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Optimizing a Hydraulic Regenerative Braking System for a 20" Bicycle Wheel
Executive Summary
With a growing concern of climate change and decreasing availability of fossil fuels, the U.S. Environmental Protection Agency (EPA) has been researching hydraulic hybrid transportation systems. For seven years, the EPA and ME450 students at The University of Michigan (U-M) have collaborated on projects developing Hydraulic Regenerative Braking Systems (HRBS) for bicycles. These systems conserve energy that is normally lost during friction braking. The bike's kinetic energy is used to drive hydraulic fluid into an accumulator via a pump, braking the vehicle. This stored energy is later released to accelerate the bike forward.
This semester we have refined previous HRBS designs by optimizing the mechanical systems and improving safety. A key goal for our team was to build a functioning prototype 20" wheel that weighs less and has fewer moving parts than previous generations. Our team has made minimal changes to the extant hydraulic system, as the parts have been well-researched and recommended by our sponsor, David Swain of the EPA. Working with Mr. Swain, we created a list of customer requirements for this project. Table 1 below lists many of our key engineering specifications that were created to meet these requirements, as well as the final characteristics of the prototype. Our four categories for engineering specifications are safety, cost, weight, and functionality. Due to the conflicting nature of these specifications, it has been difficult to improve many of the bike's systems without adversely affecting others. Compromises have been necessary in order to create a feasible design.
table 1：summary of key engineering specifications
Characteristic Target prototype
Front wheel assembly weight ≤30lbs 24.75lbs
Bicycle load rating(rider weight) ≤160lbs ＞200lbs
System pressure as limited by relief vale ≤4200psi ≤4200psi
Bicycle deceleration target 3.4m/s2—2.6m/s2 not available
Bicycle acceleration target 2.0m/s2—2.5m/s2 not available
Number of moving/ rotating parts inside hub ＜11 7
Prototype cost ≤$1400 $1338
Many of the main hydraulic components have long acquisition lead times. To meet our goal of having a functional prototype by the end of the term, we expedited concept generation and selection so as to leave enough time to order and receive these parts. We created a detailed plan for the semester based on expected task requirements as well as these lead times.
In reducing the weight of the prototype compared to previous designs, we have significantly reduced the number of gears, replaced the bulky fiberglass hub support system with a lightweight aluminum spoke system, and removed excess material from the internal support plate ("superbracket"). These modification choices were made from a broad number of concepts, based on a thorough analysis of the forces and torques required of each of the components. The main engineering obstacles to implementing these design improvements have been dealing with the nonstandard interface between metric and non-metric components, and determining the routing of the hydraulic circuit.
1 Abstract
The U.S. Environmental Protection Agency (EPA) is researching hydraulic hybrid transportation systems in an effort to address the growing concerns about global climate change and insatiable fossil fuel demands. Hydraulic hybrid vehicles use regenerative braking to store energy in pressurized fluids. This energy is then released to assist in vehicle acceleration. For the past seven years, ME450 students at The University of Michigan (U-M) have been developing designs for hydraulic hybrid bicycle systems. This semester we refined the design of a hydraulic hybrid system enclosed in a 20" bicycle wheel, with a focus on decreasing weight, improving safety, and reducing the number of moving parts.
2 Introduction
This section outlines the origins of the hydraulic hybrid bicycle system concept at the EPA as well as the driving force for its development. A brief outline of the project's scope for the Winter 2009 semester of ME450 is also presented below.
2.1 Background and Motivation
Founded in 1970, the United States Environmental Protection Agency is a federal body tasked with correcting environmental damage and establishing guidelines to help protect the natural environment of the United States [1]. Research into clean energy, particularly for use in transportation, is the focus of several of the EPA's efforts [2]. In cooperation with Eaton Corporation, United Parcel Service, Ford, International, and the U.S. Army, the EPA has developed several hydraulic hybrid vehicles for the purposes of improving fuel economy and reducing environmental impact [3].
The primary concept of hydraulic hybrid technology is to capture and utilize the energy that would otherwise be lost during braking and use it to accelerate the vehicle. As the vehicle brakes, a hydraulic pump connected to the drivetrain pumps hydraulic oil into the high-pressure accumulators. During vehicle acceleration, the energy stored in the accumulators is released back into the drivetrain, as the fluid flows through a hydraulic motor. This significantly lowers the amount of fuel needed to accelerate back to normal operating speeds [3]. The result of this regenerative braking is a marked improvement in fuel economy - a feature that is not just better for the environment, but also reduces fuel costs for the owner. A diagram showing this hydraulic regenerative braking system (HRBS) is shown in Figure 1 on page 6.
Figure 1: The hydraulic fluid's path in an HRBS [4]
The use of bicycles for commuting reduces fossil fuel use, greenhouse gas emissions, roadway congestion, and vehicle miles traveled while increasing the user's physical health [5]. The EPA has demonstrated 20-40 percent fuel economy improvements by installing HRBS on vehicles with internal combustion engines [3]. The possibility of clean, efficient transportation with hydraulic assistance bears exploration. The EPA has been working with U-M students on hydraulic bicycle implementation since 2002, but the project has produced only one functional product.
2.2 Project Description
The goal of this project is to develop a hydraulic regenerative braking system for a children's 20" bicycle. Due to the difficult nature of scaling down a hydraulic system, and the comparative ease of scaling upwards, the intent of using a 20" bicycle is to analyze the weight, force, and torque issues inherent to the HRBS on a small scale.
The EPA has been working on HRBS bicycles with ME450 students for the past seven years. Previous ME450 teams have worked on fitting these systems in 26" and 20" bicycle wheels. The primary focus of our work on the HRBS is refining the existing designs by improving safety, reducing weight, ensuring functionality, and lowering cost. We are designing an HRBS for a 20" wheel. Notably, one of the main goals is to reduce the device weight to 30 lbs without sacrificing mechanical robustness or safe pressure containment. We plan to retain the majority of the hydraulic components from past designs, as this technology has been well-researched and documented by David Swain and previous teams. By focusing on reducing moving parts, decreasing weight, and improving safety, we are further developing the understanding and implementation of HRBS technology through the fabrication of a functional prototype.
3 Information Search
To gain a better understanding of hydraulic hybrid systems, our team surveyed a broad collection of information including research papers, previous ME450 reports, and EPA resources. This section of the report discusses the information we found regarding hydraulic hybrid vehicle technology.
Hydraulic systems are used in a variety of applications such as machinery, braking systems, and energy storage. They are often used because of their ability to transfer large forces and convert kinetic energy into potential energy efficiently. To safely utilize this technology, many precautions must be taken to prevent high-pressure systems from rupturing.
The EPA, U-M, and companies such as Eaton and Ford have been developing hydraulic hybrid systems for transportation applications including cars, trucks, and bicycles. Hydraulic hybrid bicycle technology has been pioneered through a partnership between the EPA and U-M. For seven years, ME450 students at U-M have been researching, designing, and building hydraulic hybrid bicycle systems using HRBS. These systems require improvements in safety, functionality, and performance.
4 Project Requirements & Engineering Specifications
To outline the specifications for this project, we began by defining our customer requirements. We then translated these requirements into engineering specifications. This section of the report details these requirements and the resulting specifications.
4.1 Customer Requirements
The customer requirements for this term, as outlined by our sponsor David Swain, are continuations of the past two semesters with an added emphasis on three major underlying themes-safety, performance, and cost- to guide the formation of our engineering specifications. Table 1 on page 11 shows a listing of our customer's requirements, as grouped by the three major themes and their relative importance in each.
4.2 Engineering Specifications
When translating the customer requirements into engineering specifications, cost and safety translated directly. However, performance split into weight and functionality, as we find both categories of high enough importance to be separate. The resultant engineering specifications are described in the following list.
5 Concept Generation
To effectively generate a broad collection of concepts, we began by decomposing the main subsystems of the HRBS. After breaking down the subsystems, we listed the main components of each. Each team member then created a list of concepts for each of the components. We then met as a team to build on one another's ideas and we created a master concept list
5.1 Functional Decomposition
Based on the unique history and relative complexity of our project, we followed a slightly different concept generation process than most teams. We began by decomposing the bicycle HRBS into five functional subsystems. These subsystems are hydraulics, powertrain, hub, superbracket, and user interface. Each of these subsystems contained at a minimum two major components. Figure 3 is a functional decomposition tree showing which components fall under which subsystem.
Figure 3: Functional decomposition tree outlining main components of each subsystem
After completing the functional decomposition, we generated concepts for each of the subsystem components. By individually creating concepts and analyzing them as a team, we were able to attack each design problem from multiple angles.
5.2 Hydraulics
The subsystem most refined by previous teams is hydraulics. This is also the subsystem with the longest lead-time items. As a result, many of our hydraulic
5.2 Hydraulics
The subsystem most refined by previous teams is hydraulics. This is also the subsystem with the longest lead-time items. As a result, many of our hydraulic components including the pump, motor, high pressure accumulator, tubing & fittings, and low pressure reservoir till remain the same as those specified by previous teams.
In addition to the systems used on previous generations, it is important to include a pressure relief system to prevent over-pressurizing the system. This can be achieved by including a variable pressure relief valve or a burst disc.
The valves category is made up of a check valve preventing high pressure flow from entering the pump and a directional valve to start and stop the launch process. There are various types of check valves that respond better to different pressures. The directional valve could either be a two-way or a three-way electronic valve. There are different types of each of these valves that vary in their sealing method. Poppet valves seal quite well, leaking only a few drops per minute; spool valves can leak multiple milliliters per minute.
5.3 Powertrain & Packaging
Powertrain decomposes into only two component categories, but it is very complicated due to the packaging constraints of a 20" bicycle wheel. In the past, the mechanical reduction was created using steel spur gears. We generated many concepts including plastic gears, phenolic gears, sprockets & chain, cogged belts, cables & pulleys, and friction rollers like those used to launch roller coasters.
The second powertrain category is clutch mechanisms. A system is needed to disengage the pump and motor from the rotating hub when braking and launching are not engaged. Concepts to complete this task included electromechanical clutches (benchmark), mechanical clutches, roller clutches, and a custom clutch utilizing a linear actuator.
5.4 Hub
The hub's main roles on the bike are to support the rim, to interface with the mechanical reduction, and to enclose the system's moving components. This hub rotates around the bike's axle, which is stationary. Previous teams have created hubs made of carbon fiber and fiberglass. We included these in our concept list as well as aluminum sheet metal, vacuum formed plastic, and spokes with a thin cover. We developed another concept by combining the spoke and vacuum form designs. In this design a rigid skeletal structure would be used to support the bicycle and a thin plastic cover would enclose the system.
5.5 Superbracket
The superbracket subsystem is made up of the superbracket and the bike's axle. These components are rigidly connected together. The hub rotates on the axle and electric wiring exits the hub through the center of the axle. Designing the superbracket is a material selection and thickness optimization problem. The bracket needs to support the hydraulic and mechanical components and prevent the pump and motor's output/input shafts from being loaded radially. To meet these criteria we created a list of potential materials, including steel, aluminum, fiberglass, tooling board, wood, carbon fiber, and plastic. Along with material selection we have discussed methods of increasing the bracket's stiffness by using dimple dies, adding gussets, and adding angle iron reinforcements.
5.6 User Interface and Controls
Previous designs incorporated a switch box for controlling the brake and launch functions. This box was mounted on the frame of the bike directly in front of the seat. While functional, this forces the rider to let go of the handlebars with at least one hand to activate either system. In the event of a system braking failure, the rider would have to quickly adjust his hand position to activate the hand brake on the handlebar. One concept that could potentially solve this problem is to integrate the switch and the preexisting hand brake. This could be done by splicing a toggle switch into the cable. A light squeeze on the hand brake could activate the HRBS, while a hard squeeze would be enough to engage the friction brakes. Another option, provided that the bike is equipped with front and rear brakes, is to leave the rear hand brake unmodified and splice a toggle switch into the front hand brake cable. The launch activation could potentially be switched via a toggle switch mounted on the handlebars, or a pushbutton mounted on the handlebars. If two switches are wired in parallel, there is the advantage that both switches must be activated for the launch to be triggered - this could be beneficial from a safety standpoint.
6 Conclusion
This semester we designed and built a hydraulic regenerative braking system enclosed in a 20" bicycle wheel. We used hydraulic hybrid technology that was proven by the EPA and previous ME450 teams. Using the vast resources available to our team, we redesigned the mechanical and electrical systems on the bike. The hydraulic component specifications did not change from previous iterations of the bicycle. We reduced weight, improved safety, and increased functionality with our design and were motivated by those driving factors during manufacturing and assembly. We were able to meet the deadlines of our project by sourcing parts aggressively and scheduling proactively throughout the semester. In such a short design cycle, adherence to a methodical and thoughtful approach was necessary to avoid confusion and misguided efforts. It also allowed for each team member to have an intimate knowledge of the system and its components, resulting directly in a significant leap forward in the evolution of this project.
20英寸自行車輪液壓濕式制動系統(tǒng)的優(yōu)化設(shè)計
摘要
隨著氣候變化和減少使用化石燃料日益受到關(guān)注，美國環(huán)境保護(hù)署（EPA）已研發(fā)了液壓混合動力運輸系統(tǒng)。在7年時間中，EPA和美國密西根大學(xué)（UM）的學(xué)生組成的機(jī)械工程450團(tuán)隊（ME450）為發(fā)展自行車液壓再生制動系統(tǒng)（HRBS）進(jìn)行了項目合作。一般來說，這些系統(tǒng)節(jié)約了在摩擦制動過程中丟失的能源。它采用了將自行車的動能通過泵液壓油的流動進(jìn)入蓄能器，從而制動車輛。而此次儲存的能量將釋放來加速自行車向前行駛。
這學(xué)期，我們通過優(yōu)化機(jī)械系統(tǒng)和改善其安全來改進(jìn)以前HRBS設(shè)計。我們團(tuán)隊的主要目標(biāo)是建立一個正常運作的原型20"輪，它重量更輕，并具有運動部件少。比前幾代，作為它部分的設(shè)計已精心研究，我們的團(tuán)隊取得了現(xiàn)存的液壓系統(tǒng)的微小變化。此次設(shè)計由我們的贊助商，大衛(wèi)環(huán)保局史懷恩先生指導(dǎo)，我們建立了一個滿足客戶的要求項目清單。下面的表1列出了許多我們的重點工程，并創(chuàng)造了滿足這些要求的規(guī)格，以及最后的樣機(jī)特征。我們的四大類工程規(guī)格為，安全性、成本、重量和功能性。由于這些規(guī)格與其性質(zhì)的沖突，它一直難以改善自行車的許多系統(tǒng)而對他人設(shè)計產(chǎn)生不利影響。妥協(xié)以創(chuàng)造一個可行的設(shè)計是必要的途徑。
表1：重點工程規(guī)格概要
特征 指標(biāo) 樣機(jī)
前輪的裝配重量 ≤30磅 24.75磅
自行車額定負(fù)載（騎手體重） ≤160磅 ＞200磅
限量系統(tǒng)壓力安全閥 ≤4200psi ≤4200psi
自行車減速指標(biāo) 3.4m/s2—2.6m/s2 無法取得
自行車加速指標(biāo) 2.0m/s2—2.5m/s2 無法取得
移動/旋轉(zhuǎn)樞紐內(nèi)部零部件數(shù)量 ＜11 7
樣機(jī)成本 ≤1400美元 1338美元
許多主要的液壓元件需要長期收購多次。為了完成我們?nèi)纹诮Y(jié)束時作出一個擁有這些功能的樣機(jī)的目標(biāo)，我們加快觀念的生成和選擇，從而以留出足夠的時間訂購和接收這些部件?；陬A(yù)期的任務(wù)要求以及交貨時間，這學(xué)期我們創(chuàng)造了一個詳細(xì)的計劃。
與以前的設(shè)計相比，我們已經(jīng)大大降低了齒輪的數(shù)量來減輕樣機(jī)的重量，用一個輕型鋁合金發(fā)言系統(tǒng)來取代笨重的玻璃纖維樞紐的支撐系統(tǒng)，并從內(nèi)部支撐板（“superbracket”）去除多余的材料。這些修改的選擇來自于一個基于深入分析每個組件所需的力量和扭矩廣泛的概念。實現(xiàn)這些涉及公制和非公制元件之間的非標(biāo)準(zhǔn)接口這項主要工程的障礙進(jìn)行改進(jìn)設(shè)計，并確定液壓回路的路線選擇。
1摘要
美國環(huán)境保護(hù)署（EPA）正在努力研究液壓混合動力運輸系統(tǒng)以解決有關(guān)全球氣候變化和貪得無厭的化石燃料的需求得到越來越多的關(guān)注。在過去的7年里，ME450在美國密西根大學(xué)（UM）的學(xué)生為液壓混合動力自行車系統(tǒng)設(shè)計作出發(fā)展，液壓混合動力汽車使用再生制動儲存能量加壓液體這種能量的釋放可以協(xié)助車輛加速。這學(xué)期，我們提出20英寸自行車輪內(nèi)的液壓混合動力系統(tǒng)的設(shè)計，重點是降低重量，提高安全性，并減少移動部件的數(shù)量。
2引言
本節(jié)概述了液壓混合動力系統(tǒng)的概念、自行車環(huán)保局的起源以及其發(fā)展的原動力。此項目在2009年冬季學(xué)期ME450范圍內(nèi)進(jìn)行，概要介紹如下。
2.1背景和動機(jī)
美國環(huán)境保護(hù)局成立于1970年，是一個負(fù)責(zé)糾正損害環(huán)境和建立指引的聯(lián)邦機(jī)構(gòu)，以幫助保護(hù)美國的自然環(huán)境[1]。主要研究清潔能源，特別是運輸是EPA的努力的幾個重點[2]。與伊頓公司、聯(lián)合包裹運送服務(wù)公司、福特、國際和美國軍隊保持合作，環(huán)保局已開發(fā)出幾種改善燃油經(jīng)濟(jì)性和減少對環(huán)境的影響為目的的液壓混合動力汽車[3]。
液壓混合動力車技術(shù)的主要概念是捕捉和利用，否則將失去制動過程中用它來加快車輛的能源。作為汽車制動器，液壓泵連接到傳動泵的液壓油進(jìn)入高壓蓄能器。在車輛加速過程中，在蓄能器儲存的能量被釋放到動力傳動系統(tǒng)，液體流動通過液壓馬達(dá)。這大大降低了所需的燃料，回到正常的運行速度以加速[3]。這種再生制動的結(jié)果是在燃油經(jīng)濟(jì)性上明顯的改善—不僅是對環(huán)境有益，也為業(yè)主降低燃料成本的特點。這種液壓再生制動系統(tǒng)（HRBS）（如圖1所示）的圖表在第6頁上顯示。
圖1：液壓油在HRBS上的路徑[4]
使用自行車作為交通工具上下班，減少了化石燃料的使用、減輕了溫室氣體排放、減緩了道路擁堵，同時騎行自行車數(shù)英里還提高了用戶的身體健康[5]。環(huán)保局已經(jīng)證明在內(nèi)燃發(fā)動機(jī)的車輛上安裝HRBS可改善20-40％的燃油經(jīng)濟(jì)值[3]。自2002年以來，環(huán)保局已與澳學(xué)生對液壓自行車實施狀況進(jìn)行合作，對其具有清潔、液壓助力式承擔(dān)的高效的運輸可能性進(jìn)行探索。但該項目產(chǎn)生只有一個功能性的產(chǎn)品。
2.2工程項目說明
這個項目是以開發(fā)20英寸兒童式自行車液壓再生制動系統(tǒng)為目標(biāo)。由于液壓系統(tǒng)在同比例大小進(jìn)行縮放向下的艱巨性和縮放向上較為容易性，因此選擇20英寸這個尺寸研究HRBS自行車，它在分析重量、力的大小、和固有的扭矩問題上較為合適。
在過去的7年,環(huán)保局和機(jī)械工程學(xué)450的學(xué)生一直致力于研究HRBS自行車。以前的ME450隊一直在26英寸和20英寸自行車輪子上擬合這些系統(tǒng)。我們在研究HRBS方面的主要焦點是改善現(xiàn)有的設(shè)計，以提高它的安全性、降低重量、確保功能性和降低成本。值得注意的是，我們正在設(shè)計的20英寸輪HRBS系統(tǒng)，其主要目標(biāo)之一是在不犧牲機(jī)械強(qiáng)度或壓力容器的安全的前提下將設(shè)備的重量減少到30磅。我們的計劃是保留大部分過去設(shè)計的液壓元件，因為這項技術(shù)已由大衛(wèi)_斯溫和以前的團(tuán)隊得到了很好的研究和記錄。我們正在進(jìn)一步制定一個HRBS技術(shù)的、以減少運動部件、降低重量、提高安全性為重點的功能樣機(jī)。
3信息收集
為了更好的理解液壓混合動力系統(tǒng)，我們團(tuán)隊調(diào)查了廣泛收集的信息，其中包括研究論文、以前ME450的報告和環(huán)保局的資源。在這部分報告所顯示的信息中我們找出了液驅(qū)混合動力車輛技術(shù)。
液壓系統(tǒng)在如機(jī)械、制動系統(tǒng)、儲能等各種應(yīng)用場合下受到使用。由于液壓系統(tǒng)傳輸較大的力的性質(zhì)和高效地將勢能轉(zhuǎn)換成動力的能力而受到了大家廣泛地使用。為了安全地利用這項技術(shù)，我們必須采取預(yù)防措施，以防止因高壓而爆裂液壓系統(tǒng)。
4項目要求及工程規(guī)范
該部分概述了這個項目的規(guī)格，我們首先了解我們的客戶的需求。然后我們改變工程規(guī)格要求來滿