Automotive transmissions - Role and determination of gear ratios

For over a thousand years, gear-driven systems have been used to multiply human and animal effort. Some of these ancestral systems are still in use today, for example in Egypt, where oases are equipped with "spinner wheels" to collect water from the spring. These are powered by a camel, using perpendicular gears with wooden teeth to increase speed and act as angle gears.

Other testimonies and contributions to gear systems have been made since then, whether theoretical, practical or even artistic: Dürer, Vinci, La Hire, Euler, Willis, Reuleaux...

On this basis, the first practical application of a transmission is linked to the development of the steam engine and the need to adapt the available engine power to the required use. The first steam-powered vehicles were equipped with ratchet wheels, capable of transforming the reciprocating motion of pistons into rotary motion of the wheel axle. But the need to negotiate slopes soon eliminated this type of system. In 1784, James Watt patented a two-speed constant-mesh gearbox with two dog clutches, a concept still in use today. The multi-speed transmission was born.

The production of automobiles, first steam-powered, then petrol-powered, was accompanied by a number of patents that brought transmission closer to the definition it has today in its manual form (Griffith, Bodmer, Pecqueur, Selden, Anderson, Marcus, Benz, Renault...).

If we were to take a very global view of the historical development of automotive transmissions, we could distinguish four main periods:

• 1780-1880: recognition of the need for a transmission to adapt engine power to the constraints of different driving configurations. The most common solutions were two-speed transmissions with constant mesh and clutches, or sliding gears;

• 1880-1920: search for the best solutions to improve the match between engine power and wheel power requirements. This led to the development of numerous solutions, including gear, chain, belt, friction, pneumatic, hydraulic and even electric transmissions;

• 1920-1980: widespread use of geared transmissions, renowned for their high power-to-weight ratio and efficiency. These transmissions benefited from constant progress in reliability, acoustics and driving pleasure;

• since 1980: the performance of the powertrain (engine and transmission) has become a major preoccupation for automakers, with reduced fuel consumption a priority. The number of transmission ratios increased, as did their opening. In this context, automated manual gearboxes made their appearance. Automatic converter transmissions also saw an increase in the number of gears,...