[Abstract] This paper introduces ZFST's VT1 type CVT transmission and its advantages compared to conventional automatic transmissions. It also discusses the development of an electronically controlled version of VT1 and its further development. Keywords: Metal V-Belt CVT, electronic control, driveability, fuel consumption 1 Introduction ZF Group's N.V. Sint-Truiden (ZFS), located in Belgium, is part of ZF's passenger car division. As an international company, ZF develops and manufactures a wide range of transmissions, steering systems, axles, chassis components, and provides complete systems for passenger cars, commercial vehicles, and off-highway vehicles. At ZF, the passenger car division is responsible for developing and producing automatic and manual transmissions for passenger vehicles. Currently, ZF offers three types of automatic transmissions: conventional automatic transmissions, continuously variable transmissions (CVTs), and dual-clutch transmissions (DCTs). ZFST focuses on front-wheel-drive CVTs. As a company that evolved from the renowned rubber belt transmission, ZFST began developing CVTs in 1988 and started production in 1992. From the beginning, ZFST not only launched CVTs covering all sizes from 125Nm to 210Nm, but also developed the electronically controlled VT1. This article provides an overview of the development over the past decade, showing that CVTs have significant advantages over 4-speed automatic transmissions, and describes the development stages from VT1H to VT2, whose fuel consumption is comparable to that of manual transmissions. The basic principles of CVT technology are common knowledge today and will not be repeated. 2. Technical Description of VT1 Here, only the two main components of CVT—the transmission and the control system—are briefly described. The VT1 transmission consists of the following main components (see Figure 1): • Planetary gear mechanism with multi-plate wet clutch, enabling forward and reverse movement • Continuously variable transmission (CVT) consisting of drive pulleys, driven pulleys, and a V-belt • Two-stage gear reduction • Differential • Oil pump • Hydraulic control • Transmission electronic control unit (TCU) • Technical parameters of the VT1 are shown in Table 1. The transmission control system consists of three main control systems (see Figure 2): • Belt ratio control, used to control the transmission ratio • Clutch control, used to control the multi-plate wet clutch • Clamping force control, used to control the clamping force of the steel belt to prevent slippage. 3. Hydraulically Controlled VT1H Our company's first CVT, called VT1H, was put into production in 1992. It was a fully hydraulically controlled CVT. At the time, VT1H proved that CVT technology had a bright future and great potential. ZFST conducted market research in Europe at that time, with different drivers driving the same type of car equipped with manual, 4-speed automatic, and VT1H transmissions and the same engine (results shown in Figure 3). The advantage of a CVT is that you can drive the vehicle in a more comfortable and dynamic way, requiring only a light or quick press of the throttle. A lower throttle opening results in lower engine speeds for a smooth and comfortable driving experience, while a larger throttle opening corresponds to higher engine speeds for a noticeable acceleration feel. Due to the characteristics of a CVT, you won't experience shift shocks. This makes a CVT not only easy to drive but also more fun. In terms of fuel consumption, the VT1H is better than a 4-speed automatic transmission because the CVT allows the engine to operate at its lowest fuel consumption point. However, this doesn't fully compensate for the losses in the oil pump and metal V-belt, which are not present in a manual transmission. Overall, the CVT's overall score is on par with a manual transmission, but better than a 4-speed automatic transmission. Further research has been confirmed by real-world testing, using the same vehicles and engines, with both a 4-speed automatic transmission and the VT1H installed. The results from both studies are strikingly similar. You can find these measurements in Table 2. 4. The development of the electronically controlled VT1F went through two stages: first, the VT1V, which only had electronic pulley ratio control; then, the VT1F, which added electronic clutch control and electronic clamping force control to the VT1V, achieving a fully electronically controlled VT1. The semi-electronically controlled VT1V began production in 1999, while the fully electronically controlled VT1F began production in 2001. Electronic pulley ratio control allows the CVT to program shift strategies as needed. Different CVT modes (economy mode, sport mode, etc.), manual mode with a fixed gear ratio (often called TIP mode), and automatic mode are all possible. Of course, a balance must be achieved in terms of vehicle performance, driving characteristics, and fuel economy. Electronic pulley ratio control also allows the CVT to control upshift speed very precisely, reducing the minimum upshift speed from 1800rpm to 1400rpm, resulting in better fuel economy. Electronic clamping force control allows the CVT to adjust the clamping force, not only related to the transmission ratio like the VT1H, but also adjusted according to engine torque. Therefore, the clamping force acting on the steel belt is also smaller at low engine torque, resulting in lower steel belt and oil pump losses, thus improving fuel economy. Electronic clutch control allows the CVT to control the clutch based on engine and drive pulley speeds, achieving smoother clutch performance. This system compensates for temperature effects and prevents clutch abuse. To further optimize fuel consumption, the braking signal is also used as an input signal to the transmission. When braking is applied while the engine is idling, the clutch pressure is reduced in D gear. This also improves idling vibration and the "creep" feel during parking. Compared to the VT1H, the VT1F reduces fuel consumption by 12% due to the above improvements, which is significantly better than conventional automatic transmissions, even 5-speed automatic transmissions, only slightly inferior to manual transmissions. 5. Thrust Limit: Two years later, ZFST developed the high-torque VT1F—VT1+. This transmission can deliver 210Nm (120kW) but has the same dimensions, meaning you can use either the VT1F or VT1+ in the same vehicle without changing anything. The high torque capacity is achieved by using a wider and stronger steel belt. The bandwidth has increased from 24mm to 30mm. Of course, some internal parts must be modified accordingly to accommodate this wider bandwidth and higher torque. Table 3 shows the acceleration parameters of VT1+. 6. Further Development: In addition to the above significant advantages, further improvements have resulted in the second-generation CVT, VT2. The main features of VT2 are: • Fuel consumption rate similar to manual transmissions • Suitable for both conventional transmissions and hybrid systems • Lower cost • Easy installation • Same size as VT1 To achieve these goals, a completely new control system (hydraulic, electronic, and TCU) had to be developed, and some modifications had to be made to the transmission hardware. Compatibility with hybrid systems is a significant challenge. The stop/start function of hybrid systems requires very fast and precise control of the transmission. Meanwhile, numerous tests have proven the feasibility of VT2, and one customer has even conducted tests using a VT2 prototype. VT2 will go into production in 2006 as part of a hybrid system for an Asian customer. 7. Conclusion Compared with conventional automatic transmissions, CVTs have significant advantages in driving comfort, fuel consumption, and driving pleasure. Electronic control can further enhance performance, achieving fuel consumption comparable to manual transmissions and enabling applications in hybrid vehicles. Looking at the Chinese automotive market, front-wheel-drive vehicles with an engine torque of 200 Nm are being manufactured and sold, and VT1 and VT2 are indeed products of interest to Chinese automakers and consumers.