Continuously Variable Transmissions2

7/29/2019 Continuously Variable Transmissions2

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Continuously Variable Transmissions (CVT):The Holy Grail of automotive

transmission systems?

MOHIT RAJ

Mechanical Engineering DeptAcharya institute of

technology


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Overview of a CVT:

Varies the transmission ratio continuously

Shifts automatically with an infinite number of

ratios. Seamless power delivery

Constantly changes its gear ratio to optimizeengine efficiency

Allows the engine to rev almost immediatelyto deliver maximum torque


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Main types of CVTs:

Metal Push Belt CVT.

Toroidal Drive CVT.

Variable Diameter Elastomer Belt CVT

Nutating Traction Drive CVT


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History of CVTS:

1490: Leonardo da Vinci drew sketchesof his ideas for a CVT.

1886:The first CVT was built by Daimlerand Benz.

1930s: GM developed a toroidal CVT.

1958: Daf (Netherlands) developed and

produced a belt driven CVT for a 0.6Lengine.


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The good and the bad of CVTs:

ADVANTAGES Decreases engine fatigue Allows for an infinite number

of gear ratios, maintaining theengine in its optimum powerrange

More mechanically efficientthan Automatic transmissions.

Greater fuel efficiency thanboth manual and automatictransmissions.(Fuel savings ofmore than 17% have beenachieved).

Cheaper and lighter thanAutomatic trans.

Smooth, responsive and quietto drive

Newer CVTs have a manualoption, giving the driver morecontrol, simulating a MT.

CPU can be configured to suit awide range of driving modesand styles.

DISADVANTAGES Limited torque capacity when

compared w/manualtransmissions.

Larger and more costly thanmanual transmissions.

Slipping in the drive belt or

pulleys.(NO LONGER AN ISSUEDUE TO NEW ADVANCES).

Rubber band effect.(SOLVED)

Complacency by the automobileindustry, unwilling to discardbillions of dollars in

development in MT & ATs.

CVT AT MT


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Metal Push Belt CVT: Uses a pair of axially adjustable

sets of pulley halves(Variators).

Both pulleys have one fixed and one

adjustable pulley halve.

The transmission ratio is varied by

adjusting the spacing between the

pulleys in line with thecircumference of the tapered pulley

halves.

A belt is used to transfers the

engine's power from one shaft to

another.

The variators are adjustedhydraulically.

When one pulley is varied, the other

pulley must adapt itself inversely

since the length of the belt is fixed.


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Push Belt CVT Illustrations:

A CVT can convert every point on the

engine's operating curve to acorresponding point on its ownoperating curve.

Gear_ratioro

ri

o

i

The belt rides lower

or higher along thewalls of the pulley,depending on drivingconditions, therebychanging the gearratio.


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CVT Schematic :Belt Drive


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Hondas V-Belt CVTThe acceleration clutch is placed on the driven shaft in order tocontrol torque at the closest possible place to the axle.

This allows for smooth acceleration and a modest amount ofcreep.(eliminates need for a torque converter. )


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1993 Williams/Renault FW15C CVT prototype

1994 FIA banned CVTs from F1 racing

Without as much development as the MT version, the

experimental CVT Formula One was 1 second fasterper lap.


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CVTVs.Manual TransmissionTheoretical comparison under ideal conditions.

The ContinuouslyVariable Transmission(CVT) proved 35% moreefficient than the ManualTransmission (MT).

With same car and

engine, the CVT takesonly 75% of the time toaccelerate to 100km/h,compared to the MT.

1991 FIAT UNO: M=1250kg

Torque=101.2 N-m

n=5700rpmCVT MT

8.8 sec. 11.9 sec.

0-100 km/h


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CVT Power transmission:


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CVT Simulation:

Audi A6 Multitronic

Audi A6 3.0 L0-60 mph Mileage

A6 5-speedmanual

8.2 sec9.9 litre /

100km

A6 5-speedTiptronic

9.4 sec10.6 liter100km

A6 MultitronicCVT 8.1 sec 9.7 litre /100km


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Audi Multitronic CVT:Used in Audis 1.8L A4 and 3.0L A8.

AUses a multi-plate clutch instead of a torque converter

The electronics detect uphill and downhill gradients, andassist the driver by compensating for the added load orboosting engine braking torque accordingly.


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Audis Chain Belt design:link-plate chain madeentirely from steel

As flexible as a V-belt

Composed of severallayers of platesside byside connected by highstrength pins.

The chain's tension istransmitted to thevariator pulleys at thepin ends.

Torque capacity ofaround 280 N-m.


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Toroidal Traction Drive CVT:

Uses rollers to transmit torquebetween the input disc (whichconnects the crankshaft) and outputdisc (which connects the driveshaft).

A viscous fluid with high shearstrength properties is fundamental

in transmitting torque betweenrollers and discs.

Rollers and discs never touch.

The angle of the rollers changesrelative to shaft position resulting ina change in gear ratio.

The change in angle by a rollermust be mirrored by the opposingroller.


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Basic theory of Toroidal CVT:


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Sample of gear changes in TCVTs:


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The oil is specially formulated to become highly viscousunder pressure.

When pressure is exerted at the contact points between therollers and the discs the oil resists the tendency to slide andtransmits the power effectively.

Non-Newtonian Oil in TCVT:


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Nissan Extroid Toroidal CVT:Used by Infinitys Q35 and Nissans Cedric.


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Torotrak IVT: 3D View

Transmission ratios are achived

using a typical Toroidal CVT.

The torque converter is

replaced by a Planetary

gearset/multiple-clutch system.

The Planetary gearset is

engaged or disengageddepending on the TCVT ratio.

Input gears Variator(TCVT)

Epicyclic gearset FR chain clutch


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Torotrak IVT: Toroid Drive

HIGH REGIME: LOW REGIME:

2nd Gear+ Below 2nd Gear

Eliminates need for Torque ConverterDeveloped successfully for GMs 5.4L Ford Expedition .(70 mph@1000 rpm.)


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CVT RATINGS:


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AT RATINGS:


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MT RATINGS:


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Future of CVTs:

The internal combustion (IC) engine is nearingboth perfection and obsolescence;advancementsin fuel economy and emissions have effectivelystalled.

CVTs could potentially allow IC vehicles to meet

the first wave of new fuel regulations As CVT development continues, costs will be

reduced further and performance will continue toincrease.

This cycle of improvement will ultimatelygiveCVTs a solid foundation in the worldsautomotive infrastructure.

Continuously Variable Transmissions2

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