A SEMINAR REPORT

“Achieving Optimum Performance Using Continuously Variable Transmission

(CVT)”

SUBMITTED BY

ANURAG PATEL

UNDER THE GUIDANCE

OF

Prof. V. Kumar

DEPARTMENT OF MECHANICAL ENGINEERING

ALL INDIA SHRI SHIVAJI MEMORIAL SOCIETY’S

COLLEGE OF ENGINEERING, PUNE – 01

(YEAR 2012 -13)

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ALL INDIA SHRI SHIVAJI MEMORIAL SOCIETY’S

COLLEGE OF ENGINEERING, PUNE – 01

DEPARTMENT OF MECHANICAL ENGINEERING

CERTIFICATE

This is to certify that the Seminar Report entitled

“Achieving Optimum Performance Using Continuously Variable Transmission

(CVT)”

Submitted by

Anurag Patel

Mr. Anurag Patel is a bonafide work carried out under supervision & guidance

of Prof. V. Kumar and it is approved for the partial fulfillment of the requirements of

University of Pune for the award of the Master of Mechanical Engineering (Automotive

Engineering). The Seminar Work has not been earlier submitted to any other Institute or

University for the award of any Master Degree.

Prof. V. Kumar. Prof. S.V. Chaitnya.

Guide, Head Mechanical Engineering.

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Place: Pune

ACKNOWLEDGMENT

I am deeply indebted to my In-plant Training Guide, Prof. V. Kumar

For his valuable Suggestions, Scholarly guidance, constructive criticism and constant

encouragement at every step of the Project.

I also, would like to express our deepest gratitude to Prof. S.V.CHAITYNA Head of

Mechanical Engineering Department and Dr. S.P.Danao, Principal, AISSMS College of

Engineering for granting the permission to choose this undertaking.

I would like to finally like to extend deepest of thanks to all my friends who gave me

kind co-operation and continuously provided encouragement throughout the work of

seminar report. And finally I would like to thank the people who have helped me

directly or indirectly for completion of the seminar.

Place: Pune

Date:

Mr. ANURAG PATEL

(M.E. AUTOMOTIVE

ENGINEERING,

AISSMS COE PUNE)

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CONTENTS

 SR. NO.

  TITLE

 PAGE NO.

1   Introduction 6

2   Why CVT ? 8

3   Types of CVT

3.1 mechanical 4ws 3.2 hydraulic 4ws 3.3 electro-hydraulic 4ws

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4 14

5 Challenges and limitations 15

6 

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7 Applications of 4ws

7.1 Chevrolet suburban 2500 7.2 Gm concept truck 7.3 Jeep hurricane

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8 Conclusion 20

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ABSTRACT

Over the last two decades, significant research effort has been directed towards

developing vehicle transmissions that reduce the energy consumption of an automobile.

Good ride performance is one of the most important key attribute of a passenger vehicle.

One of the methods to achieve this is by using continuously variable transmission. A

continuously variable transmission (CVT) offers a continuum of gear ratios between

desired limits, which consequently enhances the fuel economy and dynamic performance

of a vehicle by better matching the engine operating conditions to the variable driving

scenarios. The current paper reviews the state-of-the-art research on control of friction-

limited continuously variable transmissions. As CVT development continues, costs will

be reduced further and the performance will continue to improve, which in turn make

further development and application of the CVT technology desirable. Challenges and

critical issues for future research for control of such CVTs are also discussed.

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1. INTRODUCTION

With growing socioeconomic and environmental concern, automobile energy

consumption has become a key element in the current debate on global warming. Over

the past few decades, vehicle fuel economy plays a crucial role in determining the

emission of greenhouse gases from an automobile. There are three fundamental ways to

reduce greenhouse gas emissions from the transportation sector (a) increase the energy

efficiency of transportation vehicles, (b) substitute energy sources that are low in carbon

for carbon-intensive sources (i.e. the use of alternative fuel technologies), and (c) reduce

transportation activity. With tremendous growth in consumerism and urbanization, there

is little scope for emissions reduction to occur through a decrease in the amount of

vehicle use.

In order to achieve lower emissions and better performance, it is

necessary to capture and understand the detailed dynamic interactions in a CVT system

so that efficient controllers could be designed to overcome the existing losses and

enhance the fuel economy of a vehicle. There are many kinds of CVTs, each having their

own characteristics, e.g. Toroidal CVT, Belt CVT, Hydrostatic CVT Chain CVT, etc.

However, belt and chain types are the most commonly used CVTs, among all, in

automotive applications. Thus, this paper reviews the state-of-the-art research, in the

context of controls, of belt and chain CVTs for achieving the targets of increased fuel

economy and enhanced vehicle performance.

The basic configuration of a CVT comprises

two variable diameter pulleys kept at a fixed distance apart and connected by a power-

transmitting device like belt or chain. The pulley on the engine side is called the driver

pulley and the one on the final drive side is called the driven pulley. Figure 1 and Figure

2 depict the basic layout of a metal V-belt CVT and a chain CVT. In a metal V-belt CVT,

torque is transmitted from the driver to the driven pulley by the pushing action of belt

elements. Since there is friction between bands and belt elements, the bands, like flat

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rubber belts, also participate in torque transmission. Hence, there is a combined push–

pull action in the belt that enables torque transmission in a metal V-belt CVT system.

In a chain CVT system, the plates and rocker pins, as depicted in Fig. 2b,

transmit tractive power from the driver pulley to the driven pulley. Unlike a belt CVT,

the contact forces between the chain and the pulleys are discretely distributed in a chain

CVT drive. This leads to impacts as the chain links enter and leave the pulley groove.

Hence, excitation mechanisms exist, which are strongly connected to the polygonal

action of chain links. This causes vibrations in the entire chain CVT system, which

further affects its dynamic performance. Both belt and chain CVT systems fall into the

category of friction-limited drives as their dynamic performance and torque capacity rely

significantly on the friction characteristic of the contact patch between the belt/chain and

the pulley.

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2. WHY CONTINUOUSLY VARIABLE TRANSMISSION SYSTEM?

A Continuously variable transmission (CVT) is a transmission which can change

steplessly 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. The flexibility of a CVT allows the

driving shaft to maintain a constant angular velocity over a range of output velocities.

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 order to enact 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.

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3. TYPES OF CONTINUOUSLY VARIABLE TRANSMISSION

There are mainly three types of continuously variable transmission systems:

3.1 Variable diameter pulley CVT

3.2 Toroidal CVT

3.3 Hydrostatic CVT

3.1 Variable Diameter Pulley CVT

In this most

Figure 1. Variable diameter pulley CVT

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Most CVTs only have three basic components:

A high-power metal or rubber belt

A variable-input "driving" pulley

An output "driven" pulley

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. V-belts get their name from the fact that the

belts bear a V-shaped cross section, which increases the frictional grip of the belt.

As shown in fig. 2 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.

Figure 2. working of Variable diameter pulley CVT

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.

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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.

Figure 3. Metal belt design

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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.

3.2 Toroidal CVT

Figure 4. Toroidal CVT

Another version of CVT – the toroidal CVT system – replaces the belt and pulley with

discs and power rollers 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:

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.

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Rollers, or wheels, located between the discs act like the belt, transmitting power

from one disc to the other.

Fig 5. Working of Toroidal CVT

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.

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3.3 Hydrostatic CVT

Figure 6. Hydrostatic CVT

Hydrostatic transmissions use a variable displacement pump and a

hydraulic motor. All power is transmitted by hydraulic fluid. These types can generally

transmit more torque, but can be sensitive to contamination. Some designs are also very

expensive. 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 is relatively easy to use because all forward and reverse speed can be

accessed using single lever.

An integrated hydrostatic transaxle (IHT) uses a single housing for both hydraulic

elements and gear-reducing elements. This type of transmission, most commonly

manufactured by Hydro-Gear, has been effectively applied to a variety of inexpensive

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and expensive versions of ridden lawn mowers and garden tractors. Many versions of

riding lawn mowers and garden tractors propelled by a hydrostatic transmission are

capable of pulling a reverse tine tiller and even a single bladed plow.

One class of riding lawn mower that has recently gained in popularity with consumers is

zero turning radius mowers. These mowers have traditionally been powered with wheel

hub mounted hydraulic motors driven by continuously variable pumps, but this design is

relatively expensive. Hydro-Gear, created the first cost-effective integrated hydrostatic

transaxle suitable for propelling consumer zero turning radius mowers

Some heavy equipment may also be propelled by a hydrostatic transmission; e.g.

agricultural machinery including foragers, combines, and some tractors. A variety of

heavy earth-moving equipment manufactured by Caterpillar Inc., e.g. compact and small

wheel loaders, track type loaders and tractors, skid-steered loaders and asphalt

compactors use hydrostatic transmission. Hydrostatic CVTs are usually not used for

extended duration high torque applications due to the heat that is generated by the

flowing oil .

The Honda DN-01 motorcycle is the first road-going consumer vehicle with hydrostatic

drive that employs a variable displacement axial piston pump with a variable angle

swashplate

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5. ADVANTAGES OF CONTINUOUSLY VARIABLE TRANSMISSION

OVER MANUAL TRANSMISSION.

Certainly, the clunk of a shifting transmission is familiar to all drivers. By contrast, a continuously variable transmission is perfectly smooth—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.

Table (1) Efficiency vs. Gear Ratio for Automatic Transmission Table (2) Efficiency of Various CVT Designs

Gear Efficiency Range CVT Mechanism Efficiency Range 1 60-85% Rubber Belts 90-95% 2 60-90% Steel Belts 90-97% 3 85-95% Toroidal Traction 70-94% 4 90-95% Nutating Traction 75-96% 5 85-94% Variable Geometry 85-93%

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 Honda’s Civic. A Civic with atraditional 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.

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4. Challenges & Limitations

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 togears; friction drives are inherently prone to slip, especially at high torque. The simple solution to this problem has been to use CVTs only in cars with relativelylow-torque engines. Another solution is to employ a torque converter (such as those used in conventionalautomatics), but this reduces the CVT’s efficiency.

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6. APPLICATIONS OF CONTUNUOUSLY VARIABLE TRANSMISSION

Now a days CVTs are used widely in Harvesters , tractors, Motor scooters , Snow

Mobiles, Aircraft electrical power generating system, Drill presses , milling machines,

Designing area etc.

7. 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

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REFERENCES

1. Efficiency Optimization of the Pushbelt CVT - 2007-01-1457

2. Development of High Performance CVT Components - 04CVT-40

3. “An Overview of CVT Research Past, Present, and Future” Kevin R. Lang May

3, 2000

4. http:\\www.howstuffworks.com

5. http://en.wikipedia.org/wiki/Continuously_variable_transmission

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