- Jan 19, 2012
Techsplanations: Modern Automatic Transmissions
Automatic transmissions in today's vehicles are very different from the "slushboxes" of years past. In fact, in many new cars and trucks, automatic transmissions actually return higher fuel economy than manuals. While the first automatic transmissions only had two forward gears, and even as late as the 1990's it was common to see three-speed automatics, these transmissions have been entirely supplanted by modern designs. The automatics in today's cars use computers to control when they shift, and they almost always have at least five forward gears, though six- and even eight-speed automatics are becoming the norm.
How does it work?
Inside a conventional automatic transmission, there are what's called "planetary gears," and clutches and bands, a pump and valves, and a whole lot of transmission fluid. If that sounds complicated, it is – the whole operation is actually too complex to fully explain here, but we will describe the basics.
Conventional automatic transmissions use a hydraulic fluid, the transmission fluid, to connect the engine of the car to the driveshaft as well as operate the transmission. It's this fluid that serves to transfer the mechanical energy of the engine's rotating crankshaft through the transmission. This in turn spins the driveshaft, which turns the differential, which turns the axle, which rotates the wheels of the car.
The automatic transmission is capable of changing the gears for you, which is why automatics don't have clutch pedals and you just shift the car into "D" and go. This is accomplished thanks to the torque converter, which is what actually makes the fluid coupling to the engine. It also helps increase power when accelerating, by using the difference in the rotational speed of the fluid in the torque converter versus the speed of the engine's crankshaft to multiply the torque of the engine. At higher speeds, the torque converter will "lock," that is, rotate at the same speed as the engine. This is desirable for efficiency's sake, as one of the biggest ways in which automatic transmissions are inherently less efficient than manual transmissions is because of the torque converter.
Why would I want it?
Besides not having to coordinate the clutch pedal and stick shift, which many people don't want to have to mess with, automatics often return better fuel economy today, which is a real turnabout from years past. To better understand why, we spoke with Torsten Karnahl, the general manager for product strategy at Volkswagen. He told us that there are really two ways that automatic transmissions have so dramatically improved.
The first is rather simple: With more gears in modern automatics, the transmissions are better able to match the engine's torque and power curve with what's needed while driving. Or to put it more simply, extra gears allow an engine to operate more efficiently under the same driving demand. The second way in which automatics have improved is through computer control, which not only affects when the transmissions shift, but how quickly the torque converter locks for greater efficiency. "Computer-controlled modern gearboxes let the machine do the work to decide," Karnahl told us.
He said that when comparing manual transmissions to automatics, drivers of manuals often run the engines at higher revolutions per minute, or rpms, than is ideal. This consumes more fuel, and it's one reason why cars sold in Europe are required to have a shift indicator, he said, which recommends that you shift as fast as possible into the highest gear.
Once again, computers do the job better than humans. Depressing, but true.
"There's a rule of thumb," Karnahl said, "the fewer rpms you use to move the car, the better it is for fuel economy."
Allowing a modern, six-speed automatic transmission to determine that is almost always going to produce better fuel economy than doing it yourself.
Is there any downside?
Automatic transmissions can be less durable and more costly to repair than manual transmissions. Automatics are generally considered less sporty to drive, although some modern automatics have shift-it-yourself capabilities, which can even include steering-wheel-mounted paddle shifters.
What vehicles offer it?
Almost every vehicle on the market that's not a high-powered sports car has an automatic available, and automatics are standard on most cars sold in the U.S. Manufacturers tell us that even on models they offer with manuals, about 95 percent of buyers choose automatic transmissions. That said, some automakers have been adding a "manual" version to some new cars where previously they didn't because of the image it presents. The new Buick Regal, for example, comes in a version that includes a manual shifter.
Being able to operate your car while you drink a coffee is great, especially when you're saving enough on gas to pay for the cup of joe.