Osu 1308287500

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System Dynamics Modeling and Development of a Design Procedure for Short-term Alternative Energy Storage Systems THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Joshua John McDonough Graduate Program in Mechanical Engineering The Ohio State University 2011 Master's Examination Committee: Professor Marcello Canova, Advisor Professor Yann Guezennec

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Transcript of Osu 1308287500

  • System Dynamics Modeling and Development of a Design Procedure for Short-term

    Alternative Energy Storage Systems

    THESIS

    Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in

    the Graduate School of The Ohio State University

    By

    Joshua John McDonough

    Graduate Program in Mechanical Engineering

    The Ohio State University

    2011

    Master's Examination Committee:

    Professor Marcello Canova, Advisor

    Professor Yann Guezennec

  • Copyright by

    Joshua John McDonough

    2011

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    Abstract

    Recovering and storing a vehicles kinetic energy during deceleration and the subsequent

    use of the stored energy during acceleration has lead to significant increases in vehicle

    efficiency. Current production hybrid electric vehicles (HEVs) convert the energy and

    store it using electric machines and electro-chemical batteries. While these systems can

    be configured to provide substantial benefits in addition to kinetic energy recovery,

    significant limitations exist which hinder the performance and market penetration.

    Converting mechanical energy to electricity then storing it chemically leads to

    considerable losses during storage. The path must be followed in the opposite direction

    during release, compounding the losses. Current HEV batteries, while very effective at

    storing large quantities of energy, have longevity driven power limitations which drive up

    cost and weight. As a result of these limitations, investigations have been made into

    alternative means to recover and store kinetic energy on board vehicles.

    This thesis investigates two such methods of energy recovery and storage, a hydraulic

    system with accumulator energy storage and a purely mechanical system with flywheel

    energy storage. Both systems are of parallel hybrid architecture and offer high power

    capacity at relatively low cost. The hydraulic system consists of a pump/motor to convert

    mechanical work to fluid power and a high-pressure accumulator to store the energy.

    The mechanical system transmits the vehicles kinetic energy to a flywheel through

    changing the ratio of a continuously variable transmission linked between the flywheel

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    and the drivetrain. System dynamics models are created for each of the systems

    components and coupled to allow for analysis over simulated drive cycles. An iterative

    design method is proposed for both the hydraulic and mechanical systems, based on drive

    cycle analysis, performance in simulation, and system properties, such as mass and

    estimated cost. The systems are compared and contrasted with each other in order to

    evaluate the relative strengths and weaknesses of the various kinetic energy recovery

    methods.

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    Dedication

    This document is dedicated to my family and fiance.

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    Acknowledgments

    I would like to thank the many people who made this research possible. First, thank you

    to my advisor Professor Marcello Canova for his guidance and direction throughout the

    work. Thank you to Professor Giorgio Rizzoni for the opportunity to perform my

    graduate research at the Center for Automotive Research.

    Additionally I would like to thank Dr. Fabio Chiara for his assistance and insight

    throughout the project, General Motors for sponsoring the project, and Professor Yann

    Guezennec for teaching excellent hybrid vehicle classes and serving on my defense

    committee.

    Also making my graduate school education possible was the teaching assistant

    opportunity provided by the department of mechanical and aerospace engineering. Thank

    you to the faculty and staff who made that experience possible. I truly enjoyed my time

    as a TA.

    Thank you to my family for their inspiration and continued support and for instilling

    passions for engineering and automobiles. Finally, I cannot thank my fiance, Arden,

    enough for her love and support. Her encouragement was always there when I needed it.

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    Vita

    July, 1987 .......................................................Born Coffeen, Illinois

    May, 2009 ......................................................B.S. Mechanical Engineering, Rose-Hulman

    Institute of Technology, Terre Haute, IN.

    August, 2009 to present ................................Graduate Teaching and Research Associate,

    Department of Mechanical Engineering and

    Center for Automotive Research, The Ohio

    State University

    Publications

    McDonough, J., Jebakumar, K., Chiara, F., Canova, M., Koprubasi, K., Raghavan, M.

    System Dynamics Modeling of Alternative Energy Storage Systems for Hybrid Vehicles. ASME Dynamics Systems and Control Conference, 2011.

    Bolletta, A., Chiara, F., Canova, M., McDonough, J., Koprubasi, K., Raghavan, M. A Design Procedure for Alternative Energy Storage Systems for Hybrid Vehicles. ICE International Conference on Engines & Vehicles, 2011

    Fields of Study

    Major Field: Mechanical Engineering

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    Table of Contents

    Abstract ............................................................................................................................... ii

    Dedication .......................................................................................................................... iv

    Acknowledgments............................................................................................................... v

    Vita ..................................................................................................................................... vi

    List of Tables ...................................................................................................................... x

    List of Figures ................................................................................................................... xii

    Chapter 1: Introduction ...................................................................................................... 1

    1.1 Motivation ................................................................................................................. 1

    1.2 Kinetic Energy Recovery .......................................................................................... 2

    1.3 Hybrid Electric Vehicles (HEVs) .............................................................................. 4

    1.4 Alternative Energy Storage Systems for Hybrid Vehicles ........................................ 8

    1.4.1 Mechanical Energy Storage ................................................................................ 9

    1.4.2 Hydraulic Energy Storage ................................................................................ 13

    1.5 Design Considerations............................................................................................. 14

    1.6 References ............................................................................................................... 17

    Chapter 2: State of the Art ................................................................................................ 18

    2.1 Introduction ............................................................................................................. 18

    2.2 Mechanical Energy Storage Systems ...................................................................... 18

    2.3 Hydraulic Hybrid Energy Storage Systems ............................................................ 23

    2.4 Drive Cycle Analysis Methodologies for Energy Storage System Design ............. 27

    2.5 Conclusions ............................................................................................................. 34

    2.6 References ............................................................................................................... 37

    Chapter 3: System Dynamics Modeling for Alternative Energy Storage Systems .......... 39

    3.1 Introduction ............................................................................................................. 39

    3.2 Conventional Vehicle Component Models ............................................................. 39

    3.3 Modeling Scheme for Alternative Energy Storage Systems ................................... 50

    3.3.1 Model of Mechanical AESS ............................................................................. 54

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    3.3.2 Model of hydraulic AESS................................................................................. 70

    3.4 Conclusion ............................................................................................................... 92

    3.5 References ............................................................................................................... 93

    Chapter 4: Proposed Method for Design of Short-Term AESS ........................................ 95

    4.1 Introduction ............................................................................................................. 95

    4.2 Drive Cycle Analysis .............................................................................................. 98

    4.2.1 Generation of Vehicle Based Drive Cycle Statistics ...................................... 101

    4.3 Statistical Weighting Process ................................................................................ 108

    4.4 Definition of Target System Behavior .................................................................. 110

    4.5 Preliminary AESS Design Procedure .................................................................... 122

    4.5.1 Mechanical Energy Storage System Design ................................................... 122

    4.4.2 Hydraulic Energy Storage System Design ..................................................... 130

    4.6 Correlations between Design Parameters and Physical Properties ....................... 136

    4.6.1 Mechanical ESS Correlations ......................................................................... 137

    4.6.2 Hydraulic ESS Correlations ............................................................................ 143

    4.6.3 Combining the Correlations to Form Properties ............................................. 154

    4.7 Vehicle Simulator for Evaluation .......................................................................... 155

    4.7.1 Overview of Vehicle Simulator ...................................................................... 156

    4.7.2 High Level Torque Split Control Strategy ..................................................... 158

    4.7.3 Vehicle Information Needed for Simulation .................................................. 161

    4.8 Cost Function Definition and Design Optimization .............................................. 162

    4.9 Design Method Conclusions ................................................................................. 165

    4.10 References ........................................................................................................... 167

    Chapter 5: Application of Design Method to Mechanical and Hydraulic AESS ........... 168

    5.1 Introduction ........................................................................................................... 168

    5.2 Vehicle Details and Cycle Statistics...................................................................... 169

    5.3 Preliminary Design Procedure .............................................................................. 171

    5.3.1 Mechanical ESS Preliminary Design ............................................................. 172

    5.3.2 Hydraulic ESS Preliminary Design ................................................................ 175

    5.4 Evaluation of Preliminary Designs ....................................................................... 180

    5.4.1 Evaluation Procedure ...................................................................................... 180

    5.4.2 Mechanical ESS Design Evaluation ............................................................... 183

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    5.4.3 Hydraulic ESS Design Evaluation .................................................................. 192

    5.5 Design Optimization ............................................................................................. 200

    5.5.1 Mechanical ESS Design Optimization ........................................................... 201

    5.5.2 Hydraulic ESS Design Optimization .............................................................. 205

    5.6 Validation on Alternate Drive Cycle ..................................................................... 208

    5.6.1 Mechanical ESS Design Validation ............................................................... 209

    5.6.2 Hydraulic ESS Design Validation .................................................................. 213

    5.7 Conclusions ........................................................................................................... 218

    5.8 References ............................................................................................................. 219

    Chapter 6: Conclusions and Future Work ....................................................................... 220

    6.1 Conclusions ........................................................................................................... 220

    6.2 Future Work .......................................................................................................... 222

    References ....................................................................................................................... 223

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    List of Tables

    Table 1. CVT Model Inputs .............................................................................................. 60 Table 2. CVT Model Outputs ........................................................................................... 60

    Table 3. CVT Model Parameters ...................................................................................... 61 Table 4. Shape-factor K for different planar stress geometries 0 ..................................... 63 Table 5. Data for Different flywheel rotor materials 0 ..................................................... 63 Table 6. Frictional Loss Test Conditions .......................................................................... 65

    Table 7. Frictional Loss Test Results ................................................................................ 65 Table 8. Clutch and Flywheel Model Inputs ..................................................................... 69

    Table 9. Clutch and Flywheel Model Outputs .................................................................. 69 Table 10. Clutch and Flywheel Model Parameters ........................................................... 70

    Table 11. Pump/Motor Model Inputs................................................................................ 80 Table 12. Pump/Motor Model Outputs ............................................................................. 80 Table 13. Pump/Motor Model Parameters ........................................................................ 80

    Table 14. Accumulator Model Inputs ............................................................................... 85 Table 15. Accumulator Model Outputs ............................................................................ 85

    Table 16. Accumulator Model Parameters ....................................................................... 86 Table 17. PRV Model Inputs ............................................................................................ 88 Table 18. PRV Model Outputs ......................................................................................... 88

    Table 19. PRV Model Parameters .................................................................................... 89

    Table 20. Reservoir Model Inputs .................................................................................... 91 Table 21. Reservoir Model Outputs .................................................................................. 91 Table 22. Reservoir Model Parameters ............................................................................. 91

    Table 23. Sample of Possible Drive Cycle Statistics ...................................................... 100 Table 24. Sample Velocity Based Statistics for Regulatory Cycles ............................... 100

    Table 25. Sample Vehicle Parameters ............................................................................ 102 Table 26. Relevant Braking Statistics for Mid-Sized SUV on FTP-75 Cycle ................ 110

    Table 27. Mechanical ESS Design Parameters ............................................................... 123 Table 28. Mechanical ESS Design Constraints .............................................................. 125 Table 29. Hydraulic ESS Design Parameters ................................................................. 132 Table 30. Hydraulic ESS Design Constraints ................................................................. 132 Table 31. Required Vehicle Information for Simulation ................................................ 162

    Table 32. Vehicle Details ................................................................................................ 169 Table 33. Design Relevant Drive Cycle Statistics for 2009 Saturn VUE on Synthetic

    Cycle ............................................................................................................................... 171 Table 34. Mechanical ESS Design Constraints .............................................................. 173 Table 35. Mechanical ESS Design Parameters ............................................................... 173 Table 36. Mechanical ESS Preliminary Design Parameters ........................................... 175

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    Table 37. Hydraulic ESS Design Constraints ................................................................. 176

    Table 38. Hydraulic ESS Design Parameters ................................................................. 176 Table 39. Hydraulic ESS Preliminary Designs ............................................................... 179 Table 40. Cost Function Weights.................................................................................... 181

    Table 41. Mass of Mechanical Components ................................................................... 183 Table 42. Volume of Mechanical Components .............................................................. 184 Table 43. Mechanical ESS Preliminary Design Results ................................................. 188 Table 44. Mechanical ESS Preliminary Design Cost Function Values .......................... 189 Table 45. Mass of Hydraulic Components ..................................................................... 192

    Table 46. Volume of Hydraulic Components ................................................................. 193 Table 47. Hydraulic ESS Preliminary Design Results .................................................... 199 Table 48. Hydraulic ESS Preliminary Design Cost Function Values ............................. 199 Table 49. Optimized Mechanical ESS Design Parameters ............................................. 204

    Table 50. Comparison of Optimized Design to Weighted Mean.................................... 208

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    List of Figures

    Figure 1. Chevrolet Volt Range Extension Hybrid Powertrain Layout [4] ........................ 4 Figure 2. Specific Power versus Specific Energy for Various Short-Term Energy Storage

    Systems [5].......................................................................................................................... 9 Figure 3. Flow of Power for Flywheel Mechanical Energy Storage System [5] .............. 10 Figure 4. Toroidal CVT Variator Example [11] ............................................................... 12 Figure 5. Comparison of Toroidal CVT Technology with other Automotive

    Transmissions [5] .............................................................................................................. 20 Figure 6. Toroidal CVT Variator Behavior [26] ............................................................... 21

    Figure 7. Component Details for Jaguar with FHSPV [7] ................................................ 22 Figure 8. Sample Hydraulic Launch Assist Drivetrain Layout [12] ................................. 26

    Figure 9. Component Sizes over Various Driving Cycles [14] ........................................ 29 Figure 10. Comparison of Motor Power Distributions between UDDS and US06 [14] .. 30 Figure 11. Cumulative Braking Energy vs Power, FTP-75 Cycle [15] ............................ 31

    Figure 12. Braking Energy Distribution over Speed FTP-75 Cycle [15] ......................... 32 Figure 13. Energy Dispersion over Braking Events for a Real World Cycle [40] ........... 33

    Figure 14. Information Flow in the Vehicle Simulator..................................................... 40 Figure 15. Block Diagram Representation of a Forward Vehicle Simulator. ................... 42 Figure 16. Engine Model 0 ............................................................................................... 43

    Figure 17. Sample Fuel Consumption Map for the Engine Model. .................................. 44

    Figure 18: Torque Converter Model 0 .............................................................................. 46 Figure 19. Wheel and Tire Model. .................................................................................... 49 Figure 20. General Layout of Renewable Energy Storage System for Parallel Hybrid

    System. .............................................................................................................................. 52 Figure 21. Overview of Control Hierarchy for RESS....................................................... 53

    Figure 22. Mechanical Hybrid Powertrain Layout ........................................................... 55 Figure 23. CVT Model ...................................................................................................... 56

    Figure 24. Normalized CVT Efficiency............................................................................ 57 Figure 25. Scheme of flywheel to drivetrain power chain ................................................ 58 Figure 26. Block Diagram of Clutch and Flywheel Model .............................................. 61 Figure 27. Wet Clutch Coefficient of Friction [12] .......................................................... 68 Figure 28. Hydraulic Hybrid Powertrain Layout .............................................................. 71

    Figure 29: Hydraulic diagram of the ESS ......................................................................... 72 Figure 30. Hydraulic Pump Model ................................................................................... 73

    Figure 31. Axial Piston Pump ........................................................................................... 75 Figure 32: Flow and Overall Efficiency Map of P1 028 Axial Piston Pump ................... 76 Figure 33: Identification of Pump Flow Model ................................................................ 77

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    Figure 34: Pump Volumetric Efficiency Model at Max. Displacement (Left) and at

    Variable Displacement (Right) ......................................................................................... 78 Figure 35: Pump Mechanical Efficiency Model ............................................................... 79 Figure 36. Block Diagram of the Displacement Controller Logic.................................... 80

    Figure 37. Bladder-Type Accumulator ............................................................................. 82 Figure 38. Block Diagram of the Accumulator Model ..................................................... 83 Figure 39. Poppet Valve Model ........................................................................................ 86 Figure 40: Discharge Coefficient Curve for the Pressure Relief Valve ............................ 87 Figure 41. Block Diagram of the PRV Controller Logic .................................................. 88

    Figure 42. Block Diagram of the Accumulator Model ..................................................... 90 Figure 43. Proposed AESS Design Flowchart .................................................................. 97 Figure 44. US FTP-75 Test Cycle Velocity Profile [1] .................................................... 99 Figure 45. Velocity and Power Profiles for Mid-sized SUV over FTP-75 Cycle [2] ..... 103

    Figure 46. Sample Power Sign Changes on FTP-75 Cycle [2]....................................... 105 Figure 47. Distribution of Braking Event Energy, FTP-75 Drive Cycle ........................ 107

    Figure 48. Net Traction and Braking Power FTP-75 Cycle ........................................... 113 Figure 49. Energy per Braking Event FTP-75 ................................................................ 114

    Figure 50. Maximum Braking Power per Event FTP-75 ................................................ 115 Figure 51. Energy Distribution for Energy Storage Capacity FTP-75 ........................... 116 Figure 52. Energy Distribution for Maximum Braking Power ....................................... 117

    Figure 53. Event Maximum Power compared to Event Energy FTP-75 ........................ 118 Figure 54. Effect of Energy Storage Capacity on Total Energy Storage ........................ 119

    Figure 55. Effect of Maximum Power on Total Energy Storage .................................... 120 Figure 56. Effect on Maximum Speed on Total Energy Storage .................................... 121 Figure 57. Mechanical ESS Design Configuration ......................................................... 123

    Figure 58. Hydraulic ESS Design Configuration ........................................................... 131

    Figure 59. Pre-charge pressure Energy Storage Relationship ........................................ 134 Figure 60. Sample CVT Torque to Mass Correlation (Torotrak CVT) ......................... 140 Figure 61. Sample CVT Torque to Volume Correlation (Torotrak CVT) ...................... 141

    Figure 62. Sample Pump Length (Parker P1 18cc/rev, End Port Design) ...................... 144 Figure 63. Sample Pump Width and Height (Parker P1 18cc/rev, End Port Design) .... 145

    Figure 64. Pump Displacement to Volume Correlation.................................................. 146 Figure 65. Pump Displacement to Mass Correlation ...................................................... 146

    Figure 66. Accumulator Mass Correlation...................................................................... 147 Figure 67. Accumulator Outer Dimensions (Bosch HAB-5X) [10] ............................... 148 Figure 68. Accumulator Actual Volume Correlation ..................................................... 149 Figure 69. Accumulator Maximum Fluid Volume, Adiabatic Compression [10] .......... 150 Figure 70. Accumulator Fluid Volume Correlation (1400 PSI Pre-charge) ................... 151

    Figure 71. Accumulator Fluid Volume (5000PSI Max, Varying Pre-charge Pressure) . 152 Figure 72. Reservoir Mass Correlation ........................................................................... 153

    Figure 73. Diagram of Vehicle Simulator....................................................................... 157 Figure 74. AESS Brake Control Flowchart .................................................................... 160 Figure 75. AESS Traction Torque Control Flowchart .................................................... 161 Figure 76. 2009 Saturn VUE .......................................................................................... 169

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    Figure 77. Sample of Synthetic Driving Cycle ............................................................... 170

    Figure 78. Accumulator Energy ...................................................................................... 177 Figure 79. Mechanical ESS Vehicle Speed Profile Synthetic Cycle ........................... 185 Figure 80. Engine Torque and Speed on Synthetic Cycle .............................................. 186

    Figure 81. Flywheel Speed Synthetic Cycle ................................................................... 186 Figure 82. ESS State of Energy Synthetic Cycle ............................................................ 187 Figure 83. CVT Ratio Synthetic Cycle ........................................................................... 187 Figure 84. Clutch Mode Synthetic Cycle ........................................................................ 187 Figure 85. Power Split at Coupling Point Synthetic Cycle ............................................. 188

    Figure 86. Clutch Slip Example Low Numerical Gear Ratio ......................................... 190 Figure 87. Clutch Slip Example High Numerical Gear Ratio ........................................ 191 Figure 88. Desired and Actual Speed Trace Synthetic Cycle ......................................... 194 Figure 89. Engine Torque and Speed over Synthetic Cycle ........................................... 195

    Figure 90. Hydraulic Pump/Motor Torque and Speed.................................................... 196 Figure 91. Hydraulic Pump/Motor Flowrate .................................................................. 196

    Figure 92. Hydraulic Pump/Motor Displacement ........................................................... 197 Figure 93. Hydraulic Accumulator Pressure ................................................................... 197

    Figure 94. Hydraulic Accumulator and Reservoir Volumes .......................................... 198 Figure 95. Hydraulic Accumulator State of Energy ....................................................... 198 Figure 96. Mechanical ESS Design Parameter Interaction Plot ..................................... 202

    Figure 97. Mechanical ESS Design Parameters Main Effects Plot ................................ 203 Figure 98. Main Effects Plots for Cost Function Value.................................................. 206

    Figure 99. Interaction Effects for Hydraulic Design Parameters .................................... 207 Figure 101. Mechanical ESS Vehicle Speed over 75% Urban 25% Highway Cycle ..... 209 Figure 102. Mechanical ESS Power Split at Coupling Point over 75% Urban 25%

    Highway Cycle................................................................................................................ 210

    Figure 103. Flywheel Speed over 75% Urban 25% Highway Cycle .............................. 211 Figure 104. Mechanical ESS State of Energy over 75% Urban 25% Highway Cycle ... 211 Figure 105. CVT Ratio over 75% Urban 25% Highway Cycle ...................................... 212

    Figure 106. Clutch Mode over 75% Urban 25% Highway Cycle .................................. 212 Figure 107. Hydraulic ESS Vehicle Velocity over 75% Urban 25% Highway Cycle ... 213

    Figure 108. Hydraulic ESS Torques at Coupling Points over 75% Urban 25% Highway

    Cycle ............................................................................................................................... 214

    Figure 109. Hydraulic ESS State of Energy over 75% Urban 25% Highway Cycle ...... 215 Figure 110. Accumulator Pressure over 75% Urban 25% Highway Cycle .................... 215 Figure 111. Hydraulic Accumulator and Reservoir Volumes over 75% Urban 25%

    Highway Cycle................................................................................................................ 216 Figure 112. Hydraulic Pump/Motor Displacement over 75% Urban 25% Highway Cycle

    ......................................................................................................................................... 217

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    Chapter 1: Introduction

    1.1 Motivation

    In 2009, the United States alone consumed 13.28 million barrels of oil per day for

    transportation [1]. As the price of oil and fuel economy standards for automobile

    manufacturers increase, great demand is placed on ways to reduce vehicle fuel

    consumption. In the United States, CAFE standards for vehicle fuel economy are set to

    increase by 29% for passenger cars and over 24% for light trucks between 2011 and 2016

    [2]. Aside from government regulation, consumers are demanding higher fuel economy

    due to the rising fuel prices, causing OEMs to constantly look for ways to meet both the

    demand and regulation while maintaining performance, consumer acceptance and

    remaining competitive in the market.

    The increasing cost for energy and the desire to meet government regulations has caused

    automotive OEMs to investigate the application of traditionally non-automotive

    technologies to vehicles in the hopes improving fuel economy and overall vehicle

    efficiency. Many of these technologies involve fitting the vehicle with a means to supply

    traction force in addition to the internal combustion engine. Such vehicles are commonly

    referred to as hybrid vehicles, and are gaining increasing OEM focus and market share as

    external pressure and consumer preference increase demand for vehicles with higher fuel

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    economy [3]. Hybrid vehicles offer ways to increase vehicle efficiency by recovering

    normally wasted energy and/or allowing the engine to operate in a more efficient manner.

    1.2 Kinetic Energy Recovery

    A significant portion of a vehicles fuel is spent accelerating from rest. The kinetic

    energy of the vehicle is then dissipated by resistive forces and the use of traditional

    friction brakes during the subsequent deceleration. This inherent energy loss caused by

    friction braking leads to significantly higher fuel consumption for vehicles with

    conventional powertrains during city driving when compared to highway only driving

    despite lower road loads. One method for improving fuel economy is to recovery and

    store as much of the vehicles energy as possible during deceleration, then use the stored

    energy to accelerate the vehicle at a later time. The process of storing the energy is

    commonly known as regenerative braking. Ideally, regenerative braking could recover all

    of the energy traditionally wasted by friction brakes leading to significant increases in

    fuel economy.

    In order to successfully implement regenerative braking functions on a vehicle, a specific

    system needed to recover, store and release the energy. To date, several options for

    regenerative braking have been developed for vehicle use. One such method involves

    using an electric machine (generator) to convert the vehicles mechanical energy to

    electrical during braking and chemically store it in a battery. When necessary, the electric

    machine draws the stored energy from the battery and provides additional propulsion for

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    the vehicle. Alternatively, energy can also be recovered using a hydraulic pump and

    stored in a high pressure accumulator to be used for hydraulic motoring. Energy can also

    be stored mechanically using a rotating disc (flywheel) of sufficient inertia. This method

    requires a means for increasing the flywheel speed while the vehicle speed is decreasing

    in order to store the energy. To release the energy, the flywheel must decelerate while

    vehicle speed is held constant or increased. As a result, a continuously variable ratio

    device is needed between the vehicles drivetrain and the flywheel.

    Each method for kinetic energy recovery must provide a torque to the drivetrain in the

    opposite direction of rotation for normal forward drive operation. Potentially, the braking

    torque can be applied to either set of drive wheels depending on vehicle architecture and

    desired configuration. The layout of the regenerative braking architecture does impact

    potential performance, as well as the design of the vehicle powertrain. Systems can be

    completely integrated with the powertrain, which is the case for many electric

    regenerative braking systems, or completely separate with independent operation. Some

    configurations can allow for additional benefits such as providing direct storage for

    energy produced by the engine or enabling engine off operation while the vehicle is

    stopped or at very low speeds.

    For safety reasons, regenerative braking systems cannot completely replace friction

    brakes. In cases of potential energy storage system failure or desired deceleration beyond

    the capability of the regenerative system, friction brakes must still be capable of

    providing sufficient stopping capability. Also friction brakes are necessary for anti-lock

    brake functionality as well as traction and stability control implementation.

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    1.3 Hybrid Electric Vehicles (HEVs)

    The most common type of hybrid vehicle on the road today is the hybrid electric vehicle

    (HEV). HEVs use electric machines along with on-board batteries to store energy and

    provide electric power assist. Most HEVs also provide limited engine off operation at rest

    and low speeds. Multiple levels of vehicle hybridization can be found in production

    ranging from belted starter-alternator mild hybrid systems to full-size, extended range

    hybrids capable of traveling upwards of 40 miles on battery power alone [4]. The cost

    and complexity of the systems also vary greatly depending on functionality.

    Figure 1. Chevrolet Volt Range Extension Hybrid Powertrain Layout [4]

    The electric machine in a HEV provides the capability to store and retrieve energy from

    the battery. In order to store energy, the electric machine behaves as a generator and

    charges the battery by drawing energy from the vehicle. Typically this is done during

    braking, but in certain configurations can also be performed directly by the engine. When

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    traction power is commanded by the electric vehicle, the electric machine behaves as a

    motor and provides torque to the wheels at the expense of battery charge.

    HEVs store energy in a high voltage battery pack consisting of multiple battery cells in

    series which can also be combined in parallel to increase capacity. Mild hybrid systems

    operate in the 30-50V range while more advanced full-hybrids designs use voltages over

    200. Although the most common battery type for HEVs is based on nickel metal hydride

    technology, OEMs are beginning to switch to lithium ion chemistry for higher power and

    energy density [5]. Considerations on battery life typically limit the maximum current

    and therefore the power the battery can supply. As a result, HEV batteries are designed

    to provide relatively large energy storage but limited power capability.

    Additional electronic components are necessary for HEV operation. Inverters are needed

    to convert the AC electric machine power to DC power for the battery. Voltage

    converters are also necessary to operate other electronic accessories and optimized motor

    efficiency. The functionality of the power electronics comes at the cost of reduced

    system efficiency and additional cooling requirements.

    The benefits of electric hybridization are substantial and well understood. Electric

    hybrids have the capability of improving vehicle efficiency through regenerative braking,

    engine off vehicle operation, and allowing for more efficient use of the vehicles

    powertrain. Some configurations also allow use of the electric machine to vary the gear

    ratio between the engine and the wheels, effectively behaving as an electronic

    continuously variable transmission. Engine efficiency can be improved by using the

    electric motor to change the torque and speed operating point of the engine, while still

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    providing the requested torque to the wheels. In most cases this involved forcing the

    engine to run at a higher load where efficiency is greater and absorbing the excess power

    into the battery for future use [5].

    If a hybrid electric system is designed with substantial reserve energy capacity in the

    battery and electric machines which can supply a significant portion of the drivers torque

    request, the conventional engine can be downsized, allowing for more efficient operation.

    Engine downsizing can improve both fuel economy and reduce cost for a given

    technology level.

    Utilizing all of the benefits that electric hybrid vehicles have to offer, improvements in

    EPA estimated fuel economy of 7-50 % have been attained. Production examples of

    electric hybrid vehicles show improvements in fuel economy over their contemporaries

    of up to 50% in combined city and highway fuel economy. The gains are particularly

    impressive in urban driving scenarios where the regenerative braking can be used in

    combination with engine off operation to realize city driving cycle fuel economy

    improvements of up to 80% [6],[7].

    The gains and benefits of hybrid electric vehicles are not without compromises. The

    added expense of the electric components and battery are significant. For example, the

    MSRP of a 2011 Toyota Prius over the MSRP of a comparable 2011 Toyota Corolla is

    over $3,200. On larger vehicles, the premium can be even higher. A 2011 Chevrolet

    Tahoe Hybrid costs over $6,000 more than a comparably equipped model with a

    conventional powertrain [8]. The premium in cost for a hybrid electric vehicle may be

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    recovered by the consumer, but the outcome depends heavily on the cost of fuel and the

    number of miles driven by the owner [9].

    Hybrid electric vehicles also present several technical challenges that limit effectiveness.

    The conversion between forms of energy during storage and recovery causes reduced

    efficiency. The vehicles mechanical energy is transferred to electrical energy by the

    electric machine then converted to chemical energy storage by the battery. During each

    step in the process, losses are incurred, reducing the amount of actual energy stored. The

    reverse process occurs when the batterys energy is used to propel the vehicle. Estimates

    for round trip efficiency for hybrid electric systems are close to 50% depending on the

    operating points and conditions [5]. This limits the effectiveness of the regenerative

    braking and the opportunities for improving the vehicle fuel economy.

    The batteries used for energy storage also present challenges. With current battery

    technology, in order to ensure satisfactory durability and longevity, the batterys power

    must be limited to less than the maximum capability. The depth of discharge for the

    battery must also be carefully monitored and controlled. Large swings in the batterys

    state of charge (SOC) cause reduced life [10]. For this reason, compromises are required

    in the design process where the battery must be oversized in terms of energy storage

    capacity in order to provide the desired power capability. This leads to increased costs

    and system weight. Accurately estimating the batterys SOC is also a challenge in HEV

    implementation. Over several years, as battery capacity degrades, significant errors in the

    estimation of a batterys SOC can occur, potentially causing battery damage and reduced

    system performance. Batteries also display temperature impacted performance. HEVs

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    must be designed with systems capable of preventing the battery from experiencing

    extreme temperature.

    As a result of these technical challenges and limitations, other forms of energy storage for

    vehicles are being investigated including forms of mechanical and hydraulic storage.

    1.4 Alternative Energy Storage Systems for Hybrid Vehicles

    While electrical energy storage systems offer many benefits, opportunities to store energy

    in other forms should also be investigated and understood, in order to evaluate the

    feasibility as a cost effective alternative to electric systems. Two of the most promising

    candidates for non-electric energy storage are mechanical, in the form of a rotating disc

    (flywheel), and hydraulic, via bladder-type accumulators. The capability of flywheel-

    based energy storage has become a possibility due to the development of new CVT

    designs that allow for the transfer of energy into and out of the flywheel. Hydraulic

    energy storage systems have the advantage of using commonly available, cost-effective

    and well understood components with potential for low cost.

  • 9

    Figure 2. Specific Power versus Specific Energy for Various Short-Term Energy Storage

    Systems [5]

    In addition to the cost benefits, hydraulic and flywheel mechanical energy storage

    systems also have higher specific power than batteries. Figure 2 shows the specific power

    and specific energy for some short-term energy storage systems. Hydraulic and flywheel

    mechanical systems also have the advantage of relatively little performance degradation

    over time regardless of the depth of discharge in terms of energy storage capacity. Due to

    these reasons it is important to consider these technologies for vehicle hybridization.

    1.4.1 Mechanical Energy Storage

    The concept behind mechanical energy storage is relatively simple. The goal is to

    transfer the vehicles kinetic energy to a rotating disc (flywheel) mounted on board the

  • 10

    vehicle. The flywheel increases its rotational speed as the vehicle speed decreases

    accordingly. Figure 3 shows this simple flow of energy between the flywheel and vehicle

    and the results of the energy flow.

    Figure 3. Flow of Power for Flywheel Mechanical Energy Storage System [5]

    While the concept is simple, the practical implementation is more difficult. With strictly

    mechanical coupling between the flywheel and vehicle, the ratio of speeds between the

    portion of the conventional drivetrain to which the flywheel system is attached and the

    actual flywheel itself must be allowed to change. As energy is traded from the vehicle to

    the flywheel through conventional powertrain components, the rotating speed of the

    conventional components must smoothly decrease, while the rotational speed of the

    flywheel must smoothly increase. This results in a need for a constantly changing and

    variable speed ratio between the flywheel and the drivetrain. This can be accomplished

    in a couple of different ways. A conventional gearbox could be used, with clutch plates

    which allow for constant slippage while flywheel speed is changing. However, this

    involves very inefficient operation of the flywheel system and very high clutch wear. The

    other option is to use a device which allows for constantly varying input-output ratios.

  • 11

    This is typically know as a continually variable transmission and is seeing increased

    popularity for replacing stepped gear automatic transmissions for non-hybrid vehicle use.

    Changing the CVT ratio, which in turn forces a change in the flywheel speed, allows

    energy transfer between the flywheel and vehicle.

    CVTs exist in multiple designs. The simplest designs involve two variable diameter

    pulleys with a belt between them. The pulleys are split and allow for varying diameters

    depending on the radius where the belt is riding. The ratio is changed as the pulley

    diameter is altered through sliding the halves of the pulley together and apart. This is

    typically done in unison for both pulleys to keep tension on the belt. For higher torque

    applications, chain type metal belts are used.

    Toroidal CVTs offer another method of continuous ratio change. This design uses dished

    discs connected by metal rollers to transmit torque. The metal rollers are allowed to pivot,

    effectively changing the ratio between two discs. Half-toroidal designs restrict roller

    pivoting to one direction from center while full-toroidal designs allow the roller to pivot

    over center for wider ratio spread capability. Torque is transmitted while the rollers spin

    by using traction fluid that resists shear forces while compressed between the rollers and

    discs. Some current production CVTs use the half-toroidal design, while full toroidal

    designs have primarily been seen in prototype applications such as motorsports [11].

    Figure 4 shows the basic structure of a full-toroidal cvt.

  • 12

    Figure 4. Toroidal CVT Variator Example [11]

    The flywheel itself can vary widely in design and material depending on the systems

    capability. Typically flywheels fall into two categories, low speed (30,000 rpm). Low speed flywheels can be produced from steel or other low

    cost materials and primarily use conventional ball bearings. This offers the opportunity

    for low cost energy storage. High speed flywheels use composite materials with high

    tensile strength to enable speeds above 30,000 rpm. Bearings can be high precision ball

    bearings or magnetic bearings which suspend the flywheel and offer low friction.

    Flywheel energy storage requires an enclosure surrounding the flywheel for safety

    purposes. The surrounding also offers the possibility to create a vacuum around the

    flywheel for reduced windage.

    Potential benefits to flywheel energy storage include low cost, high power, high

    durability, and low weight when compared to electrical systems. Limitations for

    flywheel based mechanical energy storage include limited low energy density compared

    to batteries and limited energy storage duration due to inherent losses from the rotating

  • 13

    components. Flywheel systems can also offer superior overall efficiency of storage and

    release of up to 70% [12].

    1.4.2 Hydraulic Energy Storage

    Hydraulic energy storage revolves around storing high pressure fluid in an accumulator

    which can later be released to perform work. When vehicle deceleration is desired, a

    pump is used to send fluid from a low pressure reservoir to a high pressure accumulator.

    To provide acceleration assist, the pump behaves as a motor and changes the potential

    energy of the pressurized fluid into mechanical work. Hydraulic controls in the form of

    solenoid operated valves regulate the fluid flow to and from the pump.

    Traditional hydraulic components can be used to create the system. Bladder type steel

    hydraulic accumulators offer fluid storage at pressure up to 350bar. The external of an

    accumulator is typically steel and cylindrical in shape with domed ends. On the inside, a

    rubber liner is used to separate the gas charge from the working fluid. As fluid enters the

    fixed volume, the gas inside of the bladder must compress, causing an increase in

    pressure. Since the fluid and the gas must have equal pressure, the fluid pressure

    increases with gas pressure. Recently, hydraulic accumulators made of composite

    material have been design to hold even higher pressures at significantly less weight than

    traditional steel designs at higher cost. In addition to an accumulator hydraulic energy

    storage systems also need a low pressure reservoir to house fluid that is not inside of the

  • 14

    accumulator. The reservoir must have sufficient volume to allow for the accumulator to

    reach maximum pressure.

    Hydraulic pumps are commonly used in industrial settings in many capacities and are

    available in a wide range of designs. Simple fixed displacement pumps offer low cost

    and high reliability at the expense of limited operating conditions. Variable displacement

    pumps allow for precise control over torque output for a given speed and higher system

    efficiency, but increase cost and complexity.

    Hydraulic controls are necessary to route fluid flow and dictate the system behavior.

    With the need for the hydraulic pump to behave in both pump and motoring modes with

    always positive vehicle velocity, fluid direction through the pump needs to be re-routed

    to allow for the pump to always rotate in the same direction. Control valves are also

    needed to prevent backflow when the system is not functioning. Pressure release valves

    are necessary to prevent over pressurizing the accumulator or reservoir.

    Benefits of hydraulic energy storage include durability, proven components and high

    power capability. Depending on the system design, low cost can be achieved. The

    drawbacks include heavy components and limited energy storage when compared to

    electrical systems.

    1.5 Design Considerations

    Also of great significance when discussing multiple options for vehicle energy storage is

    the design of the system. Difficult decisions must be made in both system architecture

  • 15

    and component specifications which will ultimately decide the overall effectiveness in

    practice. With the magnitude of importance that resides in the design, great need is

    placed on methods for configuring alternative energy storage systems for maximum

    performance at minimum cost, both financially and in terms of system weight and space.

    While the ultimate test of a designs validity resides in the real-world performance,

    OEMs neither have the time nor the resources to build working prototypes of each and

    every feasible design configuration. This has given rise to analysis lead design, which

    causes design decisions to be based on available data and performance in simulation. In

    order to evaluate a design in simulation, a prescribed velocity versus time profile is

    needed as a guide for the vehicle to follow. Common drive cycles include the regulatory

    cycles as well as real-world driving data collected by logging actual vehicles in use.

    These drive cycles become the basis for comparison of results such as fuel economy,

    performance, and component efficiency. Designs are revised according the results of

    these simulations and subsequently OEMs invest significant resources into hardware

    based development.

    The typical process of running simulations to optimize fuel economy over a range of

    cycles can be expedited with good preliminary system design. This can be accomplished

    with prior knowledge of the characteristics of the driving cycles. For instance, knowing

    the maximum braking power for a given drive cycle will help set a bound on the

    maximum useful power absorption of the regenerative braking system. Taking into

    account the frequency and distribution of cycle statistics such as energy, power,

  • 16

    acceleration, for braking and acceleration periods offers potential to begin simulations

    with designs that are closer to optimal.

    Presently, design methods exist for optimizing the design of hybrid electric vehicles.

    These methods take into account the specific design targets and constraints of the

    electrical components as well as all of the possible configurations. However, significantly

    fewer methods exist for designing short-term energy storage systems, specifically

    hydraulic and mechanical systems. The short-term systems provide radically different

    constraints on the design and can potentially operate in very different conditions in terms

    of when and how the kinetic energy is both recovered and returned to the vehicle. As a

    result, the following work provides an analysis based design method for mechanical and

    hydraulic short-term energy storage systems (ESS) which allows for the maximization of

    performance while minimizing system mass, volume, and cost.

  • 17

    1.6 References

    [1] U.S. Energy Information Administration.

    http://www.eia.doe.gov/totalenergy/data/annual/pdf/sec5_3.pdf

    [2] CAFE Standards. National Highway and Traffic Safety Administration

    http://www.nhtsa.gov/fuel-economy

    [3] HEV Sales by Model. U.S. Department of Energy Alternative Fuels and

    Advanced Vehicle Data Center.

    http://www.afdc.energy.gov/afdc/data/vehicles.html

    [4] Tortosa, N. Karbon, K. Aerodynamic Development of the 2011 Chevrolet Volt.

    SAE International Technical Paper 2011-01-0168.

    [5] Guzzella, L., Sciarretta, A. Vehicle Propulsion Systems Introduction to Modeling

    and Optimization (2nd ed.). Springer: New York.

    [6] Simopoulos, G., et al. Fuel Economy Improvements in an SUV Equipped with an

    Integrated Starter Generator. SAE Paper 2001-01-2825.

    [7] 2011 Ford Fusion Sales Brochure. http://www.ford.com/cars/fusion/brochures/

    [8] Car Price Comparison. http://www.truedelta.com

    [9] Greene, D., et al. Future Potential of Hybrid and Diesel Powertrains in the U.S.

    Light-Duty Vehicle Market. (2004). U.S. Department of Energy.

    [10] Adams, J., et al. Approach to Validation Plan Development for Advanced Battery

    Systems in Vehicle Applications. SAE International Technical Paper 2011-01-

    1366.

    [11] Cross, D., Brockbank, C. Mechanical Hybrid System Comprising of a Flywheel

    and CVT for Motorsport and Mainstream Automotive Applications. SAE

    International Technical Paper 2009-01-1312.

    [12] Boretti, A. Improvements of Vehicle Fuel Economy Using Mechanical

    Regenerative Braking. SAE International Technical Paper 2010-01-1683.

  • 18

    Chapter 2: State of the Art

    2.1 Introduction

    This chapter contains a review of current research and development of non-electric

    energy storage for vehicles, specifically hydraulic and flywheel mechanical systems with

    a focus on short-term storage systems. Attention is also given to drive cycle statistical

    analysis and current HEV design procedures.

    2.2 Mechanical Energy Storage Systems

    Investigation into the concept of storing energy in a rotating disc for passenger cars was

    spurred by rising gasoline prices in the 1970s. Dr. Andrew Frank from the University of

    Wisconsin investigated the idea of using a very large flywheel to store substantial

    amounts of energy and allow engine off operation of the vehicle [1]. University of

    Wisconsin research showed fuel economy improvement of up to 33% were possible using

    a large steel flywheel underneath the vehicle to buffer the engines energy output. The

    flywheel energy was transmitted using a 4-speed manual gearbox and hydrostatic CVT

    [1]. While promising, several drawbacks limited the potential for production use. Even

    though engine downsizing could negate much of the additional weight of the flywheel

    itself, a very heavy containment structure would be needed to contain the large flywheel

  • 19

    in the event of a crash. The efficiency of the system was severely hindered by the

    designs hydrostatic CVT. High losses in the flywheel due to bearings and windage were

    significant and present over the entire cycle.

    More recently, investigations have begun using higher speed flywheels (20,000rpm and

    greater) to obtain higher energy density. Advances in technologies such as lighter,

    compact, and more efficient continuously variable transmissions (CVTs) have allowed

    new configurations for storing energy in flywheels. New designs have allowed flywheels

    to be coupled to the drivetrain in parallel with the engine to produce very high round trip

    efficiencies in regard to storing and releasing energy. Higher speed operation reduces the

    flywheel mass and eliminates some of the packaging issues with large, low speed

    flywheels.

    The introduction of kinetic energy recovery in Formula 1 racing in the 2009 season has

    spurred the development of regenerative braking systems with high power density. Out of

    this development spawned a high speed flywheel system with a full toroidal CVT capable

    of flywheel speeds exceeding 60,000rpm and 60kW of power with a total system weight

    of only 25kg [26]. The technology developed for Formula 1 has begun to infiltrate the

    realm of production vehicles beginning with the CVT. Flybrid Systems under the license

    from a toroidal CVT designer, Torotrak LLC, has been working with OEM car

    manufacturers to bring the technology of the Formula 1 flywheel hybrid system to

    production vehicles. Torotrak and Flybrid have published several papers touting the

    benefits of flywheel energy storage for vehicle fuel economy, showing the energy

  • 20

    recovered from braking can provide up to 21% of the energy needed to propel a vehicle

    over the US-FTP75 cycle [2].

    The advancement in CVT technology in terms of specific torque output and efficiency

    has made flywheel energy storage more promising. At the heart of this improvement in

    CVT technology is the development of full-toroidal traction drive CVTs. Figure 5 shows

    a plot of torque capacity and weight of toroidal CVTs (T-CVT) compared to production

    CVTs as well as other transmission types. T-CVTs have much lower weight for a given

    torque capacity than conventional push-belt CVTs, placing near manual transmissions in

    terms of torque capacity per unit mass.

    Figure 5. Comparison of Toroidal CVT Technology with other Automotive

    Transmissions [5]

    Traction drive is accomplished through the use of the variator rollers which transmit

    torque between the toroidal discs within the transmission. The variators change the input

  • 21

    speed to output speed ratio by changing the alignment of their axis of rotation. The result

    is the effective radius of where the force is being transmitted changing. Changing the

    radius effectively changes the speed ratio.

    Figure 6. Toroidal CVT Variator Behavior [26]

    Torque is transmitted without metal to metal contact by the use of elasto-hydrodynamic

    traction fluid which resists shear while under compression. With a fluid film between the

    variators and the discs, metal to metal contact is prevented, allowing for adequate

    transmission life [5]. Traction fluids have allowed the T-CVT to become a possibility and

    much development has gone into optimizing them for T-CVT use [6].

    Current applications of flywheel hybrid systems include prototype production vehicles

    such as the Jaguar XF with FHSPV (Flywheel Hybrid System for Premium Vehicles).

    The project is being lead by a consortium of automotive companies including Jaguar

    Land Rover, Ford, Prodrive, Torotrak, Xtrac, Flybrid Systems, and Ricardo. The system

  • 22

    has a parallel configuration and is capable of providing 60kW power while improving

    fuel consumption by 20% [3]. Figure 7 shows the system uses a T-CVT coupled to the

    rear axle with a high speed flywheel for energy storage. The system is capable of storing

    over 400kJ of energy and flywheel speeds of up to 60,000rpm.

    Figure 7. Component Details for Jaguar with FHSPV [7]

    Very recently a paper was published on the topic of design of mechanical flywheel

    systems for implementation in automotive vehicles. Topics discussed included possible

    configurations, design, and CVT control for a specific case of a vehicle decelerating from

    100km/h to 60km/h. The effects of gearing between the flywheel and vehicle were

  • 23

    investigated and guidelines were provided for selecting proper gearing to maximize

    energy storage in the flywheel. Assumptions were made with regards to the initial sizing

    of the system as well as the operating limits of the system. Not included in the paper were

    energy losses due to rolling resistance and aerodynamic resistance during deceleration.

    The final conclusions showed the F1 style system, with proper design, could be

    successfully implemented and controlled in road cars [8].

    While the majority of research into flywheel mechanical hybrids for road cars has been

    done in simulation, the recent advancements in technology offer an efficient and

    lightweight solution to regenerative braking.

    2.3 Hydraulic Hybrid Energy Storage Systems

    In the late 1970s hydraulic hybrids were investigated by Buchwald et al. in their work on

    parallel hybrid systems for urban bus applications. Their research showed great potential

    for braking energy recovery and reduction in fuel consumption through engine

    downsizing and regenerative braking [4]. More recent research has focused on both series

    and parallel systems for energy recovery and improvements in average engine operating

    efficiency.

    In the mid 2000s research and development on series hydraulic hybrids was performed

    by the EPA along with Eaton Corporation and others on a project involving hybridization

    of a UPS delivery truck. The goals were reducing emissions and fuel consumptions

    through recovering braking energy and optimizing engine operation. The high amount of

  • 24

    urban driving and frequent stops made the delivery truck an excellent application for

    hydraulic hybrid technology. The results were 60-70% increases in fuel economy and

    40% reduction of carbon dioxide. With the reasonable cost of hydraulic technology, the

    payback period for the cost of the hybrid delivery truck over a conventional version was

    estimated at 3 years [9].

    Research at the University of Michigan on series hydraulic hybrids for 5-ton trucks has

    also shown in simulation that substantial gains in fuel economy of up to 68% are possible

    in urban driving conditions [10]. In the simulations, engine shut down was employed and

    regenerative braking was maximized though accumulator energy control. The research

    also addressed some of the challenges related to the relatively low energy density of the

    hydraulic accumulator used in the vehicle. Allowing the SOC to reach relatively low

    levels before recharging with the engine allowed better use of the limited energy storage

    capacity.

    While gains from series hydraulic hybrid systems can be significant on large vehicles

    where the powertrain is a relatively small portion of the overall vehicle weight and the

    primary operating environment contains limited highway operation, the gains are less

    significant on smaller vehicles with more frequent highway use. As a result, lower cost

    and lighter parallel hydraulic launch assist (HLA) designs have been considered for

    smaller vehicles. HLA systems typically have less power capability and energy storage

    than series hybrid configurations, but have the advantage of less weight added to the

    vehicle and lower cost of components while still retaining the regenerative braking

    functionality.

  • 25

    Modeling and demonstration vehicle research on HLA systems for small and mid-sized

    vehicle applications has shown improvements of anywhere from 10% to over 30% are

    possible [11],[12]. Ford Motor Company Advanced Powertrain along with the U.S. EPA

    fitted a hydraulic pump/motor in parallel with the conventional drivetrain in a mid-sized

    SUV for demonstration purposes. In additional to demonstrating improvements in fuel

    economy, it also showed the ability to smoothly blend conventional brake operation with

    regenerative braking from the hydraulic system [11]. Simulation performed at Anglia

    Ruskin University in 2008 showed gains of 7-10% were possible with only regenerative

    braking and no engine off operation [12]. In this work, the system was sized to recovery

    the amount of energy equal to the vehicles kinetic energy when stopping from 60km/h to

    0km/h. The model uses an axial piston pump/motor clutched to the drive axle along with

    a piston-style high pressure accumulator. Very short urban cycles were used to estimate

    the results. Figure 8 shows the drivetrain layout for the design used for simulation.

  • 26

    Figure 8. Sample Hydraulic Launch Assist Drivetrain Layout [12]

    Recent research promoted by the Center for Compact and Efficient Fluid Power has

    focused on developing new technologies for improving the performance of hydraulic

    systems for hybrid vehicles. Topics include hydraulic control strategies, developments in

    variable displacement pump/motor efficiency, advanced accumulator energy storage, and

    noise and vibration reduction. Future projects include implementation of parallel

    hydraulic hybrid systems on a light-duty truck and a passenger car [13].

    The advantages for hydraulic hybrids are the high power density of fluid power and the

    well known technology. Unfortunately, low energy density offsets some of the

    advantages. However, advancements in component technologies such as lightweight

  • 27

    composite accumulators have allowed hydraulic hybrids to better compete with other

    hybrid technologies.

    2.4 Drive Cycle Analysis Methodologies for Energy Storage System Design

    The design procedure of a vehicle typically begins with an analysis of the intended uses.

    This is no less true with hybrid vehicles, of any type. Knowing, or at least having an

    approximate knowledge of the duty cycle, velocities, accelerations, and grades the

    vehicle must traverse allows the design to best accomplish its intended goals of

    performance, emissions, and fuel economy.

    Traditionally, vehicles have been designed to worst case scenario standards. Engines and

    transmission components were sized for peak output in order to meet acceleration and

    grade-ability targets. With these targets met, the designers could safely assume the more

    moderate conditions of the regulatory and real-world driving cycles would be easily met.

    With the advent of hybrid vehicles and the ability to draw power from more than one

    source, the design space has opened up greatly, causing an increase in the number of

    design decisions to be made. Even relatively simple parallel hybrid vehicle designs

    introduce numerous options for powertrain configuration, power capability, energy

    storage capacity, and speed range of operation. The once popular metrics of top speed,

    acceleration times, and grade-ability are of little use in designing an alternate power

    source. Instead, new statistics surrounding available braking energy, braking power, and

    speeds at which energy is available for recovery are needed to generate optimum design

  • 28

    solutions. As a result, more specific investigation into the characteristic statistics of the

    energy and power requirements for both traction and braking over traditional drive cycles

    has been performed in order to assist with design. The following paragraphs will discuss

    some of the recent analysis. It should be noted that the majority of the work on drive

    cycle analysis has been performed for HEVs and electric technologies, but the analysis

    can also be applied to non-electric forms of energy storage.

    Argonne National Laboratory published a paper on the influence of drive cycles on plug-

    in hybrid vehicle design [14]. The paper begins with discussion on the importance of

    using analysis tools to determine the approximate power and energy metrics for a certain

    vehicle and a given drive cycle. In this work, Argonne National Laboratorys Powertrain

    Systems Analysis Toolkit (PSAT) is used to determine the power and energy

    requirements. Figure 9 shows the resulting requirements of power for the different

    system components.

  • 29

    Figure 9. Component Sizes over Various Driving Cycles [14]

    In the paper, the engine was sized to meet grade-ability, thus is constant for all cycles.

    However, the required ESS power which is related to the regenerative braking power and

    the required motoring power for all electric operation each varied for each driving cycle.

    Note that, due to the large difference in vehicle energy and power demand of each cycle,

    the motor peak power varies by up to a factor of 3. While peak values are important, if

    the hybrid system power source can be augmented by the engine, the distribution and

    frequency of the power capability needed also become important. Figure 10 shows the

    distribution for motor power between two cycles.

  • 30

    Figure 10. Comparison of Motor Power Distributions between UDDS and US06 [14]

    Looking at the distribution for UDDS, there is little need to size the electric motor above

    20kW and reductions in cost and weight could be realized by sizing less than the peak

    without significant penalties for that particular cycle. However, in the case of the US06

    cycle a large percentage of the motor usage is near peak, warranting sizing for peak

    operation. The conclusion drawn from the Argonne National Laboratory paper is that

    driving cycle statistics should be an important factor in design and multiple tradeoffs

    exist between performance, cost, fuel economy, and emissions.

    A more detailed analysis of driving cycles with regard to hybrid braking system design is

    presented in [15]. The paper begins with basic longitudinal vehicle dynamics and uses the

    equations to arrive at information about the energy and power of braking events over a

  • 31

    range of cycles. Important statistics about the distribution of energy and power are

    calculated and presented. In addition to distribution plots, cumulative distribution plots

    were also shown. Figure 11 shows the percentage of energy that is not recoverable from

    minimum to maximum braking power.

    Figure 11. Cumulative Braking Energy vs Power, FTP-75 Cycle [15]

    From the above curve, it is very easy to see where the marginal gains for increasing

    power are high, and at what point the gains begin to diminish. This is extremely helpful

    from a design perspective because it allows the designer to quickly estimate the effects of

    changing the braking power on the systems ability to recover energy. Also in [15] are

    distributions with respect to vehicle velocity. Figure 12 shows the distribution of energy

    with respect to vehicle speed.

  • 32

    Figure 12. Braking Energy Distribution over Speed FTP-75 Cycle [15]

    Looking at Figure 12, for this particular drive cycle, the majority of the braking energy

    available is located at vehicle speeds below 50km/h. The usefulness of this information

    stems from the fact that the capability of the energy storage systems typically vary with

    speed either due to efficiency differences or mechanical limitations. Relevant conclusions

    from the paper include the limited amount of energy that is available at very low and very

    high speeds and the idea that most of the braking energy is concentrated in a relatively

    small power range.

    Some investigation into drive cycle analysis and the subsequent design of flywheel

    hybrid energy storage systems has been done by Flybrid Systems LLP [16]. In [16] a real

    world cycle is taken from a specific automaker and analyzed with respect to braking

    energy distribution. Both the change in kinetic energy and the amount of recoverable

    energy (change in kinetic energy minus road loads) are shown in Figure 13.

  • 33

    Figure 13. Energy Dispersion over Braking Events for a Real World Cycle [40]

    The real world cycle shows the majority of the braking events for a 1800kg vehicle are in

    the 100kJ to 500kJ range. While the analysis in [16] addresses energy, it does not

    address the power considerations under the assumption that the system in question has

    sufficiently high power capabilities.

    Current practice for hybrid design includes running countless full vehicle simulations

    over a variety of cycles in order to sufficiently sample the design space and allow for

    optimization based on the constraints such as cost, fuel economy, performance, and

    durability among others. Clearly, the type of drives cycles which are used for simulation

    will heavily impact the results. Due to this, research into methods for randomly

    generating appropriate drive cycles based on real world driving data is being performed.

    At Ohio State Universitys Center for Automotive Research, real world drive cycle data

  • 34

    has been recorded and used to generate a Markov chain model which can then be used to

    generate random drive cycles where the length of the cycles along with the percentage of

    urban and highway driving are specified by the user [17]. Varying amounts of traffic can

    also be programmed into the generator for better coverage of the various conditions the

    vehicle will encounter. These types of developments allow the hybrid vehicle designer to

    evaluate and optimize the design over cycles which closely replicate the actual driving

    patterns.

    Drive cycle analysis gives the designer a portion of the input needed. Obviously, other

    design constraints are needed to arrive at a product which is feasible to produce and will

    function as intended. Much of the research and investigation into proper design of

    energy storage systems has been focused on electrical systems. A need is present to

    develop a methodology by which to design and size hydraulic and mechanical flywheel

    energy storage systems for light duty vehicles.

    2.5 Conclusions

    Based on current literature and research progress, mechanical and hydraulic energy

    storage systems present an effective way to employ regenerative braking. The benefits

    have been researched and the systems are understood. What is lacking is a design

    procedure for short-term energy storage systems.

    Current conventional vehicle design methods where the components are sized for the

    worst case scenario of vehicle use, do not apply to designing energy storage systems

  • 35

    since the ESS is not the primary provider to traction force nor the sole provider of

    braking force.

    Current HEV energy storage system design methods do apply to a limited extent,

    however, due to the difference in nature between the long-term and short-term energy

    storage systems only certain aspects apply. Methods of HEV design where the battery

    limitations are taken into account are not valid for systems where such limitations are not

    present. Additionally, the HEV electrical system can take on multiple configurations and

    provide additional features not offered by short-term forms of energy storage. In most

    cases, the energy storage capacity of a battery in a HEV is at least an order of magnitude

    greater than the energy storage offered by mechanical and hydraulic means; however, the

    mechanical and hydraulic systems offer potentially higher power. HEV design methods

    also account for the necessity of ensuring a particular HEV design is charge sustaining.

    Short-term energy storage systems with relatively limited energy storage capacity do not

    have the charge-sustaining constraints.

    Furthermore, battery based electrical energy storage systems have very high energy

    density which allows for numerous braking events to be stored before the battery reaches

    its maximum state of energy. This allows the HEV to selectively discharge the stored

    energy throughout the drive cycle. Short-term energy storage systems, by definition, do

    not have this capability due to the lower energy density of the system. In order to

    maximize the effectiveness of the short-term energy storage systems limited energy

    storage capacity the system must be charged and discharged more frequently in order to

    prevent the case where braking energy is available for storage, but the system cannot

  • 36

    accept additional energy. This means the design of the short-term systems must be based

    on the individual braking events.

    These intrinsic differences lead to the necessity of design methods specifically for

    mechanical and hydraulic short term energy storage systems and the specific advantages

    and constraints they provide. The proposed method involves braking event by braking

    event analysis in terms of energy and power, coupled with statistical weighting, in order

    to best size the system. This approach leads to considerable reduction in the effort needed

    to not only arrive at appropriate system sizing, but also reduces the amount of testing and

    verification that is necessary.

  • 37

    2.6 References

    [1] Frank, A. Beachley, N. Evaluation of the Flywheel Drive Concept for Passenger

    Vehicles. SAE Technical Paper 790049.

    [2] Cross, D., Brockbank, C. Mechanical Hybrid System Comprising of a Flywheel

    and CVT for Motorsport and Mainstream Automotive Applications. SAE

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    http://www.wired.com/

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    International Technical Paper 2010-01-1448.

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    Paper 2002-01-3128, 2002.

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    [12] Toulson, E. Evaluation of a Hybrid Hydraulic Launch Assist System for use in

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  • 39

    Chapter 3: System Dynamics Modeling for Alternative Energy Storage Systems

    3.1 Introduction

    This chapter contains the low-frequency dynamic models developed for aiding in

    performance prediction and design methodology development for alternative energy

    storage systems. First, lumped parameter models for conventional vehicle components

    and vehicle dynamics are presented. This is followed by the development of models for

    components specific to both the mechanical and hydraulic storage systems. The goal of

    these models is to provide time averaged prediction of behavior during vehicle operation.

    3.2 Conventional Vehicle Component Models

    In order to evaluate the behavior of conventional and hybrid vehicles, it is important to

    correctly understand the energy flows in the powertrain components. Doing so allows for

    evaluation and understanding of where efficiency can be improved. For this reason, a

    brief description of the models utilized to describe the vehicle energy flows is presented

    here. Combining these models to form a simulator is necessary for easily evaluating the

    behavior. Figure 14 shows the outline for the simulator subsystems and the information

    that is passed from one sub-system to another.

  • 40

    Figure 14. Information Flow in the Vehicle Simulator.

    Vehicle Dynamics

    The vehicle dynamics model is represents the longitudinal motion of the vehicle as well

    as the longitudinal load transfer between axles. The model results from an equilibrium

    equation, where the acceleration of the vehicle is a result of a balance between the force

    generated at the road/wheel interface by each individual tire due to traction and braking

    torque and the resistive forces acting on the vehicle (tire rolling resistance, aerodynamic

    force and grade). The resulting equilibrium equation is:

    gradeaerorollwheelvehveh FFFFdt

    dVM

    (3.1)

    where Mveh is the equivalent vehicle mass (which include the inertial effects of the

    rotating masses), Vveh is the vehicle longitudinal velocity and Fwheel represents the tractive

    and braking force generated by each tire. The resistive forces are expressed as follows:

    sin

    5.0 210

    gMF

    VACF

    gVMrgMrF

    vehgrade

    vehairfxaero

    vehvehvehroll

    (3.2)

  • 41

    where Cx is an aerodynamic friction coefficient, r0, r1 are rolling friction coefficients

    (determined experimentally), Af is the vehicle frontal area and is the road slope angle

    (positive is uphill).