With the continuous improvement of urban bus driving comfort, economy, and environmental protection requirements, the development of transmission system technology—specifically the automation of shifting and clutch operations—has become a key trend. In developed regions such as Western Europe, most city buses are now equipped with hydraulic automatic transmissions (AT). While manual transmissions still dominate in domestic markets, the adoption of new automatic transmission technologies is growing rapidly. In the long run, full shift automation for city buses is inevitable. To accelerate this transition, selecting the appropriate transmission automation technology based on market demands is essential.
One notable solution is the six-speed mechanical automatic transmission, the ZF AS Tronic Lite, which is particularly well-suited for automation from manual systems. It’s ideal for customers who are sensitive to ownership and operational costs. Figure 1 shows the basic structure of the AS Tronic Lite transmission, including components like the integrated clutch housing, shifting module, and power pack.
Due to its design, the AS Tronic Lite uses a standard dry clutch and a six-speed manual transmission base. The intelligent electronic control system enables fully automated shifting and clutch operation, reducing driver error and delivering performance comparable to that of an experienced driver. This system offers several advantages:
1. Reduces driver workload by enabling automatic shifting.
2. Has a lower initial purchase cost compared to other automatic options.
3. Can reduce fuel consumption by 3% to 5% compared to manual transmissions by eliminating driver-related inefficiencies.
4. Extends clutch life by up to two to three times.
5. Minimizes damage caused by incorrect shifting, thus protecting the transmission system and reducing component failure rates.
6. Lowers dependency on driver skill, saving on training costs and allowing even inexperienced drivers to perform well.
7. Easier to maintain than traditional automatic transmissions.
In 2008, ZF Suzhou introduced the AS Tronic Lite on Shanghai buses, and it was fully validated through various operating conditions. By 2010, it became a standard feature for new buses in Suzhou, with over 700 units deployed domestically. The system has also gained traction in export markets, including Taiwan and Eastern Europe. It is suitable for city buses ranging from 9 to 12 meters in length and has been successfully matched with both imported and domestic diesel engines, such as Cummins, Yuchai, Weichai, Shangchai, and Xichai.
In recent years, natural gas engine technology has seen significant growth due to its energy efficiency, cost-effectiveness, and environmental benefits. The production of natural gas-powered buses has increased dramatically. Both natural gas engines and AMT systems offer energy-saving and emission-reducing advantages, making their combination highly effective. Simulation results show:
1. Fuel costs can be reduced by approximately 35% compared to diesel vehicles.
2. COâ‚‚ emissions can drop by about one-third, and hydrocarbon emissions can decrease by four-fifths.
This economic advantage has driven strong market demand. However, matching gas engines with AMT systems presents technical challenges. Gas engines have different torque and response characteristics compared to diesel engines, requiring close collaboration between engine and transmission suppliers to optimize control strategies and improve system performance.
Key optimization efforts include:
- Helping engine manufacturers improve torque accuracy, response speed, and stability at low speeds.
- Adjusting clutch and shift strategies to better suit gas engine behavior, such as optimizing shift points and reducing impact during start-up and braking.
Since 2009, ZF has conducted extensive simulations, bench tests, and road trials with domestic gas engine suppliers. By 2011, high-volume applications were achieved, with positive market feedback. Over 300 buses equipped with natural gas engines and ZF AS Tronic Lite are currently in operation, offering ride comfort comparable to diesel vehicles, while significantly lowering operating costs and emissions.
In conclusion, the combination of AMT transmission and natural gas engines provides substantial economic and environmental benefits for commercial vehicle users. Its application is expected to expand beyond urban buses to long-haul passenger cars and medium- and heavy-duty trucks, offering great market potential and promising future growth.
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