Repair manual

Engine Mechanicals
5
The mechanical design and function of AVS on the
4-cylinder TFSI engine closely resembles the 6-cylinder
naturally aspirated engine. However, different
thermodynamic effects are used.
At low engine speeds, a narrow profile cam lobe contour
is used. At high engine speeds the AVS changes to a
wider profile cam lobe contour.
The narrow cam lobe contour provides very late exhaust
valve opening. This effectively prevents back-flow of
exhaust gas during the valve overlap phase due to the
pre-exhaust pulse (at the exhaust valve opening point)
of the cylinder, which is offset at 180° crankshaft angle.
Advanced intake valve timings are therefore possible.
The positive cylinder pressure gradient allows the
combustion chamber to be effectively purged. This
enhances fuel mixture by reducing the residual gas
content in the cylinder, and by facilitating advanced
intake valve timings (because less intake air is expelled
after BDC).
These improvements also result in much better response
and much higher torque at low rpm. Charge pressure can
be built up more quickly, making the torque curve steeper
and minimizing turbo lag.
Design of the Exhaust Camshaft
Cam Elements with
Internal Splines
Exhaust camshaft
with external splines