Attitude Shift: What You Should Know About Transmissions

In a never-ending quest to maximize both performance and fuel efficiency, automakers have chased innovation from many angles. Usually, efficiency gains are made at the expense of performance, but there is one innovation that placates both the go-faster and the frugal camps.

With a traditional standard gearbox (above), the clutch is engaged and disengaged while another gear (in blue) is selected. During this action, the flow of power from the engine (in green) is interrupted, causing power delivery to drop off faster than a roller coaster at Canada’s Wonderland. Professional race drivers can shorten this power gap to a fraction of a second, but dual-clutch transmissions (DCTs) accomplish a gear change even more quickly.

DCTs (above) employ two clutches—one for the even-numbered gears and the other for odd-numbered gears. The gears and clutches are linked via shafts (in red) inside the transmission. While one gear is engaged, delivering power from the engine, the next gear is pre-selected and held by the second clutch. At the driver’s signal, or at a computer-programmed shift point, the first gear is released and the next gear is engaged with hardly any delay. Think of a runner handing off a baton in a relay race.

Shifting gears in a DCT takes anywhere from eight to 200 milliseconds compared to the approximate half-second it takes the average driver to shift gears in a traditional manual transmission. This makes a vehicle accelerate more quickly, while the driveline efficiencies contribute to better fuel economy.

Want to delve deeper into the world of transmissions? Here’s an explanation of continuously variable transmissions.