CONTINUOUSLY VARIABLE TRANSMISSION

CONTINUOUSLY VARIABLE TRANSMISSION

2011-2012

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DEPARTMENT OF MECHANICAL ENGINEERING,

Has completed technical paper presentation on topic of

“CONTINUOUS VARIABLE TRANSMISSION”

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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CONTINUOUSLY VARIABLE TRANSMISSION

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