Thermal management, function

S07.10-0052-79

Function requirements for thermal management, general points

 Circuit 87M (Engine management ON)

 Engine running.

Thermal management, general

Heat management as controlled by the CDI control unit (N3/28) regulates the engine's coolant temperature, exhaust gas temperature, and the fuel pressure. The following advantages arise from this:

 Rapid reaching of the optimal operating temperature

 Reduction of the exhaust emissions

 Fuel savings

 Improved heating comfort

 Component protection for engine full load

Thermal management controlling is dependent on the following sensors and signals:

 Engine oil temperature sensor (B1)

 Left hot film mass air flow sensor (B2/6), engine load

 Left intake air temperature sensor (B2/6b1)

 Right hot film mass air flow sensor (B2/7), engine load

 Right intake air temperature sensor (B2/7b1)

 Boost pressure sensor (B5/1), engine load

 Coolant temperature sensor (B11/4)

 Charge air temperature sensor (B17/8)

 DPF differential pressure sensor (B28/8)

 Accelerator pedal module (B37/4), accelerator pedal actuation (how fast and how far  driver type calm or sporty)

 Fuel temperature sensor (B50)

 Crankshaft Hall sensor (B70), engine speed

 Temperature sensor in CDI control unit

 Electronic Stability Program (ESP) control unit (N30/4) for wheel speed

 Fully integrated transmission control (VGS) electric controller unit (N15/11), for set drive range

Function sequence for thermal management

The thermal management is described in the following points:

 Function sequence for coolant thermostat

 Function sequence for maximum heating combustion

 Function sequence for fuel preheating system

 Function sequence for fuel tank protection

 Function sequence for fan control

 Function sequence for overheating protection

Function sequence for coolant thermostat

The coolant thermostat heating element regulates the coolant temperature in the range of approx. 87 to 102 °C. The limp-home function ensures that the two-disk thermostat is completely opened from approx. 102 °C, regardless of actuation.

The two-disk thermostat can take the following positions:

 Short-circuit operation position: Coolant temperature < 87 °C; coolant flow in engine only. Flow through the passenger compartment heating is possible.

 Mixed-fuel mode position: Coolant temperature between 87   and 102°C; the two-disk thermostat is open and full flow-through of the coolant begins.

 Cooler mode position: Coolant temperature > 102 °C; the two-disk thermostat is open and full flow-through of the coolant begins.

The following advantages result from regulating the refrigerant temperature of the engine:

 Operating temperature is reached faster

 Emissions reduced

 Improved heating comfort

 Component protection for engine high load

Function sequence for maximum heating combustion

With the maximum heating combustion more heat is introduced into the refrigerant by a new combustion strategy on the engine side.

The following function conditions must be met simultaneously:

 The accelerator pedal position of the accelerator pedal sensor lies below   80%  (partial load).

 Diesel particulate filter (DPF) not in regeneration mode

 At least 90 % heating output is requested.

 The coolant temperature from the coolant temperature sensor is below 82 °C (renewed cut-in takes place at 74 °C).

During maximum heating combustion, a higher exhaust temperature is achieved by means of 2 preinjections from the fuel injectors (Y76) using a much greater fuel quantity, plus a retarded main injection. This process, in combination with exhaust gas recirculation (EGR), causes more heat to be transferred to the coolant inside the engine, allowing for more rapid attainment of the optimum operating temperature and improved warming of the vehicle interior. To do this, the exhaust gas recirculation positioner (high pressure circuit) (Y27/9) is actuated by the CDI control unit.

 The CDI control unit regulates, stabilizes and increases the idle speed depending on operating conditions and engine loads if the accelerator pedal is not actuated.

Function sequence for fuel preheating system

Fuel preheating is achieved using the following regulation strategies:

 Pressure regulating valve regulation

 2-controller concept regulation

 Quantity control valve regulation

Pressure regulating valve regulation

The fuel pressure in the rails is regulated by the pressure regulating valve (Y74) during the starting procedure and for fuel heating. The quantity control valve (Y94) is controlled to open.

Pressure regulating valve regulation takes place under one of the following conditions:

 Up to 30 seconds after engine start in neutral

 Up to a fuel temperature of 20 °C

 Pressure regulating valve regulation causes the cold fuel to be heated rapidly by the fuel being forced at high pressure through a narrow gap in the pressure regulating valve.

2-controller concept regulation

The fuel pressure is jointly regulated by the pressure regulating valve and the quantity control valve during idling and in deceleration mode.

Quantity control valve regulation

The regulation of the fuel pressure in the rails via the quantity control valve takes place as of 30 s after the engine start and as of a fuel temperature of 20 °C.

 With the 2-regulator design and with regulation by the quantity control valve, the fuel is heated less than is the case with pressure regulating valve regulation.

Function sequence for fuel tank protection

By increasing the fuel pressure via the fuel high-pressure pump from 4.5 bar to up to 1,600 bar the temperature of the fuel is also increased.

To protect the fuel tank from overheating, the CDI control unit reads in the signal of the fuel temperature sensor and thus monitors the temperature of the fuel delivered to the fuel high-pressure pump.

If the temperature of the fuel delivered to the fuel high-pressure pump increases to over 90 °C, the CDI control unit reduces the injection quantity and fuel pressure in the rails via the pressure regulating valve. This causes less fuel to be compressed.

The CDI control unit causes the excess fuel to return to the fuel tank via the quantity control valve.

If the temperature of the fuel delivered to the fuel high-pressure pump drops below 90 °C, the fuel tank protection function is deactivated by the CDI control unit.

Function sequence for fan control

The CDI control unit actuates the electric fan (M91) an. The nominal fan speed is specified via a pulse width modulated signal by the CDI control unit.

The duty cycle of the pulse width modulated signal is 10 to 90 %.

This means, for example:

 0 %: Electric fan "OFF"

 10 %: Electric fan "ON", minimum fan speed

 90 %: Electric fan "ON", maximum fan speed

A fault in the fan motor is transferred to the CDI control unit via a pulse width modulated signal (ground sampling).

A fault in the signal line (loss of frequency) by the CDI control unit switches the electric fan automatically to the maximum fan speed (fan emergency mode).

Delayed fan switch off

The fan motor runs at "ignition OFF" for up to 5 min if the following signals exceed specified threshold values:

 Coolant temperature

 Temperature of the CDI control unit

 A thermal input integral (averaged over the last 6 min) calculated from:

 Engine load

 Coolant temperature

 Vehicle speed

 Outside temperature

The duty cycle of the pulse width modulated signal is a maximum of 40 % during delayed fan switch off. If the battery voltage drops down a lot, the delayed fan switch off is suppressed.

 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

The overheating protection protects against engine damage if there is a thermal overload. At a coolant temperature above 106 °C the injection quantity is reduced based on the characteristics maps stored in the CDI control unit. Reduction occurs depending on the coolant temperature and oil temperature. To do this, the CDI control unit reads in the coolant temperature sensor and engine oil temperature sensor.

After evaluating the input signals, the CDI control unit regulates the fuel pressure in the rails via the quantity control valve and the pressure regulating valve, and controls the injection period by actuating the fuel injectors.

 If the engine oil level or coolant temperature is too high, a warning message is displayed in the multifunction display (A1/1p13) of the instrument cluster (IC) control unit (A1/1). To do this, the CDI control unit transmits a corresponding signal to the instrument cluster (IC) control unit.