The Hidden Brain in Your Car: Automatic Transmissions Decide Your Gears in Real Time
Your car feels like a trusted partner, yet inside it hides a brain you can’t see. Automatic transmissions decide when to shift in real time, and they do it with a mix of hydraulics and, increasingly, computers. Inside the older hydraulically actuated designs sits a torque converter, planetary gear sets, the bands and clutches that engage those gear sets, and a tangle of valves that route hydraulic pressure to activate them. The choice of gear depends on three inputs: vehicle speed information from the governor, the accelerator pressure on the throttle valve, and the position of the gear lever. As the output shaft speeds up, hydraulic pressure rises and a shift valve engages a clutch pack for a higher gear. If you push the accelerator to the floor, more throttle pressure can override the governor, and the transmission will hold a gear or downshift. Computers later made the process easier. Computer-controlled transmissions can take input from brakes, cruise control, stability control, and traction control—and engineers even explored using GPS data to help anticipate hills or turns."
In This Article:
Inside the Guts: The Hydraulic Heart of the Automatic
Inside an automatic, at least the older hydraulically actuated ones, the core parts are a torque converter, planetary gear sets, the bands and clutches that engage those gear sets, and a network of valves that route hydraulic pressure to activate the bands and clutches. The transmission knows which ratio to select based on vehicle speed information from the governor, accelerator pressure at the throttle valve, and the gear lever position. As the output shaft speeds up, hydraulic pressure increases, and a shift valve engages a clutch pack to move to a higher gear. If you floor the pedal, more pressure from the throttle valve can override the governor, and the transmission will hold a gear or downshift. Early automatic transmissions, such as the Sturtevant, shifted gears solely based on engine rpm, but in the more advanced hydraulic transmissions, it’s about pressure."
Planetary Gears: The Solar System Inside Your Transmission
Inside the transmission is a series of planetary gear sets. Each set has a central sun gear surrounded by planetary gears inside a ring gear. Which gears are held or moved depends on which bands and clutches are engaging, and this arrangement determines the output shaft speed. Stacking several planetary gear sets yields more ratios. These gear sets are the reason a single transmission can produce multiple speeds without a huge gear train. Computer-controlled transmissions replace much of the old hydraulic interconnection with solenoids that activate clutches and bands, allowing the computer to coordinate shifts with many other systems."
From Hydraulics to Computers: The Digital Shift Revolution
Solenoids in modern transmissions replace much of the complex, interconnected hydraulic system found in older designs, enabling the computer to control the clutches and bands. The computer can take inputs from brakes, cruise control, stability control, traction control, and more to determine when and how to shift. BMW even worked on incorporating GPS information to help transmissions prepare for hills or turns, back in 2013. Thanks to throttle-by-wire, we now live in a world where the gas pedal is a torque request pedal. The car constantly examines speed, pitch, roll, yaw, wheelspin, and steering angle to decide whether to give more engine rpm and downshift."
Torque Request Pedals and the Car as Co-Pilot
The modern drivetrain is a collaboration between human and machine. Even when we mash the gas, the car may respond by limiting acceleration or shifting to optimize safety and efficiency. Because transmissions are so integrated with safety systems, the traditional gas pedal is now a torque request pedal—the car decides when to provide acceleration, based on a wide range of data. The result is a more connected driving experience—driven by physics, hydraulics, and microprocessors behind the scenes. Behind every smooth shift lies a long chain of engineering, from hydraulic theory to computer software."
