Automatic transmissions were three-speed hydraulic units about two decades ago. Then came electronic controls and lock-up torque converters, all of which were developed in the name of improved fuel economy. Exotic cars of the day, such as Mercedes-Benz and BMW, began to use four-speed automatics.
Today, four- and five-speed automatics are common and light duty trucks are not exempt, with five- and six-speed automatics becoming the norm. This is still being done for fuel economy.
Engineers have found more than one way to achieve fuel economy. Different engines generate their torque in different ways. Some engines have broad power bands with up to 90 percent of the power available from slightly above idle to red line. Others have a narrow r.p.m. power band that’s high up the range.
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Diesels tend to be in the former category, and they’ve already produced huge power increases in medium duty trucks. A General Motors engineer told me the company had to add another gear to the Alison automatic to keep the fuel economy the same on the new high torque DuraMax.
The only way to get more power out of an engine is to put more fuel in, so fuel economy would have dropped if the company had left the transmission as a five-speed. A bit of engineering showed that the added torque would let it add another gear without losing fuel economy. The six-speed Alison was born.
Gasoline engines tend to be in the latter category with narrow power bands, which is where variable valve timing comes in on the intake and exhaust sides. By changing cam timing and valve opening overlap, engine makers were able to broaden out the torque curve.
The problem is that most gas engines use more r.p.m. than diesels, and they don’t produce anywhere near the torque of a diesel. A gas engine needs more gears to be as effective as a diesel.
For decades, larger trucks have had multi-gear choices. Recently, the trickle down of that technology has turned into a flood. BMW and Mercedes-Benz have seven-speed automatics in some cars.
In designing multi-gear transmissions, engineers had to overcome such issues as shift shock and size of gear and clutch pack. These gearboxes now function with a precision only dreamed about in the past.
Lexus recently raised the bar another notch. The new Lexus IS F sports sedan uses an eight-speed electronically controlled automatic, which makes several advances at once.
It has eight gears, and shift time is one tenth of a second so the feel inside the car is as if power is never disconnected when the shift is made. Because of the short shift time, conical clutches or other items to absorb shift shock are not needed.
To fit eight gears into the package, Lexus uses more than one set of clutches. If it had used only one set, the shafts in the transmission would have been so long that the case wouldn’t have been strong enough to take all the power the engine dished out. In addition, shift times couldn’t be reduced as much because it would take too long to move the gears.
This transmission goes into Lexus’s flagship performance car. It has paddle shifters, a shift map oriented toward sport driving, and a standard leave-it-in-drive automatic program that learns the basics of your driving style.
Cars may be slightly ahead in the gear wars for now, but the quest for improved power and fuel economy will continue. As fuel costs increase and consumers demand more power, truck and car transmissions will be different from those we have traditionally seen.
Continuously variable transmissions don’t use gears as most people think of them but use an idea based on two pulleys like snowmobile clutches.
Fuel economy and horsepower may be strange bedfellows, but they do create interesting technology.
Charles Renny is a freelance automotive columnist and a member of the Automobile Journalists Association of Canada.
