Engine – Cayenne V8
Page 1.27
an advancing camshaft. If the camshaft then tries to lag due to the high
drive torque, the check valve closes and the oil cannot escape. The
camshaft is driven by the oil cushion of the sprocket during this time, as
with a freewheel. The advancing and lagging phases of the camshafts
repeat so that the camshaft automatically shifts to advanced valve
timing in stages.
As the principle described above only functions with well sealed adjust-
ment control systems and low-friction valve drives, oil pressure is
required. To ensure that an extremely large oil pump is not required, the
principle described above is taken advantage of when the engine is hot
and at a low oil pressure through the use of the check valve. The check
valve serves to increase the adjustment speed at low oil pressures.
Valves, Valve Springs
The intake and exhaust valves on the Cayenne S and Cayenne Turbo
have a shaft diameter of 6 mm. The intake and exhaust valves are bi-
metallic, i.e. the materials used for the valve plate and the lower part of
the valve stem are different to those used for the upper part of the valve
stem. In addition, the exhaust valves on the Cayenne Turbo are filled
with sodium.
The intake valve springs on the Cayenne S and Cayenne Turbo are
identical. They are designed as a conical double valve spring set. This
gives a very compact design. The exhaust valve springs on the Cayenne
S are conical single valve springs. The Cayenne Turbo features cylin-
drical double-valve spring sets to ensure that the exhaust valves close,
even at higher pressures in the exhaust system.
Vacuum Pump
Increased engine dethrottling means that the vacuum supply is no longer
sufficient for unfavorable underlying conditions, e.g. low external air
pressure at high altitudes and highly dynamic driving. A mechanical
single-vane pump driven by the camshaft is used for this reason.
The pump delivery rate is 260cm/revolution.
1 - Intake opening
2 - Rotor
3 - Secondary load connection
4 - Outlet valve in crank chamber
5 - Vane with guide shoes
6 - Housing
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