Engine cooling: Notes
The engine cooling of the N55 engine consists of cooling the coolant and the engine oil. Depending on the version, different types of engine oil cooling are used. In the variant for hot countries, decoupling the engine oil cooler from the coolant circuit prevents a heat contribution via the engine oil to the coolant of the N55 engine.
The European version has been fitted with an additional coolant cooler on the left-hand side of the vehicle. The additional coolant cooler is connected to the coolant lines parallel to the radiator, thus enlarging the surface area for engine cooling. Engine oil cooling is implemented by means of an oil-to-water heat exchanger.
A turbocharged engine with direct fuel injection makes strong demands on the engine cooling. On the N55 engine, however, no separate coolant pump is required for the twin-scroll exhaust turbocharger.
The components highlighted in red in the following illustration are only present in the European version.
| Index | Explanation |
|---|---|
| 1 | Delivery line, heat exchanger for heating system, zone 1 |
| 2 | Delivery line, heat exchanger for heating system, zone 2 |
| 3 | Valve |
| 4 | Additional coolant cooler |
| 5 | Expansion tank |
| 6 | Oil-to-water heat exchanger |
| 7 | Short-circuit line, coolant circuit |
| 8 | Radiator |
| 9 | Thermostat |
| 10 | Electric coolant pump |
| 11 | Delivery line, coolant into engine block |
| 12 | Thermostat for transmission oil cooling |
| 13 | Transmission oil coolant heat exchanger |
| 14 | Return line, heat exchanger for heating system |
| 15 | Heat exchanger for heating system |
For the cooling system with electric coolant pump, the possibilities of the conventional cooling system are exploited. The heat management determines the current cooling requirement and regulates the cooling system accordingly.
Under certain circumstances, the coolant pump can even be switched off completely, for example to heat up the coolant in the warm-up phase.
If the engine is not running but very hot, the coolant pump will also work with the engine at a standstill.
The cooling power can be requested independent of the engine speed. The heat management now means that, over and above the map thermostat, various characteristic maps can be used for control of the coolant pump. This enables the Digital Engine Electronics (DME) to adapt the coolant temperature to the driving characteristics.
The Digital Engine Electronics (DME) regulate the following temperature range:
- Economy mode: 108 °C
- Normal mode: 104 °C
- High mode: 95 °C
- High mode and control operation by characteristic map thermostat: 90 °C.
If the Digital Engine Electronics (DME) detect the economical operating range "Economy" due to the driving characteristics, the DME regulates to a higher temperature (108 °C). In this temperature range, the engine is operated with a relatively fuel requirement. The friction inside the engine is reduced at higher temperature. The temperature increase thus favor the lower fuel consumption in the low load range. In the mode "High and control by the characteristic map thermostat" mode, the driver wants to use the optimized power output development of the engine. To allow this, the temperature in the cylinder head is lowered to 90 °C. This reduction leads to better cylinder filling, which increases the torque of the engine. The Digital Engine Electronics (DME) can now regulate to a certain operating range adapted to each driving situation.
The coolant temperature influences:
- Fuel consumption
- Power
- Factor of quality of the mixture formation
- Pollutant emission
- Mechanical load on the components.
The optimization of these variables does not permit a fixed temperature value if there are different speed and load states. The optimization requires a temperature range that corresponds to each situation. The engine cooling achieves an approximation of the optimal temperature.
Signals for calculation by the Digital Engine Electronics (DME) are:
- Engine speed
- Load
- Driving speed
- Intake air temperature
- Coolant temperature.
On the basis of the above signals, the Digital Engine Electronics (DME) calculates the optimal coolant temperature for each situation. The coolant temperature is influenced by specific heating of the wax element in the characteristic map thermostat as well as requirement-oriented activation of the electric fan. At full load, low coolant temperatures improves the cylinder filling. Furthermore, a low coolant temperature reduces the risk of the engine knocking. This can be a positive influence on the power output and torque.