Continuously Variable Transmission: A Revolutionary Design


continuously Variable Transmissions (CVT) have two major types: one does not use a belt to transmit power, such as Toyota’s Hybrid Synergy Drive, which uses mechanical gears; another type of CVT uses belts connecting two pulleys.Image result for Continuously Variable Transmission: A Revolutionary Design

The first type has the advantage of real mechanical hard connections, high efficiency and high capacity (can handle large torque), but one limitation is that the Toyota Hybrid Synergy Drive, its mechanical structure can only be used in hybrid powertrains, that transmission will not work in a traditional internal combustion engine powered car.

For the belt type CVT, it can be used in all cars with different types of power sources, and it has unlimited gear ratios; but since it depends on the friction between the belt and the pulley to delivery energy, its ability to transmit torque is limited; and its efficiency under high load is not as good as other traditional automatic transmissions.

Recently, VIT Automobile Science and Technology Ltd, a company specialized in transmission technology, based in Beijing, China, released some information about their latest product: moveable teeth CVT (they called it “HN-CVT”). Below is one of its production units. The HN-CVT combines the benefits of the above two mentioned CVTs. According to VIT, they already have one model of the HN-CVT with the torque rating of 738 lb-ft (1000Nm). What’s more, the HN-CVT has an astonishing high-efficiency ratio of 96% (it only loses 4% of the energy coming from the engine flywheel, before putting the power to the car’s wheels). For comparison, traditional automatic transmissions usually have an efficiency ratio between 80% – 90%. The principle of the HN-CVT (moveable teeth CVT) is based on the fundamental of how a car transmission works.

Regardless what type of a transmission is, they all work in this way: the input side is paired to a gear with a certain amount of teeth, and the output side is paired to another gear with another amount of teeth. If the tooth amount of the two gears are different, the he input side and the output side will rotate at a different speed.

For the HN-CVT, it still has a belt. Buthe t different from other CVTs, the belt has fixed teeth on it. The HN-CVT also has a pulley, but it is a special pulley: it has many small steel sheets packed on its surfaces. Those small sheets can go up and down individually (controlled by the transmission ECU). If some sheets go up at the same time, then the convex shape it formed becomes one tooth. For all the steel sheets on the pulley, the transmission ECU can control their action to form multiple convex shapes, therefore it essentially becomes a gear. The key point is: by controlling the combination of the up-down shape of the steel sheets, it can become a gear with any tooth amount it wants. So the fixed-amount-tooth belt meets the variable-amount-tooth pulley, and different transmission ratio can be realized. Because it is tooth-to-tooth contact, this transmission can handle a large amount of forces without causing sliding and frictional wear.

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