📚 Year 12 WJEC Engineering: Case Study Practical Exercise | 案例分析实战演练
Case study analysis forms the backbone of the WJEC Year 12 Engineering assessment. It challenges students to apply theoretical knowledge to a realistic design problem, demonstrating skills in research, calculation, material selection, manufacturing, and sustainable thinking. This article walks you through a complete practice case study—designing a portable bicycle-powered mobile phone charger for rural communities in developing regions—showing how to structure your response, justify decisions, and think like an engineer. By mastering this approach, you will be well-prepared to tackle any WJEC engineering case study with confidence.
案例分析是WJEC Year 12 工程学科评估的核心。它要求学生将理论知识应用于真实的设计问题，展现研究、计算、材料选择、制造工艺和可持续思维等综合能力。本文将通过一个完整案例——为发展中国家偏远地区设计便携式自行车驱动手机充电器——带你一步步拆解答题思路、论证决策过程，并培养工程师思维。掌握这套方法，你就能从容应对 WJEC 工程案例分析中的任何挑战。
1. Understanding the Case Study Scenario | 案例情景解析
The scenario is set in a rural village with limited access to grid electricity. The brief requires a device that can be attached to any standard bicycle, convert human pedalling into electrical energy, and charge a typical smartphone via USB. Key user requirements include portability, a target output of 5V DC at 1A, simple installation, durability in dusty conditions, and affordability with locally available materials. The device must also be safe, lightweight, and capable of withstanding daily use by multiple family members.
场景设定在一个电网覆盖不足的偏远村庄。设计要求是：装置可安装于任何标准自行车，将人力踩踏转换为电能，通过 USB 接口为常见智能手机充电。主要用户需求包括便携性、输出 5V 直流 1A、安装简便、在多尘环境下耐用，以及尽可能利用当地易获取材料以控制成本。同时，装置必须安全、轻便，并能承受整个家庭日常轮换使用的强度。
2. Deconstructing the Design Brief | 拆解设计概要
Before sketching any ideas, it is essential to extract the explicit and implied specifications. Explicitly, the output must be 5V DC, 1A (5W). The mechanism should be mechanically driven by the bicycle wheel or chain. Implicitly, voltage regulation is needed to avoid damage to the phone; an ergonomic interface is required for users with limited technical literacy; and the generator must perform efficiently at typical cycling cadences of 60–80 rpm. Weight should stay under 2 kg for portability, and the total manufacturing cost should be below £30 to remain accessible.
在开始绘制草图之前，必须提取明确和隐含的规格。明确要求是 5V 直流 1A（5 W）输出，由自行车轮或链条驱动。隐含需求包括：需要稳压以避免损坏手机；界面需符合人体工程学，便于技术知识有限的用户操作；发电机需在常见骑行踏频 60–80 rpm 范围内高效工作。整机重量应低于 2 kg 以便携带，制造成本需控制在 30 英镑内以维持可负担性。
3. Research and User‐Centred Data Collection | 研究及用户数据收集
Effective engineering solutions begin with thorough research. We examined existing pedal‐powered generators, studied typical daily phone energy demands (a 3000 mAh battery at 3.7V requires roughly 11.1 Wh per charge), and surveyed bike usage patterns. Interviews with users highlighted the importance of a non‐slip roller interface that does not wear out tyres, a clear LED indicator showing charging status, and compatibility with multiple phone models. Ergonomic data also suggested that a friction‐drive roller pressed against the rear tyre is the easiest to retrofit.
优秀的工程方案始于充分的研究。我们考察了现有脚踏发电机产品，研究了手机的日常能耗需求（一块 3000 mAh、3.7V 的电池每次充电约需 11.1 Wh），并调查了自行车使用习惯。用户访谈凸显了几个关键点：不磨损轮胎的防滑摩擦滚轮、清晰显示充电状态的 LED 指示灯，以及兼容多种手机型号。人因数据还表明，压在后轮胎上的摩擦驱动滚轮是最容易后期加装的方案。
4. Generating and Evaluating Design Concepts | 设计概念生成与评估
We brainstormed three distinct concepts: (A) a chain‐driven generator mounted on the rear axle, (B) a friction roller system clamped to the seat stay, and (C) a hub dynamo integrated into the front wheel. Using a weighted decision matrix, we scored each against criteria such as ease of installation (30%), cost (25%), maintenance (20%), efficiency (15%), and portability (10%). Concept B scored highest due to its simple clamp‐on design, minimal modification to the bicycle, and lower cost compared to replacing a wheel hub.
我们构想出三种不同方案：A) 安装在后轴的链传动发电机，B) 夹在座管上的摩擦滚轮系统，C) 集成在前轮的花鼓发电机。利用加权决策矩阵，我们依据安装便捷性（30%）、成本（25%）、维护性（20%）、效率（15%）和便携性（10%）进行评分。概念 B 得分最高，因其夹装式设计简单，对自行车改动最小，且成本远低于更换花鼓轮毂。
5. Material Selection and Justification | 材料选择与论证
Material choice directly influences performance, cost, and environmental impact. The main structural bracket must be stiff yet light; we selected aluminium alloy 6061 for its high strength‐to‐weight ratio and corrosion resistance. The roller required high friction with low abrasion on rubber tyres, so polyurethane with a Shore A hardness of 80 was chosen. For electrical housing, recycled ABS plastic was picked to reduce environmental footprint while providing sufficient impact resistance. The table below summarises the comparison.
材料选择直接影响性能、成本和环境影响。主结构支架需同时兼顾刚度和轻量，我们选择了 6061 铝合金，因其强度重量比出色且耐腐蚀。滚轮需要在橡胶轮胎上提供高摩擦力而自身磨损低，因此选用了邵氏硬度 A80 的聚氨酯。电子元件外壳则采用回收 ABS 塑料，以减小环境足迹并保证足够的抗冲击性。下表总结了各项对比。
| Component | Material | Key property | Reason |
|---|---|---|---|
| Bracket | Al 6061 | σᵧ ≈ 276 MPa, ρ = 2.7 g/cm³ | High stiffness, low mass |
| Roller | Polyurethane (Shore A80) | Coefficient of friction μ ≈ 0.8 | Grip without tyre damage |
| Housing | Recycled ABS | Impact strength ≈ 200 J/m | Durable, eco‐friendly |
These materials were further justified by a cost analysis: the aluminium bracket can be cut and bent from off‐the‐shelf sheet metal, the polyurethane roller can be cast in a simple mould, and recycled ABS is widely available from local recycling hubs, driving down the unit price.
这些材料选择还经过成本分析进一步论证：铝支架可利用市售板材进行切割和折弯加工，聚氨酯滚轮可用简易模具浇铸成型，而回收 ABS 可从当地回收中心大量获取，从而压低单价。
6. Key Engineering Calculations | 关键工程计算
To ensure the generator meets the 5W output target, we performed a series of calculations. Assuming a cycling cadence of 70 rpm and a rear wheel diameter of 660 mm, the linear tyre speed v is given by:
为保证发电机达到 5W 输出目标，我们进行了一系列计算。假设骑行踏频 70 rpm，后轮直径 660 mm，轮胎线速度 v 计算如下：
v = π × D × (rpm / 60) = π × 0.66 × (70 / 60) ≈ 2.42 m/s
If the friction roller has a diameter of 40 mm, its rotational speed nroller is:
如果摩擦滚轮直径为 40 mm，其转速 nroller 为：
nroller = v / (π × 0.04) × 60 ≈ (2.42 / 0.1257) × 60 ≈ 1155 rpm
orque T at the roller shaft is derived from the power equation P = T × ω, where ω = 2π × nroller .041 N·m. This low torque confirms the friction interface will not slip under normal contact force.
一款额定电动势常数为 0.005 V/rpm 的直流发电机在此转速下可产生约 5.78 V 电压，经稳压电路降至 5V。滚轮轴所需转矩 T 由功率方程 P = T × ω 导出，其中 ω = 2π × nroller / 60 ≈ 121 rad/s，故 T = 5 / 121 ≈ 0.041 N·m。如此低的扭矩证明摩擦界面在正常接触压力下不会打滑。
7. Manufacturing Processes | 制造工艺选择
batch production. The polyurethane roller will be produced via gravity casting into a two‐part silicone mould; this low‐ n transitioned to injection moulding if demand scales up. All processes were selected to minimise tooling costs while maintaining acceptable tolerances of ±0.5 mm for non‐critical fits.
铝支架将采用数控激光切割和折弯工艺制造，确保重复精度，适合批量生产。聚氨酯滚轮通过重力浇注到两件式硅胶模具中成型；这种低成本方法允许多次翻模使用。ABS 外壳在原型阶段可真空吸塑或 3D 打印，若产量放大则转为注塑成型。所有工艺均以降低模具成本为目标，同时维持非关键配合公差 ±0.5 mm。
8. Design for Sustainability and Lifecycle | 可持续性与全生命周期设计
Sustainability is embedded from the outset. The device uses mechanical friction coupling instead of chemical batteries, thereby eliminating hazardous waste. The aluminium alloy is fully recyclable, and the polyurethane roller, while not biodegradable, can be mechanically recycled into new rollers or other products. The modular design allows users to replace individual components (e.g. the roller or voltage regulator) without discarding the entire unit, extending the product lifespan. A quick lifecycle assessment estimates a carbon footprint of 8.5 kg CO₂ equivalent per unit over a 5‐year period, significantly lower than a comparable diesel generator phone charger.
可持续性设计从项目之初就被纳入考量。装置采用机械摩擦耦合代替化学电池，杜绝了有害废弃物。铝合金可完全回收；聚氨酯滚轮虽不可生物降解，但可机械回收再造为新滚轮或其他制品。模块化设计使用户能够单独更换组件（如滚轮或稳压模块），无需废弃整机，从而延长产品使用寿命。基于快速生命周期评估，每台设备在 5 年使用周期内的碳足迹约为 8.5 kg CO₂ 当量，远低于同类柴油发电机充电方案。
9. Testing and Evaluation Strategy | 测试与评估策略
A rigorous test plan validates that the prototype meets the design brief. Firstly, electrical performance is verified using a multimeter and an oscilloscope: the output must stay within 4.75–5.25 V under varying loads (0.1–1.0 A), with ripple voltage below 100 mV. Secondly, mechanical durability is tested by running the roller against a tyre for 10,000 cycles, after which the roller diameter loss must not exceed 0.2 mm. A safety test confirms the casing temperature remains under 60°C after 1 hour of continuous operation, preventing burns. User trials in a simulated rural environment gather qualitative feedback on ease of installation and usability.
严谨的测试计划用于验证样机是否满足设计概要。首先，使用万用表和示波器检测电气性能：变负载（0.1–1.0 A）下输出电压需稳定在 4.75–5.25 V，纹波电压低于 100 mV。其次，机械耐久性测试让滚轮与轮胎对磨 10 000 次，要求滚轮直径磨损不超过 0.2 mm。安全测试确认连续运行 1 小时后外壳温度不超过 60°C，防止烫伤。在模拟农村场景下的用户试用则收集关于安装便利性和使用体验的定性反馈。
10. Critical Reflection and Future Improvements | 批判性反思与改进方向
Reflecting on the design, the friction roller interface, while simple, does introduce a slight slippage under wet conditions, reducing efficiency by up to 12%. A possible improvement is to add a spring‐loaded tensioner with a textured metal roller for rainy climates. The current voltage regulator uses a linear IC, which dissipates excess energy as heat; switching to a buck converter could increase overall electrical efficiency from 78% to over 90%. Cost analysis also revealed that by sourcing aluminium from local scrap yards, the bracket cost could fall by a further 40%. These iterative improvements demonstrate the continuous nature of engineering refinement.
反思该设计，摩擦滚轮界面虽简单，但在潮湿条件下存在轻微打滑，效率最多可下降 12%。一个潜在改进方案是为多雨地区配备弹簧张紧器和带纹理的金属滚轮。目前电压调节器使用线性稳压 IC，多余能量以热耗散；改用降压变换器可将系统电气效率从 78% 提升至 90% 以上。成本分析还表明，若从当地废品站回收铝材，支架成本可再降 40%。这些迭代改进方向恰恰体现了工程设计持续优化的本质。
Published by TutorHao | Engineering Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图