When designing a car, a key factor is the transmission. Desired performance, fuel economy and cost drive the number and selection of gears. An ideal transmission has many gears to optimize engine performance and fuel economy. But more gears cost more money. A car model with three available engines may require three different transmissions.
Imagine a transmission with infinite gears that under computer control could change gear ratios to suit conditions. Such a transmission would have gears that could change shape on command. Science fiction? No. It is called a Continuous Variable Transmission (CVT). A CVT consists of two pulleys (gears) that can change diameter, with a “belt” connecting the two. The diameters of the pulleys are changed in unison to move from low (left picture) to high (right). Power from the engine is delivered to the top pulley, while the bottom drives the wheels.
The first CVTs were made popular on snowmobiles. Looking much like the basic concept illustration, they use a rubber belt and control pulley diameter with springs and weights using engine torque and RPMs to force the drive pulley closed for higher gear ratio. This means that specific converters must be chosen for horsepower and application.
One evolution is the computer-controlled Mercedes CVT used in the new B-200, as well as the A class available in Europe. Instead of just mechanical engine torque, a computer controls the width of the drive pulley with a high-pressure hydraulic pump. Sensors provide information to the program to select the appropriate gear ratio. Data is delivered on engine speed and torque, wheel speed, gas pedal level, user setting (comfort vs sport), condition of the transmission and engine. Instead of selecting a gear from a list, the computer sets any ratio in the range.
Another improvement is the belt. Originally rubber, the B-200 CVT belt has 412 steel thrust links and 24 stainless steel bands. These are constantly bathed in an oil stream. While snowmobile transmissions require replacing the rubber belt on a regular basis, the steel assembly is designed to last the life of the car. The B-200 CVT does require a simple fluid change every 60,000 kilometers.
The Mercedes CVT, like a traditional automatic transmission, is equipped with a hydraulic torque converter. This provides hill-holding idling in gear with the vehicle stopped. Combining the use of a torque converter along with a sophisticated program, the result is a transmission that feels so much like a conventional automatic, that a well-known automobile journalist had a B-200 for a week and never realized that it wasn’t conventional.
I drove the B-200 Turbo to Cleveland, Ohio, through a blizzard in Erie, Pennsylvania. What amazed me was how well the CVT worked in the snow. With no “shifting,” the wheels never broke loose.
The CVT’s flexibility enables performance, fuel economy and smooth operation. CVTs have been used only in lower automotive horsepower applications. Honda, Toyota, Saturn, Audi and Nissan all offer a CVT transmission now.
I am a long-time fan of manual and automated manual transmissions (AMT) for smaller engines, but loved the feel and performance of this unit. It provided good fuel efficiency, responded well to changing conditions, accelerated smoothly and kept the engine in its optimum operating range. I can see a CVT in my garage some day.
Jan 14, 2006 – Toronto Sun Autonet Drive