What is A CVT Transmission?
Overview
A CVT transmission (Continuously Variable Transmission) is an automatic transmission system that can change seamlessly through an infinite number of effective gear ratios between maximum and minimum values. This allows the engine to operate at its most efficient speed and power range, providing advantages in fuel economy, emissions, and performance compared to traditional automatic transmissions with fixed gear ratios.
Key Components
The main components of a CVT include:
- Primary pulley (driver pulley) connected to the engine
- Secondary pulley (driven pulley) connected to the wheels
- Hydraulic or electro-hydraulic actuators to adjust pulley sheave radii
- Metal push belt or chain transferring power between pulleys
How Does A CVT Transmission Work?
CVT Operating Principle
A CVT uses two pulleys connected by a metal belt or chain. The pulleys have adjustable sheave radii, allowing for a continuous variation of the gear ratio by changing the effective diameters of the pulleys. This enables an infinite number of gear ratios between the minimum and maximum values, providing seamless acceleration without discrete gear shifts.
Ratio Control Mechanism
The transmission ratio is controlled by adjusting the pulley sheave radii using hydraulic pressure or electro-hydraulic actuators. As the primary pulley radius decreases, the secondary pulley radius increases, causing the belt to ride higher on one pulley and lower on the other, effectively changing the ratio. This ratio adjustment is continuously variable and controlled by an electronic control unit based on driving conditions.
Pros and Cons of CVT Transmission
Pros of CVT Transmissions
- Improved Fuel Efficiency: CVTs can continuously adjust the gear ratio to keep the engine operating at its most efficient rpm range, resulting in better fuel economy compared to traditional automatic transmissions.
- Smooth Operation: Since there are no distinct gear shifts, CVTs provide a smooth and seamless driving experience without any shift shock.
- Better Acceleration: The ability to continuously vary the gear ratio allows CVTs to maintain optimal power delivery, enabling better acceleration performance.
- Compact Design: CVTs have a relatively simple and compact design compared to conventional automatic transmissions with multiple gear sets.
Cons of CVT Transmissions
- Limited Torque Capacity: Due to the reliance on friction between the pulleys and belt/chain, CVTs have a lower torque capacity compared to gear-based transmissions, making them less suitable for larger vehicles or high-performance applications.
- Belt/Chain Wear: The belt or chain used in CVTs can wear out over time, especially under high-load conditions, leading to potential slippage and reduced transmission efficiency.
- Delayed Throttle Response: There can be a slight delay in throttle response when accelerating from a stop due to the time required for the pulleys to adjust the gear ratio.
- Noise and Vibration: Depending on the design, some CVTs may produce a characteristic “drone” noise or vibration, particularly under high loads or when accelerating.
While CVTs offer advantages in terms of fuel efficiency, smooth operation, and compact design, their limitations in torque capacity, potential belt/chain wear, and throttle response should be considered. Ongoing research and development aims to address these drawbacks and further improve the performance and durability of CVT systems.
Which Cars have CVT Transmission?
CVTs are commonly found in smaller vehicles, hybrids, and SUVs from Japanese automakers like Honda, Nissan, Subaru, and Toyota. Some examples include:
- Honda models like Civic, Accord, CR-V, and HR-V
- Nissan models like Altima, Sentra, Versa, and Rogue 4
- Subaru models like Outback, Forester, and Impreza
- Toyota models like Corolla, Prius, RAV4, and Camry
While less common, some non-Japanese automakers like Audi, Ford, and Hyundai have also offered CVT options on certain models.
Applications of CVT Transmission
Automotive Applications
CVTs are widely used in passenger vehicles, particularly compact and mid-size cars, SUVs, and hybrid vehicles. Their ability to provide an infinite range of gear ratios results in improved fuel efficiency and smoother acceleration compared to traditional automatic transmissions. Major automakers like Nissan, Honda, Toyota, and Subaru have extensively adopted CVTs in their vehicle lineups.
Off-Road and Recreational Vehicles
CVTs have found applications in off-road vehicles such as snowmobiles, all-terrain vehicles (ATVs), and utility vehicles. Their stepless gear ratio adjustment and ability to maintain constant engine speed make them well-suited for these applications. However, some modifications may be required to enable reverse torque transmission (RTT) mode for engine starting in remote locations.
Electric and Hybrid Vehicles
In electric and hybrid vehicles, CVTs are often integrated with electric motors and other components to form specialized transmission systems like E-CVTs. These systems aim to optimize power distribution between the internal combustion engine and electric motors, further enhancing efficiency and performance.
Industrial and Specialized Applications
Beyond automotive applications, CVTs have been explored for use in industrial machinery, agricultural equipment, and other specialized applications where precise speed control and smooth power delivery are desirable. Additionally, advancements in materials like ceramics have enabled the development of high-performance CVTs capable of handling higher loads and operating under demanding conditions.
It’s important to note that while CVTs offer numerous advantages, their adoption is influenced by factors such as cost, performance requirements, and compatibility with specific vehicle platforms. Ongoing research and development efforts aim to further improve the efficiency, durability, and cost-effectiveness of CVT systems, potentially expanding their applications across various industries.
Latest Technical Innovations of CVT Transmission
Direct Shift CVT
A new direct shift CVT system has been developed to address the limitations of existing CVTs . It features a compact and simple structure with a variator, traction rollers, input/output disks, and preload components that apply radially inward force on the rollers for frictional contact. This design aims to provide efficient operation and high power transmission capability suitable for larger vehicles.
Electronically Controlled CVT
Conventional CVTs with centrifugal weights only allow one shifting characteristic. Emerging electronically controlled CVTs enable variable shifting characteristics based on operational parameters, offering improved performance and efficiency for modern two-wheelers and quad bikes.
Engine Braking System
A novel CVT design incorporates a braking assembly with a thrust member and flanged sleeve . This allows the CVT to operate in reverse, using the ground-engaging members to brake and drive the engine, enabling engine braking functionality.
Thermal Management
Overheating and belt slippage are major issues affecting CVT efficiency and longevity. Researchers are exploring the use of advanced materials like ceramics and optimized sheave materials to improve thermal management and extend belt life.
Novel CVT Designs
Innovative CVT designs are being developed, such as the hemispherical CVT using a novel gear combination of a hemisphere and face gear, and a sphere-based CVT enabling independent output position and impedance variation for human-robot interfaces.
Control and Modelling
Advanced control strategies like model predictive control (MPC) and estimation techniques like extended Kalman filters are being employed to optimize CVT operation, improve drivability, enforce safety constraints, and account for slip dynamics.
These latest innovations in CVT technology aim to address existing limitations, improve efficiency, extend component life, enable new applications, and provide better control and modelling capabilities, driving the continued development and adoption of CVTs in various industries.
Application Case
| Product/Project | Technical Outcomes | Application Scenarios |
|---|---|---|
| Nissan Xtronic CVT | Improved fuel efficiency by up to 10% compared to conventional automatic transmissions. Smoother acceleration and reduced noise, vibration, and harshness (NVH) levels. | Compact and mid-size passenger vehicles, SUVs, and hybrid vehicles where fuel efficiency and driving comfort are priorities. |
| Honda CVT | Utilises a high-capacity steel belt and pulley system for enhanced durability and reliability. Offers a wider ratio range for improved acceleration and fuel economy. | Passenger vehicles, particularly compact and mid-size models, where a balance of performance and efficiency is desired. |
| Toyota Hybrid Synergy Drive | Seamlessly integrates a CVT with an electric motor, enabling smooth and efficient power delivery. Contributes to improved fuel economy and reduced emissions. | Hybrid electric vehicles, where the CVT complements the electric motor for optimal energy management and driving experience. |
| BRP CVT | Designed for high-torque applications, featuring robust components and advanced clutch systems. Enables precise control and responsiveness in varying terrain conditions. | Off-road and recreational vehicles, such as snowmobiles, all-terrain vehicles (ATVs), and utility vehicles, where ruggedness and performance are essential. |
| Subaru Lineartronic CVT | Incorporates a chain-type transfer system for improved efficiency and durability. Offers a wider ratio range and smoother gear changes compared to traditional automatics. | Subaru’s all-wheel-drive vehicles, where the CVT’s capabilities complement the brand’s performance and handling characteristics. |
Technical challenges
| Improving CVT Belt Life and Durability | Developing advanced materials and coatings for the CVT belt and pulleys to reduce friction, heat generation, and wear, thereby extending the belt’s lifespan and improving overall transmission durability. |
| Optimising CVT Ratio Control Strategy | Enhancing the control strategy and algorithms for CVT ratio adjustment to optimise engine operating conditions, improve fuel efficiency, and reduce emissions while maintaining drivability. |
| Reducing CVT Noise, Vibration, and Harshness (NVH) | Mitigating noise, vibration, and harshness (NVH) issues in CVTs through improved damping mechanisms, pulley design, and advanced control strategies for smoother operation. |
| Enhancing CVT Torque Capacity | Increasing the torque capacity of CVTs through innovative pulley and belt designs, enabling their application in larger vehicles and higher-performance applications. |
| Integrating CVT with Hybrid Powertrains | Developing seamless integration strategies for CVTs in hybrid electric vehicle powertrains, optimising power flow and energy management for improved efficiency and performance. |
To get detailed scientific explanations of CVT transmission, try Patsnap Eureka.
