| Continuously
variable transmission (CVT) |
Definition
After more than a century
of research and development, the internal combustion (IC) engine is nearing both
perfection and obsolescence: engineers continue to explore the outer limits of
IC efficiency and performance, but advancements in fuel economy and emissions
have effectively stalled. While many IC vehicles meet Low Emissions Vehicle standards,
these will give way to new, stricter government regulations in the very near future.
With limited room for improvement, automobile manufacturers have begun full-scale
development of alternative power vehicles. Still, manufacturers are loath to scrap
a century of development and billions or possibly even trillions of dollars in
IC infrastructure, especially for technologies with no history of commercial success.
Thus, the ideal interim solution is to further optimize the overall efficiency
of IC vehicles. One potential solution to
this fuel economy dilemma is the continuously variable transmission (CVT), an
old idea that has only recently become a bastion of hope to automakers. CVTs could
potentially allow IC vehicles to meet the first wave of new fuel regulations while
development of hybrid electric and fuel cell vehicles continues. Rather than selecting
one of four or five gears, a CVT constantly changes its gear ratio to optimize
engine efficiency with a perfectly smooth torque-speed curve. This improves both
gas mileage and acceleration compared to traditional transmissions.
The fundamental theory behind CVTs has undeniable potential, but lax fuel regulations
and booming sales in recent years have given manufacturers a sense of complacency:
if consumers are buying millions of cars with conventional transmissions, why
spend billions to develop and manufacture CVTs?
Although CVTs have been used in automobiles for decades, limited torque capabilities
and questionable reliability have inhibited their growth. Today, however, ongoing
CVT research has led to ever-more robust transmissions, and thus ever-more-diverse
automotive applications. As CVT development continues, manufacturing costs will
be further reduced and performance will continue to increase, which will in turn
increase the demand for further development. This cycle of improvement will ultimately
give CVTs a solid foundation in the world's automotive infrastructure. CVT
Theory & Design Today's automobiles
almost exclusively use either a conventional manual or automatic transmission
with "multiple planetary gear sets that use integral clutches and bands to
achieve discrete gear ratios" . A typical automatic uses four or five such
gears, while a manual normally employs five or six. The continuously variable
transmission replaces discrete gear ratios with infinitely adjustable gearing
through one of several basic CVT designs.
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