Article | REF: BM2582 V1

Automotive transmissions - Role and determination of gear ratios

Authors: Elian BARON, Philippe PESCAROU

Publication date: July 10, 2011, Review date: June 29, 2023

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ABSTRACT

The history of civilizations has shown that gear systems have always been used in order to multiply human or animal efforts. Automotive transmission also follows this rule and so does the transmission system of current heat engines. This article explains the reasons why the gearbox is essential in order to adapt the available power to the requirements of the axle. The driving stresses and the various resistances (aerodynamic, linked to the driving, slopes, etc.) which apply to vehicles modify the driving conditions. Implementing several gear ratios into the gearbox is the answer to this issue.

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AUTHORS

  • Elian BARON: Engineer-Doctor - Powertrain expert, Renault Automobiles

  • Philippe PESCAROU: Engineer-Doctor - Powertrain Expert-Leader, Renault Automobiles

 INTRODUCTION

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,...

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