49
Glossary
This glossary explains all terms indicated in italics or marked with
an asterisk (*) in this Self-Study Programme.
Blow-by gases
Blow-by gases are also known as "leakage gases". When the engine
running, blow-by gases flow from the combustion chamber into
the crankcase, bypassing the piston. They are produced by the high
pressures inside the combustion chamber and the absolutely nor-
mal leakage that occurs around the piston rings. Blow-by gases are
extracted from the crankcase by a PCV system and admitted into
the combustion chamber.
Event duration
Besides the valve opening cross-section (valve lift), the shape of
the cam on a camshaft determines the opening duration of the
valve. The valve opening duration (event duration) is given in
° crank angle.
Downsizing
Increased efficiency through synergy effects. This means that less
material and equipment is needed to achieve the same perform-
ance level.
Box piston
Unlike pistons with round skirts, box pistons (or window pistons)
are recessed in the area of the skirt which is towards the gudgeon
pin. This gives the piston skirt higher rigidity and allows the use of
a shorter gudgeon pin.
Wastegate
To control the charge pressure in a turbocharger, a wastegate is
integrated in the exhaust gas flow. If the charge pressure rises
above a certain limit, an actuator opens the wastegate. The
exhaust gases bypass the turbine and are admitted directly into
the exhaust system, thus preventing the turbine speed from rising
any further.
PVD process
PVD = Physical Vapour Deposition is a coating process in which the
coating material is physically vapour deposited (by kinetic or
impact energy) on a material in a vacuum environment. The advan-
tage of this process over the conventional electroplating process is
the greater diversity of coating materials (sputter bearings).
PCV
This abbreviation stands for "Positive Crankcase Ventilation".
In this system, fresh air is mixed with the blow-by gases inside the
crankshaft chamber. The fuel and water vapours in the blow-by
gases are absorbed by the fresh air and discharged via the crank-
case ventilation system.
Annex
DNA
This abbreviation stands for "deoxyribonucleic acid". Deoxyribonu-
cleic acid is a biomolecule which occurs in all living organisms and
is the carrier of genetic information. It is used in everyday speech
to explain the origin or basis of a concept.
DFS ring
Top-bevelled oil control ring with coil expander
Polyswirl
TM
The Polytec Group has developed a passive separation system
which considerably reduces the amount of residual oil in the
blow-by gases despite its very compact design. Both Audi and Lam-
borghini currently fit the so-called Polyswirl in their V10 and V8
rocker covers.
Viscous damper
To reduce the radial vibration caused by the impulse-like transmis-
sion of force from the piston to the crankshaft via the gudgeon pin
and the connecting rod, torque is briefly increased to peak levels.
The result: noise and wear. Torsional vibration dampers attenuate
this radial vibration. High-viscosity silicones and oils – so-called vis-
codampers – are used for this purpose.
Nitriding or nitrocarburizing
Surface treatments improve the running-in behaviour of the pis-
ton rings and minimise wear on the piston lining and flanks.
