A continuously variable transmission, or CVT, lets a car change its gear ratio smoothly instead of jumping between fixed gears. This matters because the engine can stay closer to its most efficient or most powerful speed while the vehicle changes speed. In a belt-and-pulley CVT, two variable-width pulleys and a strong belt replace the usual set of fixed gears.
The result is the feeling of gears without steps.
Key Facts
- A CVT changes transmission ratio continuously, not in fixed steps like 1st, 2nd, and 3rd gear.
- Speed ratio can be estimated by output speed / input speed = drive pulley radius / driven pulley radius.
- When the drive pulley effective radius is small and the driven pulley radius is large, the CVT gives high torque for starting.
- When the drive pulley effective radius is large and the driven pulley radius is small, the CVT gives higher wheel speed for cruising.
- Power flow is engine input shaft to primary pulley to belt or chain to secondary pulley to output shaft to wheels.
- For ideal motion with no belt slip, belt speed is the same at both pulleys, so v = 2πrN for each pulley.
Vocabulary
- Continuously variable transmission
- A transmission that can smoothly vary its ratio over a range instead of using a small number of fixed gears.
- Drive pulley
- The pulley connected to the engine input shaft that sends power into the CVT belt or chain.
- Driven pulley
- The pulley connected to the output shaft that receives power from the belt or chain and sends it toward the wheels.
- Effective radius
- The working radius where the belt contacts a pulley, which changes as the pulley halves move together or apart.
- Transmission ratio
- The relationship between input speed and output speed that determines how much torque or speed the transmission provides.
Common Mistakes to Avoid
- Thinking a CVT has many tiny fixed gears is wrong because the pulley radius changes smoothly over a continuous range.
- Assuming the belt simply stretches to change ratio is wrong because the ratio changes mainly when pulley halves move and force the belt to ride at different radii.
- Reversing the starting condition is wrong because a car needs a small drive pulley radius and large driven pulley radius to multiply torque at low speed.
- Ignoring belt slip and friction is wrong because real CVTs lose some energy as heat, so output power is always less than input power.
Practice Questions
- 1 A CVT drive pulley has an effective radius of 3 cm and the driven pulley has an effective radius of 9 cm. If the engine input speed is 2400 rpm and there is no slip, what is the output speed?
- 2 During cruising, the drive pulley effective radius is 8 cm and the driven pulley effective radius is 4 cm. If the output shaft spins at 3000 rpm, what is the input shaft speed?
- 3 Explain why a CVT can keep the engine near an efficient rpm while the car speeds up, even though a traditional transmission must shift between fixed gears.