968 Cabriolet L4-2990cc 3.0L DOHC (1993)
Timing Belt Tensioner: Description and Operation
Hydraulic Toothed Belt Tensioner
DESIGN AND OPERATION
Tensioning force is produced by pressure spring (1) and tensioner piston (2). When the belt tension decreases, return valve (3) opens and the oil is free
to flow from the reservoir chamber (4) into the pressure chamber (5) until an equilibrium of forces between tensioning lever and tensioner piston is
reestablished.
If the belt tension increases e.g. due to heat expansion of the engine housing, the oil must be forced from the pressure chamber into the reservoir
chamber across drain gap (6) between piston (7) and the housing since the return valve does not permit flow here. This throttling action ensures
gradual reduction of the tensioning force.
The engine-specific vibrations of the belt drive are detected by the hydraulic tensioning system. As a result, the belt tensioner controls the setting to
increase the force, thus reducing movement considerably. The main benefit of the hydraulic tensioner becomes evident when the engine has warmed
up since the tensioning force remains virtually constant independent of the engine temperature.
Since a high basic preload of the toothed belt is not required, bearing and belt loads as well the resultant running noise are reduced accordingly.
