Green by Design
Part 2: Gearing Up

One key element affecting the efficiency of a vehicle is its transmission. The job of a transmission is to change the speed ratio between the vehicle's engine and wheels. This allows the engine to run in its narrow operating band, while still accommodating the needs of the driver. Conventional transmissions have multiple gears to cover different operating ranges: some gears offer excellent acceleration at low speeds, but are only capable of moving the vehicle so fast; other gears can carry the vehicle to higher speeds, but provide very poor low-speed acceleration. The number of gears a transmission has affects not only how smooth the ride will be, but also how efficiently the drivetrain will operate. Also affecting the efficiency is how effectively the transmission shifts from gear to gear.

Not too many years ago, automatic transmissions had only three speeds. Today's are primarily four- or five-speed. Some of the more advanced automatic transmissions on the market today have six or, in a few cases, even seven speeds. These more sophisticated transmissions not only make for a smoother ride, but also yield efficiency improvements. Ford Motor Company, who has 6-speed transmissions on the Ford Fusion, Mercury Milan, and Lincoln Zephyr among others, claims the wider span between the highest and lowest gear ratios accounts for a 4-8 percent improvement in fuel efficiency.

One of the relative newcomers to the transmission market is the continuously variable transmission, or CVT. This type of transmission made its U.S. production vehicle debut in a Subaru Justy in the late 1980s. But it was only recently that materials proved durable enough to put CVTs into the larger, more powerful vehicles in which they are being placed today.

While CVTs can be designed in a variety of mechanical configurations, the most common CVT design on the market today uses a steel belt connected to a pair of variable diameter pulleys. As the pulleys expand and contract, the size of the "gears" at either end of the belt change. This allows the transmission to produce a continuous (some say "infinite") range of gear ratios instead of being limited to a handful of discrete gear ratios, as found on conventional transmissions. Multiple benefits result from this setup. First, because the CVT can vary its gear ratio to meet the performance needs of the vehicle, the engine can be kept in its efficient operating window more often, saving fuel. Second, frictional losses that occur during shifts in today's fluid-coupled automatics (the "jolts" we're used to feeling during a gearshift) are disposed of, resulting in smoother acceleration and the facilitation of lower emissions.

In 2006, 16 different nameplates in the U.S. carried CVTs under the hood, including the Mini Cooper compact car, Ford Five Hundred sedan, and Nissan Murano SUV. A number of hybrids, such as the Ford Escape Hybrid, Honda Insight, and Toyota Highlander Hybrid, also incorporated these transmissions. The efficiency improvement attributable to CVTs varies depending upon the model and CVT design, but fuel economy improvements between 6 and 12 percent have been cited.

Continue to Part 3: A New Day for Diesel?