by Chris Chase
The transmission is an integral part of any modern car. It provides low gear ratios that maximize power in acceleration and high gears that allow for efficient highway cruising. And if you drive a vehicle with an automatic transmission, you know that the car decides which ratio is best for any situation.
Automakers use three types of automatic transmissions in modern cars. The oldest and most common design is the planetary transmission, but many of today’s cars and crossovers use continuously variable and dual-clutch automatics. All three types do the job of transmitting the engine’s power to the car’s wheels, but use different technological tricks to do so.
Here’s an overview of how all three versions operate.
How does a planetary automatic transmission work?
The first place your engine’s power goes is into a large donut-shaped device called a torque converter, which takes the place of a manual transmission’s clutch. It’s filled with transmission fluid and contains two parts that look a bit like the blades in a plane’s jet engine. One of those is called the impeller, or pump, which is attached to the car’s engine. The other piece is the turbine, and it’s connected to the transmission.
Initially, there’s no contact between the impeller and turbine. Instead, the engine turns the impeller, and the impeller agitates the transmission fluid, which, in turn, spins the turbine to power the transmission and the wheels.
At cruising speeds, the impeller and turbine lock together, eliminating slip between the engine and transmission to improve fuel economy. As your car slows down, the torque converter unlocks so that the engine doesn’t stall.
The torque converter has another important job: It generates hydraulic pressure that keeps the transmission lubricated and allows it to smoothly shift through its gear ratios.
Inside the transmission itself are a number of planetary gear sets that provide the transmission’s ratios. Each consists of a sun gear, planet gears that mesh with and revolve around the sun gear, and a ring gear around the whole assembly that meshes with the planet gears. Clutches around the ring gear can lock the sun, planet and ring gears together or let them turn independently to provide different gear ratios.
The more planetary gear sets are in the transmission, the more gear ratios it can provide. A computer and electronic sensors open and close valves to direct hydraulic pressure to the clutches that engage the right ratio for any driving scenario.
Here’s an animation that shows how the power flows through a planetary automatic transmission.
How does a continuously variable transmission work?
Continuously variable transmissions (CVTs) have become common in many mainstream cars and crossovers. A CVT promises smooth performance and improved fuel economy because it can provide an almost infinite range of ratios between the low and high ends, rather than being limited to a fixed number of gears.
Like a planetary transmission, a CVT is typically connected to the engine by a torque converter. But beyond that, a CVT’s inner workings are very different, swapping out gears for a pair of pulleys and a metal belt.
One pulley takes power from the engine to drive the belt, and the second pulley takes power from the belt to turn the car’s wheels. The belt fits into a groove between the two halves of each pulley. Those two halves can move farther apart or squeeze close together, which changes the pulley’s diameter. Here’s a simple animation of how a CVT works. (https://www.youtube.com/watch?v=QqTWY0lbT1A)
In the CVT’s lowest ratio, the two parts of the drive pulley are far apart, making its diameter smaller. Meanwhile the two halves of the output pulley (the one connected to the car’s wheels) are close together so that its diameter is larger. In this setting, the engine will be turning quickly but the car won’t be moving very fast. Think of this like the gear you’d use to ride a bike up a steep hill.
As the car accelerates, the two parts of the drive pulley squeeze together to create a larger diameter, while the output pulley does the opposite. This lets the car move at highway speeds while the engine turns at a relatively relaxed pace. On a bicycle, this is the gear you would use for flat or downhill stretches of riding.
If you press on the gas pedal at cruising speed, the CVT’s pulleys will adjust to increase the engine’s RPM so the car can accelerate more easily.
How does a dual-clutch transmission work?
Dual-clutch transmissions, or DCTs, are mostly used in sporty vehicles. Porsche uses its PDK (an acronym for the German translation of dual-clutch transmission) in the 911, Cayman and Boxster sports cars, the Panamera sedan and the Macan SUV. Volkswagen offers a dual-clutch transmission in the GTI, and you can get one from Hyundai in sporty versions of the Kona crossover and Elantra compact car. Ford also offered a DCT in its compact Focus and Fiesta models in the 2010s.
Inside a DCT are gears arranged on parallel shafts connected to the car’s engine and its wheels, respectively. Engaging different pairs of gears on the two shafts creates the ratios that allow the car to accelerate from a stop to cruising speeds.
As its name suggests, a DCT has two clutch packs made up of multiple discs of grippy friction material. If you cut one in half, it would look a bit like a mechanical version of a round mille-feuille dessert with a hole in the middle.
Both clutches are driven by the engine, but at the other end, one is connected to the transmission’s odd-numbered gears, while the other directs power to the even-numbered ratios.
When you shift a DCT into drive, the transmission actually selects both first and second gears at the same time, but only engages the first-gear clutch to get the car moving. When it’s time for second gear, that clutch engages, while the clutch that engaged first gear disconnects and the transmission pre-selects third gear to get ready for the next shift. This process continues as the car accelerates.
In everyday driving, a DCT’s main appeal is its ability to shift extremely smoothly. Enthusiastic drivers like DCTs because the shifts are also very quick and well-suited to racetrack driving. Click here to see an animation of how a dual-clutch transmission works.