07-08-2012, 09:58 AM
A Continuously Variable Transmission (CVT)
Continuously Variable Transmission (CVT).docx (Size: 329.67 KB / Downloads: 39)
INTRODUCTION
A continuously variable transmission (CVT) is a transmission that can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that offer a fixed number of gear ratios. The flexibility of a CVT allows the driving shaft to maintain a constant angular velocity over a range of output velocities. This can provide better fuel economy than other transmissions by enabling the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds. Alternatively it can be used to maximize the performance of a vehicle by allowing the engine to turn at the RPM at which it produces peak power. This is typically higher than the RPM that achieves peak efficiency. Finally, a CVT does not strictly require the presence of a clutch, allowing the dismissal thereof.
If you've read about the structure and function of automatic transmissions, then you know that the job of the transmission is to change the speed ratio between the engine and the wheels of an automobile. In other words, without a transmission, cars would only have one gear - the gear that would allow the car to travel at the desired top speed. Imagine for a moment driving a car that only had first gear or a car that only had third gear. The former car would accelerate well from a complete stop and would be able to climb a steep hill, but its top speed would be limited to just a few miles an hour. The latter car, on the other hand, would fly at 80 mph down the highway, but it would have almost no acceleration when starting out and wouldn't be able to climb hills. So the transmission uses a range of gears - from low to high - to make more effective use of the engine's torque as driving conditions change. The gears can be engaged manually or automatically.
CHARACTERISTICS
CVT Basics
A Continuously Variable Transmission (CVT) is a method of transferring the power from the engine, which maybe a combustion engine or an electrical motor. In the case of an electrical motor, these are usually controlled electronically and so the CVT is of more benefit when used with combustion engines as a method of transferring power to the driven components. Successful CVT will resolve the compromises in reliability, durability, efficiency, and controllability with low cost. Implementation of commercially produced CVT’s transmits drive through friction.
Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears, which means they don't have interlocking toothed wheels. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability between highest and lowest gears with no discrete steps or shifts. Although CVTs change this ratio without using a set of planetary gears, they are still described as having low and high "gears" for the sake of convention. Fig 3.1 shows the different types of CVT model.
CVT allows for the operation at the lowest possible speed and highest possible load, partially avoiding the low efficiency region of the engine map. A continuously variable transmission (CVT) transfers power through a range of speed/torque ratios from engine input to output, continuously without interruption. Contrast with either manual or conventional automatic transmissions that use discrete ratios and normally disengage when changing ratio. The CVT category includes infinitely variable transmissions (IVT) that give a zero output speed within the operating range.
Toroidal CVTs
Another version of the CVT - the toroidal CVT system - replaces the belts and pulleys with discs and power rollers. Although such a system seems drastically different, all of the components are analogous to a belt-and-pulley system and lead to the same results – a continuously variable transmission as shown in fig 3.2.2.
How the Toroidal CVT type works:
• One disc connects to the engine. This is equivalent to the driving pulley.
• Another disc connects to the drive shaft. This is equivalent to the driven pulley.
• Rollers, or wheels, located between the discs act like the belt, transmitting power from one disc to the other.
Hydrostatic CVTs
Both the pulley-and-V-belt CVT and the toroidal CVT are examples of frictional CVTs, which work by varying the radius of the contact point between two rotating objects. There is another type of CVT, known as a hydrostatic CVT, that uses variable-displacement pumps to vary the fluid flow into hydrostatic motors. In this type of transmission, the rotational motion of the engine operates a hydrostatic pump on the driving side. The pump converts rotational motion into fluid flow as shown in fig 3.2.3. Then, with a hydrostatic motor located on the driven side, the fluid flow is converted back into rotational motion.
Continuously Variable Transmission (CVT).docx (Size: 329.67 KB / Downloads: 39)
INTRODUCTION
A continuously variable transmission (CVT) is a transmission that can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This contrasts with other mechanical transmissions that offer a fixed number of gear ratios. The flexibility of a CVT allows the driving shaft to maintain a constant angular velocity over a range of output velocities. This can provide better fuel economy than other transmissions by enabling the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds. Alternatively it can be used to maximize the performance of a vehicle by allowing the engine to turn at the RPM at which it produces peak power. This is typically higher than the RPM that achieves peak efficiency. Finally, a CVT does not strictly require the presence of a clutch, allowing the dismissal thereof.
If you've read about the structure and function of automatic transmissions, then you know that the job of the transmission is to change the speed ratio between the engine and the wheels of an automobile. In other words, without a transmission, cars would only have one gear - the gear that would allow the car to travel at the desired top speed. Imagine for a moment driving a car that only had first gear or a car that only had third gear. The former car would accelerate well from a complete stop and would be able to climb a steep hill, but its top speed would be limited to just a few miles an hour. The latter car, on the other hand, would fly at 80 mph down the highway, but it would have almost no acceleration when starting out and wouldn't be able to climb hills. So the transmission uses a range of gears - from low to high - to make more effective use of the engine's torque as driving conditions change. The gears can be engaged manually or automatically.
CHARACTERISTICS
CVT Basics
A Continuously Variable Transmission (CVT) is a method of transferring the power from the engine, which maybe a combustion engine or an electrical motor. In the case of an electrical motor, these are usually controlled electronically and so the CVT is of more benefit when used with combustion engines as a method of transferring power to the driven components. Successful CVT will resolve the compromises in reliability, durability, efficiency, and controllability with low cost. Implementation of commercially produced CVT’s transmits drive through friction.
Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears, which means they don't have interlocking toothed wheels. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability between highest and lowest gears with no discrete steps or shifts. Although CVTs change this ratio without using a set of planetary gears, they are still described as having low and high "gears" for the sake of convention. Fig 3.1 shows the different types of CVT model.
CVT allows for the operation at the lowest possible speed and highest possible load, partially avoiding the low efficiency region of the engine map. A continuously variable transmission (CVT) transfers power through a range of speed/torque ratios from engine input to output, continuously without interruption. Contrast with either manual or conventional automatic transmissions that use discrete ratios and normally disengage when changing ratio. The CVT category includes infinitely variable transmissions (IVT) that give a zero output speed within the operating range.
Toroidal CVTs
Another version of the CVT - the toroidal CVT system - replaces the belts and pulleys with discs and power rollers. Although such a system seems drastically different, all of the components are analogous to a belt-and-pulley system and lead to the same results – a continuously variable transmission as shown in fig 3.2.2.
How the Toroidal CVT type works:
• One disc connects to the engine. This is equivalent to the driving pulley.
• Another disc connects to the drive shaft. This is equivalent to the driven pulley.
• Rollers, or wheels, located between the discs act like the belt, transmitting power from one disc to the other.
Hydrostatic CVTs
Both the pulley-and-V-belt CVT and the toroidal CVT are examples of frictional CVTs, which work by varying the radius of the contact point between two rotating objects. There is another type of CVT, known as a hydrostatic CVT, that uses variable-displacement pumps to vary the fluid flow into hydrostatic motors. In this type of transmission, the rotational motion of the engine operates a hydrostatic pump on the driving side. The pump converts rotational motion into fluid flow as shown in fig 3.2.3. Then, with a hydrostatic motor located on the driven side, the fluid flow is converted back into rotational motion.