A few posts ago we talked about the NuVinci continuously variable transmission during our series on gas mileage technologies. The NuVinci design is an innovative type of continuously variable transmission that has not yet been used by a major automaker. However, there are other types of conventional continuously variable transmissions that have been used in cars. These are conventional in comparison with the NuVinci, but advanced compared to the common manual or automatic transmissions in almost all of our cars.
Everyday transmissions adjust the ratio between the rotational speed or RPM of the engine and the wheels by choosing from four or five fixed metal gears. Current continuously variable transmissions use a pair of variable diameter pulleys instead of the gears. Each pulley is formed out of two cones with their tips or apexes pointing towards each other. These cones can move closer together or farther apart. A hydraulic or spring system is used to control the spacing between the cones.
A belt runs between the pulleys, fitting in the groove between the two cones. Older designs used rubber belts that were shaped like a V to better contact the sides of the cones. Now with newer metal alloys there are metal belts. The metal belts are stronger and allow for transmissions that can support much higher torques. The idea is that as the cones move farther apart, the belt can slip farther down between them and get closer to the pulley axis. The lower down the groove the belt is the smaller the effective diameter of the pulley.
It is necessary to have variable diameter pulleys in pairs. If one pulley pushed the cones closer together forcing the belt up the groove and increasing the belt diameter the belt would have to stretch if the other pulley did not simultaneously decrease its diameter the same amount. One pulley of the pair is connected to the engine crankshaft and the other to the vehicle driveshaft. As they change their diameters in lockstep the ratio of crankshaft diameter to driveshaft diameter can vary continuously. The ratio of these diameters is in inverse proportion to the ratio of their rotational speeds or RPM. If for example the largest diameter (when the cones are close together) is double the smallest diameter (when the cones are far apart) then the output (driveshaft) RPM can vary continuously between one half the input (crankshaft) RPM and double the input RPM.
The result is better gas mileage due to two main factors. One is the fact that the engine can run closer to its most efficient RPM more of the time. The second is that the transmission is very simple so the losses due to friction within the mechanisms are reduced. These factors give a 6% increase in fuel economy.