07-08-2012, 11:30 AM
Continuously Variable Transmission
ABSTRACT
As the U.S. government enacts new regulations for automotive fuel economy and emissions, the continuously variable transmission, or CVT, continues to emerge as a key technology for improving the fuel efficiency of automobiles with internal combustion (IC) engines. CVTs use infinitely adjustable drive ratios instead of discrete gears to attain optimal engine performance. Since the engine always runs at the most efficient number of revolutions per minute for a given vehicle speed, CVT-equipped vehicles attain better gas mileage and acceleration than cars with traditional transmissions.
CVTs are not new to the automotive world, but their torque capabilities and reliability have been limited in the past. New developments in gear reduction and manufacturing have led to ever-more-robust CVTs, which in turn allows them to be used in more diverse automotive applications. CVTs are also being developed in conjunction with hybrid electric vehicles. As CVT development continues, costs will be reduced further and performance will continue to increase, which in turn makes further development and application of CVT technology desirable.
INTRODUCTION
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.
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.
Push Belt
This most common type of CVT uses segmented steel blocks stacked on a steel ribbon, as shown in Figure (1). This belt transmits power between two conical pulleys, or sheaves, one fixed and one movable . With a belt drive:
Other CVT Varieties
Several other types of CVTs have been developed over the course of automotive history, but these have become less prominent than push belt and toroidal CVTs. A nutating traction drive uses a pivoting, conical shaft to change “gears” in a CVT. As the cones change angle, the inlet radius decreases while the outlet radius increases, or vice versa, resulting in an infinitely variable gear ratio . A variable geometry CVT uses adjustable planetary gear-sets to change gear ratios, but this is more akin to a flexible traditional transmission than a conventional CVT.
BACKGROUND & HISTORY
To say that the continuously variable transmission (CVT) is nothing new would be a gross understatement: Leonardo da Vinci sketched his idea for a CVT in 1490 . In automotive applications, CVTs have been around nearly as long as cars themselves, and certainly as long as conventional automatics. General Motors actually developed a fully toroidal CVT in the early 1930s and conducted extensive testing before eventually deciding to implement a conventional, stepped-gear automatic due to cost concerns. General Motors Research worked on CVTs again in the 1960s, but none ever saw production . British manufacturer Austin used a CVT for several years in one of its smaller cars, but “it was dropped due to its high cost, poor reliability, and inadequate torque transmission” . Many early CVTs used a simple rubber band and cone system, like the one developed by Dutch firm Daf in 1958 .
Challenges & Limitations
CVT development has progressed slowly for a variety of reasons, but much of the delay in development can be attributed to a lack of demand: conventional manual and automatic transmissions have long offered sufficient performance and fuel economy. Thus, problems encountered in CVT development usually stopped said progress. “Designers have … unsuccessfully tried to develop [a CVT] that can match the torque capacity, efficiency, size, weight, and manufacturing cost of step-ratio transmissions”. One of the major complaints with previous CVTs has been slippage in the drive belt or rollers.
This is caused by the lack of discrete gear teeth, which form a rigid mechanical connection between to gears; friction drives are inherently prone to slip, especially at high torque. With early CVTs of the 1950s and 1960s, engines equipped with CVTs would run at excessively high RPM trying to “catch up” to the slipping belt. This would occur any time the vehicle was accelerated from a stop at peak torque: “For compressive belts, in the process of transmitting torque, micro slip occurs between the elements and the pulleys. This micro slip tends to increase sharply once the transmitted torque exceeds a certain value …”
Future Prospects for CVTs
Much of the existing literature is quick to admit that the automotive industry lacks a broad knowledge base regarding CVTs. Whereas conventional transmissions have been continuously refined and improved since the very start of the 20th century, CVT development is only just beginning. As infrastructure is built up along with said knowledge base, CVTs will become ever-more prominent in the automotive landscape. Even today’s CVTs, which represent first-generation designs at best, outperform conventional transmissions. Automakers who fail to develop CVTs now, while the field is still in its infancy, risk being left behind as CVT development and implementation continues its exponential growth.
Moreover, CVTs are do not fall exclusively in the realm of IC engines.
CVTs & Hybrid Electric Vehicles
While CVTs will help to prolong the viability of internal combustion engines, CVTs themselves will certainly not fade if and when IC does. Several companies are currently studying implementation of CVTs with HEVs. Nissan recently developed an HEV with “fuel efficiency … more than double that of existing vehicles in the same class of driving performance”. The electric motor avoids the lowspeed/ high torque problems often associated with CVTs, through an innovative double-motor system. At low speeds:
CONCLUSION
Today, only a handful of cars worldwide make use of CVTs, but the applications and benefits of continuously variable transmissions can only increase based on today’s research and development. As automakers continue to develop CVTs, more and more vehicle lines will begin to use them. As development continues, fuel efficiency and performance benefits will inevitably increase; this will lead to increased sales of CVT-equipped vehicles. Increased sales will prompt further development and implementation, and the cycle will repeat ad infinitum. Moreover, increasing development will foster competition among manufacturers—automakers from Japan, Europe, and the U.S. are already either using or developing CVTs—which will in turn lower manufacturing costs. Any technology with inherent benefits will eventually reach fruition; the CVT has only just begun to blossom.
ABSTRACT
As the U.S. government enacts new regulations for automotive fuel economy and emissions, the continuously variable transmission, or CVT, continues to emerge as a key technology for improving the fuel efficiency of automobiles with internal combustion (IC) engines. CVTs use infinitely adjustable drive ratios instead of discrete gears to attain optimal engine performance. Since the engine always runs at the most efficient number of revolutions per minute for a given vehicle speed, CVT-equipped vehicles attain better gas mileage and acceleration than cars with traditional transmissions.
CVTs are not new to the automotive world, but their torque capabilities and reliability have been limited in the past. New developments in gear reduction and manufacturing have led to ever-more-robust CVTs, which in turn allows them to be used in more diverse automotive applications. CVTs are also being developed in conjunction with hybrid electric vehicles. As CVT development continues, costs will be reduced further and performance will continue to increase, which in turn makes further development and application of CVT technology desirable.
INTRODUCTION
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.
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.
Push Belt
This most common type of CVT uses segmented steel blocks stacked on a steel ribbon, as shown in Figure (1). This belt transmits power between two conical pulleys, or sheaves, one fixed and one movable . With a belt drive:
Other CVT Varieties
Several other types of CVTs have been developed over the course of automotive history, but these have become less prominent than push belt and toroidal CVTs. A nutating traction drive uses a pivoting, conical shaft to change “gears” in a CVT. As the cones change angle, the inlet radius decreases while the outlet radius increases, or vice versa, resulting in an infinitely variable gear ratio . A variable geometry CVT uses adjustable planetary gear-sets to change gear ratios, but this is more akin to a flexible traditional transmission than a conventional CVT.
BACKGROUND & HISTORY
To say that the continuously variable transmission (CVT) is nothing new would be a gross understatement: Leonardo da Vinci sketched his idea for a CVT in 1490 . In automotive applications, CVTs have been around nearly as long as cars themselves, and certainly as long as conventional automatics. General Motors actually developed a fully toroidal CVT in the early 1930s and conducted extensive testing before eventually deciding to implement a conventional, stepped-gear automatic due to cost concerns. General Motors Research worked on CVTs again in the 1960s, but none ever saw production . British manufacturer Austin used a CVT for several years in one of its smaller cars, but “it was dropped due to its high cost, poor reliability, and inadequate torque transmission” . Many early CVTs used a simple rubber band and cone system, like the one developed by Dutch firm Daf in 1958 .
Challenges & Limitations
CVT development has progressed slowly for a variety of reasons, but much of the delay in development can be attributed to a lack of demand: conventional manual and automatic transmissions have long offered sufficient performance and fuel economy. Thus, problems encountered in CVT development usually stopped said progress. “Designers have … unsuccessfully tried to develop [a CVT] that can match the torque capacity, efficiency, size, weight, and manufacturing cost of step-ratio transmissions”. One of the major complaints with previous CVTs has been slippage in the drive belt or rollers.
This is caused by the lack of discrete gear teeth, which form a rigid mechanical connection between to gears; friction drives are inherently prone to slip, especially at high torque. With early CVTs of the 1950s and 1960s, engines equipped with CVTs would run at excessively high RPM trying to “catch up” to the slipping belt. This would occur any time the vehicle was accelerated from a stop at peak torque: “For compressive belts, in the process of transmitting torque, micro slip occurs between the elements and the pulleys. This micro slip tends to increase sharply once the transmitted torque exceeds a certain value …”
Future Prospects for CVTs
Much of the existing literature is quick to admit that the automotive industry lacks a broad knowledge base regarding CVTs. Whereas conventional transmissions have been continuously refined and improved since the very start of the 20th century, CVT development is only just beginning. As infrastructure is built up along with said knowledge base, CVTs will become ever-more prominent in the automotive landscape. Even today’s CVTs, which represent first-generation designs at best, outperform conventional transmissions. Automakers who fail to develop CVTs now, while the field is still in its infancy, risk being left behind as CVT development and implementation continues its exponential growth.
Moreover, CVTs are do not fall exclusively in the realm of IC engines.
CVTs & Hybrid Electric Vehicles
While CVTs will help to prolong the viability of internal combustion engines, CVTs themselves will certainly not fade if and when IC does. Several companies are currently studying implementation of CVTs with HEVs. Nissan recently developed an HEV with “fuel efficiency … more than double that of existing vehicles in the same class of driving performance”. The electric motor avoids the lowspeed/ high torque problems often associated with CVTs, through an innovative double-motor system. At low speeds:
CONCLUSION
Today, only a handful of cars worldwide make use of CVTs, but the applications and benefits of continuously variable transmissions can only increase based on today’s research and development. As automakers continue to develop CVTs, more and more vehicle lines will begin to use them. As development continues, fuel efficiency and performance benefits will inevitably increase; this will lead to increased sales of CVT-equipped vehicles. Increased sales will prompt further development and implementation, and the cycle will repeat ad infinitum. Moreover, increasing development will foster competition among manufacturers—automakers from Japan, Europe, and the U.S. are already either using or developing CVTs—which will in turn lower manufacturing costs. Any technology with inherent benefits will eventually reach fruition; the CVT has only just begun to blossom.