VOL.203 MAY 2025
[SPRING SPECIAL ISSUE] VARIOUS VARIETIES OF CHERRY BLOSSOMS IN JAPAN (PART 2): ADMIRING THE CHERRY BLOSSOMS AT CASTLES IN JAPAN [Science & Technology] Newly Developed Advanced CVT Belt Reduces CO₂ Emissions

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Part of an advanced CVT belt. It consists of a pair of rings and approximately 400 elements.
Photo: Honda

Transmissions, which effectively convert engine power into a force that turns the axle shaft, are an indispensable part of any car. In addition to conventional transmissions, which shift gears in steps, continuously variable transmissions (CVTs) have emerged in recent years. CVTs can shift through a continuous range of gear ratios, allowing a smoother transfer of engine power. By developing a belt that functions as a key component of these CVTs, Japanese automaker Honda has not only significantly enhanced fuel efficiency and reduced CO₂ emissions, but has also improved CVT durability. Here, we present the development of this CVT belt, for which Honda received the 2024 Award for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology.

The function of the transmission is to adjust the rotational speed of the engine and deliver the optimum power for the speed of the car. This function allows the car to accelerate and decelerate smoothly, making the most efficient use of engine power. CVT stands for Continuously Variable Transmission, a type of transmission used in cars, motorcycles, and other vehicles. CVTs differ not only from manual transmissions (MTs), in which the driver changes gears manually, but also from automatic transmissions (ATs), in which the car changes gears automatically, as CVTs do not change gears in fixed steps. They use two variable-diameter pulleys connected by a metal belt to change the speed (see photo). Since they function in a stepless manner, CVTs deliver a smooth ride with extremely low gearshift shock and vibration. They are particularly suitable for compact cars used in city traffic where stop-start driving is common. CVTs enable automatic gear shifting, thus keeping engine revolutions at an optimum level at all times and improving fuel efficiency. Naturally, this also helps reduce CO₂ emissions. As CVTs have fewer components than AT, they can also help lower production costs.

CVTs have been on the market in Japan since the mid-1990s, but initially the metal belts, a key component of the CVT, were only manufactured in the Netherlands. Honda was one of the first automakers to undertake in-house CVT production, including manufacturing the metal belts, and successfully launched mass CVT production in Japan in 2001. Since then, the company has continued to develop advanced CVT belts with the aim of improving CVT efficiency.

The structure of a CVT belt consists of about 400 plates, called elements, held in place by a set of two rings inserted on both sides of the elements. Since all these components are made of metal, there is inevitably some loss of energy due to slippage and friction between the rings and the elements and between the elements themselves. To reduce this energy loss as much as possible, Honda has been working on improving the design specifications of the elements and developing advanced precision shearing¹ technology to put these improvements into practice. By applying a thin groove to the material during shearing, Honda has been able to prevent material distortion and reduce warping and bending of the cut surface. In addition, shearing has traditionally been performed at a large die² penetration angle. However, by performing multi-stage shearing at smaller penetration angles, Honda engineers have reduced the frictional flow with the material and have realized a more precise funnel flow type shearing technology (see Figure 1), taking advantage of the plastic flow³ within the material. These technologies have made it possible to process the surface of the elements more smoothly and reduce their misalignment with the belt caused by friction and slippage, which has resulted in the completion and practical utilization of an advanced CVT belt with improved power transmission efficiency.

Compact cars that use this advanced metal belt are sold mainly in Thailand, and results show that their fuel economy, defined as kilometers per liter of gasoline, has improved from 20 km/L (conventional type) to more than 23.25 km/L, while the amount of CO₂ emitted during driving has improved from 120 g/km to less than 100 g/km. In addition, 1.25 million CVT belts made using the funnel flow type shearing technology are sold annually.

According to YAGASAKI Toru, Honda’s expert engineer who led the development of this advanced CVT belt, it is also historically significant that the advanced metal belt was developed in Japan, the country with the world’s highest distribution rate of metal belt CVTs. “Not only have we improved the power transmission efficiency of the metal belt CVT, but we hope that our rapid material testing method and new shearing method will continue to contribute to the development of technology around the world,” says YAGASAKI.

• 1. A technology for cutting a material by applying pressure to it using a pair of upper and lower cutting edges and utilizing a displacement force.
• 2. The fixed lower cutting edge under the material in the shearing process. In shearing, the material is placed on the die, and the punch pushes the material from above against the die to the point of fracture.
• 3. A phenomenon in which a material under severe stress starts to behave as a fluid.

By FUKUDA Mitsuhiro
Photo: Honda

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