change time AUDI S4 1998 B5 / 1.G Engine Manual

27
The solenoid valve for charge pressure control
N75 changes the opening time to atmospheric
pressure according to the signals it receives
from the engine control unit (duty cycle).
Thus, a control pressure is produced by
modulating the charge pressure and
atmospheric pressure. This pressure acts on
the pressure unit for the wastegate.
The wastegate is kept closed in a
depressurised state by a spring inside the
pressure unit . The entire exhaust gas flow is
routed via the turbine, and a charge pressure is
built up.
The control pressure counteracts this spring
force and opens the wastegate. Part of the
exhaust gas flow is fed from the wastegate
past the turbine, and the charge pressure stops
rising.
If there is no flow, N75 is closed and the charge
pressure acts directly on the pressure unit. The
waste gate opens even if the charge pressure
is low.
If the charge pressure control fails, the charge
pressure is thus limited to a “basic charge
pressure“ in order to prevent the maximum
permissible charge pressure being exceeded.
This results in a loss of performance.
The “basic charge pressure“ is the charge
pressure (approx. 300 - 400 mbar) which is
achieved without regulation (mechanical
charge pressure).

SSP 198/66

Turbine wheel
to catalytic converter
Impeller
Exhaust gas from
combustion chamber
Wastegate flap
(open)
Control pressure from solenoid
valve for charge pressure control
pressure unit N75charge pressure to solenoid valve
for charge pressure control N75
Intake air
to
combustion
chamber

SSP 198/67

Atmospheric
pressure from
distributor piece
Solenoid valve for
charge pressure
limitation N75
Charge pressure from compressor housing
RestrictorPassage in no
flow state
Control
pressure to
pressure unit