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1. INTRODUCTION1.1 What Is CVT?CVT stands for continuously variable transmission. In manual transmission instead of power being transmitted through a series of gears, the drive goes through two ‘variators’ and a ‘v’-section steel belt.1.2 CVT BASICS:Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears, which means they don't have interlocking toothed wheels. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability between highest and lowest gears with no discrete steps or shifts.1.3 HOW IT WORKSAlthough 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 variators resemble larger versions of conventional v-belt pulleys that you find driving your car’s alternator. There’s one crucial difference: the variators is split into halves. As they move closer together under hydraulic control, the diameter of the ‘pulley’ increases; as they move further apart, it decreases. Upon acceleration, the engine-side variators increases in diameter, raising the gear ratio. Fig no.1 CVT IN A TWO WHEELER2. TYPES OF CVT:2.1 PULLEY BASED CVTs: Peer into a planetary automatic transmission, and you'll see a complex world of gears, brakes, clutches and governing devices. By comparison, a continuously variable transmission is a study in simplicity. Most CVTs only have three basic components: • 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. 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. 2.2 TOROIDAL CVT:Another version of the CVT -- the toroidal CVT system -- replaces the belts and pulleys 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 othe
Citation preview
CONTINUOUSLY VARIABLE TRANSMISSION
2011-2012
Certificate
DEPARTMENT OF MECHANICAL ENGINEERING,
Has completed technical paper presentation on topic of
“CONTINUOUS VARIABLE TRANSMISSION”
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CONTINUOUSLY VARIABLE TRANSMISSION
ABSTRACT
In today’s world automatic transmission are already ruling the roost in
developed markets. The next generation auto boxes that have already
forayed into our country are trying to bridge the gap between manual and
automatics both in terms of fuel economy and performance.
CVTs promise, both as a boon to fuel economy and as a low-cost alternative
to conventional transmissions. At the top end of market however the dice is
heavily loaded for the automatics as the technology is getting better and
better.
With the introduction of improved materials, such as high-density rubber belts,
advanced hydraulics and, more recently, high-speed sensors and
microprocessors, the stage was set for CVT’s rise in automobile.
This paper reveals the basic idea behind CVT technology further the various
types of CVTs available are mentioned. A small case study on Honda’s CVT
called as “Hondamatic” shows the latest development in CVT technology. In
the case study the working of it is studied and also its implementation in the
vehicles is specified. The advantages of the technology shows how it is more
superior than compared to the conventional manual type transmissions.
At last mentioning some disadvantages and area of improvement tells you
how the technology can emerge as a boon to shiftless driving.
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CONTINUOUSLY VARIABLE TRANSMISSION
INDEX
SR. NO. CONTENT PAGE NO.
1 INTRODUCTION 05
2 WORKING 07
3 TYPES OF CVT 08
4 DIFFERTIAL GEAR RATIO 11
5 PUMP SYSTEM 13
6 COMPARISON OF CVT 16
7 EFFICENCY 17
8 ADVANTAGES 18
9 LIMITATIONS 19
10 CONCLUSION 20
11 REFERANCE 21
LIST OF FIGURES
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1. CVT IN A TWO WHEELER 7
2. TOROIDAL CVT 8
3. HYDROSTATIC CVT 9
4. CVT FOR MINI BAJA VEHICLE 10
5. DIFFERTIAL GEAR 12
6. PUMP SYSTEM 14
7. COMPARISON OF CVT 17
1. INTRODUCTION
1.1 What Is CVT?
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CONTINUOUSLY VARIABLE TRANSMISSION
CVT stands for continuously variable transmission. In manual transmission
instead of power being transmitted through a series of gears, the drive goes
through two ‘variators’ and a ‘v’-section steel belt.
1.2 CVT BASICS:
Unlike traditional automatic transmissions, continuously variable
transmissions don't have a gearbox with a set number of gears, which means
they don't have interlocking toothed wheels. The most common type of CVT
operates on an ingenious pulley system that allows an infinite variability
between highest and lowest gears with no discrete steps or shifts.
1.3 HOW IT WORKS
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 variators resemble larger versions of conventional v-belt pulleys that you
find driving your car’s alternator. There’s one crucial difference: the variators
is split into halves. As they move closer together under hydraulic control, the
diameter of the ‘pulley’ increases; as they move further apart, it decreases.
Upon acceleration, the engine-side variators increases in diameter, raising the
gear ratio.
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CONTINUOUSLY VARIABLE TRANSMISSION
Fig no.1 CVT IN A TWO WHEELER
2. TYPES OF CVT:
2.1 PULLEY BASED CVTs:
Peer into a planetary automatic transmission, and you'll see a complex world
of gears, brakes, clutches and governing devices. By comparison, a
continuously variable transmission is a study in simplicity. Most CVTs only
have three basic components:
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. V-belts get
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CONTINUOUSLY VARIABLE TRANSMISSION
their name from the fact that the belts bear a V-shaped cross section, which
increases the frictional grip of the belt.
2.2 TOROIDAL CVT:
Another version of the CVT -- the toroidal CVT system -- replaces the belts
and pulleys 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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CONTINUOUSLY VARIABLE TRANSMISSION
Fig no.2 Toroidal CVT
2. HYDROSTATIC CVT:
Both the pulley-and-V-belt CVT and the toroidal CVT are examples of
frictional CVTs, which work by varying the radius of the contact point between
two rotating objects. There is another type of CVT, known as a hydrostatic
CVT that uses variable-displacement pumps to vary the fluid flow into
hydrostatic motors. In this type of transmission, the rotational motion of the
engine operates a hydrostatic pump on the driving side. The pump converts
rotational motion into fluid flow. Then, with a hydrostatic motor located on the
driven side, the fluid flow is converted back into rotational motion.
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CONTINUOUSLY VARIABLE TRANSMISSION
3. CVT FOR MINI BAJA VEHICLE
In the case of the Virginia Tech Mini Baja vehicle, the particular
CVT used is a rubber V-belt type. Such a CVT consists of two pulleys
connected by a rubber V-belt. The primary sheave is driven directly by the
engine while the secondary sheave provides input to a secondary
reduction. This is a common arrangement in small vehicles that
incorporate CVTs. Fig. No. 4.1 shows the actual photograph and Fig. No.
4.2 shows working arrangement of CVT for Mini Baja vehicle.
The main components of the primary pulley are fixed and movable
sheaves, a set of two flyweights, and a compression spring. As the engine
speed increases, the flyweights tend to swing open and push the movable
sheave inward toward the fixed
Sheave. However, this movement is not possible until the force
created by the flyweights is able to overcome the force caused by the
primary spring. Once this is occurs, the flyweights must also overcome the
resisting force caused by friction between the belt and the sheaves as well
as the spring forces in both the primary and secondary pulleys.
FIG NO. 3.1 PHOTOGRAPH OF DRIVE ARRANGEMENT IN MINI BAJA
VEHICLE.
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CONTINUOUSLY VARIABLE TRANSMISSION
The secondary pulley includes fixed and moveable sheaves and a
spring loaded in compression. As the primary begins to shift, its two
sheaves move closer together. This, in turn, narrows the width of the v-slot
driving the belt to a larger diameter. The normal force caused by the
wedging of the belt between the sheaves forces the secondary sheaves
apart, which allows the belt to move to a smaller diameter on the
secondary. This is the process that shifts the system to a higher gear ratio.
In an ideal case, such a combination is capable of producing an overdrive
gear a ratio of 0.75:1 in the CVT. The initial reduction provided by the CVT
is 3.5:1.
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CONTINUOUSLY VARIABLE TRANSMISSION
FIG NO. 3.2 WORKING OF CVT FOR MINI BAJA VEHICLE
3 DIFFERENTIAL GEAR RATIOS WITH THE HELP OF CVT
MECHANISM
When the speed of front pulley increases, the two halves are pulled together,
allowing the V belt to rise the pulley away from the axis of rotation. Therefore, the
belt moves faster. Just the opposite occurs in the secondary clutch where the belt sinks
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CONTINUOUSLY VARIABLE TRANSMISSION
deeper allowing the lower gear ratio and therefore the belt moves slowly. This is the
arrangement for differential gear box.
Fig. No.5
4.1.1 Gear Ratio Control
Gear ratio is controlled by a pre-set 3D map with car speed, throttle position, and
ideal engine rpm as its 3 axis. The difference between current and ideal engine rpm on
the chart is continuously fed back to the ECU. Using linear solenoids, the four-way
valves controlling pulley width are activated.
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CONTINUOUSLY VARIABLE TRANSMISSION
In order to prove useful in all situations, there are three 3D maps the user can
select from. In D(rive) mode, the upper power band is avoided, resulting in excellent
fuel economy. In S(port) mode, the user can redline the engine.
Fig. No.8
4.1.2 Acceleration Control
The acceleration clutch's power transfer amount is controlled via the clutch piston's
oil pressure. Additionally, the amount of creep is also controlled here as well. The
"smart" creep has two settings - one with the brakes on, and one with the brakes off.
While the brake pedal is depressed, there is very little to no creep, while releasing the
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CONTINUOUSLY VARIABLE TRANSMISSION
brake pedal will engage creep again. By dropping the creep level while the
automobile is stopped.
Additional fuel efficiency is realize
4.1.3 Side pressure control
In addition to the standard step-less transmission's oil-pressure controlled layout,
we considered the effects of torque on the pulleys and belt. By applying linpressure.
Friction is lowered, and the oil pump does not have to work as hard. Both yield in
high fuel efficiency and better endurance of the transmission.
2.2 Pump System
The research lead to radial piston pump, which has been further developed for
transmission application. The radial piston pump is suction controlled to improve
efficiency. This newly developed highly efficient oil pump has additional advantage in that
it can be installed in line with the drive shaft to reduce installation volume and costs.
Fig. No. 3
2.3 Forward And Reverse Driving System Device
For forward and reverse double pinion planetary gear type gear set is used. The forward
mode planetary gear set is blocked to avoid the rolling and friction losses and ensure the
high efficiency.
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2.4 Variator System
Variator is designed for high load and wide range ratio. The variator is driven by newly
developed 30 mm VDT push belts. This was developed for low noise and low efficiency.
2.5 Final Drive Gear Set And Differential
Final drive gear set consists of two ratios first ratio is produced by gear on
secondary shaft and the gear on the drive shaft. These gears are flexible and easily
changeable to obtain different final ratio for different applications. The drive shaft and
the differential are the most expensive parts; therefore they were left unchanged for all
applications to increase production volume and reduce cost.
1.2 Efficiency
Use of CVT in the automotive application has begun to increase; however,
because of their relative newness and previous use in non-automotive applications
broad base of technical information on various types of CVT’s does not exist.
Currently there are number of CVT technologies that have been used in automotive,
off roads and industrial applications. The most of the current production automobiles
use either conventional manual transmission or automatic transmission configured
with multiple planetary gear sets that use the integral clutches and bands to achieve
the discrete gear ratios. The most common current production automatic transmission
have four gear ratios while newly introduced transmissions have five gear ratios
typically the additional fifth gear is introduced to improve the vehicle starting
characteristic. The characteristics of automatic transmission are that they have large
efficiency variation throughout the wide dynamic range of torque and speed
conditions that constitute a typical automotive operation as given below.
Gear Efficiency percentage
1 60-85 %
2 60-90 %
3 85-95 %
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CONTINUOUSLY VARIABLE TRANSMISSION
4 90-95 %
5 85-94 %
The overall
efficiency rating for
automatic
transmission is 86% which most current production units achieve. This is in contrast
to the manual transmission that has the
similar rating of 95%. In general CVT offers promise of better efficiency than the
conventional transmission particularly at part
CVT Mechanism Mechanism efficiency
Belts
Steel
Rubber
90-95 %
90-97 %
Traction
Toroidal
Nutating
70-94 %
75-96 %
Epicyclic 85-93 %
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4. COMPARISON OF CVT WITH OTHER METHOD FOR FUEL ECONOMY
There are several technique used to reduces the fuel consumption
of current vehicle. These methods are as follows.
1. Stop at idle: - Switching off the engine at idle condition.
2. CDA:- Cylinder deactivation method
3. CAI: - Controlled auto ignition method.
4. LIVC: - Load control using intake valve closing method.
If all these methods are applied in standard engine and result is
tabulated for manual transmission and continuously variable transmission
as shown in graph which greatly increase the fuel economy.
FIG NO4 SUMMARIES OF PREDICTED NEDC FUEL ECONOMY
BENEFITS.
The graph shows the New European drive cycle (NEDC) fuel
consumption results for the vehicle when fitted with the CVT. Fitting a CVT
to the vehicle whilst retaining the standard engine gives a predicted fuel
economy improvement of 12% as compared to manual transmission.
Additionally the 0-100 km/h acceleration time is predicted to be improved
by 0.4 seconds. The addition of a CVT with the engine modifications
discussed previously also yields a similar benefit in all cases. The most
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CONTINUOUSLY VARIABLE TRANSMISSION
significant improvement is seen when a CVT is used in conjunction with
CAI. The best overall fuel consumption performance is achieved when the
CVT is used with the proposed LIVC engine, giving a predicted
improvement of almost 25%.
4.1 EFFICIENCY LOSS AND ITS CONTROL
In order to assess the efficiency improvement potential of the proposed
modifications first the power losses occurring in the reference transmission
were carefully modeled. The main power losses are caused by the hydraulic
pump and by the variator and were obtained by means of measurements.
Losses in the power reduction were calculated by means of literature data
and models. These loss components will be treated separately in the
following sections.
(b) Efficiency LOSS DUE TO SLIP AND ITS CONTROL
The variator efficiency losses occur due to tendency of slip of belt.
To reduce power loss due to slip the optimum clamping force is consider.
For the optimum clamping force safety factor Sf is consider as which is
ratio of actual clamping force to the minimum clamping force required for
transfer of a specific input torque at a specific variator ratio. The following
fig4.2 shows that the Sf must be reduce to lowest possible value in order
to obtain variator efficiency. Usually, lowering of the safety factor is
severely limited, especially for part load conditions. Low safety value at
low nominal torque increases the risk of variator slip caused by torque
peaks originating from road irregularities. This forms another reason why
safety values are seen to be considerably higher than 1 in practice.
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CONTINUOUSLY VARIABLE TRANSMISSION
FIG. NO4.2 EFFICIENCY VERSUS SAFETY FACTOR (OVER-
DRIVE, 1500rpm).
5.INHERENT ADVANTAGES & BENEFITSCertainly, 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, CVT’s offer improved efficiency and performance.
TABLE (1) EFFICIENCY VS. GEAR RATIO FOR AUTOMATIC
TRANSMISSION
G
ear
Efficiency Range
1 60-85%
2 60-90%
3 85-95%
4 90-95%
5 85-94%
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TABLE (2) EFFICIENCY OF VARIOUS CVT DESIGNS
CVT Mechanism Efficiency Range
Rubber Belts 90-95%
Steel Belts 90-97%
Toroidal Traction 70-94%
Nutating Traction 75-96%
Variable Geometry 85-93%
Table (1) shows the power transmission efficiency of a typical five-
speed automatic, i.e. the percentage of engine power translated through
the transmission. By comparison, Table (2) above gives efficiency ranges
for several CVT designs
The potential for fuel efficiency gains can also be seen in the CVT
currently used in Honda’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.
In general CVT has the following advantages:
Continuously changing reduction ratio, which keeps the
engine work in optimum rate.
Great decreasing of fuel consumption.
Smooth and impact less transmission of power to the driving
wheels.
Maximum utilization of power because there is no need of
temporary decoupling of the engine and the transmission
for changing from one gear to another.
Less dangerous substances in output gases and less
contamination of environment.
Safe and pleasant driving as the driver is concentrated on the
road conditions and does not distract his attention for shifting.
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6. LIMITATIONS OF THE EXISTING CVT
CVT have the following disadvantages
Transmit motion by friction, witch leads to a greater wearing and a
short exploitation period;
Cannot transmit great powers;
Require great forces of pressure between the cone sheaves and the
belt and between the toroid cylinders and disks, witch frau its behalf
leads to greater dimensions and a special and sophisticated
hydraulic system for governing;
have small diapason of changing of the transmission ratio and
require installing of hydraulic transformer between the engine and the
transmission which by itself is a sophisticated and expensive
aggregate;
Require special oils and materials;
Sophisticated and expensive technologies for their production;
7. CONCLUSION
Unlike traditional automatic transmissions, continuously variable
transmissions don't have a gearbox with a set number of gears, which
means they don't have interlocking toothed wheels. The most common
type of CVT operates on an ingenious pulley system that allows an
infinite variability between highest and lowest gears with no discrete steps
or shifts.
There are some drawbacks of this technology such as overtaking is
quite difficult in this and also the because of slipping of the belt there is a
power loss but as the technology progresses there will be changes and
also solutions for the problems will available.
8. REFERANCES
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Christopher Ryan Willis, Dr. Charles F. Reinholtz, Chair, Dr. Richard M.
Goff, Dr. Mehdi Ahmadian,
“A Kinematic Analysis and Design of a Continuously Variable
Transmission”, 19 January 2006 Blacksburg, VA.
1. Mohan Gangadurai, . Harikrishanan and B. Sreekumar,
“Development of an Analytical Design Concept of
Mechanically Controlled Continuously Variable Transmission”,
SAE paper 2005-26-069
3 AUTODRIVE JUNE ‘05
4 OVERDRIVE AUG’05
5 AUTOMOBILE ENGINEERING by Crouse Anglin
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