The flywheel stores kinetic energy and helps the crankshaft to overcome empty strokes and dead centres in case the engine features less than 4 cylinders. In case of an increasing strain it provides for a slower drop in performance of the engine. The dual mass flywheel diminishes, in addition, rotary vibrations between clutch and drive.
The flywheel takes up the starter gear rim and the clutch. In the past it contained notches for the ignition timing. Later also some special material accumulations were added, e.g., for the electric TDC transducer as pulse generator. In the age of the inductive pickup a gear rim from metal sheet might have remained. The measurement of the crankshaft position is more exact here than at the V-belt pulley because the diameter is bigger.
A round, outside heavy metal disc is fastened on one side to the crankshaft and on the other via the clutch with the gearbox (final drive). It absorbs rotary vibrations and helps to overcome dead centres with the piston-stroke engine. For four-stroke engines with less than four cylinders it holds ready enough moment of inertia to help get through the empty strokes. This is the bigger, the further outside the flywheel masses are positioned. Light flywheels are suitable for very sporty engines, heavy ones keep the rev of the engine in the mountain according to the weight to power ratio.
The dual mass flywheel (see picture) is bipartite. One part still influences the vibrations of the crankshaft, the other part helps avoid vibrations in the gearbox and further in the drive train. The two parts are connected by inside-coil springs (see figure) and/or gearwheels with each other. In return, the torsion springs in the clutch disc are perhaps obsolete. Picture 3 displays once more the complete (cut open) dual mass flywheel with additional planet drive.
Vibrations can have different sources. There are rotary vibrations which are ignition-caused, an especially unequal torque distribution for the diesel engine can be another source. Also abrupt, strong changes in the load from the operation of the car can create vibrations. The vibrations are absorbed by flywheel masses assigned to the vibrating parts and decoupling them from the drive train. 05/08
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