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5 Things You Need to Know About Fuel Efficient Transmissions
By Bill Siuru
Transmissions Affect Fuel Economy Too
Transmissions get little focus in today's evolving discussion of high gas prices and fuel efficiency. Yet, these drivetrain components are increasingly critical to achieving higher mpg. Gasoline and diesel engines are most efficient – that is, produce the greatest horsepower and torque using the least amount of fuel – over a relatively narrow rpm range. The more gear ratios in a transmission, the greater the ability to operate an engine in this optimum rpm range at all road speeds. Thus, a smaller engine with more gear ratios can provide the same, or even greater, low-speed torque for quick acceleration as a larger and more fuel-thirsty engine while still being able to cruise easily at high speeds.
How Many Speeds are Enough?
Transmission manufacturer ZF recently quantified the improvement in fuel economy that comes with more gear ratios. According to ZF, a four-speed automatic transmission brings a six percent fuel economy improvement over a three-speed automatic; a five-speed brings a two percent improvement over a four-speed; a six-speed brings a five percent improvement over a five-speed; a second generation six-speed brings a three to six percent improvement over a first generation six-speed; and an eight-speed brings a five percent improvement over a six-speed. To illustrate how far we've come in recent years, consider that an eight-speed automatic transmission results in a 21 to 24 percent fuel economy improvement over a three-speed automatic trans …independent of any other engine or drivetrain improvements.
Geared for Both High- and Low-End Cars
ZF plans to offer an eight-speed automatic transmission in 2010, most likely for Mercedes and BMW models. Lexus already offers an eight-speed automatic. Even low-priced entry level cars have at least four speeds today. Many higher priced models have automatics with five and even six speeds. Nine-speed automatics could appear in future years. Some experts say we may be reaching a point of diminishing returns because internal parasitic (energy draining) losses can cancel out improvements made in efficiency.
Other Advantages
Because an engine doesn't have to work as hard, not only does fuel economy improve but the engine is likely to last longer. Reduced engine rpms at high cruising speeds means quieter engines and less engine wear and stress. Today's multi-speed automatic transmissions make imperceptible gear changes even under pedal-to-the-metal, maximum throttle action. Multi-speed transmissions have eliminated the fuel economy and performance compromises that were previously inherent with "shiftless" driving. Today, EPA mileage ratings for many vehicles are identical when equipped with automatic and manual transmissions, and in some same cases automatics are better. Plus, today's high performance automatics can shift through the gears more effectively than all but most accomplished drivers.
What About CVTs?
A continuously variable transmission, or CVT, effectively has an infinite number of gear ratios. These advanced transmissions use two metal pulleys inside the transmission usually linked by a metal drive belt. The width of the pulleys is varied with engine and road speed. This changes the belt location to points higher or lower on the pulleys to change the gear ratio. Because the belt can be positioned in an infinite number of positions on the pulleys, gear ratios are continuously variable. While they may appear much simpler than five- and six-speed automatic transmissions, it takes considerable care in programming an on-board control computer to instantly and smoothly provide the correct gear ratio for all driving conditions. Done correctly, a CVT can provide manual transmission-like fuel economy without the need to manually shift gears.
Bill Siuru is a retired USAF colonel who has been writing about automotive technology for nearly 40 years. He has a bachelor's degree in automotive engineering, a PhD in mechanical engineering and has taught engineering at West Point and the U.S. Air Force Academy.
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