The continuously-variable transmission is also an automatic transmission system, which changes the diameters of input shaft and output shaft directly, instead of going through several gears to perform gear ratio change. This design can generate an infinite number of possible gear ratios. Unlike the complicated planetary automatic transmission system, a CVT only has three major parts; a drive pulley connected to the input shaft, a driven pulley connected to the output shaft, and a belt. Figure 21 shows the structure of the most common pulley based CVT systems. If two cones of the pulley fall apart, the diameter of the pulley becomes small; while they are close, the diameter of the pulley is large. Because the length of the driving belt is fixed, when the diameter of the drive pulley becomes small, the diameter of the driven pulley must become large by closing two cones of the pulley together, and vice versa. Based on this infinite number of gear ratios design, it is said that CVT is the most efficient transmission system in the world.
Another popular CVT is Nissan Extroid toroidal CVT. It uses two discs instead of pulleys and two rollers (wheels) instead of a belt, compared with a pulley based CVT. Figure 22 shows that the two rollers spin around the horizontal axis, and also tilt in or out around the vertical axis. When the two rollers tilt to the upper disc in Figure 22, they touch the upper disc with a larger diameter, while touching the bottom disc with a smaller diameter. This works according to the same idea as pulley based CVT, by generating an infinite number of gear ratios.
Even though the pulley based CVT is currently the most common CVT in the world, the belt is under extremely large stress between pulleys when they are spinning. Therefore, in the past the CVT could only be used for some low torque engines like lawn mowers or snow blowers. It was also used for some small engine vehicles under 1,500 cc. With the progress of material technology, Nissan has been able to put a pulley based CVT into a 3,500 V6 Altima, which dramatically improves the fuel efficiency for a large engine vehicle.
Because CVT is basically designed to perform automatic transmission without manually engaging/disengaging the clutch, it generates another issue: How to prevent the engine from stalling when the car is not moving? CVT has several clutch designs to achieve this goal. Figure 23 is a CVT clutch assembly normally used for a scooter. The CVT clutch for an automobile is much more complicated but the idea is similar. The CVT clutch has two parts, a clutch disc with frictional materials attached on the side, and a clutch cover (drum). The cover encircles the disc, with the frictional materials barely touching the inside wall. The cover is connected to the drive pulley, while the disc is connected to the engine. When the engine is in idle, the clutch disc spins slowly, and the frictional pad doesn't contact with the inside wall of the cover. At high speed, the centrifugal force causes the frictional pad to move away from its spinning axis to touch the inside wall of the cover, and drives the cover to spin. That's why for most of CVT vehicles, when the gear is shifted to "D", the car is still not moving when the engine is in idle, which is different from an automatic transmission with a torque converter.
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