Many industry analysts predicted that the introduction of the automatic transmission, more than 50 years ago, would mark the beginning of the end for the manual transmission. After all, who would want to do all the work of shifting gears if a hydraulically powered automatic transmission could do it for you?
Well, it turned out that those early automatics ate up considerable amounts of power and a small, but important, segment of drivers still insisted on a manual transmission for better fuel economy, higher levels of performance, plus greater driver involvement. But engineers kept working on the automatic transmission, refining its performance and improving its efficiency.
During the 1980s and '90s, the marriage of computer control to the automatic's hydraulics resulted in significant improvements. Suddenly, the automatic was a valid choice, even for cars with extremely high performance or drivers looking for good fuel economy.
Just What Does a Manual Do?
Manual transmissions are pretty simple - their gears are located along parallel shafts inside the transmission housing. Power flows when gears are meshed. During gear changes, or when the car is stationary and the engine is idling, a clutch is used to interrupt the flow of power from the engine to the transmission.
When it is time to shift, the driver slides one gear forward or back until it engages a different gear on the other shaft. Gear speeds can be synchronized to aid in their engagement. But for an automatic transmission, a whole new type of gear system was required.
How Does an Automatic Work?
Planetary Gears
The heart of an automatic transmission is its planetary gear set. A planetary gear set consists of a sun gear on a shaft in the center, an outer ring gear with internally facing gear teeth, and planet gears that engage between the sun gear and the ring gear. Each of these three elements can act as an input or an output gear, or may be held stationary, allowing the other gears to move around it.
Because the three gear elements are all spin around the same axis, friction bands can be used to limit the motion of one of the elements while allowing the other two to transmit torque. In addition, the engagement of different elements can take place without interrupting power flow. It sounds complicated, but over the years it has been perfected to work smoothly and seamlessly.
Instead of a Clutch
The function of a clutch in an automatic transmission is handled by a torque converter. This device consists of an impeller driven by the engine and a turbine connected to the transmission, facing each other and separated by a fraction of an inch. (Think of two pinwheels placed face to face.) Between these two parts is a light oil automatic transmission fluid (ATF). While the engine is idling, the impeller turns slowly, and the force created by the motion of the ATF is not enough to move the car.
As engine speed is increased, however, greater force is transmitted through the ATF to the turbine, and power flows into the transmission. The faster the engine revs, the faster the impeller spins, imparting more force to the turbine. Depending upon the configuration of the impeller and turbine, the torque converter can also multiply the torque from the engine.
Built-in Inefficiencies
Hydraulically-operated automatic transmissions became quite sophisticated in their ability to choose and shift gears. But there were two problems; the torque converter used up a lot of energy as the ATF slipped past the turbine, and most early automatics only had two or three forward gears. All of this wasn't too critical when fuel was cheap and most American cars had large-displacement V8 engines, but it became a real problem in 1973 during the first fuel crisis. For a time, the automatic transmission seemed doomed, but several innovations overcame these problems.
First, a clutch was added inside the torque converter to lock the impeller to the turbine when the car reached a certain speed. By locking the torque converter at higher speeds, no energy was wasted in torque converter slippage.
To solve the problem of limited gear ratios, additional planetary gear sets were fitted in a line, so that the output from one became the input for the next, multiplying the number of gear ratios available. Instead of two or three forward speeds, typical automatics now had four, with the top gear being an overdrive for better highway fuel economy. In addition, the hydraulic/mechanical automatics gained electronic controls to more precisely select the right gear at the right time, and to further smooth out the shifting. These transmission computers also controlled the lockup clutches in the torque converter, allowing this action to take place even at very low speeds. All of this greatly increased the efficiency of the automatic transmission.
End of the Manual?
It's a good thing that automatic transmissions have gotten so good, because the traditional manual transmission has started to fade into history, mainly as a result of government regulations. More stringent standards for fuel economy and exhaust emissions threaten the practice of shifting for yourself. Engineers charged with meeting these standards would much rather program a transmission control computer for every conceivable driving need than let an unpredictable driver make those choices. Fortunately, there are now new technologies that are giving shifting for yourself a reprieve...albeit in a much different form.
Manually Controlled Automatics
Even the earliest automatic transmission could be manually controlled by the driver for better performance or to hold lower gears while descending steep hills. In the 1980s, engineers began to develop better manual controls for automatics so that a sporting driver could manipulate the shifting more readily.
Ultimately, this has led to sophisticated computer-controlled automatic transmissions that can be operated in fully automatic mode or can be switched into a driver-controlled manual mode. Manual shifts are accomplished through either a console-mounted lever (often pushed forward or back) or by upshift and downshift buttons or paddles located on the steering wheel. Although these advanced transmissions are still automatics, which mean they still rob some power during their operation, they now allow the driver significant vehicle control.
Semi-Automatics
A new breed of transmission has been introduced recently, which is far more of a hybrid between traditional automatic and traditional manual transmissions. Instead of using a torque converter, which absorbs energy, a standard clutch from a manual transmission is fitted and its operation is fully automated. This new electro-hydraulic system eliminates the need for a clutch pedal, but allows full manual shifting and, with advanced computer control, fully automatic shifting.
The driver controls the entire system through upshift and downshift buttons on the steering wheel or with a console-mounted lever. While manual shifting with these systems is immediate and satisfying, in automatic mode sensitive drivers may notice a lack of smoothness in the shifts compared to the latest automatics. Still, these systems are usually far more involving for the shift-it-yourself enthusiast compared to driving manually controlled automatics.
Four, Five, Six, Seven
Today, manuals and automatics are available with five or six forward speeds. In one case, a luxury automaker now offers a seven-speed automatic. Why the increase in the number of forward speeds? The idea is to provide an optimum gear ratio for almost any combination of road condition and engine speed, to provide scintillating performance while also meeting the latest exhaust emissions and fuel economy standards.
A modern six-speed transmission for example - whether manual or automatic - might have overdrive ratios on the two top gears so the engine will rev more slowly at highway speeds for better fuel economy. For the enthusiast driver, this increase in the number of forward gears also means that the gear ratios of the lower gears can be "shorter" (giving higher engine revs at lower speeds) and more closely spaced to provide faster acceleration and higher performance. Performance and fuel economy: The best of both worlds!
A Shifting Future
In many ways, the elements for future transmission technologies are already in place. It's hard to imagine the need for more than six or seven speeds (although more than five seemed absurd just a decade or two ago). Technologies that provide nearly seamless shifting and higher efficiencies are already here. Refinement of the existing technologies to further improve control and shift quality will undoubtedly remain the primary goals for powertrain engineers during the upcoming decade.
For those who like to use a traditional manual transmission, sadly, availability will continue to decline. The good news is that the application of new automatic transmission technologies will still give those who want to the ability to shift for themselves.
