31-01-2012, 10:56 AM
CHAPTER1-INTRODUCTION
Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears. Continuous Variable Transmission (CVT), which is now replacing planetary automatic transmission in some of the automobiles. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability be-tween highest and lowest gears with no discrete steps or shifts.
CVTs offer a continuum of infinitely variable gear ratios by changing the location of pulley sheaves. As a result, CVTs have the potential to increase the overall vehicle efficiency and reduce the jerk usually associated with manual and automatic transmissions.
CHAPTER2-TRANSMISSION BASICS
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. Example Car with 1st gear would have high torque and low speed. And car with 3rd gear would have high speed and low torque.
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 run 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.
In a traditional automatic transmission, the gears are literally gears interlocking, toothed wheels that help transmit and modify rotary motion and torque. A combination of planetary gears creates all of the different gear ratios that the transmission can produce, typically four forward gears and one reverse gear. When this type of transmission cycles through its gears, the driver can feel jolts as each gear is engaged.
CHAPTER3-CVT’s BASICS
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. The word "gear" still appears in the explanation of a CVT, since gear refers to a ratio of engine shaft speed to driveshaft speed. 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.
As shown in the figure 3 below Essentially, a CVT transmission operates by varying the working diameters of the two main pulleys in the transmission. CVT continuously controls the O-pulley diameter and changes the gear change ratio. CVT makes for optimal tuning between the smaller pulley piston, discharge rate reducing oil pump, and the direct control torque converter by which an efficient engine output transfer and a smooth ride. Based on accelerator travel information from actual drivers under actual driving conditions, the optimal efficiency point between the engine and the CVT was obtained and a supple and smooth driving feel is obtained under numerous conditions while also achieving low fuel consumption.
The pulleys have V-shaped grooves in which the connecting belt rides. One side of the pulley is fixed; the other side is moveable, actuated by a some means. When actuated, the cylinder can increase or reduce the amount of space between the two sides of the pulley. This allows the belt to ride lower or higher along the walls of the pulley, depending on driving conditions, thereby changing the gear ratio.
CHAPTER4-TYPES OF CVT
Continuous Variable Transmission are classified as frictional CVTs and hydrostatic CVT.
1. Frictional CVT.
a. Pulley-based CVT.
b. Toroidal CVT.
2. Hydrostatic CVT
4.1.PULLEY-BASED CVT
Pulley based CVT only have three basic components
• A high power metal or rubber belt.
• A variable input driving pulley.
• An output driven pulley.
The figure 4 below shows the arrangement of drive pulley and driven pulley arrangement in a pulley based CVT. The variable-diameter pulleys are the heart of a CVT. Each pulley is made of two 20-degree cones facing each other. A belt rides in the groove between the two cones. V-belts are preferred if the belt is made of rubber. V-belts get their name from the fact that the belts bear a V-shaped cross section, which increases the frictional grip of the belt.
When the two cones of the pulley are far apart (when the diameter increases), the belt rides lower in the groove, and the radius of the belt loop going around the pulley gets smaller. When the cones are close together (when the diameter decreases), the belt rides higher in the groove, and the radius of the belt loop going around the pulley gets larger. CVTs may use hydraulic pressure, centrifugal force or spring tension to create the force necessary to adjust the pulley halves.
4.2.TOROIDAL CVT
Another version of the CVT the toroidal CVT system replaces the belts and pulleys with discs and power rollers. The figure 4 below shows the arrangement of a Toroidal CVT.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. One disc connects to the engine. This is equivalent to the driving pulley. Another disc con-nects to the drive shaft. This is equivalent to the driven pulley. Rollers, or wheels, located be-tween the discs act like the belt, transmitting power from one disc to the other.
The wheels can rotate along two axes. They spin around the horizontal axis and tilt in or out around the vertical axis, which allows the wheels to touch the discs in different areas. When the wheels are in contact with the driving disc near the center, they must contact the driven disc near the rim, resulting in a reduction in speed and an increase in torque (i.e., low gear). When the wheels touch the driving disc near the rim, they must contact the driven disc near the center, resulting in an increase in speed and a decrease in torque (i.e., overdrive gear). A simple tilt of the wheels, then, incrementally changes the gear ratio, providing for smooth, nearly instantaneous ratio changes.
Unlike traditional automatic transmissions, continuously variable transmissions don't have a gearbox with a set number of gears. Continuous Variable Transmission (CVT), which is now replacing planetary automatic transmission in some of the automobiles. The most common type of CVT operates on an ingenious pulley system that allows an infinite variability be-tween highest and lowest gears with no discrete steps or shifts.
CVTs offer a continuum of infinitely variable gear ratios by changing the location of pulley sheaves. As a result, CVTs have the potential to increase the overall vehicle efficiency and reduce the jerk usually associated with manual and automatic transmissions.
CHAPTER2-TRANSMISSION BASICS
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. Example Car with 1st gear would have high torque and low speed. And car with 3rd gear would have high speed and low torque.
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 run 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.
In a traditional automatic transmission, the gears are literally gears interlocking, toothed wheels that help transmit and modify rotary motion and torque. A combination of planetary gears creates all of the different gear ratios that the transmission can produce, typically four forward gears and one reverse gear. When this type of transmission cycles through its gears, the driver can feel jolts as each gear is engaged.
CHAPTER3-CVT’s BASICS
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. The word "gear" still appears in the explanation of a CVT, since gear refers to a ratio of engine shaft speed to driveshaft speed. 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.
As shown in the figure 3 below Essentially, a CVT transmission operates by varying the working diameters of the two main pulleys in the transmission. CVT continuously controls the O-pulley diameter and changes the gear change ratio. CVT makes for optimal tuning between the smaller pulley piston, discharge rate reducing oil pump, and the direct control torque converter by which an efficient engine output transfer and a smooth ride. Based on accelerator travel information from actual drivers under actual driving conditions, the optimal efficiency point between the engine and the CVT was obtained and a supple and smooth driving feel is obtained under numerous conditions while also achieving low fuel consumption.
The pulleys have V-shaped grooves in which the connecting belt rides. One side of the pulley is fixed; the other side is moveable, actuated by a some means. When actuated, the cylinder can increase or reduce the amount of space between the two sides of the pulley. This allows the belt to ride lower or higher along the walls of the pulley, depending on driving conditions, thereby changing the gear ratio.
CHAPTER4-TYPES OF CVT
Continuous Variable Transmission are classified as frictional CVTs and hydrostatic CVT.
1. Frictional CVT.
a. Pulley-based CVT.
b. Toroidal CVT.
2. Hydrostatic CVT
4.1.PULLEY-BASED CVT
Pulley based CVT only have three basic components
• A high power metal or rubber belt.
• A variable input driving pulley.
• An output driven pulley.
The figure 4 below shows the arrangement of drive pulley and driven pulley arrangement in a pulley based CVT. The variable-diameter pulleys are the heart of a CVT. Each pulley is made of two 20-degree cones facing each other. A belt rides in the groove between the two cones. V-belts are preferred if the belt is made of rubber. V-belts get their name from the fact that the belts bear a V-shaped cross section, which increases the frictional grip of the belt.
When the two cones of the pulley are far apart (when the diameter increases), the belt rides lower in the groove, and the radius of the belt loop going around the pulley gets smaller. When the cones are close together (when the diameter decreases), the belt rides higher in the groove, and the radius of the belt loop going around the pulley gets larger. CVTs may use hydraulic pressure, centrifugal force or spring tension to create the force necessary to adjust the pulley halves.
4.2.TOROIDAL CVT
Another version of the CVT the toroidal CVT system replaces the belts and pulleys with discs and power rollers. The figure 4 below shows the arrangement of a Toroidal CVT.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. One disc connects to the engine. This is equivalent to the driving pulley. Another disc con-nects to the drive shaft. This is equivalent to the driven pulley. Rollers, or wheels, located be-tween the discs act like the belt, transmitting power from one disc to the other.
The wheels can rotate along two axes. They spin around the horizontal axis and tilt in or out around the vertical axis, which allows the wheels to touch the discs in different areas. When the wheels are in contact with the driving disc near the center, they must contact the driven disc near the rim, resulting in a reduction in speed and an increase in torque (i.e., low gear). When the wheels touch the driving disc near the rim, they must contact the driven disc near the center, resulting in an increase in speed and a decrease in torque (i.e., overdrive gear). A simple tilt of the wheels, then, incrementally changes the gear ratio, providing for smooth, nearly instantaneous ratio changes.