Digital Media Concepts/Continuous Variable Transmission (CVT)

A Continuously Variable Transmission or CVT is a form of automatic transmission that offers smooth and seamless transitions through gear ratios as apposed to other forms of transmissions that involve a fixed number of gear ratios. Instead of "stepping through the gears", CVTs are built to allow infinite variability between highest and lowest gears. The variability results in smooth acceleration and enhanced fuel efficiency. Recent emission laws caused this transmission configuration to adopted by many automobile manufactures to increase fuel economy in their late-model vehicles and pass these laws. Manufacturers have also realized that a CVT would be able to access most of their economy engines' power which resulted in a better driver experience.

History

Early Development

Despite its modern intricacy, the idea of a continuously variable transmission has been around for almost 500 years, being first conceptualized by Leonardo Da Vinci in 1490 and officially patented in 1886 by Benz and Daimler in Europe and filed another one in 1935 in the U.S. The racing world would see the first application of CVT being used by Zenith Motorcycles. However, it was deemed unfair and was no longer allowed in hill climb events due to its efficiency.

Technological Advancements & Early Applications

DAF & Hub van Doorne

DAF's Variomatic Transmission

It wouldn't be until the 1950s where CVT was offered in production automobiles manufactured by DAF and its company founder, Hub van Doorne. In 1958, DAF's 600 model was equipped with the company's Variomatic transmission which showcased what is now the universal layout of a continuously variable transmission: a belt driven by two pulleys, each composed of two conical-shaped pulleys whose effective diameter could be changed to either spin close to the rim of the pulley or near the spindle, depending on the distance created by the two cones. The 600 would be DAF's first production model in addition to the first form of automatic transmission in the Netherlands'. DAF continued to put this transmission in their vehicles until the passenger car division of DAF was sold to the Swedish and Van Doore's patents were transferred to his newly founded company Van Doorne Transmissie. Him and new company fabricated the steel belts used in CVTs. The Swedish car manufacturer Volvo would go onto use DAF's CVT transmission in their 340 model.

Subaru

Subaru Justy was the first car to be sold in the U.S. that offered a CVT.

Japanese automobile manufacturer Subaru also had a footprint in CVTs' history. Subaru's Justy GL became the first production automobile to offer a CVT in the U.S in 1989. 30 years later, they still use of continuously variable transmission called Lineartronic. Subaru's introduction of the CVT to the U.S. opened the door for other automobile manufacturers such as Saturn, Ford, and Nissan as they would also produce vehicles with CVT. Now, almost every manufacturer has at least one model that utilizes a continuously variable transmission.

Timeline

Historic Development of CVT

Year	Event
1490	Leonardo da Vinci sketches a gearless, continuously variable transmission
1886	first toroidal CVT patent filed by Benz and Daimler
1935	Adiel Dodge receives U.S. patent for toroidal CVT
1958	DAF produces small car with a continuously variable transmission
1989	Subaru Justy GL gets introduced the USDM, the first production vehicle in the U.S. with CVT
2002	Saturn Vue is sold with CVT
2004	Ford offers CVT in many of their models

Types of CVT

A continuously variable transmission can take on many forms, but all of they follow the same concept; they provide an infinite variability of gears between highest and lowest without the discrete steps or shifts a driver would feel in a standard automatic transmission. Despite a CVT not having gears, the term gears when used to define CVTs is used to describe the ratio of the engine's crankshaft speed compared to the car's driveshaft speed. Currently, there are three different kinds of CVTs: pulley-based, toroidal, and hydrostatic.

Pulley-Based CVT

Subaru's modern Lineartronic, pulley-based CVT with a clear casing.

As the name suggests, this transmission's main components are a belt and pulleys. There is one pulley, the driving pulley, that is the variable input/input from the crankshaft and a second pulley, the driven pulley, that is the output to the driveshaft. Both the driving and driven pulley have two 20-degree cones facing each other. There is then a belt, V-Belt, that rides in the groove between the two pulleys. As the cones move away from one another, the V-Belt rides lower in the groove. Oppositely, when the cones move closer to one another, the V-Belt rides higher in the groove. Many CVT manufacturers have different methods of controlling the halves of the pulleys, including centrifugal force, spring tension, or hydraulic pressure. As one pulley increases in radius, the other pulley decreases in order to keep the belt tight and vice versa. This is what causes the infinite number of gear ratios in a pulley-based CVT. For example, at low gear, the crankshaft would cause the drive pulley's radius to lower. Then, in order to keep the V-Belt tight, the driven pulley's radius would increase, which causes the rotational speed of the driveshaft to be low. This type of CVT is the most commonly used in modern automotive production.

Toroidal CVTs

In the toroidal variation of the CVT, discs and power rollers replace the belts and pulleys of the pulley-based CVT. There is one disc that connects to the engine and another that connects to the driveshaft. These two conical discs face each other, and the power rollers located in between them transmit power from one disc to the other. The power rollers can either tilt in or out, which allow the wheels the touch the discs in different areas. Similar to the pulley-based CVT, when power rollers contact the crankshaft disc near the center, the power rollers would, in turn, make contact with the driveshaft rollers near the rim resulting in a low speed with an increase in torque and vice versa. This kind of CVT is commonly used by late model Nissans.

Hydrostatic CVTs

Instead of utilizing frictional CVTs, such as the previous two, hydrostatic CVTs use hydrostatic motors controlled by variable displacement pumps. Rotational motion of the crankshaft is used to operate a hydrostatic pump on the driving side, converting the motion into fluid flow. Then, on the driveshaft side, fluid flow is converted back to rotational motion. Usually, this system is paired with a planetary gearset and clutches resulting in a hybrid system called hydromechanical transmission. At lower speeds, power is transmitted from the driving side to the driven side hydraulically. At higher speeds, power is transmitted mechanically. The middle area of these two ends calls for the involvement of both systems. This type of CVT is typical for heavy-duty applications such as agricultural tractors and all-terrain vehicles.

Vs. Automatic Transmission

Standard Automatic Transmission

Comparison and Contrast of CVT and Automatic Transmission

Field	CVT	Automatic
Gear Ratios	Infinite gear ratios	Fixed number of gears
Weight	Lighter than automatic transmission; less mechanical parts than automatic transmission	Heavy; involves many mechanical parts: Gear sets Hydraulic system Torque converter
Fuel Efficiency	Continuous change in gear ratios allow engine to always perform at peak efficiency in addition to weight saved from transmission	Limited efficiency due to fixed gears
Driver Experience	Advantages: Very smooth driving; no abrupt shifting Smooth uphill driving Less interior noise when at constant speed Better emissions Disadvantages: Absence of connection between accelerator and engine High cost of repair/replacement Low acceleration Loud when accelerating Relatively fast wear; low durability of whole mechanism	Advantages: More connected feeling between engine and speed of vehicle Less expensive than CVT to maintain Better sense of control Lasts longer than CVT Fastest acceleration among other transmission types Disadvantages: Higher production of emissions "Gear hunting" when driving uphill Cost more upfront
Presence in Current Market	Relatively new technology CVT Professionals are not that common	Tried and true Very common in vehicles Automatic Transmission Technicians are common