Cooling System

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The cooling system for engine B4204 differs in several ways compared to the previous engine variants. Instead of a mechanical water pump there is a 400W electric water pump. The thermostat is also electrically heated.

With an electric water pump the water pump can be demand controlled as required. This results in a lower load on the engine, resulting in lower fuel consumption and better power. With electric heated thermostat one can regulate the thermostat opening temperature as needed.

Electric water pump, CWP (Coolant Water Pump) 

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The demand controlled electric water pump is controlled by the ECM (Engine Control Module) via LIN communication. The speed varies between 750-5800 r/min depending on the cooling demand. Maximum speed demand is prioritized. The pump can supply a maximum of approximately 2 l/second. On certain occasions, such as cold starts or at idle, the water pump does not usually run. This partly reduces the load on the engine, thereby reducing fuel consumption. In other cases, so that the engine can reach operating temperature faster. For example, when the pump starts:

• If heat to the passenger compartment is requested, the electric water pump starts in order to supply water to the heat exchanger.
• At high output, the water pump starts in order to prevent local heating peaks in e.g. the cylinder head and turbo.
• When the coolant temperature approaches the ideal temperature (90° C or 105° C degrees, depending on the driving conditions) the water pump starts. This is to create a uniform temperature distribution in the engine and across the thermostat when the thermostat opens.
• If cooling is required, after the engine has been switched off, the water pump always starts together with the cooling fan to cool the coolant.
• The electric water pump is also used to circulate the coolant during Start/Stop.

Thermostat 

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On engines B4204T9/T10/T11/T12/T15 the thermostat is installed as a complete unit together with the thermostat housing. The thermostat consists of a traditional wax thermostat and an electrically heated heating element. In the wax thermostat's core (in the wax) there is a heating element that is demand controlled by the ECM. This causes the engine temperature to be regulated as required. The thermostat opens in two different ways:

• Traditional opening via the wax element.
• Demand controlled opening via addition of extra heat in the wax element by the electrically heated wax element.

During normal driving, outdoor temperature below approx. 30 °C and with non-activated sport mode, the thermostat opens in the traditional manner at 105 °C. At higher opening temperatures this results in cleaner exhaust gases and lower fuel consumption.

At higher loads, outdoor temperatures above about 30 °C or sport mode selected, the thermostat opens at 90 °C using the heating element. At higher loads, more cooling is required.

Sensor, coolant temperature 

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The sensor is an NTC resistor (Negative Temperature Coefficient) which is powered with 5 V by the engine control module (ECM). Feedback is an analogue 5 V signal where the voltage level corresponds to a certain temperature. The resistance depends on the temperature according to the following: