Engineers are just wizards with college degrees. No, I take that back, they're powers are stronger than mere magic. I don't care what spells Harry Potter knows; nothing in his wizarding world is as impressive as the inner workings of automatic transmissions. With manual gearboxes, you choose which gear the transmission goes into. You're constantly judging vehicle speed and engine revs in anticipation of your next move, whether that's slowing down to enter your driveway, accelerating down a straight, or heel-toe shifting while navigating a tight hairpin. But automatic transmissions don't have brains or eyes, and yet they also have to figure out which gear you need and pronto.
Computers absolutely make this process easier, and don't worry, we'll discuss how they handle shifts. But it's the analog transmission of yesteryear that is worth exploring first, because like French cooking, you stare at it and wonder how anyone could have come up with such a complex solution.
Inside an automatic transmission, at least an old hydraulically actuated one, is a torque converter, planetary gearsets, the bands and clutches that engage those gearsets, and a series of valves that route hydraulic pressure to activate the bands/clutches. Now, the transmission knows which ratio to select based on vehicle speed information from the governor, accelerator pressure on the throttle valve, and what position the gear lever is in. The gear lever tells the transmission which shift valves it can and can't actuate. Pressure in the system increases as the governor senses the output shaft moving faster, and it will activate a shift valve to engage a clutch pack for a higher gear. If you put the pedal to the metal, more pressure from the throttle valve will negate the governor's input, and the transmission will hold a gear or even kick down.
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That is the ridiculously simplified explanation, but it gives you the gist. For a more concise overview, check out Sabin Civil Engineering's video, "Automatic Transmission, How it works?" on YouTube. Once the car is in drive, your right foot is all the input your slushbox needs to choose the right gear at any given point. The car slows down as your foot comes off the gas, the governor informs the transmission about it by reducing the flow of hydraulic fluid, and the corresponding decrease actuates the appropriate bands/clutches to select a lower gear. 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.
If it sounds odd that hydraulic fluid can act that fast and be responsible for so many processes in your transmission, remember, these devices are the result of really smart engineers using fluid dynamics to do some serious left-brained stuff. Take the torque converter, for instance. While a lockup torque converter has a clutch that will create a direct mechanical connection between the engine and transmission at cruising speed to maximize efficiency, much of the time, it's just spinning goo that transmits power.
A super simple explanation of the torque converter is that it's a donut-shaped case filled with a viscous fluid, and a set of fan blades coming off the engine spin the fluid with enough force that it causes a turbine attached to the transmission's input to rotate. This is why transmissions and torque converters have very specific fluid weights and viscosities. Too thick and the fluid will gum up the works. Too thin, and the turbine won't get any rotational energy.
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Inside the transmission is a series of planetary gear sets. These gearsets contain a central sun gear surrounded by planetary gears set inside a ring gear. Picture the sun with planets revolving around it inside the solar system, and you'll understand how it got its name. Depending on whether the bands/clutches hold the ring gear, the sun gear, or force them to all move together, determines output shaft speed. Stacking planetary gear sets will give you more ratios.
In computer-controlled transmissions, solenoids activate the hydraulically actuated clutches/bands and have replaced much of the complex, interconnected hydraulic systems found in older designs. Instead of a governor and throttle valve giving simple commands, the computer can take inputs from every system, including the brakes, cruise control, stability control, and traction control. BMW was even working on incorporating GPS info to help transmissions prepare for varying conditions, such as hills or turns, back in 2013.
Computers have given us plenty of unnecessary and annoying modern car features, but their biggest automotive contribution is arguably in the relationship between your transmission and the gas pedal. Thanks to throttle-by-wire, we now live in an "I'm sorry, Dave, I'm afraid I can't do that" world. In Jason Cammisa's Know It All series on Hagerty's YouTube channel, he explains that because our automatic transmissions are so integrated with our car's safety systems, we no longer have gas pedals, but rather torque request pedals. We can mash the pedal, and we might get the acceleration we want, but the car is constantly examining all conditions, including speed, pitch, roll, yaw, wheelspin, and steering wheel angle. If the car deems us worthy, it may give us more engine rpm and a downshift, to boot.
