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CONTENTS
SR. NO.
TITLE
PAGE NO.
1 Introduction6
2 Why CVT ?
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3 Types of CVT
3.1 mechanical 4ws
3.2 hydraulic 4ws3.3 electro-hydraulic 4ws
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4
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5 Challenges and limitations15
6
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7
Applications of 4ws
7.1 Chevrolet suburban 2500
7.2 Gm concept truck7.3 Jeep hurricane
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Conclusion 20
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. Overthe 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
owncharacteristics, 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-
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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
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 polygonalaction 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 changesteplessly 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
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and performance will continue to increase, which in turn makes further development and
application of CVT technology desirable.
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
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In this
most
Figure 1. Variable diameter pulley CVT
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
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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.
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 drivenpulley, 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.
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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
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
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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.
Rollers, or wheels, located between the discs act like the belt, transmitting power
from one disc to the other.
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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 ahydraulic 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 avariety of inexpensiveand expensive versions of ridden lawn mowers and garden tractors. Many versions of
riding lawn mowers and garden tractors propelled by a hydrostatic transmission are
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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 theflowing 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, acontinuously variable transmission is perfectly smoothit 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, asthe 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 thepower 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 Range1 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 vehiclespeed, 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 morethan 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 Hondas 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 inthe 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 amuch 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 orrollers.
This is caused by the lack of discrete gear teeth, which form a rigid mechanical connectionbetween to
gears; 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
conventional
automatics), but this reduces the CVTs 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 todays 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 manufacturersautomakers from Japan,
Europe, and the U.S. are already either using or developing CVTswhich 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-403. An Overview of CVT Research Past, Present, and FutureKevin R. Lang May
3, 2000
4. http:\\www.howstuffworks.com
5. http://en.wikipedia.org/wiki/Continuously_variable_transmission
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