AT5 Constantly Variable Transmission (CVT)

Constantly Variable Transmission (CVT)

Constantly Variable Transmission (CVT) • Unconventional transmission design. • No fixed forward speeds. • The gear ratio varies with engine speed. • Fitted with a one-speed reverse gear. • CVT uses belts and pulleys.

Constantly Variable Transmission (CVT) • The constantly variable transmission (CVT) uses a pair of cone-shaped pulleys connected by a metal belt. • The key to the operation of a CVT system is a high-friction drive belt . • The belt, made from high performance steel, transmits drive by thrust rather than tension. • The ratio of the rotations, or the gear ratio, is determined by how far the belt rides from the centres of two pulleys.

Pitch Radius

Pitch Radius

Constantly Variable Transmission (CVT)

Constantly Variable Transmission (CVT) • The transmission can produce an unlimited number of ratios. • As the car changes speed, the ratio is continuously adjusted. • Cars with this system are said to use fuel more efficiently than cars with set gear ratios. • In the gearbox, hydraulic control is used to move the pulleys and hence change the ratio.

Constantly Variable Transmission (CVT) • To achieve forward and reverse, a standard epicyclic gear set is used. • The drive belt transmits torque from the primary cone pulley to the secondary cone pulley unit . • The belt is V-shaped and consists of several hundred steel elements held together by steel strips.

Operation (CVT)

Operation (CVT)

Operation (CVT)

Constantly Variable Transmission (CVT) • 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.

Constantly Variable Transmission (CVT) • The distance between the center of the pulleys to where the belt makes contact in the groove is known as the pitch radius. •

When the pulleys are far apart, the belt rides lower and the pitch radius decreases.

• When the pulleys are close together, the belt rides higher and the pitch radius increases. • The ratio of the pitch radius on the driving pulley to the pitch radius on the driven pulley determines the gear.

Constantly Variable Transmission (CVT) • 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.

Constantly Variable Transmission (CVT) • 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. • 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.

Constantly Variable Transmission (CVT) • One pulley is the driven member and the other is the drive. • Each pulley has a movable face and a fixed face. • When the movable face moves, the effective diameter of the pulley changes. • The change in effective diameter changes the effective pulley (gear) ratio. • A steel belt links the driven and drive pulleys.

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 Vs. Manual Transmission Theoretical comparison under “ideal” 1991 FIAT UNO: M=1250kg conditions. 0-100 km/h Torque=101.2 N-m n=5700rpm

CVT

MT

8.8 sec.

11.9 sec.

•

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.

Types of Belts(CVT) Van Doorne Steel Belt LuK-PIV chain

Types of Belts(CVT) Van Doorne Steel Belt

The first metal belt system was introduced into the market place in 1987 as the Transmatic, by Van Doorne’s Transmissie (VDT) of the Netherlands.

Types of Belts(CVT) Van Doorne Steel Belt

Types of Belts(CVT) Van Doorne Steel Belt

Van Doorne Steel Belt : 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 .

Types of Belts(CVT) LuK-PIV chain

Types of Belts(CVT) LuK-PIV chain

Types of Belts(CVT) LuK-PIV chain •

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.

Types of Belts(CVT) LuK-PIV chain