The overall engine is slower but produces higher torque. It is compact, turbocharged or two-stage turbocharged, and paired with a multi-speed transmission, a small gear ratio axle, and energy-efficient tires. For buses, this appears to be the general direction of powertrain system development in recent years. In fact, several domestic diesel engine manufacturers have focused on low-speed engine technology.
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 personnel from bus companies in Beijing, Shanghai, and Changchun, discussed technical challenges related to engine matching from the user's perspective and the bus operating environment. They also explored the future direction of bus-specific engines from both production and usage angles.
**Bus Engines Enter the Era of Low-Speed, High-Torque Development**
Jin Qiang, deputy manager of the passenger car power division at Weichai Power Co., Ltd., mentioned that the rated engine speed for Weichai’s passenger car market has dropped from 2,300 rpm to 2,100 rpm, and further to 1,900 rpm. The power range now spans from 240 to 460 horsepower, marking significant breakthroughs in economy, performance, reliability, and comfort.
With the rated speed reduced to a lower range, the fuel-efficient area of the engine expands. When matched with a transmission and axle featuring a small gear ratio, the overall transmission efficiency improves, leading to even lower fuel consumption. From a reliability standpoint, low-speed engines reduce the linear speed of piston movement and the rotational speed of the crankshaft, gear train, and belt system, enhancing engine durability. The overhaul interval has now exceeded 1.2 million kilometers.
In terms of comfort, low-speed power development helps prevent the engine from operating in high-frequency vibration zones, reducing noise. Additionally, optimized power matching and the use of exhaust brakes lead to fewer shifts, improving the driving experience. As a result, overall comfort has been significantly enhanced.
"By properly matching the engine, transmission, axle, and tires, vehicle fuel consumption can drop by about 10%," said Jin Qiang. Wang Hui, technical director of Yuchai Engineering Research Institute, and Tang Wei, deputy director of FAW Jiefang Xichai’s R&D department, expressed similar opinions. It’s clear that bus engines are entering a new era of low-speed, high-torque development.
**Engine Miniaturization and Weight Reduction**
Alongside improvements in powertrain matching and fuel efficiency, engine miniaturization and weight reduction have become a prominent trend. Adding more gears often signals this shift, but turbocharging remains the most visible sign of compact and lightweight engine design.
For instance, Yuchai’s bus engines utilize two-stage turbocharging. A smaller turbocharger handles low-speed acceleration, while a larger one engages at higher speeds to boost power and improve high-speed performance. This effectively resolves the conflict between low- and high-speed power demands. Moreover, the low-fuel-consumption area of the two-stage turbocharged engine is adjusted to stabilize within the commonly used speed range, further improving fuel economy.
According to Tang Wei, variable nozzle turbochargers (VNT) significantly enhance power output. Their use leads to noticeable fuel savings, especially below 1,500 rpm. Additionally, VNT allows for better optimization of the vehicle’s size and weight, giving more confidence in choosing smaller engines. "Although a VNT might cost an extra 2,000 to 3,000 yuan, it enables the use of a smaller engine, which results in substantial cost savings through replacement and reduction," Tang explained.
Ball Valve,Mini Ball Valve,Two Way Mini Ball Valve,Stainless Steel Ball Valve
Ningbo Wenhan Fluid Equipments Co., Ltd. , https://www.wenhanvalves.com