Automatic gearbox

In European automobile engineering, manual gear-changing is being superseded by automatic gear-changing in the larger cars. American private cars already nearly all have automatic transmission systems.

The automatic gearbox has no gear lever for selecting the various speeds, or ratios. Speed control is entirely effected by the position of the accelerator pedal (gas pedal). Automatic gearboxes of this kind embody a combination of hydraulic and mechanical transmission components.
The hydraulic part comprises fluid couplings and torque converters; the mechanical part comprises planetary (or epicyclic) gears.

The working principle of a fluid coupling can most conveniently be explained with reference to the behaviour of two fans placed together: one of the fans is driven and produces a current of air which sets the blades of the other fan, which is not driven, in motion.

The transmission of the rotary motion in this case is not effected by friction (as in the friction clutch), but through the agency of a medium. In the example of the fans the medium is air; oil is used in a fluid coupling.

A coupling of this kind comprises two rotating parts fitted with vanes, one of which is the driving member (impeller) and the other is the driven member (turbine). The impeller itself is driven by the engine. The oil with which the coupling is filled is flung outward and, since it cannot escape, it is forced in between the vanes of the turbine, where it is deflected and flung back into the impeller.

The circulating oil drives the turbine round at an increasing speed until it is rotating at the same speed as the impeller.

A fluid coupling thus provides a smooth ‘take-up’, so that the vehicle moves off entirely without any jerking. This transmits the same torque as that developed by the engine, except for minor losses due to so-called ‘slip’, which is due to the fact that the turbine is not mechanically locked to the impeller but runs with a certain amount of lag in relation to it. At low speeds this loss is relatively far greater than at high speeds.

Other important components of automatic gearboxes are the planetary gears. They comprise a centrally mounted sun wheel and one or more planet wheels which engage with it and which furthermore engage with an external toothed ring (annulus).