Overview
Mainstream automatic transmissions currently include AT transmission, DCT (or DSG) transmission, and CVT transmission. AT transmissions are the most common. DCT (or DSG) transmissions are mainly used in German vehicles or by independent manufacturers, while CVT transmissions are commonly used in Japanese vehicles; some independent manufacturers also use them.
There are two world-class AT transmission manufacturers, Aisin and ZF, which indicates that Japan does not lack AT transmission technology. Nonetheless, many Japanese vehicles, especially economical models, prefer CVT. This preference is closely related to Japan's development philosophy.
Fuel Economy and Transmission Influence
Japanese vehicles generally have relatively low fuel consumption. Many factors affect vehicle fuel consumption, including the engine, transmission, final drive ratio, vehicle weight, aerodynamic drag coefficient, and tires. Among these, the engine and transmission are the two primary factors. Japanese engine technology is well developed. How much does the transmission, particularly a CVT, affect fuel consumption?
CVT Structure and Operation
CVT, also called continuously variable transmission, has a relatively simple structure. Its main components are the driving pulley assembly, driven pulley assembly, metal belt, and hydraulic pump. The metal belt is made of two sets of metal rings and hundreds of metal plates. Both the driving and driven pulley assemblies consist of movable and fixed discs. The pulley on the side closer to the hydraulic cylinder can slide along the shaft, while the other side is fixed.
The movable and fixed discs have conical surfaces that form a V-shaped groove to engage the V-shaped metal drive belt. Power from the engine output shaft is first transmitted to the CVT driving pulley, then via the V-shaped drive belt to the driven pulley, and finally through the reduction gear and differential to the wheels to drive the vehicle.
The CVT shifts by axially moving the movable discs of the driving and driven pulleys to change the working radii where the pulleys engage the V-shaped belt, thereby changing the transmission ratio. The axial movement of the movable discs is controlled by adjusting the hydraulic pressure in the driving and driven pulley cylinders via the control system. Because the working radii of the driving and driven pulleys can be adjusted continuously, the transmission achieves a continuously variable ratio.
Advantages of CVT
Several advantages are apparent from the CVT shift process. First, fuel economy: because a CVT can implement continuously variable ratios over a wide range, it can achieve optimal matching between the transmission and the engine operating conditions, improving overall fuel efficiency.
Second, performance: the continuously variable nature of a CVT allows the selection of transmission ratios that maximize available reserve power, so its performance is clearly superior to manual transmissions and conventional AT transmissions in certain conditions.
Third, emissions: the wide ratio range enables the engine to operate closer to its optimal condition, improving combustion and reducing exhaust emissions.
Finally, cost: CVT systems have a simple structure and fewer parts than AT systems, so once a manufacturer reaches large-scale production, CVT costs can be lower than those of AT.
