Audi 4.2-litre V8 FSI, Service Training

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Function of fine oil separator
Blow-by gas volume is dependent on engine load
and RPM. The fine oil ("oil spray") is separated by
means of a cyclone separator. Cyclone separators
only have a high separation efficiency over a small
volumetric range. For this reason, one, two or three
cyclones of the fine oil separator operate in parallel
depending on the gas flow rate.
The cyclones are released by the control piston.
The displacement of the control piston against its
spring force is dependent on the gas flow rate.
Piston ring flutter at very high engine RPM and low
engine load can result in a very high gas flow rate.
The crankcase internal pressure is set by the two-
stage pressure control valve.
The bypass valve together with the control piston
ensures that the cyclones operate at the optimum
operating point (if the volumetric flow rate is too
high or too low, this will impair the functioning of
the cyclones).
When the bypass valve opens, a fraction of the
blow-by gas flows to the engine untreated, but the
remainder is optimally treated by the cyclones.
The separated oil is collected in an oil reservoir
beneath the cyclones. The oil cannot drain out of
the reservoir until the oil drain valve is opened.
The oil drain valve is closed as long as the pressure
in the crankcase, i.e. below the valves, is higher
than in the oil separator. The valve opens auto-
matically due to gravity only at very low engine RPM
or when the engine is at a standstill, because the
pressure conditions above and below the valve are
in equilibrium.
The crankcase ventilation system also includes the
crankcase breather. Air is extracted downstream of
the air filter and flows through a non-return valve
into the crankcase from above.
The non-return valve is located at the end of the
vent line and is bolted between the two cylinder
banks in the engine block.
A damping chamber is located below the non-return
valve in the engine block. This prevents non-return
valve flutter and eliminates noise.
A restrictor bore connects this chamber to the
inner chamber of the crankcase. It has the task of
supplying only a defined volume of fresh air to the
crankcase.
Control piston
Oil reservoir
Oil drain valve
Triple cyclones