Technologies

Battery Electric Bus Body: Structural Design and Strength Optimization To advance the adoption of electric vehicles and fully capitalize on the energy-saving and emission-reduction benefits of battery electric buses (BEBs), a lightweight electric bus body design is essential. This article presents an overview of topology optimization theory and explores key material options for electric bus body construction, including high-strength steel, aluminum alloys, and carbon fiber composites. It highlights the advantages of advanced materials in improving structural performance and outlines methodologies for analyzing structural loads, optimizing topology, and ensuring strength and reliability. These insights provide valuable guidance for the development, design, and manufacturing of next-generation electric buses, supporting both performance and sustainability goals. Introduction The electric vehicle (EV) sector is undergoing rapid development, particularly in the public transportation segment, where battery electric buses offer significant advantages such as zero emissions, reduced operating costs, and simpler maintenance. However, one of the primary challenges hindering wider adoption is limited driving range. Reducing curb weight – while maintaining safety and structural integrity – is a proven strategy to extend range and improve overall vehicle efficiency. Since the body structure contributes more than one-third of a bus’s total weight, lightweighting the body is a critical focus. By leveraging topology optimization theory and finite element analysis (FEA), engineers can minimize material use while maintaining required levels of stiffness and strength. This approach not only enhances vehicle range but also contributes to long-term energy efficiency and emission reduction goals. 1. Topology Optimization Principles When applying topology optimization to electric bus body design, shape optimization techniques are used to find the most efficient material layout under one or more loading conditions. This approach targets minimum structural stiffness. It starts with developing a digital base model of the bus body, then uses optimization algorithms to remove unnecessary components while retaining critical structural elements. This improves the structural layout’s rationality and reliability. Basic mathematical model of battery electric bus In practice, topology optimization is performed with the following considerations: Analysis Type: Static analysis Element Types: First-order and second-order tetrahedral elements Material Behavior: Liner elastic, elastoplastic, hyperelastic Loading Conditions: Concentrated force, pressure, torque, gravity Boundary Conditions: Displacement constraints Contact Conditions: Bonded and surface contact Connection Types: Rigid and beam connections Objective Function: Maximize stiffness Design Variables: Topology variables Constraints: Volume fraction If the total number of elements is denoted as a, then each element can be represented as y(i = 1, 2, 3, …, a). During the structural optimization process, if the h-th element is determined to be non-essential, it will be assigned a value of yₕ = 0; if it is essential and should be retained, it will be assigned yₕ = 1. The structural optimization design of a pure electric bus body can be carried out using finite element analysis (FEA) combined with variable design methods. 2. Main Types of Electric Bus Materials The evolution of bus body materials reflects a broader shift toward lightweight and high-performance materials. Initially dominated by steel (over 90% of body structure), the industry has gradually shifted toward aluminum alloys and carbon fiber composites. This shift is driven by the demand for lightweight, energy-efficient, and low-emission vehicles. High-Strength Steel (HSS): Created by adding trace elements to low-carbon steel and undergoing specialized rolling processes. It offers tensile strength up to 420 N/mm² and excellent deep-drawing properties, making it a strong candidate for lightweight structural components. Aluminum Alloys: Compared to steel, aluminum alloys have a lower density (2.7 g/cm³), higher specific strength, good corrosion resistance, thermal stability, and recyclability. These advantages make aluminum alloys widely used for lightweight applications. Carbon Fiber Composites: Composed of carbon fiber bundles and resin, these materials offer exceptional high tensile strength (often over 3500 MPa), high stability, and resistance to deformation during impact. Carbon fiber composites can significantly enhance passive safety and are more than twice as durable as steel. With increasing demand for extended range, pure electric buses have transitioned from using high-strength steel to aluminum alloy bodies, which reduce body weight by 25%-35%. Modern aluminum alloy components are often assembled using rivets and bolts rather than traditional welding, and stamped aluminum parts have replaced conventional steel panels. Aluminum alloys also offer high recyclability, with recovery rates exceeding 85% when vehicles are retired. Skeleton layout of 10.5 m battery electric bus aluminum alloy roof Skeleton layout of 10.5 m pure electric bus aluminum alloy bone around on the left side Skeleton layout of 10.5 m pure electric bus aluminum alloy bone around on the right side 3. Performance and Cost of Advanced Automotive Materials Advanced materials and manufacturing processes are key drivers of automotive innovation. High-Strength Steel: Offers 15%-25% higher strength than standard steel, with better balance and 20% improved corrosion resistance. However, it requires additional anti-corrosion treatments to meet durability requirements. Aluminum Alloys: In a 10-meter bus, switching from steel to aluminum components can reduce body weight by over 450 kg. Aluminum also provides superior thermal insulation and vibration damping compared to steel. Manufacturing methods such as hot forming, die casting, and precision machining enhance part performance and structural integrity. Carbon Fiber Composites: Carbon fiber’s strength and stiffness per unit weight surpass all other commonly used materials. It absorbs 4-5 times more impact energy than steel and can reduce body weight by over 40%, while enhancing safety and aesthetic appeal. However, its high cost restricts its applications to high-end or specialized vehicles. Relative Costs: Steel: Lowest cost Aluminum Alloys: Moderate cost Carbon Fiber Composites: Highest cost   4. Structural Load Analysis and Topology Optimization of Battery Electric Bus Body The structural design of a battery electric bus must comply with the requirements outlined in relevant national standards and the specific design brief. This includes determining the vehicle’s primary exterior dimensions – such as length, width, height, wheelbase, front overhang, and rear overhang – while ensuring a balanced and harmonious overall appearance.  Effective spatial arrangement of the driver’s cabin and passenger area is essential for optimal internal layout. The number, position, and minimum dimensions of passenger doors and emergency

Integrated Electric Bus Powertrain Axle Assembly Design Introduction to Electric Bus Powertrain Development Electric buses have increasingly been recognized worldwide as a leading direction in EV development. As the consumption of non-renewable resources accelerates, the automotive industry’s shift toward electrification is becoming even more pronounced. However, due to current limitations in energy storage technology, electric vehicles continue to lag behind traditional vehicles in areas such as driving range, battery lifespan, and charging convenience. Consequently, pure battery electric buses (BEBs) tailored to specific routes and operating conditions are at the forefront of pilot programs. In recent years, the electrification of public transportation fleets in major cities around the world has gained significant momentum. With substantial investments flowing into the pure electric bus sector, it is critical to harness emerging technologies to strengthen product competitiveness. Among these advancements, the integration of electric motor and drive axle design has notably enhanced the performance of electric buses and has become a key focus in EV safety development. Overview of Integrated Electric Bus Powertrain Axle Assembly Traditional bus powertrains consist of an engine, transmission, driveshaft, and axle assemblies. As the electric bus powertrain industry continues to evolve, many manufacturers initially adopted transitional solutions — replacing the internal combustion engine with an electric motor while largely retaining the traditional mechanical layout, as illustrated in Figure 1. However, this approach introduces several challenges: Low transmission efficiency: Overall system efficiency remains below 80%. Poor system integration: The design conflicts with the industry’s ongoing lightweighting initiatives. Limited regenerative braking performance: Due to the inherent limitations of traditional bevel gear axles, brake energy recovery rates are typically below 30%. The new generation electric bus powertrain layout proposed in this paper effectively overcomes these shortcomings (Figure 2). In this new design, the electric motor is fully integrated into the drive axle assembly, eliminating the need for separately mounted motors, suspension components, and driveshafts within the vehicle frame. This integrated configuration simplifies vehicle layout and installation. The suspension system mounts directly to the axle housing, and the battery packaging is optimized, significantly improving internal space utilization. Nevertheless, engineering challenges remain. Precisely determining the torque axis is complex due to factors such as vehicle and drivetrain layout, packaging constraints, and the mechanical characteristics of rubber isolators, all of which affect the compression-to-shear stiffness ratio and complicate accurate positioning of the fore-aft mounting centers along the torque shaft. System Design of the Integrated Electric Bus Powertrain Axle Assembly Taking an 8.5-meter pure electric bus as a design example: Current mainstream design: Uses a low-speed, permanent magnet synchronous, water-cooled electric motor. Max power: 300 kW Max torque: 2070 Nm Speed: 4000 rpm Axle load: 8 tons Gear ratio: 5.857 Motor weight: 550 kg Driveshaft weight: 40 kg Maximum wheel driving torque: 2070 × 5.857 = 12100 Nm The suspension system’s natural frequency is below 30 Hz. New high-speed design: Introduces a high-speed permanent magnet synchronous, water-cooled motor: Power: 205 kW Torque: 500 Nm Max speed: 12,000 rpm Motor weight: 102 kg Axle weight: 580 kg (with no driveshaft required) Gear ratio: 24.8 Maximum wheel driving torque: 500 × 24.8 = 12400 Nm In traditional axles, the engine layout perpendicular to the driving direction necessitates the use of bevel gears to redirect power. Bevel gear manufacturing limitations mean that convex and concave gear sides have different accuracies. During regenerative braking, if over 30% of braking energy is recovered, the concave gear side’s insufficient precision can cause gear surface damage or even failure. To address this, the new solution arranges the motor parallel to the vehicle’s driving direction, eliminating the need for bevel gears. All gears are cylindrical gears, which are easier to design and manufacture with high precision, improving durability and supporting more efficient brake energy recovery. Weight Comparison Category Weight (kg) Traditional pure electric design 300 + 550 + 40 = 890 kg Integrated axle motor design 102 + 580 = 682 kg Weight saving 890 – 682 = 208 kg Application of Integrated Electric Bus Powertrain Axle Assembly By integrating the motor and axle into a unified powertrain system, there is no longer a need to reserve separate space for the motor. This design reduces the rear suspension space requirements, as shown below, and significantly improves passenger cabin space by increasing the distance between the front and rear doors. Performance Comparison Between Integrated and Traditional Electric Bus Powertrains The integrated electric bus powertrain offers the following advantages over traditional designs: Higher system integration:The axle self-assembles with the powertrain, simplifying vehicle layout and improving transmission efficiency. Enhanced regenerative braking:Brake energy recovery capacity can reach up to 100%. Significant weight reduction:Lightweighting of approximately 208 kg for an 8.5-meter electric bus. Improved vibration isolation and vehicle stability:Experimental tests confirm enhanced ride quality and comfort. Conclusion This study explores an innovative electric bus powertrain layout that significantly improves electric vehicle performance. Through ultra-short rear suspension designs and system lightweighting, the proposed integrated axle motor assembly enhances vehicle energy efficiency (EKG indicators) and extends the continuous driving range. The results demonstrate higher transmission efficiency, superior regenerative braking capabilities, and reveal the future electrification trends for heavy-duty vehicles. Discover our integrated electric bus powertrain (electric bus axle) here: https://brogenevsolution.com/electric-axle-for-bus/ Business inquiry: contact@BrogenEVSolution.com Or you can complete the table below to get in touch with us. Contact Us Get in touch with us by sending us an email, using the Whatsapp number below, or filling in the form below. We usually reply within 2 business days. Email: contact@brogenevsolution.com Respond within 1 business day Whatsapp: +8619352173376 Business hours: 9 am to 6 pm, GMT+8, Mon. to Fri. LinkedIn channel Follow us for regular updates > YouTube channel Ev systems introduction & industry insights > ContactFill in the form and we will get in touch with you within 2 business days.Please enable JavaScript in your browser to complete this form.Please enable JavaScript in your browser to complete this form. Name * FirstLast Work Email *Company Name *Your Project Type *– Please select –Car, SUV, MPVBus, coach, trainLCV (pickup truck, light-duty truck, etc.)HCV (heavy-duty

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.