How Stuff Works: Fluid flywheels explained with Fuzz

Fuzz Townshend explains how fluid’s able to replace friction between engine and gearbox

Even at the dawn of the motor car the question of easy gear changing preoccupied the minds of engineers and manufacturers.

For much of the first three decades of motoring, the selection of gears lay somewhere in between an art and a skill, meaning that the ‘charming’ sound of grinding mis-engaged gears was a regular feature of the sonic tapestry of early 20th century streets. Not only did this grate on sensitive ears, it did the same to precision-cut gears.

Synchronisation of existing gearbox type’s internal whirring components was one solution, ending in the full-synchro’ gearboxes we know and love to this day. But backroom boffins had their eye on a much greater prize; the clutchless gearchange.

The answer lay in epicyclic gear trains, which offered a means of permanent pre-engagement of all gear ratios, with their actuation by use of friction brake bands. This brought the promise of crunchfree, smooth gear selection, but not without the further question of easy and smooth engagement from standstill and between ratios.

This latter problem could have been overcome by continued use of a dry-plate friction clutch, but then the added expense of an epicyclic gearbox would have rendered it an interesting engineering white elephant.

The epicyclic gearbox, having brought about the possibility of clutchless gear selection and changing, required a further link in the drive train and in practise there were two options.

The first was a centrifugal clutch. These assemblies utilised bobweights to operate a friction clutch.

At engine idle, the bob-weight operated clutch was disengaged from the engine flywheel but, on acceleration, the weights engaged the clutch and so drive was obtained. On releasing the throttle, the weights disengaged the clutch and smooth ratio changes were possible.

The second, much more common and smoother method was called the fluid flywheel.

Fluid flywheels featured three main components, filled with a moderately light oil. The first component was a dished steel flywheel attached to the rear of the engine crankshaft, with a gasket and sealing washers preventing flywheel oil from leaking out from the mounting point.

’The fluid flywheel got rid of a solid drive medium engine and gearbox’

The second and third components comprised of an assembly, commonly known as a ‘flywheel bun’. This consisted of an internally vaned aluminium impeller and similar turbine, the former being bolted to the rim of the steel flywheel.

The turbine was supported in the centre of the steel flywheel on a roller bearing and was bolted to an output shaft and flange, the shaft of which passed through the centre of the impeller with oil being retained by a seal known as a flywheel gland.

The output flange of the flywheel was attached to the input flange of the epicyclic gearbox, if the latter was remotely mounted. Some vehicles featured the gearbox closecoupled to the engine, in which case the flywheel was often described as a charged coupling.

With the engine running, the impeller, bolted to the steel flywheel, was directly turned. The oil contained within the fluid flywheel was rotated by the action of the impeller vanes and forced into a shear action by the proximity of the turbine’s vanes.

This shear action allowed an amount of ‘slip’ between drive and driven and therefore the cushioning needed to be able to engage gears from standstill and to change between ratios with the vehicle moving.

The fluid flywheel thus eliminated a solid drive medium between the engine and gearbox.