65
N55 Engine
Function of the twin scroll turbocharger
The system is designed so that constant exhaust gas pressure is rearly applied to the tur-
bocharger. At low engine speeds, the exhaust reaches the turbine in tuned pulsed form.
Due to this pulsation, a higher pressure ratio is temporarily reached in the turbine. Since
the efficiency increases as the pressure rises, the pulsation improves the boost pressure
progression and thus the torque progression of the engine. This is the case particularly at
low engine speeds.
To limit the back pressure and ensure that the individual cylinders do not mutually
influence each other during the cylinder charging process, cylinders 1 - 3 (bank 1) and
cylinders 4 - 6 (bank 2) are combined to form two exhaust channels. The exhaust gas
pulses in the exhaust channels (1 and 2) are directed into two scrolls (spirals) within the
turbocharger to drive the turbine wheel. This design layout makes it possible to optimally
use the exhaust pulsations for generating boost pressure based on the firing order of the
engine. This improves engine efficiency by enhancing throttle response and limiting
unwanted turbo lag.
The wastegate valve is used for the purpose of limiting the boost pressure and is already
known from previous BMW turbo engines. It is vacuum operated and electronically con-
trolled through a vacuum control solenoid by the DME (ECM).
Diverter valve
The basic function of the diverter valve remains the same. The difference compared to
the N54 engine is that the diverter valve is not operated pneumatically. The diverter valve
on the N55 engine is an electric actuator that is controlled directly by the DME.
The number of components has been greatly reduced by positioning the diverter valve on
the turbocharger compressor housing.
The diverter valve is designed to release unwanted pressure in the intake by connecting
the pressure side of the induction system to the inlet side under deceleration. The unde-
sirable peaks in the boost pressure that can occur when the throttle valve is quickly
closed are reduced. This means the diverter valve plays an important role in terms of the
engine acoustics while protecting the components of the turbocharger.
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