continuously variable transmission CVT seminar or presentation report
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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.
This paper evaluates the current state of CVTs and upcoming research and development, set in the context of past development and problems traditionally associated with CVTs. The underlying theories and mechanisms are also discussed.
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 worldâ„¢s automotive infrastructure.
CVT THEORY & DESIGN
Todayâ„¢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.
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:
In essence, a sensor reads the engine output and then electronically increases or decreases the distance between pulleys, and thus the tension of the drive belt. The continuously changing distance between the pulleysâ€their ratio to one anotherâ€is analogous to shifting gears. Push-belt CVTs were first developed decades ago, but new advances in belt design have recently drawn the attention of automakers worldwide.
These transmissions use the high shear strength of viscous fluids to transmit torque between an input torus and an output torus. As the movable torus slides linearly, the angle of a roller changes relative to shaft position, as seen in Figure (2). This results in a change in gear ratio .
Variable Diameter Elastomer Belt
This type of CVT, as represented in Figure (2), uses a flat, flexible belt mounted on movable supports. These supports can change radius and thus gear ratio. However, the supports separate at high gear ratios to form a discontinuous gear path, as seen in Figure (3). This can lead to the problems with creep and slip that have plagued CVTs for years .
This inherent flaw has directed research and development toward push belt CVTs.
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 .
However, the Daf CVT could only handle a 0.6 L engine, and problems with noise and rough starts hurt its reputation . Uninspired by these early failures, automakers have largely avoided CVTs until very recently, especially in the United States.
Inherent Advantages & Benefits
Certainly, the clunk of a shifting transmission is familiar to all drivers. By contrast, a continuously variable transmission is perfectly smoothâ€it naturally changes gears discreetly and minutely such that the driver or passenger feels only steady acceleration. In theory, a CVT would cause less engine fatigue and would be a more reliable transmission, as the harshness of shifts and discrete gears force the engine to run at a less-than-optimal speed.
Moreover, CVTs offer improved efficiency and performance. Table (1) below shows the power transmission efficiency of a typical five-speed automatic, i.e. the percentage of engine power translated through the transmission. This yields an average efficiency of 86%, compared to a typical manual transmission with 97% efficiency . By comparison, Table (2) below gives efficiency ranges for several CVT designs.
These CVTs each offer improved efficiency over conventional automatic transmissions, and their efficiency depends less on driving habit than manual transmissions . Moreover:
Because the CVT allows an engine to run at this most efficient point virtually independent of vehicle speed, a CVT equipped vehicle yields fuel economy benefits when compared to a conventional transmission. Testing by ZF Getriebe GmbH several years ago found that the CVT uses at least 10% less fuel than a 4- speed automatic transmission for U.S. Environmental Protection Agency city and highway cycles.
Moreover, the CVT was more than one second faster in 0-60 mph acceleration tests . The potential for fuel efficiency gains can also be seen in the CVT currently used in Hondaâ„¢s Civic. A Civic with a traditional automatic averages 28/35 miles per gallon (mpg) city/highway, while the same car with a CVT gets 34/38 mpg city/highway . Honda has used continuously variable transmissions in the Civic for several years, but these are 1.6 liter cars with limited torque capabilities. Ongoing research and development will inevitably expand the applicability of CVTs to a much broader range of engines and automobiles.
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 Â¦ 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 Â¦
For many years, the simple solution to this problem has been to use CVTs only in cars with relatively low-torque engines. Another solution is to employ a torque converter (such as those used in conventional automatics), but this reduces the CVTâ„¢s efficiency.
Perhaps more than anything else, CVT development has been hindered by cost. Low volume and a lack of infrastructure have driven up manufacturing costs, which inevitably yield higher transmission prices. With increased development, most of these problems can be addressed simply by improvements in manufacturing techniques and materials processing. For example, Nissanâ„¢s Extroid is derived from a century-old concept, perfected by modern technology, metallurgy, chemistry, electronics, engineering, and precision manufacturing.
In addition, CVT control must be addressed. Even if a CVT can operate at the optimal gear ratio at any speed, how does it know what ratio to select? Manual transmissions have manual controls, where the driver shifts when he or she so desires; automatic transmissions have relatively simple shifting algorithms to accommodate between three and five gears. However, CVTs require far more complex algorithms to accommodate an infinite division of speeds and gear ratios.
RESEARCH & DEVELOPMENT
While IC development has slowed in recent years as automobile manufacturers devote more resources to hybrid electric vehicles (HEVs) and fuel cell vehicles (FEVs), CVT research and development is expanding quickly. Even U.S. automakers, who have lagged in CVT research until recently, are unveiling new designs:
General Motors plans to implement metal-belt CVTs in some vehicles by 2002.
The Japanese and Germans continue to lead the way in CVT development. Nissan has taken a dramatic step with its Extroid CVT, offered in the home-market Cedric and Gloria luxury sedans. This toroidal CVT costs more than a conventional belt-driven CVT, but Nissan expects the extra cost to be absorbed by the luxury carsâ„¢ prices. The Extroid uses a high viscosity fluid to transmit power between the disks and rollers, rather than metal-to-metal contact. Coupled with a torque converter, this yields exceptionally fast ratio changes. Most importantly, though, the Extroid is available with a turbocharged version of Nissanâ„¢s 3.0 liter V6 producing 285 lb-ft of torque; this is a new record for CVT torque capacity.
Audiâ„¢s new CVT offers both better fuel mileage than a conventional automatic and better acceleration than even a manual transmission. Moreover, Audi claims it can offer the CVT at only a slight price increase. This so-called multitronic CVT uses an all-steel link plate chain instead of a V-belt in order to handle up to 280 lb-ft of torque. In addition, Audi claims that the multitronic A6 accelerates from 0-100 km/h (0-62 mph) 1.3 s quicker than a geared automatic transmission and is 0.1 s quicker over the same speed than an equivalent model with optimum use of a five speed manual gearbox. If costs were sufficiently reduced, a transmission such as this could be used in almost any automobile in the world.
Many small cars have used CVTs in recent years, and many more will use them in the near future. Nissan, Honda, and Subaru currently use belt-drive CVTs developed with Dutch company Van Doorne Transmissie (VDT) in some of their smaller cars. Suzuki and Daihatsu are jointly developing CVTs with Japanese company Aichi Machine, using an aluminum/plastic composite belt reinforced with Aramid fibers. Their CVT uses an auxiliary transmission for starts to avoid low-speed slip. After about 6 mph, the CVT engages and operates as it normally would. The auxiliary geartrainâ„¢s direct coupling ensures sufficiently brisk takeoff and initial acceleration. However, Aichiâ„¢s CVT can only handle 52 lb-ft of torque. This alone effectively negates its potential for the U.S. market. Still, there are far more CVTs in production for 2000 than for 1999, and each major automobile show brings more announcements for new CVTs.
New CVT Research
As recently as 1997, CVT research focused on the basic issues of drive belt design and power transmission. Now, as belts by VDT and other companies become sufficiently efficient, research focuses primarily on control and implementation of CVTs.
Nissan Motor Co. has been a leader in CVT research since the 1970s. A recent study analyzing the slip characteristics of a metal belt CVT resulted in a simulation method for slip limits and torque capabilities of CVTs. This has led to a dramatic improvement in drive belt technology, since CVTs can now be modeled and analyzed with computer simulations, resulting in faster development and more 8 efficient design. Nissanâ„¢s research on the torque limits of belt-drive CVTs has also led to the use of torque converters, which several companies have since implemented. The torque converter is designed to allow creep, the slow speed at which automatic transmission cars drive without driver-induced acceleration. The torque converter adds improved creep capability during idling for improved driveability at very low speeds and easy launch on uphill grades. Nissanâ„¢s Extroid uses such a torque converter for smooth starting, vibration suppression, and creep characteristics.
CVT control has recently come to the forefront of research; even a mechanically perfect CVT is worthless without an intelligent active control algorithm. Optimal CVT performance demands integrated control, such as the system developed by Nissan to obtain the demanded drive torque with optimum fuel economy. The control system determines the necessary CVT ratio based on a target torque, vehicle speed, and desired fuel economy. Honda has also developed an integrated control algorithm for its CVTs, considering not only the engineâ„¢s thermal efficiency but also work loss from drivetrain accessories and the transmission itself. Testing of Hondaâ„¢s algorithm with a prototype vehicle resulted in a one percent fuel economy increase compared to a conventional algorithm. While not a dramatic increase, Honda claims that its algorithm is fundamentally sound, and thus will it become one of the basic technologies for the next generationâ„¢s powerplant control.
Although CVTs are currently in production, many control issues still amount to a tremendous number of trials and errors . One study focusing on numerical representation of power transmission showed that both block tilting and pulley deformation meaningfully effected the pulley thrust ratio between the driving and the driven pulleys . Thus, the resultant model of CVT performance can be used in future applications for transmission optimization. As more studies are conducted, fundamental research such as this will become the legacy of CVT design, and research can become more specialized as CVTs become more refined.
As CVTs move from research and development to assembly line, manufacturing research becomes more important. CVTs require several crucial, high-tolerance components in order to function efficiently; Honda studied one of these, the pulley piston, in 1998. Honda found that prototype pistons experienced a drastic thickness reduction (32% at maximum) due to the conventional stretch forming method. A four-step forming process was developed to ensure a greater and more uniform thickness increase and thus greater efficiency and performance. Moreover, work-hardening during the forming process further increased the pulley pistonâ„¢s strength .
Size and weight of CVTs has long been a concern, since conventional automatics weigh far more than manual transmissions and CVTs outweigh automatics. Most cars equipped with automatic transmissions have a curb weight between 50 and 150 pounds heavier than the same cars with manual transmissions. To solve this problem, Audi is currently developing magnesium gearbox housings, a first for cars in its class. This results in nearly a 16 pound weight reduction over conventional automatics.
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 todayâ„¢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 Â¦ 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:
A low-power traction motor is used as a substitute mechanism to accomplish the functions of launch and forward/reverse shift. This has made it possible to discontinue use of a torque converter as the launch element and a planetary gearset and wet multiplate clutches as the shift mechanism.
Thus use of a CVT in a HEV is optimal: the electric portion of the power system avoids the low-speed problems of CVTs, while still retaining the fuel efficiency and power transmission benefits at high speeds. Moreover, the use of a CVT capable of handling high engine torque allows the system to be applied to more powerful vehicles. Obviously, automakers cannot develop individual transmissions for each car they sell; rather, a few robust, versatile CVTs must be able to handle a wide range of vehicles.
Korean automaker Kia has proposed a rather novel approach to CVTs and their application to hybrids. Kia recently tested a system where the CVT allows the engine to run at constant speed and the motor allows the engine to run at constant torque independent of driving conditions. Thus, both gasoline engine and electric motor always run at their optimal speeds, and the CVT adjusts as needed to accelerate the vehicle. Kia also presented a control system for this unified HEV/CVT combination that optimizes fuel efficiency for the new configuration.
Â¢ Tractors just as cars have the need for a flexible system to convey power from their engine to their wheels. The C.V.T. will provide just this and at high fuel savings with low atmospheric pollution.
Â¢ Golf Carts stand to benefit from the C.V.T. as well in the way electric cars do. that is: Large range of speeds, longer driving range between charges, Fewer baterries, lower maintenance cost, less weight.
Â¢ Ride on Lawn Mowers like small tractors are gas powered and contribute to the air pollution problem. The C.V.T. approach can prevent ride-ons to pollute the air to the extend they currently do.
Â¢ Motorized Wheelchairs. Battery run, speed controlled by a rheostat. Going up a ramp slowly, causes a drop in power (when it's most needed). C.V.T. is a form of transmission, lower speed means MORE POWER.
Â¢ Bicycles. Ever try to shift gears while pedaling uphill? Good news; the KINESIS C.V.T. will automaticaly select the appropriate for the situation "gear" ratio. No hasle, no trouble. End of story.
Â¢ Power tools and household appliances, that vary from benchtop drills to wash machines and blenders need to depart from the centuries old belt and pulley configuration for smoother operation and more reliability.
Â¢ Industrial Equipment and production machinery often use either gears or cumbersom belt and pulley configurations. C.V.T. can do away with all that and additionaly give them infinite ratios.
Â¢ Minimachines. Small devices that need to operate in a wide range of speeds, as the need arises. Our unique design allows the production of an inexpensive miniature C.V.T. to enable them do just that!.
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 todayâ„¢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 manufacturersâ€automakers from Japan, Europe, and the U.S. are already either using or developing CVTsâ€which will in turn lower manufacturing costs. Any technology with inherent benefits will eventually reach fruition; the CVT has only just begun to blossom.
1. http//audimultitronic, 2001.
2. Avery, G., Tenberge, P., Electromechanical Hybrid Transmission â€œ concept, design, simulation, Proc. Integrated Powertrains and their Control, University of Bath, IMechE, 2000.
3. Brace, C.J., Deacon, M., Vaughan, N.D., Horrocks, R.W., Burrows C.R., "An Operating Point Optimiser for the Design and Calibration of an Integrated Diesel / CVT Powertrain", Proceedings of The Institution of Mechanical Engineers Journal of Automobile Engineering (Part D) Vol 213, May 1999, pg 215-226, ISSN: 0954-4070, 1999.
4. Brace, C.J., Deacon, M., Vaughan, N.D., Horrocks, R.W., Burrows C.R., "Integrated Passenger Car Diesel CVT Powertrain Control for Economy and Low Emissions", IMechE International Seminar S540 'Advanced Vehicle Transmissions and Powertrain Management' 25 -26 Sept 1997.
5. Torotrak, homepage, http//torotrak.com, 2001.
First of all I thank the almighty for providing me with the strength and courage to present the seminar and presentation.
I avail this opportunity to express my sincere gratitude and outset thank to my seminar and presentation guide and head of mechanical engineering department
Dr. T.N. Sathyanesan , for permitting me to conduct the seminar and presentation and for his inspiring assistance, encouragement and useful guidance. And I also thank staff incharge Asst. Prof. Mrs. Jumailath Beevi. D., for their inspiring assistance, encouragement and useful guidance.
I am also indebted to all the teaching and non- teaching staff of the department of mechanical engineering for their cooperation and suggestions, which is the spirit behind this report. Last but not the least, I wish to express my sincere thanks to all my friends for their goodwill and constructive ideas.
Chindu A. Kharim
1. INTRODUCTION 1
2. CVT THEORY AND DESIGN 3
3. BACKGROUND & HISTORY 6
4. RESEARCH & DEVELOPMENT 10
5. OTHER APPLICATIONS 17
6. CONCLUSION 19
7. REFERENCES 20
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CONTINUOUSLY VARIABLE TRANSMISSION
A Seminar Report
MECHANICAL ENGINEERING DIVISION
SCHOOL OF ENGINEERING
COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY
Continuously Variable Transmissions (CVT).pdf (Size: 1.02 MB / Downloads: 734)
Continuously Variable Transmissions (CVT) offer a continuum of gear ratios
between desired limits. This allows the engine to operate more time in the optimum range
given an appropriate control of the engine valve throttle opening (VTO) and transmission
ratio. In contrast, traditional automatic and manual transmissions have several fixed
transmission ratios forcing the engine to operate outside the optimum range. The present
research focuses on developing models to understand the micro slip behavior and to define
an operating regime of a metal pushing V-belt CVT. Slip is modeled on the basis of gap
redistribution between the elements. Studies were conducted to observe the influence of
loading conditions (i.e. axial forces and torques) on the slip behavior and torque
transmitting ability of the CVT. The model also investigates the range of axial forces
needed to initiate the transmission and to successfully meet the load requirements. The
mathematical model and the results corresponding to different loading scenarios are
discussed. CVT (Continuously Variable Transmission) is a system that makes it possible to
vary progressively the transmission ratio and it allows engine to run at its optimum speed.
In this paper a CVT using a belt with changing thickness is described. Developments in
clamping force control for the push belt Continuously Variable Transmission (CVT) aim at
increased efficiency in combination with improved robustness. Current control strategies
attempt to prevent macro slip between elements and pulleys at all times for maximum
robustness. CVT efficiency increases which will lead to an improvement in fuel
consumption up to 5%.
3. Why there is a need for transmission?
4. Continuously Variable Transmission
5. Why we use a CVT?
6. How does CVT Work?
7. Belt design
8. Types of CVT
9. Advantage and drawbacks
10. Implementation- Example
The overwhelming majority of transmissions in road going vehicles are either
manual or conventional automatic in design. These transmissions use meshing gears that
give discrete ratio steps between engine and the vehicle speed .However, alternative
designs exist that can transmit power and simultaneously give a step less change of ratio; in
other words a Continuously Variable Transmissions. Continuously Variable Transmissions
are a type of automatic transmission that provides an uninterrupted range of speed ratios,
unlike a normal transmission that provides only a few discrete ratios.
Continuously variable transmissions (CVT) offer a continuum of gear ratios
between desired limits. The present research focuses on developing models to understand
the micro slip behavior and to define an operating regime of a metal pushing V-belt CVT.
Slip is modeled on the basis of gap redistribution between the elements. Studies were
conducted to observe the influence of loading conditions (i.e. axial forces and torques) on
the slip behavior and torque transmitting ability of the CVT. The model also investigates
the range of axial forces needed to initiate the transmission and to successfully meet the
load requirements. CVT is an emerging automotive transmission technology that offers a
continuum of gear ratios between high and low extremes. Today, Continuously Variable
Transmissions have lured a great deal of automotive manufacturers and customers. Several
car companies like Honda, Toyota, Ford, Nissan, etc., have been doing intensive research to
exploit the advantages of a CVT. The chief advantage of a CVT is its ability to offer an
infinite range of gear ratios with fewer moving parts, and consequently this influences
engine efficiency, fuel economy, and cost.
Continuously Variable Transmissions (CVT’s) have developed notably indifferent
applications over the past years. This is especially true in the automobile field because of
advantages in terms of car handling and efficiency on urban roads. The advantages of a
Continuously Variable Transmission are in terms of its power and efficiency. A
Continuously Variable Transmission is different from the conventional automatic
The CVT (Continuously Variable Transmission) design was developed by inventor
Dr Jan Naude as a result of a long felt need for an all gear non-traction fluid, non-hydraulic,
continuously variable gearbox/transmission. CVTs were first patented in Europe in the
19th century, but are just now coming into their own in a big way in the American market
place. The CVT results in a more efficient and versatile drive train in comparison to
conventional drive trains and has been designed as an alternative to existing gearboxes,
providing greater efficiency for equivalent cost. The formation of the company is the
culmination of negotiations between partners Dr Jan Naude and Marinus van den Ende and
Barloworld Equipment. This Barloworld Equipment Smart PartnershipTM, Varibox CVT
(Pty) Ltd trading as Barloworld CVT Technologies was created to expand and bring to the
market the innovative inventions of Dr Jan Naude.
According to TOROTRAK, the first patent for a toroidal CVT was filed at the end
of the century was designed and built by Dutch Hub van Doorne, co-founder of DAF, in the
late 1950s, specifically to produce an automatic transmission for a small, affordable car.
The first DAF car using van Doorne's CVT was produced in 1958. Van Doorne's patents
were later sold to Volvo along with DAF's car business
3. WHY THERE IS A NEED FOR TRANSMISSION?
According to some engineers, the transmission is a large, expensive bracket to stop
the engine dragging on the road. In reality transmissions are much more interesting than the
other, less significant, parts of the power train. Essentially, the transmission takes the power
from the engine to the wheels, in doing so actually makes the vehicle usable. The functions
that enable this include:
Ø Allow the vehicle to start from rest, with the engine running continuously.
Ø Let the vehicle stop by disconnecting the drive when appropriate.
Ø Enable the vehicle to start at varied rates, under a controlled manner.
Ø Vary the speed ratio between the engine and wheels.
Ø Allow this ratio to change when required.
Ø Transmit the drive torque to the required wheels.
The transmission needs to perform all of the above functions and others\refined manner.
The structural aspects of the transmission, predominantly the casting, often contribute
significantly to the structure of the power train and the vehicle as a whole. This is important
when it comes to engineering for the lowest noise and vibration. The stiffness of the power
train assembly itself is important in determining the magnitude and frequency of the
vibrations at the source (the engine). This stiffness (and indeed the strength) can also be
important to the integrity of the vehicle in a crash. Particularly with front wheel drive
vehicles, the way in which the body collapses on impact has to be engineered very
carefully, and the presence of a large rigid lump such as the power train has a critical
influence on the way this occurs. The size, shape and orientation of the unit also affect the
intrusion into the passenger space after an impact.
4. CONTINUOUSLY VARIABLE TRANSMISSIONS (CVT)
Continuously variable transmissions (CVTs) have an infinitely variable ratio,
which allows the engine to operate more time in the optimum range given an appropriate
control of the engine valve throttle opening (VTO) and transmission ratio .In contrast,
traditional automatic and manual transmissions have several fixed transmission ratios
forcing the engine to operate outside the optimum range With growing interest in
improving the fuel economy in the moving vehicles continuously variable transmission s
has attracted a great deal of interest. This type of transmission provide continuously
variable reduction ratio that enables the engine to operate under the most economical
conditions over a wide range of vehicle speed.
Two representative type of continuously variable transmission are Van Doorne belt
system and Perbury (Trotrak) system. Van Doorne system, it was installed in since 1955.
It has pair of conically faced pulleys, as shown in figure. The effective radius of the pulley,
and hence the reduction ratio, can be varied by adjusting distance between the two sides of
the pulley. on the original system the reduction ratio can be controlled by mechanical
means through centrifugal weight on the driving pulley and engine vacuum actuator .more
recently microprocessor based control system has-been developed .this type continuously
variable trans mission can achieve a reduction ratio, ranging from 4 to 6 . The
mechanical efficiency of the transmission varies from load and speed. The variation in
efficiency with input torque and speed at a reduction ratio 1 for a system designed for a
light weight passenger car. To improve the efficiency with reduce noise and wear, a
segmented steel belt or a push belt system has been developed. it comprises a set of belt
element about 2 mm thick , with slots on each side to fit two high tensile steel bands which
hold them together .unlike the conventional v-belt ,it transmit power by the compressive
force between the belt elements ,instead of tension . The Van Doorne system is most
suitable for low power applications, mainly in front wheel drive vehicles and has been used
in small size passenger cars and snow mobiles.
The continuously variable transmission (CVT) has been around as long as the
automobile. Engineers have always recognized its theoretical advantage over the multi ratio
gearbox. A CVT enables the engine to run at its most fuel-efficient or most power-efficient
speed while driving the vehicle at any speed desired.
With a CVT engine speed and vehicle speed are no longer connected by a series of
discrete ratios. Instead, they can function independently across a wide and step less band
according to engine characteristics and performance requirements. The advantages of this
infinite ratio selectivity are enormous. Most obvious in the IC engine application is that the
engine can be loaded into its most fuel-efficient region at cruising speeds, then allowed to
accelerate into its region of greatest output when peak power is needed, regardless of
vehicle speed. Practical problems have consistently plagued the design. hut the CVT is now
coming of age.
Continuously Variable Transmissions (CVT)
Van Doorne belt system
The Perbury System is shown in fig. the key component of the system is variator,
which consists of three disks .with the outer pair connected to the input shafts and the inner
one is connected to the output shaft .the inner surface of the disk are of the toroidal shape
Perbury (Trotrak) systems
Continuously Variable Transmissions (CVT) is typically composed of two
hydraulically actuated variable radii pulleys and a metal pushing belt (see Figure 1). CVTs
offer a continuum of infinitely variable gear ratios by changing the location of pulleys
heaves. As a result, CVTs have the potential to increase the overall vehicle efficiency and
reduce the jerk usually associated with manual and automatic transmissions. Although
many criteria characterize performance, the focus is on fuel consumption and vehicle
longitudinal dynamics (jerk) during vehicle acceleration One shortcoming however, is their
difficulty in transmitting high torque at low operating speeds, which so far has limited their
use to small vehicles. Alternatively, power-split CVTs (PSCVTs) offer both fixed gears and
adjustable pulleys and are able to extend the torque transmission capability significantly. A
CVT relies on a flexible metal belt and pulleys to constantly shift gear ratios, boosting fuel
economy as much as 10 percent. They are also smaller, lighter, cheaper and easier to build
and install than traditional stick-shift or automatic transmissions. For all these reasons,
CVTs are starting to get a lot of attention from car-makers worldwide, as they continuously
look for ways to help their cars and trucks get better mileage, and be lighter, less expensive
to build and easier to repair.
Cross Section Of CVT Pulley
5. WHY USE A CVT?
Traditional transmissions use gears, friction plates and hydraulic fluid to transfer
power from an engine to a drive shaft. Continuously variable transmissions use a simple
belt and pulley system, creating a "continuously variable" gear ratio that is more fuelefficient
CVT technology replaces the gears and friction plates in traditional transmissions
with a belt and pulley system to transfer the power smoothly from a motor to the wheels.
The pulleys inside a CVT are typically cone-shaped, and the belt that runs between
them slides between the narrow and wide ends of each pulley. That creates a "continuously
variable" gear ratio to transfer power from the engine to the wheels.
A CVT is an ideal power transmission device for a snowmobile for one main reason
it allows power from the engine to be transmitted continuously to the ground. In contrast, a
standard gear-box transmission takes time to shift gears, time in which the engine and
wheels are disconnected, and results in a loss of momentum. Because of the terrain that
snowmobiles often encounter (snow, and specifically unbroken snow), it is not practical to
experience a loss of momentum while operating a snowmobile. The time spent in changing
gears would allow the high drag properties of deep snow to overcome most of the vehicle’s
momentum. By using a CVT, snowmobiles overcome this dilemma and deliver
uninterrupted engine power to the ground. Furthermore, the two and four-stoke engines
common to snowmobiles have a smaller range of deliverable power than those used in
geared transmissions, so use of a CVT allows for the engine to operate at a constant speed,
namely that speed which produces the maximum power.
6. HOW DOES A CVT WORK?
Although there are different variations on the CVT theme, most passenger cars use a
similar setup. Essentially, a CVT transmission operates by varying the working diameters
of the two main pulleys in the transmission. The pulleys have V-shaped grooves in which
the connecting belt rides. One side of the pulley is fixed; the other side is moveable,
actuated by a hydraulic cylinder. When actuated, the cylinder can increase or reduce the
amount of space between the two sides of the pulley. This allows the belt to ride lower or
higher along the walls of the pulley, depending on driving conditions, thereby changing the
gear ratio. If you think about it, the action is similar to the way a mountain bike shifts gears,
by "derailing" the chain from one sprocket to the next — except that, in the case of CVT,
this action is infinitely variable, with no "steps" between.
The "stepless" nature of its design is CVT's biggest draw for automotive engineers. Because
of this, a CVT can work to keep the engine in its optimum power range, thereby increasing
efficiency and gas mileage. A CVT can convert every point on the engine's operating curve
to a corresponding point on its own operating curve
With these advantages, it's easy to understand why manufacturers of high-mileage vehicles
often incorporate CVT technology into their drive trains. Look for more CVTs in the
coming years as the battle for improved gas mileage accelerates and technological advances
further widen their functionality.
A CVT operates on dynamic principles which are based on three main mechanical
components — flyweights or cams, springs, and ramps. Essentially, these three componenttypes
work in conjunction to transmit engine power and torque to the ground, while
maintaining a constant engine speed. There are certain mechanical feedback systems,
created by the CVT components, which govern how the transmission behaves, and
ultimately how the snowmobile performs.” The A+CVT developed by Larry Anderson uses
flexible sprocket bars along the pulley shafts to create a more efficient and durable positive
drive system rather than the typical friction drive of a belt system.
Continuously Variable Transmissions (CVT) Unit
CVT Unit Incorporated In The Vehicle
7. BELT DESIGN
7.1 Push Belt Loading During Variator Operation
The heart of a CVT system is the variator, i.e. the push-belt/pulley system illustrated
in Figure 1. In the variator, torque or power is transmitted from the primary to the
secondary pulley via friction between the push-belt elements and the pulley sheaves
.Stepless shifting between the extreme LOW (under drive) and OD (overdrive) ratios is
achieved by varying the pulley clamping forces and thereby changing the axial position of
the moveable pulley sheaves, modifying the effective running radius. An example of a Van
Doorne push-belt is shown in Figure 2.
During operation of the variator, the push-belt and pulleys undergo cyclic (fatigue)
loading. Stress levels in the push-belt elements and pulleys are determined by the applied
pulley clamping forces, rotational speeds, and torque levels. Experience has shown that
fatigue loading of the elements and pulleys is less critical in practice than ring fatigue
loading. The push-belt rings are mainly subjected to bending and tensile stresses, although
in a rather complex manner. In general, the bending stresses are determined by the applied
running radii (transmission ratio) and the ring thickness. The tensile stresses are mainly
determined by the applied pulley clamping forces, rotational speeds, and torque.
Example Of A Variator And Its Working Principle
Push Belt Loading And Doorne Push Belt
7.2 CVT Using A Belt With Changing Thickness
Continuously variable transmissions (CVTs) have an infinitely variable ratio, which
allows the engine to operate more time in the optimum range given an appropriate control
of the engine valve throttle opening (VTO) and transmission ratio. In contrast, traditional
automatic and manual transmissions have several fixed transmission ratios forcing the
engine to operate outside the optimum range. There are various types of CVTs but in this
opinion a new concept for a CVT using a belt with changing thickness is described. If the
thickness of belt is not negligible as compare to radius of pulley, the neutral axis of rotation
is taken as radius of pulley plus half the thickness of belt. Therefore the effective diameter
of the pulley will change. If the belt is made up such that its thickness decreases when
stretched & increases when compressed .Consider that the smaller pulley is fixed and
position of larger pulley can be changed and Power is transmitted from smaller pulley to
larger pulley. If distance between pulleys is increased, the thickness of belt decreases &
vice-versa. Normally the Transmission ratio (G) is given by
G = D/d,
Where d and D are diameters of smaller and larger pulley respectively.
But due to considerable thickness’t’ of the belt ‘G’ changes to
G = (D + t) / (d + t),
Now G will vary continuously as‘t’ varies. If the variation in transmission ratio is
not sufficient then it can be enlarged by increasing no. of belts in series. It can also be
increased using power split transmission.
Most commonly used another type of transmission is chain type. It is described
below the new A+CVT Steel Block Chain was designed specifically for use with the dualcone
A+CVT. This chain consists of steel block links connected with steel pins and side
plate links. Each block link has a drive lug protruding from the inner side, which meshes
with the floating sprocket bars. The drive lug is designed so that there is a single vertical
line of contact with the floating sprocket bars. This allows for a single speed ratio at each
point. This would be impossible with a belt, since a belt has multiple speed ratios across its
width, and a CVT cannot function with multiple speed ratios simultaneously. A U.S. Patent
for the A+CVT Chain has been applied for.
The original A+CVT used a beaded chain for demonstration purposes. The new
A+CVT Steel Block Chain was developed in response to concerns expressed by some
engineers that a beaded chain would be too weak for heavy-duty applications. We remain
convinced that a beaded chain of sufficient quality and strength could be developed for use
with the A+CVT. However, our attention has shifted to the new A+CVT Steel Block Chain,
as it can be scaled to meet any torque requirements more easily
CVT stands for continuously variable transmission. This type of transmission
allows for a change in ratios without stopping or disengaging the gears. Most CVT's are
friction drive, with some means of varying the relative diameters of the driving components
while driving. These friction drive transmissions are simple, but can only transmit a limited
amount of torque before the wheels start slipping. There are some CVT's that use gears and
cranks that offer positive drive without a chance for slippage, but these are much more
complicated. My Lego CVT is a friction drive assembly, and it is really just a quick concept
that wouldn't be very practical in real applications.
8. TYPES OF CVT
8.1 Pulley Based CVT
Pulley Based CVT
This type of CVT uses pulleys pulley is a wheel with a groove along its edge, for holding a
rope or cable. Pulleys are usually used in sets designed to reduce the amount of force
needed to lift a load. However, the same amount of work is necessary for the load to reach
the same height as it would without the pulleys. The magnitude of the force is reduced, but
it must act through a longer distance. Pulleys are usually considered one of the simple
machines. A chain Roller chain or bush roller chain is the type of chain most commonly
used for transmission of mechanical power on bicycles, motorcycles, and in industrial and
agricultural machinery. It is simple, reliable, and efficient (as much as 98% efficient under
ideal conditions), but requires more attention to maintenance than may be desired by
potential owners; therefore there has been of late a tendency towards the use of other modes
of other modes of power transmission such as the cog belt.
8.2 Roller-Based CVT
Consider two almost-conical parts, point to point, with the sides dished in such that
the two parts could fill the central hole of a torus is a doughnut-shaped surface of revolution
generated by revolving a circle about an axis coplanar with the circle. The sphere is a
special case of the torus obtained when the axis of rotation is a diameter of the circle.
If the axis of rotation does not intersect the circle, the torus has a hole in the middle
and resembles a ring doughnut, a hula hoop and an inflated tire (U.K. tyre). The other case,
when the axis of rotation is a chord of the circle, produces a sort of squashed sphere
resembling a round cushion. Torus was the Latin word for a cushion of this shape. One part
is the input, and the other part is the output (they do not quite touch). Power is transferred
from one side to the other by one or more rollers. When the roller's axis is perpendicular to
the axis of the almost-conical parts, it contacts the almost-conical parts at same-diameter
locations and thus gives a 1:1 gear ratio. The roller can be moved along the axis of the
almost-conical parts, changing angle as needed to maintain contact. This will cause the
roller to contact the almost-conical parts at varying and distinct diameters, giving a gear
ratio of something other than 1:1.
8.3 Hydrostatic CVT
Some continuously variable transmissions instead use a variable displacement pump
variable displacement pump is a device that converts mechanical energy to hydraulic (fluid)
energy. Some of these devices can also be reversible, meaning that they can act as a
hydraulic motor and generate mechanical energy from fluid energy. The displacement can
be adjusted to increase or decrease the amount of fluid pumped and a hydraulic motor to
transmit power. These types can generally transmit more torque, but they are very
expensive to buy and maintain. However, they have the advantage that the hydraulic motor
can be mounted directly to the wheel hub, allowing a more flexible suspension system and
eliminating efficiency losses from friction in the drive shaft and differential components.
This type of transmission has been effectively applied to expensive versions of light duty
ridden lawn mower
8.4 Intermeshing Cones
The basic principle here is that two cones are fashioned such that they can slide in
and out of one another, effectively creating a v-shaped pulley that varies in size as the
cones are moved in and out of one another. Anderson's idea is a modification of the dualcone
CVT. It uses floating sprocket bars along the length of each cone to engage a chain.
This creates a more durable, positive drive system instead of using simple friction to move
a belt up and down the cones. Belt-driven CVTs are friction dependent - and that result in
power loss and a less efficient system, he said.
"There's not much more auto companies can do (to improve fuel efficiency) by
shaving weight and using plastic in bumpers and fenders," Anderson said. "The
transmission is one place where they can go to improve fuel efficiency
The toroidal transmission consists of two sets of planetary type, steerable rollers
housed between an inner and outer toroidal-shaped disc, one driving and the other driven.
By tilting the steerable rollers, the relative diameters of engagement of the input and output
toroidal discs can be varied to achieve a desired speed ratio. Very high contact pressures
exist at the point of contact between the steerable rollers and the toroidal discs. Torque is
transferred at the point of contact under a high shear stiffness traction fluid placed under
extremely high pressures, causing the fluid to become glass-like. In this mode, its behavior
is described as elastrohydrodynamic. With the fluid operating in the elastrohydrodynamic
region, metal to metal contact between the rollers and toroidal discs is prevented.
The toroidal CVT transfers torque with the help of a traction fluid, which becomes
glass-like under extremely high pressures. This configuration handles extremely high
torques at high efficiencies. SWRI engineers use computer models to model CVTs and
other transmissions to determine optimal sizing, estimate expected performance levels, and
evaluate many different options in a timely manner before beginning hardware fabrication -
all of which reduce overall development time and costs.
8.6 Super Simple
Anderson's idea is a modification of the dual-cone CVT. It uses floating sprocket
bars along the length of each cone to engage a chain. This creates a more durable, positive
drive system instead of using simple friction to move a belt up and down the cones. Beltdriven
CVTs are friction dependent and that result in power loss and a less efficient system,
he said. "There's not much more auto companies can do (to improve fuel efficiency) by
shaving weight and using plastic in bumpers and fenders," Anderson said. "The
transmission is one place where they can go to improve fuel efficiency.
Anderson's Dual-Cone CVT
9. ADVANTAGES AND DRAWBACKS
CVTs have much smoother operation than hydraulic automatic transmissions
automatic transmission is an automobile gearbox that can change gear ratios automatically
as the car or truck moves, thus freeing the driver from having to shift gears manually. The
frictional force is a function of the force pressing the surfaces together and the coefficient
of friction between the materials. In particular: and strength tensile strength of a material is
the maximum amount of tensile stress that it can be subjected to before it breaks. This is an
important concept in engineering, especially in the fields of material science, mechanical
engineering and structural engineering.CVTs can smoothly compensate for changing
vehicle speeds, allowing the engine speed to remain at its level of peak efficiency. This
improves both fuel economy and exhaust.CVT's design advantages lie not only in its
efficiency but its simplicity .It consists of very few components. A continuously variable
transmission typically includes the following major component groups:
§ A high-power/density rubber belt
§ A hydraulically operated driving pulley
§ A mechanical torque-sensing driving pulley
§ Microprocessors and sensors
Because of this simplicity in design, CVT offers some advantages over traditional
transmissions, although it also has certain Drawbacks. For instance, its belt-driven
orientation limits its application; until recently, cars with engines larger than 1.2 liters were
considered incompatible with CVT. More and more, however, CVTs are becoming
available that can handle more powerful engines, such as the V6 power plants found in
some Nissan and Audi vehicles. CVTs offer a continuum of infinitely variable gear ratios
by changing the location of pulleys heaves. As a result, CVTs have the potential to increase
the overall vehicle efficiency and reduce the jerk usually associated with manual and
automatic transmissions. One shortcoming, however, is their difficulty in transmitting high
torque at low operating speeds, which so far has limited their use to small vehicles. Other
disadvantages include its larger size and weight. Still, in the right situation, CVT's
advantages outweigh its disadvantages. Less complexity and moving parts theoretically
mean fewer things to go wrong and maintain.
Modified CVT Efficiency Map
10. EXAMPLES- IMPLEMENTATION
Many small tractors for home and garden use have simple CVTs, as do most
snowmobiles. Almost all motor scooters today are equipped with CVT.
1. Automobile 2.Trucks 3 Military vehicles 4. Heavy construction equipments 5.
Bicycle 5. Motor cycles 6. Industrial machinery 7.Any type of motor
Possibly the largest vehicle currently sold with a CVT is the Nissan Murano, a midsize
sport utility vehicle with a 3.5L V6 engine. The CVT is also available for Audi, Fiat,
Honda, Mercedes-Benz and Mini Cooper cars.
Some combines have CVT. The machinery of a combine is adjusted to operate best
at a particular engine speed. The CVT allows the forward speed of the combine to be
adjusted independently of the machine speed. This allows the operator to slow down and
speed up as needed to accommodate variations in thickness of the crop.
Automobiles Equipped With CVT
§ Audi A4 2.0/1.8T/2.4/3.0/2.5 TDI
§ Audi A6 2.0/1.8T/2.4/3.0/2.5 TDI
§ Fiat Punto 1.2
§ Ford Escape Hybrid 2.3 4cyl
§ Ford Focus C-MAX 1.6 TDCi 110ps
§ Honda Civic Hybrid 1.3 4cyl
§ Honda HR-V 1.6
§ Honda Insight 1.0 3cyl
1. M J Nunney, Light and Heavy Vehicle Technology, Elsevier Publishers
2. W D Erickson, Belt Selection and Application for Engineers, Marcel Dekker Publishers
3. Heinz Heisler, Advanced Vehicle Technology, Elsevier Publishers
5. William B Ribbens, Understanding Automotive Electronics
6. ‘Belt Slip-A Unified Approach’ by Gerbert.G, ASME Journal of Mechanical Design,
7. howstuffs works.com
9. theaudimultitronic CVT
summer project pal|
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CONTINUOUSLY VARIABLE TRANSMISSION(CVT)
ROLL NO. 19
CONTINUOUSLY VARIABLE TRANSMISSION.ppt (Size: 1.86 MB / Downloads: 575)
NEED OF TRANSMISSION
TYPES OF CVT
USE OF CVT
HOW DOES CVT WORK?
ADVANTAGES AND DRAWBACKS
CVT IS AN EMERGING AUTO MOTIVE TECHNOLOGY THAT
OFFERS A CONTINUUM OF GEAR RATIOS BETWEEN HIGH
AND LOW EXTREMS. TODAY CONTINUOUSLY VARIABLE
TRANSMISSION (C V T) HAVE LURED A GREAT DEAL OF
AUTOMOTIVEMANUFACTURERS AND CUSTOMERS.
CONTINUOUSLY VARIABLE TRANSMISSION(CVT)
DESIGN WAS DEVELOPED BY INVENTER DR. JAN NAUDE.
CVT WAS FIRST PATENTED IN EUROPE IN 19TH CENTURY
THE FIRST PATENT FOR A TOROIDAL CVT WAS FILED AT
THE END OF THE CENTURY WAS DESIGNED AND BUILT
BY DUTCH HUB VAN DOORNE.
NEED OF TRANSMISSION
THE TRANSMISSION TAKES THE POWER FROM THE ENGINE TO THE WHEEL,IN DOING SO ACTUALLY MAKES THE VEHICLE USABLE.
Allow the vehicle to start from rest, with the engine running continuously.
Let the vehicle stop by disconnecting the drive when appropriate.
Enable the vehicle to start at varied rates, under a controlled manner.
Vary the speed ratio between the engine and wheels.
Allow this ratio to change when required.
Transmit the drive torque to the required wheels.
CONTINUOUSLY VARIABLE TRANSMISSION (CVT)
Continuously variable transmissions (CVT) offer a continuum of gear ratios between desired limits. Which allows the engine to operate more time in the optimum range given an appropriate control of the engine valve throttle opening (VTO) and transmission ratio. CVT transmission operates by varying the working diameters ofthe two main pulleys in the transmission .or by changing belt thickness
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A+CVT_REPORT.doc (Size: 2.41 MB / Downloads: 221)
The primary function of a transmission is to transmit mechanical power from a
power source to some form of useful output device. Since the invention of the internal
combustion engine, it has been the goal of transmission designers to develop more
efficient methods of coupling the output of an engine to a load while allowing the engine
to operate in its most efficient or highest power range. Conventional transmissions allow
for the selection of discrete gear ratios, thus limiting the engine to providing maximum
power or efficiency for limited ranges of output speed. Because the engine is forced to
modulate its speed to provide continuously variable output from the transmission to the
load, it operates much of the time in low power and low efficiency regimes. A
continuously variable transmission (CVT) is a type of transmission, however, that allows
an infinitely variable ratio change within a finite range, thereby allowing the engine to
continuously operate in its most efficient or highest performance range, while the
transmission provides a continuously variable output to the load.
The development of modern CVTs has generally focused on friction driven
devices, such as those commonly used in off-road recreational vehicles, and recently in
some automobiles. While these devices allow for the selection of a continuous range of
transmission ratios, they are inherently inefficient. The reliance on friction to transmit
power from the power source to the load is a source of power loss because some slipping
is possible. This slipping is also a major contributor to wear, which occurs in these
To overcome the limitations inherent in the current CVT embodiments employing
friction, a conceptual, continuously variable, positive engagement embodiment has been
proposed for investigation at Brigham Young University. This concept proposes utilizing
constantly engaged gears which transmit power without relying on friction. Because the
proposed embodiment is new, no engineering analysis has yet been performed to
determine its kinematic and meshing characteristics, an understanding of which are
necessary to validate the proposed concept as a viable embodiment. This research will
investigate both the kinematic and meshing characteristics of this and related concepts.
The objective of this research is also to analyze the family of positive engagement
CVTs. Although the CVT embodiment that has been proposed for investigation is new,
other embodiments belonging to this family have been developed and published. The
embodiments in this family do not rely on friction based power transmission. All
embodiments in this family, however, have been based on overcoming a distinct problem
which manifests itself seemingly regardless of the embodiment and will hereafter be
referred to as the non-integer tooth problem. This research describes the nature of the
non-integer tooth problem and details the occurrence of the problem in the proposed
concept, as well as three published embodiments, and details solutions to the non-integer
tooth problem as embodied in the three published embodiments. The presentation of
some published solutions to the non-integer tooth problem clarifies the nature of the noninteger
tooth problem, as well as aids in the development of characteristics of a general
solution to the non-integer tooth problem applying to all members of the positive
engagement CVT family.
Continuously variable transmissions have been in use for many years. Near the
beginning of the twentieth century, cars like the Sturtevant, Cartercar, and Lambert
featured friction dependent CVTs (Puttré, 1991). These friction drive CVTs were
common in automotive use until engines capable of producing higher torques became
common and necessitated the move to geared, fixed-ratio transmissions capable of high
torque transfer and having better wear characteristics than friction dependent CVTs.
Only in the past few years, with the advent of advanced materials and technology, have
friction dependent CVTs returned to commercial application in the automotive industry.
To provide a foundation and motivation for the research presented, this chapter
first presents a definition of a continuously variable transmission. For background
purposes, a review of the current literature on CVTs is included. The families in which
various embodiments can be classified are presented, along with a description of the
operating principles in each family. A new family of embodiments of the positive
engagement classification is also presented, along with the principles governing this new
classification. This research focuses most heavily on embodiments in the final
DEFINITION AND TERMINOLOGY
A transmission is a device which allows the transmission of power from a rotating
power source to a rotating load. Conventional transmissions allow for the selection of
discrete gear ratios, thus limiting the engine to providing maximum power or efficiency
for limited ranges of transmission output speed. A continuously variable transmission,
however, is a type of transmission that allows an infinitely variable ratio change within a
finite range, thereby allowing the engine to continuously operate in its most efficient or
highest performance range.
Beachley and Frank, 1979, present a sub-classification of the continuously
variable transmission called the infinitely variable transmission (IVT). While the two
terms are often used interchangeably, there is a distinct difference between them. While
a CVT allows an infinitely variable ratio change within a finite range, an IVT must be
capable of producing an output speed of zero for any input speed, thus giving an infinite
There are several classifications of CVTs. The following five are most relevant to
the current research: hydrostatic, friction, traction, variable geometry, and electric.
Hydrostatic transmissions are commonly used in off-road vehicles and
agricultural machinery. Many commercial riding lawn mowers commonly employ
hydrostatic transmissions in their drivetrains. These transmissions use high-pressure oil,
commonly at pressures up to 5000 psi, to transmit power. They are composed of a
hydraulic pump and hydraulic motor (see Figure 2.1), which are connected by hydraulic
lines (not labeled in Figure 2.1). The hydraulic pump, which is generally driven by the
engine, provides power to the hydraulic motor, in the form of high-pressure fluid. The
hydraulic motor, in turn, converts the hydraulic power into mechanical power, which is
transferred to a load.
The continuously variable nature of this transmission comes in the ability of the
hydraulic pump to adjust the pressure and flow of hydraulic fluid that it supplies to the
hydraulic motor by changing its displacement. Hydrostatic transmissions will almost
always have a ratio range of infinity, i.e., be IVT’s. This is accomplished because the
stroke of the pump can be varied from zero to its maximum. Also, because the stroke of
the pump can generally be reversed, the hydraulic motor can have both positive and
negative rotation, thus providing forward and reverse rotations of the output.
An advantage of the hydrostatic transmission is the ability that it has to transmit
high torque from the input to the output, which allows for its application in a wide range
of devices. This is enhanced by the ability hydrostatic transmissions have for precise
speed control. One major disadvantage of hydrostatic CVTs is their moderate efficiency
(between 60 and 80%), which offsets the efficiency gains of allowing the engine to
operate in its most efficient regime.
The friction CVT is one of the most common forms of CVTs in use today. These
CVTs are characterized by the use of friction to transmit power. Traction drives use a
form of friction to transmit power, but are classified separately and will be discussed
later. In the friction CVT family, there are several different embodiments. These include
rubber V-belts, metallic V-belts, flat rubber belts and chain drives.
The common characteristic of the V-belt drives is the use of a drive and driven
sheave, each with variable diameters. The effective diameter of the sheave is adjusted by
varying the distance between the two halves of the sheave (see Figure 2.2). Each sheave
consists of one mobile and one stationary half, and the two sheaves are positioned at a
fixed center distance. As the halves of the sheave move together, the belt is forced up to
a larger diameter on the sheave. As the halves of the sheave move apart, the belt returns
to a smaller diameter. The ability to continuously vary the diameter of the drive and
driven sheaves allows for a continuously varying transmission ratio.
The sheave diameters can be varied in several ways, depending on the type of
control desired and the ratio range needed. Figure 2.3 shows a common CVT used in
snowmobile and ATV applications. It consists of two sheaves, referred to as the driver or
primary clutch, and the driven or secondary clutch, and a composite v-belt. In this
application, the control of the CVT is automatic. The primary clutch is actuated by
engine rotation, using centrifugal force on flyweights that produce an axial force on the
mobile half of the sheave, causing it to move toward the stationary half of the sheave.
The secondary sheave is referred to as a torque sensing sheave, and is spring loaded to
maintain proper belt tension.
Rubber V-belt CVTs are also commonly used in machine tools. The control in
this case, however, is a mechanical system that determines the spacing of the two halves
of one of the sheaves. Because the belt length remains constant, the second pulley must
be spring loaded, allowing it to adjust automatically.
It is common for slipping to occur in both rubber V-belt CVT applications
presented. This is because the driving force is transmitted through friction between the
sides of the V-belt and the inside surfaces of the sheaves. While this negatively affects
efficiency, it can have a positive safety effect in machine tools, especially when the
machine becomes overloaded.
An advantage of the rubber v-belt CVT is the high ratio range that it can provide,
as well as the ability for automatic speed control, which is what makes it so desirable for
use in ATVs where an expensive control system is not desirable. Some disadvantages of
this type of CVT are its low torque capability and the significant wear that develops due
to belt slipping. This wear inhibits the ability of the CVT to shift ratios properly. Belt
slipping also contributes to the moderate efficiency of the device, which is usually
between 70% and 80%.
Another common belt-type CVT is the metal push belt CVT. This belt driven
CVT is different from the previously mentioned rubber belt versions in that power is
transmitted through the belt by way of compression. The first company to commercially
develop this concept was Van Doorne Transmisse. This metal push belt CVT can
transmit more force, and therefore is better suited to the automotive industry. Figure 2.4
shows the XTRONIC CVT, developed by Nissan, which employs a metal push belt.
The construction of the metal push belt is shown in Figure 2.5. The belt consists
of thin, high-strength, segmented steel blocks that are held together by stacked bands of
steel. The bands are stacked into slots on both sides of the blocks, and help maintain the
shape of the belt as it passes through the sheaves. Kluger and Fussner, 1997, stated that
the load path is dependent on the complex interaction and friction between the bands and
block slots, the adjacent blocks, and the block sidewalls and the faces of the sheaves.
The advantage of the metal push belt over the rubber v-belt is its ability to
transmit higher torque, usually up to 350 N-m, which, as stated previously, makes it more
useful in higher torque situations, like in automobiles. It is also more efficient - between
80% and 90% - than the rubber v-belt, due to the reduced amount of slipping that it
allows. A disadvantage of the metal push belt CVT is the high contact stresses in the
sheaves, which requires special materials and special controls to minimize belt slip,
which would otherwise rapidly wear the sheaves.
A third type of friction CVT is the flat belt CVT. Kluger and Fussner, 1997, state
that flat belts are more efficient for transmitting power because more of the allowable belt
tension can be used for transmitting power rather than producing belt to sheave forces.
Developed originally by Kumm Industries, the flat belt CVT is composed of a flat
elastomer belt and two pulleys. The two pulleys are composed of two guideway discs on
each side. These guideway discs have logarithmic spiral guideway slots which support
the ends of the belt drive elements. The set of guideways in one disc have clockwise
curvature and the slots in the opposing disc have counterclockwise curvature
Actuation and control of the flat belt CVT is accomplished by means of a
hydraulic actuator in each of the two pulleys. This actuator rotates the inner set of discs
of each pulley relative to the outer set of discs. This causes the belt drive elements to be
positioned at a desired diameter (see Figure 2.7). Pressure is set in the hydraulic actuator
to generate the required belt tension at the desired speed ratio.
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wer r d pictures and drawings for this report
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A continuously variable transmission (CVT) is a transmission which can change sleeplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that only allow a few different distinct gear ratios to be selected. This can provide better fuel economy than other transmissions by enabling the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds
In this most common CVT system, there are two V-belt pulleys that are split perpendicular to their axes of rotation, with a V-belt running between them. The gear ratio is changed by moving the two sections of one pulley closer together and the two sections of the other pulley farther apart. Due to the V-shaped cross section of the belt, this causes the belt to ride higher on one pulley and lower on the other. Doing this changes the effective diameters of the pulleys, which changes the overall gear ratio. The distance between the pulleys does not change, and neither does the length of the belt, so changing the gear ratio means both pulleys must be adjusted (one bigger, the other smaller) simultaneously to maintain the proper amount of tension on the belt.
The V-belt needs to be very stiff in the pulley's axial direction in order to make only short radial movements while sliding in and out of the pulleys. This can be achieved by a chain and not by homogeneous rubber. To dive out of the pulleys one side of the belt must push. This again can be done only with a chain. Each element of the chain has conical sides, which perfectly fit to the pulley if the belt is running on the outermost radius. As the belt moves into the pulleys the contact area gets smaller. The contact area is proportional to the number of elements, thus the chain has lots of very small elements. The shape of the elements is governed by the static of a column. The pulley-radial thickness of the belt is a compromise between maximum gear ratio and torque. For the same reason the axis between the pulleys is as thin as possible. A film of lubricant is applied to the pulleys. It needs to be thick enough so that the pulley and the belt never touch and it must be thin in order not to waste power when each element dives into the lubrication film. Additionally, the chain elements stabilize about 12 steel bands. Each band is thin enough so that it bends easily.
If bending, it has a perfect conical surface on its side. In the stack of bands each band corresponds to a slightly different gear ratio, and thus they slide over each other and need oil between them. Also the outer bands slide through the stabilizing chain, while the center band can be used as the chain linkage.
A high-power metal or rubber belt
A variable-input "driving" pulley
An output "driven" pulley
CVTs also have various microprocessors and sensors, but the three components described above are the key elements that enable the technology to work.
The variable-diameter pulleys are the heart of a CVT. Each pulley is made of two 20-degree cones facing each other. A belt rides in the groove between the two cones. V-belts are preferred if the belt is made of rubber.
When the two cones of the pulley are far apart (when the diameter increases), the belt rides lower in the groove, and the radius of the belt loop going around the pulley gets smaller. When the cones are close together (when the diameter decreases), the belt rides higher in the groove, and the radius of the belt loop going around the pulley gets larger. CVTs may use hydraulic pressure, centrifugal force or spring tension to create the force necessary to adjust the pulley halves.
Variable-diameter pulleys must always come in pairs. One of the pulleys, known as the drive pulley (or driving pulley), is connected to the crankshaft of the engine. The driving pulley is also called the input pulley because it's where the energy from the engine enters the
transmission. The second pulley is called the driven pulley because the first pulley is turning it.
As an output pulley, the driven pulley transfers energy to the driveshaft.
Fig. 2 variable diameter pulleys
The distance between the center of the pulleys to where the belt makes contact in the groove is known as the pitch radius. When the pulleys are far apart, the belt rides lower and the pitch radius decreases. When the pulleys are close together, the belt rides higher and the pitch radius increases.
When one pulley increases its radius, the other decreases its radius to keep the belt tight. As the two pulleys change their radii relative to one another, they create an infinite number of gear ratios -- from low to high and everything in between. For example, when the pitch radius is small on the driving pulley and large on the driven pulley, then the rotational speed of the driven pulley decreases, resulting in a lower “gear.” When the pitch radius is large on the driving pulley and small on the driven pulley, then the rotational speed of the driven pulley increases, resulting in a higher “gear.” Thus, in theory, a CVT has an infinite number of "gears" that it can run through at any time, at any engine or vehicle speed.
The simplicity and stepless nature of CVTs make them an ideal transmission for a variety of machines and devices, not just cars. CVTs have been used for years in power tools and drill presses. They've also been used in a variety of vehicles, including tractors, snowmobiles and motor scooters. In all of these applications, the transmissions have relied on high-density rubber belts, which can slip and stretch, thereby reducing their efficiency.
The introduction of new materials makes CVTs even more reliable and efficient. One of the most important advances has been the design and development of metal belts to connect the pulleys. These flexible belts are composed of several (typically nine or 12) thin bands of steel that hold together high-strength, bow-tie-shaped pieces of metal.
Fig. 3 Metal belt design
Metal belts don't slip and are highly durable, enabling CVTs to handle more engine torque. They are also quieter than rubber-belt-driven CVTs.
1.3 SOME OTHER TYPES OF CVT’s
Toroidal or roller-based CVT
Toroidal CVTs are made up of discs and rollers that transmit power between the discs. The discs 4
can be pictured as two almost conical parts, point to point, with the sides dished such that the two parts could fill the central hole of a torus. One disc is the input, and the other is the output (they do not quite touch). Power is transferred from one side to the other by rollers. When the roller's axis is perpendicular to the axis of the near-conical parts, it contacts the near-conical parts at same-diameter locations and thus gives a 1:1 gear ratio. The roller can be moved along the axis of the near-conical parts, changing angle as needed to maintain contact. This will cause the roller to contact the near-conical parts at varying and distinct diameters, giving a gear ratio of something other than 1:1. Systems may be partial or full toroidal. Full toroidal systems are the most efficient design while partial toroidals may still require a torque converter, and hence lose efficiency.
Another version of the CVT -- the toroidal CVT system -- replaces the belts and pulleys with discs and power rollers
Fig. 4 Nissan Extroid toroidal CVT
Although such a system seems drastically different, all of the components are analogous to a belt-and-pulley system and lead to the same results -- a continuously variable transmission. Here's how it works: 5
• One disc connects to the engine. This is equivalent to the driving pulley.
• Another disc connects to the drive shaft. This is equivalent to the driven pulley.
• Rollers, or wheels, located between the discs act like the belt, transmitting power from one disc to the other.
The wheels can rotate along two axes. They spin around the horizontal axis and tilt in or out around the vertical axis, which allows the wheels to touch the discs in different areas. When the wheels are in contact with the driving disc near the center, they must contact the driven disc near the rim, resulting in a reduction in speed and an increase in torque (i.e., low gear). When the wheels touch the driving disc near the rim, they must contact the driven disc near the center, resulting in an increase in speed and a decrease in torque (i.e., overdrive gear). A simple tilt of the wheels, then, incrementally changes the gear ratio, providing for smooth, nearly instantaneous ratio changes.
INFINITELY VARIABLE TRANSMISSION (IVT)
A specific type of CVT is the infinitely variable transmission (IVT), in which the range of ratios of output shaft speed to input shaft speed includes a zero ratio that can be continuously approached from a defined "higher" ratio. A zero output speed (low gear) with a finite input speed implies an infinite input-to-output speed ratio, which can be continuously approached from a given finite input value with an IVT. Low gears are a reference to low ratios of output speed to 6
input speed. This low ratio is taken to the extreme with IVTs, resulting in a "neutral", or non-driving "low" gear limit, in which the output speed is zero. Unlike neutral in a normal automotive transmission, IVT output rotation may be prevented because the backdriving (reverse IVT operation) ratio may be infinite, resulting in impossibly high backdriving torque; ratcheting IVT output may freely rotate forward, though.
The IVT dates back to before the 1930s; the original design converts rotary motion to oscillating motion and back to rotary motion using roller clutches. The stroke of the intermediate oscillations is adjustable, varying the output speed of the shaft. This original design is still manufactured today, and an example and animation of this IVT can be found here. Paul B. Pires created a more compact (radially symmetric) variation that employs a ratchet mechanism instead of roller clutches, so it doesn't have to rely on friction to drive the output. An article and sketch of this variation can be found here
Most IVTs result from the combination of a CVT with a planetary gear system (which is also known as an epicyclic gear system) which enforces an IVT output shaft rotation speed which is equal to the difference between two other speeds within the IVT. This IVT configuration uses its CVT as a continuously variable regulator (CVR) of the rotation speed of any one of the three rotators of the planetary gear system (PGS). If two of the PGS rotator speeds are the input and output of the CVR, there is a setting of the CVR that results in the IVT output speed of zero. The maximum output/input ratio can be chosen from infinite practical possibilities through selection of additional input or output gear, pulley or sprocket sizes without affecting the zero output or the continuity of the whole system. The IVT is always engaged, even during its zero output adjustment.
IVTs can in some implementations offer better efficiency when compared to other CVTs as in the preferred range of operation because most of the power flows through the planetary gear system and not the controlling CVR. Torque transmission capability can also be increased. There's also possibility to stage power splits for further increase in efficiency, torque transmission capability and better maintenance of efficiency over a wide gear ratio range
An example of a true IVT is the SIMKINETICS SIVAT that uses a ratcheting CVR. Its CVR ratcheting mechanism contributes minimal IVT output ripple across its range of ratios.
Another example of a true IVT is the Hydristor because the front unit connected to the engine can displace from zero to 27 cubic inches per revolution forward and zero to -10 cubic inches per revolution reverse. The rear unit is capable of zero to 75 cubic inches per revolution
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13-07-2012, 09:25 PM
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14-07-2012, 09:48 AM
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16-07-2012, 08:29 AM
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