How the automatic gearbox won America first and Europe much later
By 1957, more than 80 percent of new American cars shifted their own gears, yet as late as 1997 only about one European car in ten did. Cheap fuel explains part of the gap, since early automatics wasted more energy. Today Britain, like America long before, buys mostly automatics.
Petrol and diesel engines only work well within a narrow band of rotation speeds, so a gearbox must stretch that band across everything from a crawl to motorway pace; electric motors largely escape this need. The 1904 Sturtevant horseless carriage gearbox is often called the first automatic. It shifted up at high engine speed and back down as the car slowed, but abrupt changes tended to break it. Planetary gearsets, used in the 1901 Wilson-Pilcher and the 1908 Ford Model T, pointed the way forward.
Inventors kept trying. Alfred Horner Munro, a steam engineer from Regina in Canada, patented a compressed-air version in 1923 that proved too weak to sell, and in 1932 José Braz Araripe and Fernando Lehly Lemos, engineers from Brazil, built the first one using hydraulic fluid. Semi-automatic designs from REO and Oldsmobile in the 1930s still needed the clutch pedal to pull away. General Motors' Hydra-Matic, fitted to 1940 Oldsmobiles, finally became the first mass-produced automatic.
Inside a typical modern unit, planetary gears replace the parallel shafts of a manual box, and a torque converter replaces the friction clutch. Pressurised transmission fluid, fed by a pump turning with the engine, applies internal clutches and bands to lock particular gears, while computer-controlled solenoids now decide when to shift. Because that pump stops with the engine, most automatics cannot be push-started.
Efficiency remains the trade-off. Conventional automatics pass along 86 to 94 percent of their input power, and 2022 road tests found manuals 2 to 5 percent thriftier in ordinary driving, or up to 20 percent with an expert. Even so, the global share of new manual cars slipped from 43 percent in 2015 to 37 percent by 2020, helped by hybrids, electric cars and automatic emergency braking.
Source: Automatic transmission
