The concept of the continuously variable transmission (CVT) was envisioned by Leonardo da Vinci more than 500 years ago. Even though a patent for the first toroidal CVT was filed in 1886, the technology is still far from replacing the traditional transmission system. However, several manufacturers are working on developing designs that incorporate the CVT.
Basic Transmission System
The purpose of the transmission is to balance required torque with the speed of the vehicle. For example, a car with only one high gear would not be able to accelerate or climb steep hills, and a car with only one low gear would be a great hill climber, but would not be able to drive more than a few kilometres an hour. The traditional transmission has a range of gears allowing the vehicle to effectively use the engine's torque.
The gears are literally gears: toothed wheels that interlock to control and modify the torque and rotary action, similar to the gears on a bike. Transmissions are either automatic or manually operated by the driver. A typical vehicle has three or four forward gears and one reverse, and no matter how smoothly a transmission operates, a distinct jolt can always be felt when the gear changes.
A continuously variable transmission doesn't have a gear box or a set number of gears. Instead, it operates on pulley system which allows for an infinite number of variations between the lowest and highest gears, without the distinctive jolt. The only reason these variations are referred to as gears is because the word "gear" is used to explain the ratio between engine shaft and drive-shaft. In a traditional transmission the word is also literal, but in a CVT it is only used for clarification.
CVT Pulley System
The inside of an automatic transmission looks like a complicated compilation of gears, clutch controls, brakes and hinges. By comparison, the inside of a continuously variable transmission has three simple components: metal or rubber belt, variable-input pulley and an output pulley. There are a variety of sensors and microprocessors to make these components work together properly, however these are the three key elements of the system.
The heart of the system is the variable-input pulley. Each pulley is made of two cones set face to face with a belt riding in a groove in between the cones. With rubber belts, the V-shaped type works the best because it increases friction, allowing for a better grip.
The input pulley is connected to the crankshaft, providing energy to the transmission. Pulleys always come in pairs. The pulley is the output pulley, which transfers the energy it creates to drive-shaft.
A simplified way of looking at this is to think about a crank radio. As the crank is turned, the input pulley is activated, transferring energy to the output pulley which then makes the electricity needed to power the radio.
When the belt on one pulley rides up higher, the radius on the other is decreased to keep the belt tight. The pulleys maintain an equal ratio, which in turn changes the gears. For example, when the radius on the input pulley is small, the radius on the output pulley is large, meaning that the car is in a lower gear.
Toroidal CVT System
The toroidal uses discs and power rollers instead of belts and pulleys. It works the same as the pulley system, except the pulleys are replaced with discs and the belts are replaced with rollers.
Hydrostatic CVT System
The belt and pulley system and the toroidal system are both based on friction, which work by changing the radius of the contact point between two rotating objects. The hydrostatic CVT system uses variable-displacement pumps that change the flow of fluid entering hydrostatic motors.
With this CVT system, the rotation of the engine sets a hydrostatic pump into motion, which then converts this motion into the flow of fluid. When the fluid is on the driving side, it is input power. When on the driven side, it is output power.
A hydrostatic transmission is often combined with a traditional transmission system to create an advanced system known as the hydro mechanical transmission. In this system, the transmission is powered hydraulically at low speeds and mechanically at a high speeds.