4
Thermostat housing – small/large coolant circuit and bypass valve
5
Cylinder head – exhaust side
7
Cylinder block and cylinder head – intake side
System Operation
Cooling system function
The cooling system is controlled in 3 phases.
Phase 1
When the engine is cold, the small/large coolant circuit thermostat and the short block thermostat are closed.
The mechanical coolant pump delivers the coolant only to the exhaust-side part of the cylinder head. In this phase, the coolant in the coolant jacket around the cylinder and in the intake-side part of the cylinder head remains motionless.
Below a coolant temperature of approximately 50°C and below an engine speed of approximately 3000 rpm, only the exhaust-side part of the cylinder head is cooled.
4
Casing – Thermostat – Small/large coolant circuit and bypass valve
5
Cylinder head – exhaust side
7
Cylinder block and cylinder head – intake side
The coolant flows from the part of the cylinder head on the exhaust side into the thermostat housing in the small/large coolant circuit and bypass valve and, from there, via the heater core to the oil cooler and then back to the coolant pump.
In addition, part of the coolant flows from the short block through the electrical coolant pump and, from there, on past the exhaust gas turbocharger to the coolant expansion tank and then back to the coolant pump.
1
Coolant flow from oil cooler
2
Coolant flow to heat exchanger
3
Small/large coolant circuit thermostat
5
Coolant flow to mechanical coolant pump
6
Excessive coolant flow from exhaust-side part of the cylinder head
If the engine speed exceeds approximately 3000 rpm, the bypass valve opens. The now increased coolant pressure from the engine opens the bypass valve so that the increase pressure can escape directly to the intake side of the coolant pump.
Phase 2
The short block thermostat opens at a coolant temperature of 50 °C and is completely open at 85 °C.
As with a conventional system, this opens the small coolant circuit.
The coolant pump now also delivers the coolant to the cooling jacket around the cylinder, washes around it and enters the entire cylinder head via the bore holes.
3
Small/large coolant circuit thermostat
7
Coolant flow to the radiator
The small/large coolant circuit thermostat opens at a coolant temperature of 92 °C and is completely open at 106 °C. At the same time, the bypass valve thermostat closes. This ensures that the entire coolant flow, regardless of the coolant pressure, is always routed via the radiator.
With this engine, in addition to the usual mechanical coolant pump, an electric coolant pump is installed in the coolant circuit.
The pump is secured onto the cooling fan housing via a retaining device.
The coolant pump is only activated whenever the coolant temperatures exceeds a critical value. This can occur in particular if the engine has been operated at a high load for a long time and is then parked immediately afterwards. For example, after a long motorway journey at high speed, with subsequent parking at a service station. This can lead to an excessive build-up of heat around the engine after the engine has been switched off.
It can therefore be the case that the electrical coolant pump continues to run for a certain period of time.
