No Shifting Required: Types of Automatic Transmissions
Automatic transmissions have been widely available in automobiles since the introduction of the Oldsmobile Hydra-Matic automatic transmission in the early 1940s. Before this, most vehicles used a manual transmission to change gears. The Hydra-Matic transmission was advertised as “Motoring’s Magic Carpet,” an alternative to the cumbersome task of manually shifting gears. After its initial development, these transmissions would be used in an array of military vehicles, including tanks, in WW2. Following the war, most manufacturers introduced into their line up some sort of automatic transmission to avoid being left in the dust.
While hydraulic automatic transmissions have been improved since the introduction of the Hydra-Matic, they have been left mostly unchanged in automobiles. However, there are two other main types of automatic transmissions, which have recently become more widespread: the continuously variable and dual-clutch transmissions. But why do we need all these different types of transmissions? Aren’t they all doing the same thing? Let’s cruise.
Hydraulic (Conventional) Automatic Transmission
The current hydraulic automatic transmission remains similar to the Hydra-Matic transmission of old. It uses a torque converter to connect the engine to the transmission ala transmission fluid. This torque converter allows the wheels to be stopped and the engine to be turning at the same time without disengaging a clutch like it would in a manual transmission.
Conventional transmissions use a series of planetary gears to generate different gear ratios, which drive the wheels at different speeds. Depending on the selected gear, sometimes the “sun” gear is driven, sometimes the “ring” gear, and sometimes both. These selections generate the resulting ratio, which is transmitted to the planet carrier via the planet (also known as pinion) gears. The planet carrier for the first planetary gear set is the input and drives the ring gear of the second planetary gear set. Depending on the transmission, there may be more planetary gear sets attached in the same way. The final planetary gear set is connected to the driveshaft, which eventually turns the wheels.
The transmission determines what gear to select based on factors such as speed, engine load, and throttle position. Many modern transmissions are computer controlled and may have selectable modes, including “sport” or “economy,” which will affect how the transmission behaves. They may also have mechanisms (such as paddles by the steering wheel) to allow the driver to manually select which gear to use if they desire.
For years, automatic transmissions were not able to achieve the gas mileage numbers that their manual counterparts could. More recently, automatics have been able to do just as well or even better in gas mileage. This can be attributed to up to 10 forward gears in the design, up from three or four gears. These added ratios allow the engine to stay at its peak efficiency range for a longer amount of time. Ford has partnered with GM to invest in the automatic transmission with ten ratios which can be seen in the 2017 Ford F-150 and 2017 Chevrolet Camaro ZL1. Many other manufacturers are also continuing with automatic transmissions including Mazda, Dodge, and Rolls Royce (the Ram Pickup and Rolls Royce Phantom sharing the same transmission, interestingly enough).
Continuously Variable Transmission
Where automatic transmissions have a limited number of gear ratios to use, continuously variable transmissions (CVTs) have (theoretically) infinite gear ratios. This results in a smoother driving experience as there are no gears to change.
In a CVT, the gears are replaced with a pair of pulleys. These pulleys are conical or “V” shaped. The transmission can increase or decrease the width of the pulleys which adjusts the current drive ratio by moving the chain along a narrower or wider part of the cone. A low ratio is used for low speeds and acceleration, where a high ratio is used for fuel economy.
CVTs have become more common in cars in the last decade, with manufacturers like Nissan and Subaru using them almost exclusively in place of other automatic transmissions, even on the new Subaru WRX (which is marketed as a sporty sedan). Many other manufacturers have just a few models with CVT, such as the Honda Civic and Toyota Corolla. CVTs have been common in ATVs, snowmobiles, and lawnmowers for much longer. They tend to get better gas mileage than their automatic counterparts but are not viable for high horsepower applications (at this time).
An interesting thing about CVTs is that they seem to confuse drivers who notice the vehicle doesn’t shift gears when accelerating quickly. When doing so, the CVT keeps the engine at a continuously high speed (RPM) while the car increases speed, instead of increasing speed towards redline and decreasing as the gear changes (like transmissions have done for a hundred years). That being said, CVTs have the potential to be the fastest accelerating vehicles on the road (once they can handle more power efficiently), and to that point have been banned from F1 racing due to said efficiency.
Dual Clutch Transmissions
Manual transmissions are generally lighter and more efficient than automatic transmissions, but automatics are easier to drive and easier to sell. Porque no los dos? Enter the dual clutch transmission, basically a computer controlled manual transmission, but with two clutches instead of one. One clutch handles the odd gears and the other controls the even gears. For example, when the first gear is engaged, the second gear is readied and the only thing to do is switch engine power from the odd clutch to the even clutch, resulting in lightning quick shifts.
There are many manufacturers who use dual clutch automatics such as Audi, Porsche, and Ford, but Volkswagen has perfected the dual clutch transmission that is available in vehicles such as the Volkswagen Golf GTI. It allows for much of the fun of driving a manual transmission; e.g. selecting gears, downshifting while braking, and holding gears through corners, while still offering the ability to operate just as a regular automatic transmission for less involved driving. As an enthusiast, if I had to have a vehicle with an automatic transmission, it’d be a dual clutch.
You’ll have to try out these transmission types to determine which feels right for you. Many automakers strive to make the transmission unnoticeable to the driver, so in most cases, it will probably be the transmission you notice the least. If you are considering a manual transmission, be sure to check out a staffer comparison of manual vs. automatic, and then shift your gaze to Carsforsale.com to find the right transmission to test drive today!
Participation Time: Does the type of automatic transmission affect whether or not you’d buy a particular model?