accelerator pedal sensor (B37), rate of change of load (driver type: calm or sporty)

- Crankshaft Hall sensor (B70), engine speed

- AAC control unit (N22/1), outside air temperature and air conditioning status over the interior CAN (CAN B) and chassis CAN (CAN E)

- ESP control unit (N47-5) or regenerative braking system control unit (N30/6) (for hybrid), vehicle speed via the chassis CAN

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Thermal management function sequence

Thermal management consists of the following subfunctions:

 Heating system switch-off (for engine 272)

 Heating the three-disk thermostat

 Fan control

Heating system switch-off (for engine 272)

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

The shutoff valve is actuated if necessary by the ME-SFI [ME] control unit by means of a ground signal. When deenergized, the shutoff valve is opened by an integrated spring.

The shutoff valve for the heating system is nevertheless always open for a switched on heater (status "heating" of the AAC control unit).

Heating the three-disk thermostat

Depending on the operating conditions (performance map), the ME-SFI [ME] control unit actuates the heating element in the three-disk thermostat valve (Y110) with a ground signal. The voltage is supplied through "circuit 87 M2".

When heated, the three-disk thermostat opens in the coolant thermostat. In this way the coolant temperature of 80 to 100°C is regulated via the duty cycle.

The three-disk thermostat can be set in four positions:

 Full throttling

 Bypass mode

 Mixed mode

 Cooler mode

P20.10-2226-74

Position of the three-disk thermostat

1 Coolant from engine

2 Coolant back to engine

3 Coolant to engine radiator

A Full throttling

B Short-circuit mode position

C Mixed fuel mode position

D Cooler mode position

Full throttling

For a cold start the three-disk thermostat is not heated. The heating circuit is also switched off by the heating system shutoff valve when the vehicle heating is switched off.

The coolant in the engine now warms up very quickly. The engine oil is warmed up over the heat exchanger at the front on the engine.

The closing plate on the three-disk thermostat opens but dependent on the pressure (by the rotating coolant pump)
from about 3500 rpm (short circuit operation).

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Bypass mode

From an outside temperature >12°C and a coolant temperature of 45°C or from a coolant temperature of about 70°C, switching to short-circuit operation takes place.

For short-circuit operation the three-disk thermostat is heated less and part of the coolant flows back into the engine circuit. Short-circuit operation can be 'choked', so that the heater responds more quickly.

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Mixed-fuel mode

The mixed-fuel operation is enabled if the coolant temperature reaches 98°C for the first time after the start.

For mixed-fuel operation the three-disk thermostat is heated in such a way that the coolant flows through the engine radiator and the engine circuit.

At partial load, the three-disk thermostat regulates at an increased coolant temperature of around 100°C.

In order to prevent critical coolant temperatures from occurring, the range of increased coolant temperature is lowered under the following conditions:

- Intake air temperature >38°C

- Engine speed >3000 rpm

- Engine load > 30%

- Sporty driver

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Coolant temperature is thus lowered according to outside air temperature. If outside air temperature is below
12°C, the temperature is lowered to about 90°C and if outside air temperature is above 12°C, it is lowered to about 80°C.

Cooler mode

The three-disk thermostat is heated constantly during radiator operation. The coolant only flows through the engine radiator.

If coolant temperature is above about 110°C, the three-disk thermostat is open continuously, irrespective of heating (limp-home function).

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If the coolant temperature is too high a warning message is displayed in the multifunction display (A1p13) in the instrument cluster (A1). To do this, the ME-SFI control unit sends the corresponding signal via the chassis CAN and central CAN (CAN F) to the instrument cluster.

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Fan control

The ME-SFI [ME] control unit actuates the engine and air conditioning electric suction fan with integrated control (M4/7). The specified fan speed is set by the ME-SFI control unit with a pulse width modulated signal (PWM signal).

The duty cycle of the PWM signal is 10 to 90%.

Meaning:

10% Fan motor "OFF"

20% Fan motor "ON", minimal rpm

90% Fan motor "ON", maximum rpm

If actuation is faulty, the air fan rotates at the maximum rotational speed (fan emergency mode).

The AAC control unit (N22/1) sends the air conditioning status via the interior CAN and chassis CAN to the ME-SFI control unit.

Delayed fan switch off

With "ignition OFF", the fan motor runs on for up to 5 min. if the coolant temperature or engine oil temperature (calculated from the temperature model) has exceeded the specified maximum values.

The PWM signal duty cycle is 40 % maximum while the delayed fan switch-off is active.

If the battery voltage is dropping too much, the delayed fan switch-off is stopped.