Continuously Variable Transmission : Design, Working, Types, Differences & Its Applications

The CVT, or continuously variable transmission, is an innovative type of automatic transmission system that has been around for centuries. But the first CVT was designed in 1490 by Leonardo DaVinci, although it was not patented until 1886 by Daimler. Many innovations have made CVT efficient. However, they initially achieved fame in 1910 while an efficient CVT variant was developed by Zenit motorcycles. This technology is very popular because of its potential for a smoother driving experience and enhanced fuel efficiency. A continuously variable transmission, or a CVT, is a main feature in practically every model by manufacturers. This article elaborates on a CVT, or continuously variable transmission, working, and its applications.

What is a Continuously Variable Transmission?

A continuously variable transmission or a variable diameter pulley system is used to change gear ratios continuously with a chain or a belt connected to two variable pulleys, with a seamless and smooth change within gear ratios. This transmission system transmits rotating power from a single shaft to another shaft. CVT includes two significant components: a driving pulley & a driven pulley, which are interconnected through a belt to facilitate motion transfer. CVTs meet challenges while handling extensive torque within automotive applications. So it exhibits lower efficiency in power transmission as compared to traditional gearboxes.

Continuous Variable Transmission Design

The CVT or continuously variable transmission main parts include pulleys and push belts, which provide seamless changes within gear ratios by eliminating the requirement of traditional gears.

Pulleys

Gears play a significant role while the engine is connected to the wheels in a traditional transmission, However, vehicles frequently need different power & torque levels, requiring the use of a variety of gear ratios like low torque-high speed & high torque-low speed. These paradigm shifts, like conical pulleys in a CVT system, change the conventional gear arrangement to mark pulley transmission. This transmission is available in two types: primary pulley and secondary pulley.

The primary pulley is directly connected to the crankshaft of the engine, whereas the secondary pulley is connected to the drive shaft, which finally leads to the wheels of the vehicle. These pulleys have both static & movable sheaves, where each pulley can also have its axle shaft, which runs parallel to the other.
>When a belt drives the pulleys, then infinite gear ratios can be achieved where their diameters differ in response to the sheaves’ movement. Most importantly, at both ends, the expansion and contraction directions of these pulleys change.

Push Belt

The push belt works like a medium to transmit torque to the driven pulley from the driving pulley in the CVT system. Push belts are primarily available in two types: V-shape Kevlar rubber belts and steel belts, which are discussed below.

The V-shape Kevlar rubber belt is flexible, frequently used in scooters to provide greater flexibility as compared to standard rubber belts. Its V-shape facilitates smooth travel and positioning between the two pulleys.

Steel belts are used broadly in continuously variable transmissions because of their strong construction. So these belts have separate thin steel rings with a bunch of small-toothed steel elements. These belts have a unique design that provides flexibility and strength by enabling them to powerfully pass through and connect through the pulleys across an extensive range of RPMs.

Working

A continuously variable transmission works with a dual-pulley mechanism to provide an infinite number of gear ratios. The pulleys in CVTs are connected through a drive belt with a primary pulley, which is connected to the engine, whereas the secondary pulley is connected to the drive shaft. Every pulley includes a movable sheave that expands & contracts at the same time.

When the engine turns, then both pulleys’ diameters will be changed because of the sheave’s motion. Once the size of one pulley is reduced, the other size will be increased. So this constant change within pulley size will be changed into a seamless variation within diameters, thus creating a continuous range of gear ratios.

Lest discusses how this system works in two cases, at low speed and high speed.

At Low Speed

The CVT system comes into play when the vehicle needs more torque during initial acceleration. So the belt drives the pulleys to make the variable sheave of the primary pulley go away from the stationary sheave, which results in a minor diameter for the driving pulley. At the same time, the secondary pulley will be increased in diameter and generate greater torque at the starting position of the vehicle.

At High Speed

When the vehicle requires higher speed but not much torque in acceleration, then the diameter of the secondary pulley will be decreased. So this diameter reduction will occur when the movable sheave moves away from the static sheave. The primary pulley simultaneously increases in diameter to set up a specific gear ratio between them. In this state, the single revolution at the driving pulley communicates with four revolutions.

Types of CVTs

CVTs are available in different types, and each type has a unique mechanism to achieve a continuous and smooth range of gear ratios, which are explained below.

Pulley-based CVTs

A pulley-based CVT is a type of automatic transmission that works with a belt/chain and two pulleys to modify gear ratios continuously. So this transmission allows for stepless and smooth acceleration to improve fuel economy.

The pulley diameter in this transmission can be adjusted to modify the gear ratio. When the diameter of the driving pulley is small, whereas the diameter of the driven pulley is large, then the gear ratio will be high, which allows acceleration. Similarly, the gear ratio is low when the diameter is reversed, which allows for efficient cruising. The applications of pulley-based CVTs include: cars, Tractors, and Motor scooters.

These are the most common types, using two variable-diameter pulleys and a belt or chain to transmit power. The distance between the two pulleys is changed, effectively altering the gear ratio and allowing for a continuous range of speeds.

Toroidal CVTs

A toroidal CVT is a type of automatic transmission that uses a set of discs & rollers to change gear ratios by allowing for continuous and smooth changes in power delivery. So these types of CVTs are well known for their ability and high efficiency to transmit high torque.

Generally, these CVTs include two main components: an input disc & an output disc, where the rollers are arranged between them. The power roller angle can be adjusted to modify the gear ratio by allowing for continuous and smooth transitions between various speeds.

Toroidal CVTs can be used in a variety of applications, like industrial machinery, heavy-duty equipment, and automotive vehicles. The best example of a Toroidal CVT used vehicle is the Y34 Nissan Cedric.

Hydrostatic CVTs:

Hydrostatic CVT uses motors and hydraulic pumps to change fluid flow and creates the required rotating motion to adjust gear ratios. It provides smoother gear changes & decreases friction-related problems.
>The working of this CVT is that the engine-driven pump delivers pressurized hydraulic fluid to the motor to change hydraulic energy into rotating motion.

The pump displacement can be changed to control the torque and speed delivered to the electric motor and the wheels of the vehicle. All the pump’s output in a closed system can be directed to the electric motors & the speed ratio can be decided by the variation in their displacements. Hydrostatic CVTs are used in construction equipment, agricultural equipment, some vehicles, etc.

Difference between CVT & Automatic Transmission

The continuously variable transmission & automatic transmissions both are automatic, however, they significantly change in how they vary gear ratios. The following table provides a comparison between both CVT & automatic transmissions.

Signs of CVT

The signs of CVT transmission failure are discussed below.

• Overheating is a common problem caused by the continuous gear ratio adjustment with two pulley systems.
• Fluid leakage is one more problem where the CVT transmission depends on transmission fluid.
• Low transmission fluid levels can lead to component injury, which results in delayed motion and issues with set points. The transmission fluid with contamination of dirt can affect its performance, which causes the vehicle to exhibit strange lurches, surges, jumps, shakes, etc.
• CVT malfunction can produce odd noises when the car is also in neutral. So these noises may signal automatic deterioration of transmission components or inadequate transmission fluid.
• A CVT transmission may experience an unexpected acceleration loss, mainly when the system overheats.
• Gear slippage is another sign of CVT problems while driving a car.
• Check your car’s engine light, whether it illuminates or not, so it may indicate either major or minor CVT transmission issues.

Maintenance

A continuously variable transmission needs frequent maintenance, mainly when fluid changes, to ensure longevity and best performance. So the step-by-step maintenance steps are discussed below.

• Use the proper CVT fluid.
• Fluid change must be done based on the suggested schedule of the manufacturer, which is for every 30K to 60K miles.
• Check the condition of the fluid frequently if your vehicle includes a CVT dipstick.
• Limit extreme weight and heavy towing because it can damage the CVT and lead to overheating.
• Stay away from jerky movements and aggressive acceleration, which can put additional stress on the transmission.
• Frequent maintenance can extend CVT lifespan, improve performance, avoid problems, reduce costly repairs, etc.
• Frequent maintenance is considered in heavy towing, stop-and-go driving, steep inclines, etc.
• Sudden braking and acceleration can pressure the transmission and shorten its lifetime. So, need to drive smoothly and stay away from hard stops and starts.
• Pay attention to any strange noises, difficulty accelerating, or slipping gears because these could indicate an issue that requires attention.
• Use the right type of CVT fluid specified by the manufacturer.
• Check the temperature of transmission fluid & avoid prolonged heavy driving periods, particularly in hot weather.

Advantages & Disadvantages

The continuously variable transmission provides many benefits over the traditional automatic transmission, which include the following.

• The continuously variable transmission maintains the RPM of the engine in an optimal and narrow range for improved fuel economy than traditional transmissions with partial gear ratios.
• They provide a nonstop range of gear ratios by removing the associated noticeable jerks and shifts with traditional automatic transmissions.
• Its simplified design with fewer moving parts can provide less deterioration, potentially needing less maintenance.
• CVTs allow the engine to work at its most efficient RPM by constantly changing the gear ratio to optimize fuel efficiency and power output.
• CVTs are suitable for city driving in stop-and-go conditions due to seamless and smooth operation, which has the ability to maintain the engine in its best RPM range.
• It gives an effortless and smooth driving experience on long highway drives.

The disadvantages of continuously variable transmission include the following.

• CVTs are not suitable for towing heavy loads.
• Some continuously variable transmission models can be less durable as compared to conventional transmissions, which potentially leads to improved wear and tear.
• The smooth and continuous gear ratios within CVTs can seem less engaging for some vehicle drivers who desire the gear shift sensation.
• These are complex and need specialized training and knowledge for repairs.
• Some CVTs generate a clear noise, mainly in acceleration, when the belts enlarge and contract to change the gear ratio.
• CVTs can be responsive to high temperatures and may get hot in prolonged driving under difficult conditions.
• The CVT unit, in some cases, might wrongly interpret engine inputs to cause the engine to rev needlessly to a higher gear, which leads to performance problems.

Continuously Variable Transmission Applications

The applications of continuously variable transmission include the following.

• The continuously variable transmission is used in automobiles, particularly in smaller vehicles that prioritize fuel economy.
• CVTs can be used in hybrid electric vehicles to manage the power delivery from the electric motor & internal combustion engine.
• CVTs can also be found in motor scooters & snowmobiles for efficient power delivery and smooth acceleration.
• These are used in some bicycles to allow for continuous and smooth gear changes by eliminating the need for fixed CVT shifting.
• Hydraulic CVTs can be used in heavy off-highway vehicles, construction equipment, and farm tractors.
• CVTs can also be found in earthmoving equipment to provide variable and smooth power delivery.
• CVT is used as an advanced transmission system in automotive vehicles.

Thus, this is an overview of a continuously variable transmission or CVT. It provides a smooth and fuel-efficient driving experience by removing the requirement of fixed gear shifts. So these are the best choices for vehicle drivers, particularly in city driving conditions. The best CVT Cars in India are: the Toyota Corolla Altis, Honda Civic, Honda Amaze VX,, etc. Here is a question for you: What is the manual transmission?