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

Salim Nasser

Mechanical Design II

Fall Semester 2002

Mechanical Engineering Dept.

Florida International University

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 optimize

engine efficiency Allows the engine to rev almost immediately

to deliver maximum torque

Main types of CVT’s:

Metal Push Belt CVT.Toroidal Drive CVT. Variable Diameter Elastomer Belt CVT Nutating Traction Drive CVT

History of CVT’S:1490: Leonardo da Vinci drew sketches

of his ideas for a CVT.1886:The first CVT was built by Daimler

and Benz.1930’s: GM developed a toroidal CVT.1958: Daf (Netherlands) developed and

produced a belt driven CVT for a 0.6L engine.

Austin 18. Used a CVT1934

The good and the bad of CVT’s: ADVANTAGES Decreases engine fatigue Allows for an “infinite” number

of gear ratios, maintaining the engine in its optimum power range

More mechanically efficient than Automatic transmissions.

Greater fuel efficiency than both manual and automatic transmissions.(Fuel savings of more than 17% have been achieved).

Cheaper and lighter than Automatic trans.

Smooth, responsive and quiet to drive

Newer CVT’s have a “manual” option, giving the driver more control, simulating a MT.

CPU can be configured to suit a wide range of driving modes and styles.

DISADVANTAGES Limited torque capacity when

compared w/manual transmissions.

Larger and more costly than manual transmissions.

Slipping in the drive belt or pulleys.(NO LONGER AN ISSUE DUE TO NEW ADVANCES).

Rubber band effect.(SOLVED) Complacency by the automobile

industry, unwilling to discard billions of dollars in development in MT & AT’s.

CVT AT MT

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 the circumference of the tapered pulley halves.

A “belt” is used to transfers the engine's power from one shaft to another.

The variators are adjusted hydraulically.

When one pulley is varied, the other pulley must adapt itself inversely since the length of the belt is fixed.

Push Belt CVT Illustrations:

•A CVT can convert every point on the engine's operating curve to a corresponding point on its own operating curve.

Gear_ratior o

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•The belt rides lower or higher along the walls of the pulley, depending on driving conditions, thereby changing the gear ratio.

CVT Schematic :Belt Drive

Hondas’ V-Belt CVT•The acceleration clutch is placed on the driven shaft in order to control torque at the closest possible place to the axle.

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

Van Doorne Steel Belt (1):oAlmost all of today’s belt driven CVT’s use this design invented by Dutch CVT specialist Van Doorne.

o Maximum torque it can withstand is around 150 lb-ft (190hp).

Used in:

•Honda Civic HX

• Nissan Primera

• Toyota Prius

•Honda Insight

• BMW Minicooper

• Saturn Vue .

Van Doorne Steel Belt (2):Assembled/Disassembled

1993 Williams/Renault FW15C CVT prototype 1994 FIA banned CVT’s from F1 racing•Without as much development as the MT version, the experimental CVT Formula One was 1 second faster per lap.

CVT Vs. Manual TransmissionTheoretical comparison under “ideal” conditions.

The Continuously Variable Transmission (CVT) proved 35% more efficient than the Manual Transmission (MT).

With same car and engine, the CVT takes only 75% of the time to accelerate to 100km/h, compared to the MT.

1991 FIAT UNO: M=1250kg 1991 FIAT UNO: M=1250kg

Torque=101.2 N-mTorque=101.2 N-m

n=5700rpmn=5700rpmCVT MT

8.8 sec. 11.9 sec.

0-100 km/h

CVT Power transmission:

CVT Simulation:

Audi A6 MultitronicAudi A6 3.0 L

0-60 mph Mileage

A6 5-speed manual

8.2 sec9.9 litre / 100km

A6 5-speed Tiptronic

9.4 sec10.6 liter

100km

A6 Multitronic CVT

8.1 sec9.7 litre / 100km

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

A•Uses a multi-plate clutch instead of a torque converter

•The electronics detect uphill and downhill gradients, and assist the driver by compensating for the added load or boosting engine braking torque accordingly.

Audi’s Chain Belt design:•link-plate chain made entirely from steel

•As flexible as a V-belt

•Composed of several layers of platesside by side connected by high strength pins.

•The chain's tension is transmitted to the variator pulleys at the pin ends.

•Torque capacity of around 280 N-m.

Toroidal Traction Drive CVT: Uses rollers to transmit torque

between the input disc (which connects the crankshaft) and output disc (which connects the driveshaft).

A viscous fluid with high shear strength properties is fundamental in transmitting torque between rollers and discs.

Rollers and discs never touch. The angle of the rollers changes

relative to shaft position resulting in a change in gear ratio.

The change in angle by a roller must be mirrored by the opposing roller.

Basic theory of Toroidal CVT:

Sample of gear changes in TCVT’s:

•The oil is specially formulated to become highly viscous under pressure.

•When pressure is exerted at the contact points between the rollers and the discs the oil resists the tendency to slide and transmits the power effectively.

Non-Newtonian Oil in TCVT:

Nissan Extroid Toroidal CVT:Used by Infinity’s Q35 and Nissan’s Cedric.

Torotrak IVT: 3D ViewTorotrak 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 disengaged depending on the TCVT ratio.

Input gears Variator(TCVT)

Epicyclic gearset FR chain clutch

Torotrak IVT: Toroid Drive

HIGH REGIME: LOW REGIME:

2nd Gear+ Below 2nd Gear

Eliminates need for Torque Converter•Developed successfully for GMs’ 5.4L Ford Expedition . (70 mph@1000 rpm.)

CVT RATINGS:

AT RATINGS:

MT RATINGS:

Future of CVT’s: The internal combustion (IC) engine is nearing

both perfection and obsolescence; advancements in fuel economy and emissions have effectively stalled.

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 to increase.

This cycle of improvement will ultimately giveCVTs a solid foundation in the world’s automotive infrastructure.