Engine Cooling: Notes
Because of the turbocharging and indirect charge air cooling, the N74 engine has the same cooling requirements as the N63 engine. Consequently it too has two separate cooling circuits. One is for cooling the engine and exhaust turbochargers, the other is for charge air cooling and for cooling the two engine control units.
The engine cooling system performs the task of drawing heat off the engine and maintaining the operating temperature as constant as possible. As on the N54 and N63 engines, the two exhaust turbochargers are also cooled.
On the N74 engine, the coolant passages have been integrated mainly in the engine block. Optimizations to the engine cooling circuit have enabled a significant reduction in the coolant quantity for the bypass mode, thus shortening the warm-up phase.
The coolant feed line downstream of the coolant pump is routed directly beside the engines main oil duct. The oil in the main oil duct flows in the opposite direction to the coolant. This enhances the heat exchange between the two media, and has a positive effect on the engine oil temperature. The overall cooling effect is comparable with that of an engine oil-coolant heat exchanger.
Engine cooling circuit of the N74 engine
| Index | Explanation |
|---|---|
| 1 | Radiator |
| 2 | Radiator for transmission cooling |
| 3 | Coolant temperature sensor at radiator outlet |
| 4 | Electric cooling fan |
| 5 | Characteristic map thermostat |
| 6 | Electric auxiliary coolant pump for turbocharger cooling |
| 7 | Coolant pump (mechanical) |
| 8 | Exhaust turbocharger |
| 9 | Heater core |
| 10 | Duo Heater valve |
| 11 | Electric auxiliary coolant pump for vehicle heating |
| 12 | Coolant temperature sensor at engine outlet |
| 13 | Filling canister |
| 14 | Expansion tank |
| 15 | Vent line |
| 16 | Transmission fluid thermostat and the fluid-to-coolant heat exchanger |
The coolant passages in the cylinder heads are similar to the N63 engine. The coolant flows through the cylinder heads diagonally from the outside to the inside, whereby it flows in at the rear (outside) and flows out at the front (inside). This is also known as diagonal cooling.
As on the N63 engine, an additional electric coolant pump is used which supplies the bearings of the exhaust turbochargers with coolant.