BMW N55, Manual, Page: 91 / 100

91
N55 Engine
A diverter valve is installed on the compressor housing of the turbocharger. It connects
the pressure side to the inlet side of the induction system and is controlled directly by the
engine management. The diverter valve eliminates undesirable peaks in the boost pres-
sure that can occur when the throttle valve is quickly closed. Therefore it has a decisive
influence on the engine acoustics while protecting the turbocharger and its related com-
ponents.
A pressure wave is built up from the throttle valve to the turbocharger compressor wheel
when the throttle valve is closed. This pressure wave acts against the throttle plate and
the compressor blades pressing them against the bearings. The diverter valve reduces
this pressure wave and thus the load on these components by “diverting” air pressure
from the pressure side to the suction side of the compressor housing. This also maintains
the turbocharger spooled (up to speed) for the next acceleration and reduces turbo lag.
Engine cooling
The engine cooling system utilizes an electric coolant pump. The heat management
determines the current cooling requirement and controls the cooling system accordingly.
Under certain circumstances, the coolant pump can be completely switched off, e.g. to
rapidly heat up the coolant during the warm-up phase. The coolant pump continues to
operate when the hot engine is shut down. The coolant capacity can therefore be varied
regardless of the engine speed. In addition to the characteristic map thermostat, the heat
management makes it possible to use various characteristic maps for controlling the
coolant pump. In this way the engine control unit can adapt the engine temperature to
the driving conditions.
The engine control unit regulates the following temperature ranges:
• 108°C/226°F = Economy mode
• 104°C/219°F = Normal mode
• 95°C/203°F = High mode
• 90°C/194°F = High mode and control with characteristic map thermostat
The engine management sets a higher temperature (108°C) when, based on vehicle
operation, the engine control unit detects ”Economy” mode. The engine is operated with
relatively low fuel requirements in this temperature range. The internal engine friction is
reduced at higher temperatures. The increase in temperature therefore results in low fuel
consumption in the low load range. The driver wishes to utilize the optimum power devel-
oped by the engine in “High and control with characteristic map thermostat” mode. For
this purpose, the temperature in the cylinder head is reduced to 90°C. This temperature
reduction promotes improved volumetric efficiency, thus resulting in an increased engine
torque. Adapted to the relevant driving situation, the engine control unit can now regulate
a defined operating range. In this way it is possible to influence the fuel consumption and
power output through the cooling system.