In recent years, the development of bus powertrain systems has increasingly focused on engines with lower speeds and higher torque. These engines are typically compact, equipped with supercharging or two-stage supercharging systems, paired with multi-speed transmissions, small gear ratio axles, and energy-efficient tires. This approach not only enhances fuel efficiency but also improves reliability and driving comfort. Several domestic diesel engine manufacturers have aligned themselves with low-speed engine technology, indicating a clear industry trend.
On August 22nd, during a small seminar on bus-specific engines hosted by the China Internal Combustion Engine Industry Association, experts from Weichai, Yuchai, and FAW Jiefang Xichai, along with representatives from bus companies in Beijing, Shanghai, and Changchun, discussed various technical challenges related to engine matching from both user perspectives and real-world bus operating conditions. They also explored future directions for the development of bus-specific engines from both production and usage angles.
The bus engine sector is now entering a stage of low-speed, high-torque development. Jin Qiang, deputy manager of Weichai Power’s passenger car division, noted that the rated engine speed for the passenger car market has dropped from 2,300 rpm to 2,100 rpm, and even further to 1,900 rpm. The power range now covers 240–460 horsepower, marking significant advancements in economy, power, reliability, and comfort.
When the engine operates at a lower speed, its fuel consumption is reduced over a wider range. By pairing it with a transmission and axle with a smaller gear ratio, the overall transmission efficiency increases, leading to lower fuel consumption. From a reliability standpoint, lower engine speeds reduce the linear speed of piston movement and the rotational speed of components like the crankshaft, gears, and belt system, thereby enhancing engine durability. Some engines now achieve overhaul intervals exceeding 1.2 million kilometers.
In terms of comfort, low-speed operation helps avoid high-frequency vibration zones, reducing noise levels. Additionally, optimized power matching and the use of an exhaust brake system help reduce shifting frequency, improving the overall driving experience. As Jin Qiang stated, "With proper matching of the engine, transmission, axle, and tires, vehicle fuel consumption can drop by about 10%."
Yuchai’s Wang Hui and FAW Jiefang Xichai’s Tang Wei echoed similar sentiments, confirming that the bus engine industry is clearly moving toward low-speed and high-torque configurations.
Another key trend in engine development is miniaturization and weight reduction. While more gears are being added, boosting—especially through technologies like two-stage supercharging—has become a prominent sign of this shift. For example, Yuchai's bus engines use two-stage supercharging: a small turbocharger handles low-speed acceleration, while a larger one engages at higher speeds to improve power performance. This effectively resolves the conflict between low- and high-speed power demands and expands the low-fuel-consumption area, optimizing fuel economy.
Tang Wei also highlighted the benefits of variable nozzle turbochargers (VNT). These systems significantly enhance low-speed torque, resulting in noticeable fuel savings, especially below 1,500 rpm. In addition, VNTs allow for a more compact and lighter vehicle design, giving engineers more confidence in using smaller engines. “Although a VNT may cost 2,000 to 3,000 yuan more, it allows for the use of a smaller engine, which leads to substantial cost savings in replacement and maintenance,†Tang explained.
Overall, the evolution of bus engines reflects a broader industry shift towards efficiency, reliability, and user comfort, driven by technological innovation and a deeper understanding of real-world operating conditions.
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