Technologies

Battery Thermal Management for Electric Buses: An Overview This article explores the structure and working principles of common battery thermal management in battery electric buses (BEB), offering valuable insights for their design. The performance of the traction battery system is a key factor in a battery electric bus’s overall efficiency, range, and reliability. Since battery temperature directly affects performance, lifespan, and safety, a well-designed thermal management system is crucial for optimizing operation and ensuring long-term durability. 1. Different Types of Battery Thermal Management Effective battery thermal management ensures optimal performance and safety by regulating temperature under varying conditions. This includes cooling the battery during high temperatures to prevent overheating and heating it in low temperatures to maintain efficiency and reliability. 1.1 Battery Cooling Methods Battery cooling is essential for maintaining performance and safety in electric buses. The most common cooling methods for EV  batteries include natural air cooling, forced air cooling, liquid cooling, and direct refrigerant cooling. The air cooling method features a simple structure, lightweight design, low cost, and no risks of harmful gas accumulation or liquid leakage. However, its drawbacks include low heat dissipation efficiency, difficulty in sealing design, and poor dustproof and waterproof performance. Depending on whether additional devices are used to introduce cooling air, the air cooling system is divied into natural air cooling and forced air cooling:  Natural Air Cooling & Forced Air Cooling Natural Air Cooling: It’s a method of utilizing the wind generated by the vehicle’s movement to flow through a diversion pipe and directly cool the battery pack. This approach requires no auxiliary motors, offers a simple structure, and is easy to use. However, the cooling airflow is subject to instability due to fluctuations in vehicle speed, resulting in variable cooling performance. Additionally, air has low heat capacity and thermal conductivity, which limits the efficiency of heat transfer. Natural air cooling is best suited for vehicles with low discharge rates and minimal heat generation in their power batteries. Forced Air Cooling: It directly introduces cabin air, natural air, or external convection air into the battery compartment to cool the battery pack. This cooling method has relatively poor performance, the largest system volume, and a high risk of water ingress. However, it is lightweight, easy to control, has low energy consumption, relatively low system costs, is relatively easy to implement, and offers high process reliability. Liquid Cooling Liquid Cooling: It utilizes convective heat transfer through a coolant to dissipate the heat generated by the battery, effectively lowering its temperature. It’s currently the most widely adopted cooling solution in the market. Compared to air cooling, liquid cooling systems – using coolant as the heat transfer medium – offer significantly higher specific heat capacity and thermal conductivity. Additionally, in low-temperature environments, the system can also provide heating for the battery pack. However, the primary drawbacks of liquid cooling include increased structural complexity, added battery system weight, and higher overall system costs. Direct Refrigerant Cooling Direct Refrigerant Cooling: Also known as direct cooling or refrigerant-based cooling, integrates the battery thermal management system with the vehicle’s air conditioning system. In this approach, an evaporator in the refrigerant loop functions as the battery’s direct cooling plate, simplifying the overall system. It minimizes heat exchange losses, resulting in a rapid thermal response. And the design eliminates the need for a separate battery cooling loop, reducing components like the coolant pump, piping, and chiller, leading to a more space-efficeint, lightweight thermal management system. A comparison of different cooling methods is as follows: Comparison Item Natural Cooling Forced Air Cooling Liquid Cooling Direct Cooling Working Principle Natural air convection Forced air convection Forced liquid convection Phase-change cooling Heat Transfer Coefficient (W/m²K) 5-25 25-100 500-15000 2500-25000 Heat Dissipation Efficiency Poor High High Very High Temperature Uniformity Good without external heat sources, otherwise poor Poor (especially at inlet and outlet) Good Good Installation Environment Adaptability Poor (requires external insulation and ventilation) Fair (dependent on inlet and outlet air structure) Good Good Suitability for High/Low Temperatures Poor (conflict between auxiliary heating/insulation and heat dissipation) Poor (conflict between auxiliary heating/insulation and heat dissipation) Good (capable of both heating and cooling) Poor (requires additional auxiliary heating, difficult to control heat pump) Complexity Simplest Moderate Complex Complex Energy Consumption None High Low (easy to implement insulation) Low (high efficiency) Cost Low Relatively High High (can be optimized) High 1.2 Battery Heating Methods To maintain optimal performance in cold conditions, EV traction batteries require heating. The most common battery heating methods include: Integrated Electric Heating Film: A thin electric heating film integrated inside the battery pack directly heats the battery cells. Performance: Above 0°C: works well, requires no additional space, consumes no energy when inactive, and is cost-effective. Below 0°C: heating efficiency drops significantly, making it unsuitable for extremely cold environments. Advantages: Space savings and easy to implement; low system cost and no additional control needed. Limitations: Ineffective in freezing temperatures, leading to limited adoption in colder climates. Liquid-Based Heating System: An electric liquid heater is integrated into the thermal management system’s coolant circuit to heat the antifreeze, which then circulates to warm the battery pack. Performance: Provides efficient and uniform heating, making it the preferred method in colder regions. Advantages: Compact system design with minimal space requirements; mature and reliable technology with well-established control mechanisms; high heating efficiency, ensuring stable battery operation in low temperatures Limitations: Higher system cost compared to electric heating films. Due to its efficiency and reliability, liquid heating is currently the most commonly used battery heating solution. Comparison Item Electric Heating Film Liquid Heating Heating Characteristics Constant power heating Convective/conductive heating Space constraints (Thickness) 0.3~2 mm Integrated into the liquid heating system Heating Rate 0.15~0.3°C/min 0.3~0.6°C/min Uniformity (Battery Temperature Difference) ≈8°C ≤5°C 2. Structure and Working Principles of Battery Thermal Management Systems To maintain optimal performance and longevity, EV traction batteries in electric buses operate within an ideal temperature range of 25°C ± 5°C, regardless of seasonal variations. In winter, the battery thermal management system heats the coolant to maintain the target

This 120kW/240kW electric motor is designed for 10-12 meter battery-electric buses and 8-10 meter electric coaches.

Electric Bus Powertrain: Wheel-Side Drive Motor – Introduction and Analysis Electric buses have traditionally relied on centralized drive systems. Two main configurations have dominated the market: Direct-Drive Motor System In these systems, the electric motor directly replaces the internal combustion engine. This approach offers a simple and straightforward system architecture, with mature vehicle structural design technology that has made it the mainstream configuration for battery electric buses. Motor + Gearbox Systems This configuration, which combines an electric motor with an Automatic Manual Transmission (AMT) gearbox, delivers superior performance on steep slopes, meeting the demands of mountainous or scenic areas. Although it borrows extensively from conventional fuel vehicle technology and is relatively mature, including an automatic shifting mechanism can compromise reliability, leading to practical challenges in real-world operations. As electric bus powertrain systems evolve toward higher speeds, greater integration, and lightweight designs, distributed electric drive technology—exemplified by wheel-side drive motors—has begun to enter the practical stage. Compared with centralized drive systems, wheel-side drive configurations eliminate the need for transmission shafts, main reduction gears, and differentials. This results in a shorter power transmission chain, higher transmission efficiency, and a more compact structure.  In addition, precise control of wheel-side motor speed and torque enables integrated functions such as vehicle propulsion, braking, differential action, and energy recovery. Owing to its high degree of freedom and functional expandability, the wheel-side drive system has become a focal point in the research and development of pure electric bus powertrain systems. In addition, precise control of wheel-side motor speed and torque enables integrated functions such as vehicle propulsion, braking, differential action, and energy recovery. Owing to its high degree of freedom and functional expandability, the wheel-side drive system has become a focal point in the research and development of pure electric bus powertrain systems. Electric Bus Powertrain: Wheel-Side Motor Drive Configurations Wheel-side motor solutions for battery electric buses bridge the gap between centralized and hub motor designs. They typically integrate a motor with a fixed-ratio reducer mounted on the chassis, directly driving the wheels via short axles. Two primary configurations exist: A. Fixed-Motor Configuration The fixed-motor configuration usually takes the form of an integrated wheel-side drive axle. In this design, the conventional axle housing and half-shafts are eliminated, and the drive motor is mounted adjacent to the wheel. Despite maintaining a rigid axle structure, this design can employ either steel leaf springs or a combination of air springs and coil-over shock absorbers.  Brogen wheel-side drive system with fixed motors Key advantages: Reduced Weight and Space: By removing the axle housing, casing, and half-shafts, the overall structure is significantly lighter and more compact. High-Speed Motor Integration: With the use of high-speed motors paired with high reduction ratios (often through a planetary gear structure), the design minimizes motor volume and weight while increasing power density. Robust Performance: Our integrated wheel-side drive axle, for example, features two high-speed motors mounted on either side of the axle. With a two-stage reduction system, it delivers enhanced torque and is capable of handling high axle loads—ideal for heavy-duty, low-floor city buses. B. Swing-Motor Configuration In the swing-motor configuration, the traditional rigid axle is abandoned in favor of an independent air suspension system. Here, both the drive motors and reducers are mounted directly on the suspension, and the reducer can be designed as either a two-stage or planetary gear system. Brogen wheel-side drive system with swing motor and independent suspension Key advantages: Lower Unsprung Mass: The elimination of the rigid axle structure reduces unsprung mass. A well-designed suspension can effectively transfer the motor’s mass to the vehicle body, improving ride comfort and handling. Enhanced Cabin Design: This configuration enables increased interior space, a wider aisle, and lower floor heights—critical factors in the design of modern low-floor city buses. Benefits of Wheel-Side Drive Motors in Electric Buses The adoption of a wheel-side drive motor for an electric bus powertrain – where flexible electrical connections replace some mechanical linkages – offers significant benefits in electric bus design and performance. Below are the primary advantages:  1. Increased Cabin Space and Lower Floor Height Low-floor city buses are a growing trend. In these designs, the area from the front passenger door to the last axle forms a continuous, step-free zone. Lowering the interior floor not only reduces the number and height of steps – making boarding and movement inside the bus easier, safer, and more accessible for all passengers – but also increases headroom in key areas. For instance, the structural dimensions of an integrated wheel-side drive axle can reduce floor installation height by approximately 70 mm, achieving a floor clearance as low as 290 mm. This enhanced design meets both interior space and passenger safety requirements. 2. Vehicle Lightweighting Reducing the overall vehicle weight directly contributes to lower energy consumption. Research indicates that a 10% reduction in vehicle weight can lead to a 6% – 8% reduction in energy consumption. The wheel-side drive motor configuration achieves significant weight savings through: High-speed motor design: Increased motor speeds allow for lower torque requirements, which in turn reduces cost and weight. System integration: Consolidating components (motor, reducer, controller) into an integrated design reduces the need for additional attachments and cabling, thereby lowering both the weight and cost of the powertrain. 3. Enhanced Vehicle Dynamics Mainstream single-motor direct-drive configurations often struggle with steep climbs and mid-to-high-speed acceleration, while dual-motor systems face challenges in weight and cost. Additionally, motor+AMT configurations can suffer from power interruptions during gear shifts. In contrast, the wheel-side motor configuration: Dual-Motor Advantage: By deploying two motors, the power demand on each unit is reduced while maintaining overall system performance. Elimination of the Main Reducer: Replacing the main reducer with a high-reduction ratio gear system (using helical gears) not only simplifies the transmission patch but also improves strength, reduces manufacturing complexity, and lowers costs. 4. Improved Transmission Efficiency Eliminating the traditional main reducer and differential from the electric bus powertrain means that power is transmitted through a shorter chain – enhancing efficiency. The use of helical gears in the

How an e-Axle with Independent Suspension is Shaping the Future of Electric Bus Design? As the world shifts toward sustainable transportation, the demand for energy-efficient, high-performance solutions in electric buses and other Zero Emission Buses (ZEBs) is steadily increasing. One of the key innovations at the forefront of this transformation is the wheel-side e-axle with independent suspension system for public transportation electrification. This solution is reshaping the way we think about electric bus powertrains, improving overall passenger experience, and enhancing operational efficiency. The Next Level of Efficiency: Wheel-Side Distributed Drive E-Axle With Independent Suspension The wheel-side distributed drive e-axle with independent suspension system is designed to offer a lighter, more efficient, and more compact solution for EV buses. This technology effectively reduces vehicle weight and energy consumption. The result? More spacious interior, greater energy savings, and increased passenger capacity flexibility. Our e-Axle with independent suspension showcased at Automechanika Shanghai 2024 What sets this system apart is its unique design: the left and right wheel-side motors replace the traditional differential and gearbox, applying torque directly to the wheels. This not only simplifies the drivetrain but also minimizes energy loss, making the electric bus more efficient. The integration of high-speed motors and reducers ensures that the system delivers large torque in a small form factor, enhancing the vehicle’s power output. The wheel-side drive e-axle and its controller operate based on the same principles as the motor and controller systems used in other pure electric or hybrid buses. However, the key challenge in wheel-side distributed drive control for buses lies in achieving the differential speed function between the drive e-axles. As a result, the control strategy and functionality of wheel-side drive e-axles differ from those in other battery electric or hybrid buses. 1.1 Three Configurations of the ePowertrain System for Electric Buses The distributed wheel-side e-Axle solution eliminates the need for a differential, half-shaft, and shortens the transmission chain. This results in higher transmission efficiency, a lighter vehicle, and reduced energy consumption. The compact motor size and wide central aisle make it suitable for low-floor layouts. Before in-hub motor technology become fully mature, wheel-side distributed drive e-axles remain the best matching solution for electric buses. 1.2 Basic Structure of the Wheel-Side Distributed Drive e-Axle with Independent Suspension Independent Suspension System: Featuring a four-airbag double wishbone design, this system offers reduced unsprung mass, resulting in a smoother ride with improved comfort and stability. Dual Wheel-Side Motors: With high torque and power density, the dual motors provide a strong and reliable output. Reducer: Reduces the motor output power to increase torque. Motor Controller: Controls the motor to operate at the desired speed, angle, direction, and response time. Electronic Components: Includes accelerator pedal sensors, brake pedal sensors, Hall-effect wheel speed sensors, Hall-effect steering angle sensors, roll angle sensors, yaw rate sensors, and controller hardware. 1.3 Working Principles of the Wheel-Side Distributed Drive e-Axle with Independent Suspension At the heart of the wheel-side distributed drive e-axle system is the Vehicle Control Unit (VCU), which calculates the total torque demand based on the driver’s acceleration or deceleration intent. The electronic differential system allocates the total torque between the left and right drive motors based on the steering angle. vehicle posture sensors, and road surface traction coefficients. The left and right motor controllers, upon receiving the instructions, convert the energy from the battery to the necessary power for the drive emotors, ensuring the vehicle’s stability and handling. 1.4 Operating Modes of the Wheel-Side Distributed Drive e-Axle with Independent Suspension Driving Mode: The battery supplies energy to the drive motor through the motor controller, which then directly drives the wheels. Regenerative Braking Mode: The rolling wheels drive the motor (regenerative generation) to charge the battery (energy recovery). Applications of the Distributed Drive e-Axle With Independent Suspension on Electric Buses The wheel-side e-axle with 4-airbag independent suspension system is not just about technological innovation; it’s about transforming the way we experience and operate electric buses in urban environments. The system offers several key advantages: Increased Safety: With a low-floor layout, passengers, including the elderly, children, and people with disabilities, can easily board and disembark. The design also reduces the risk of accidents caused by falls on traditional steps, offering a safer and more inclusive experience for all. Enhanced Comfort: The independent suspension system significantly improves ride quality, with reduced vibration and a smoother overall journey. Improved Operational Efficiency: For an 8.8-meter electric bus, the weight of the e-powertrain system can be reduced by more than 700 kg; for a 10-meter electric bus, the weight of the e-powertrain system can be reduced by more than 800 kg, offering significant energy savings, improved range, and lower maintenance costs. Real-World Impact: Proven Performance The independent suspension e-axle has already been deployed successfully on electric buses in several countries. Statistics show that after one year of use of a city’s 200 units of electric buses, the passenger injury accident rate has dropped by about 70%, largely due to the elimination of high-step entryways and the vehicle’s stable ride quality. These buses are proving to be a safer, more efficient, and comfortable choice for city transportation. Advantages of our Electric Bus Powertrain Solution The wheel-side distributed drive electric axle system is a leading-edge solution for electric buses, offering a future-proof EV technology that enables operators to meet sustainability goals while improving passenger experience and operational performance. As the global market for electric buses expands, this innovation is paving the way for more sustainable, energy-efficient public transit systems in cities across the globe.  Optimized Interior Space – Enables an 800 mm wide aisle and a fully flat low-floor design for enhanced accessibility and passenger comfort. Enhanced Maneuverability & Safety – Integrated steering function allows the rear wheels to track the front wheels, significantly reducing right-turn accidents caused by wheel differences. Proven Reliability – Features a mature and dependable electronic differential system, with tire replacement mileage exceeding 100,000 km. Technical Parameters Item Parameters Motor type PMSM Motor power 2×55 kW / 2×90 kW Maximum motor speed 9500 rpm Motor torque 2×140

AMT Powertrain System for Battery Electric Buses: Enhancing Efficiency and Performance The transition to battery electric buses has become a key focus for bus manufacturers and cities aiming to reduce emissions and improve public transport sustainability. In recent years, some bus manufacturers have shifted from traditional direct-drive systems to e-powertrain systems that include Automated Manual Transmission (AMT) for battery electric buses (BEB). But what does this mean for electric bus performance, and how does it affect the overall driving experience? What is an AMT Powertrain System for BEB? The direct drive system delivers power directly from the motor to the rear axle, offering a simple, reliable, and efficient design. In contrast, the AMT e-powertrain system utilizes a gearshifting mechanism to transmit motor torque to the rear axle, allowing for optimized performance and energy efficiency by adjusting the gear ratios.  AMT Powertrain System vs Direct Drive System: Performance In fact, not all BEBs should be equipped with an AMT system. Compared to the conventional direct-drive system, electric buses with AMT have both advantages and disadvantages. Let’s take a 12-meter battery electric bus as an example to focus on the power performance comparison between AMT powertrain and direct-drive systems. System Parameters AMT Powertrain Direct Drive System Total Weight 260 kg 364 kg Maximum Torque 3730 N.m 2800 N.m Length 716 mm 470 mm Width 614 mm 603 mm Height 592 mm 592 mm For a 12-meter BEB, the electric motor in the AMT powertrain system has a rated power of 80 kW, whereas the direct-drive system uses a more powerful 100 kW electric motor. However, the motor in the AMT system can reach a maximum speed of 3500 rpm, which is higher than the motor speed (3000 rpm) in the direct-drive system. Electric buses with AMT powertrain systems offer better hill-climbing ability (23%) and can achieve higher speeds (14%), though the acceleration time may be slightly longer due to the shifting process. Motor Parameters AMT Powertrain System Direct-Drive System Rated Power 80 kW 100 kW Peak Power 140 kW 200 kW Rated Torque 600 N.m 1250 N.m Peak Torque 1000 N.m 2800 N.m Maximum Speed 3500 rpm 3000 rpm Maximum Gradeability 23% 14% Maximum Vehicle Speed 116 km/h 107 km/h 0-50 km/h Time 16 s 13.4 s In terms of system efficiency during normal operation, the two types of systems are quite different. When comparing system efficiency at different speeds, we find that the direct-drive system performs more efficiently in the 40-60 km/h range, with lower overall energy consumption. However, in the speed ranges below 40 km/h and above 60 km/h, the AMT powertrain system outperforms, showing lower overall energy consumption. Overall, when considering a wider range of driving conditions, the AMT system tends to offer better energy efficiency. In typical urban driving conditions, the electric bus equipped with the AMT system has a simulated energy consumption of 68.3 kWh per 100 km, with actual test results showing 68.6 kWh per 100 km. In contrast, the electric bus using a direct-drive system has a simulated consumption of 72.5 kWh per 100 km, with test results showing 73.3 kWh per 100 km. AMT Powertrain System vs Direct Drive System: Cost Considerations Cost is another significant factor for bus manufacturers. While the AMT system offers enhanced perofrmance, the cost implications depend on the bus’s torque demands. For buses with lower torque requirements, the direct drive system may be more cost-effective. However, for high-torque applications, such as large electric buses, the AMT system proves more economical. By using smaller motors with the AMT system, bus manufacturers can reduce motor costs without sacrificing power, resulting in a more affordable overall vehicle. In essence, when the required torque exceeds 1500 N.m, the AMT powertrain system becomes the more cost-effective choice. Larger buses with higher power demands benefit the most from AMT technology, offering both cost savings and better performance. AMT Powertrain System vs Direct Drive System: Comfort & Reliability Beyond performance and cost, comfort and reliability are crucial factors for any bus manufacturer. AMT powertrain systems do introduce gear shifts, which can cause slight interruptions in power delivery and reduce comfort compared to direct drive systems, which offer smoother acceleration. However, technological improvements in AMT design can mitigate these discomforts. For example, the shifting process can be optimized to minimize interruptions, thus improving the passenger experience. From a reliability perspective, the complexity of the AMT system theoretically makes it less reliable than direct drive systems. However, through rigorous design and quality control, these concerns can be addressed. For instance, AMT products can be designed with durability in mind, achieving similar reliability levels as direct-drive systems. Additionally, the AMT system’s lightweight design, compared to traditional systems, contributes to overall vehicle weight reduction. Our AMT Powertrain Systems for Electric Commercial Vehicles At Brogen, we offer e-powertrain systems based on AMT technology, utilizing PMSM for drive. The AMT is co-axially connected to the motor, taking advantage of the gearbox’s speed control and torque-boosting features. This enables a smaller motor to achieve the same performance as a larger direct-drive motor, while also enhancing the vehicle’s adaptability and efficiency. Our e-powertrain system with AMT ranges from 60 kW to 550 kW, making it suitable for applications in buses, trucks, heavy commercial vehicles, municipal vehicles, and loaders. Motor Power / Torque Transmission Maximum Motor Speed Maximum Output Torque Applicable Vehicles 300/500 N.m, 60/120 kW 2-speed: 2.73/1 5000 rpm 1365 N.m 4.5-7T truck 350/850 N.m, 75/120 kW 2-speed: 2.741/1 5000 rpm 2330 N.m 7-12T truck 500/1100 N.m, 80/160 kW 2-speed: 2.741/1 4500 rpm 3015 N.m 8-10M bus, 11-14T truck 500/1300 N.m, 120/185 kW 2-speed: 2.741/1 3500 rpm 3380 N.m 10M, 12M bus 500/1100 N.m, 100/185 kW 4-speed: 6.61/3.52/1.89/1 3500 rpm 7000 N.m 14-18T truck, specialty vehicles 850/1950 N.m, 180/300 kW 4-speed: 8.39/3.54/1.74/1 3500 rpm 16360 N.m 25-30T truck 1200/2400 N.m, 250/380 kW 4-speed: 8.39/3.54/1.74/1 3500 rpm 20136 N.m 31-49T truck 1500/2400 N.m, 270/405 kW 4-speed: 8.39/3.54/1.74/1 3500 rpm 20136 N.m 35-60T truck 1500/2400 N.m, 300/450 kW 4-speed: 8.39/3.54/1.74/1 3500 rpm 20136 N.m 60-90T truck