Design and Implementation of a Belted Alternator Starter System for the. OSU EcoCAR 3 Vehicle THESIS

Transcription

1 Design and Implementation of a Belted Alternator Starter System for the OSU EcoCAR 3 Vehicle THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Dennis Ssebina Kibalama Graduate Program in Electrical and Computer Science The Ohio State University 2017 Master's Examination Committee: Dr. Giorgio Rizzoni, Advisor Dr. Levent Guvenc Dr. Shawn Midlam-Mohler

2 Copyrighted by Dennis Ssebina Kibalama 2017

3 Abstract The transportation sector is a great contributor to overall energy consumption and emissions. Stringent regulations have been put in place to curb the emissions and regulate fuel consumption due to dependency on a finite resource, fossil fuels. This has driven OEMs to re-engineer the automotive powertrain which has led to a burst in production of PHEVs, HEVs and EVs. The U.S. D.O.E, General Motors, Argonne National Laboratory (ANL) and other industry sponsors have spearheaded (Advanced Vehicle Technology Competitions) AVTCs with a goal of training the next generation of automotive engineers by challenging collegiate teams to re-engineer stock vehicles to improve fuel consumption, reduce emissions while maintaining consumer acceptability. The latest in this AVTC series is the EcoCAR 3, a 4-year competition which challenges 16 North American university teams to re-engineer a 2016 Chevrolet Camaro into a HEV while maintaining the performance aspects of the iconic American car. The OSU EcoCAR 3 vehicle boasts a Parallel-series post transmission PHEV architecture designed by the team in Year 1 of the competition. To meet the team designed (Vehicle Technical Specification) VTS targets, the architecture includes a motor coupled to the engine, a Belted Alternator Starter (BAS) which performs engine start/stop, series operation, speed matching and torque assist. Due to the versatility of the component in ii

4 realizing the VTS targets, this thesis sets to outline the design and validation work done with regards to the BAS system. The BAS system consists of the electric machine, the engine, belt transmission, inverter and battery pack. The thesis outlines the design metrics considered in the design of the BAS system ranging from electrical, performance, mechanical and thermal considerations. The BAS chosen is a sponsor donated component that wasn't supplied with an inverter solution. This thesis details the two inverter choices adopted over Years 2 3 of the competition and the control, calibration, validation, performance and packaging carried out to realize functionality of the BAS. To accurately model the dynamics of the BAS system during engine startup, a dynamic engine model is developed to model engine, BAS and belt transmission dynamics. The underlying assumptions made to develop an accurate representation of the dynamics while minimizing calibration efforts are also outlined. This model will be used in Year 4 for development and optimization of an engine start/stop controller. The thesis also analyses the two control methods adopted for engine start; an open loop controller and a closed loop controller and evaluates the performance of the controllers in terms of rise time, engine speed overshoot, maximum jerk and root mean square acceleration. This thesis encompasses the design and validation work done to move the BAS system development work from a component/subsystem level to vehicle/system level. This sets the team in a good position heading into Year 4 of the competition to implement engine start/stop functionality in the vehicle, optimize torque assist functionality and use the BAS for speed matching for faster shift times. iii

5 Acknowledgments I am indebted to the entire support of the people at the OSU Center for Automotive Research. I would like to thank Dr. Giorgio Rizzoni and Dr. Shawn Midlam-Mohler for not only the opportunity to work with the OSU EcoCAR team but also their guidance and tutelage as I pursued graduate studies; their ideas, thought processes and drive were instrumental in my time at The Ohio State University. I'd like to acknowledge my fellow EcoCAR team members who were very resourceful and hardworking individuals dedicated to applying their engineering skills and knowledge to solve engineering challenges and achieve a well-engineered Camaro. Shout out to Aditya Modak, Andrew Huster, Andrew Johnson, Arjun Khanna, Brandon Bishop, Greg Jankord, Kristina Kuwabara, Nick Tomczack, Simon Trask and Wilson Perez. A special shout out to Andrew Huster and his family for making my experience in a new country memorable. You surely made Ohio a home away from home. I am indebted to Kiira Motors Corporation for facilitating my pursuit for higher education; none of this would have been possible without them. I am thankful for my family and friends in Uganda that have supported me throughout my pursuit of graduate studies. Finally, I'm thankful to the EcoCAR organizers and sponsors that make this an invaluable learning experience for everyone involved with AVTCs. iv

6 Vita October 24, Born Kampala, Uganda Makerere College School, Uganda June B.S. Electrical Engineering, Makerere University, Uganda August 2014 to present...graduate Research Associate, Department of Electrical and Computer Engineering, The Ohio State University. Publications D. Kibalama, A. Huster, A. Khanna, A. Modak, M. Yasko, G. Jankord and S. Midlam- Mohler. Testing and Validation of a Belted Alternator Starter System for a Post- Transmission Parallel PHEV for the EcoCAR 3 Competition. SAE Technical Paper, , Oct Fields of Study Major Field: Electrical and Computer Engineering v

7 Table of Contents Abstract... ii Acknowledgments... iv Vita... v Publications... v Fields of Study... v Table of Contents... vi List of Tables... xi List of Figures... xii List of Acronyms... xv Chapter 1: Introduction Energy Trends Analysis and Motivation AVTCs & The EcoCAR 3 Competition Vehicle Architecture Vehicle Technical Specifications Objectives Thesis Overview... 7 vi

8 Chapter 2: Literature Review Introduction Drivetrain Electrification Hybrid Electric Vehicles (HEVs) Plugin Hybrid Electric Vehicles (PHEVs) HEV & PHEV Configurations Degrees of Hybridization Inverter Control Drive Quality and NVH Engine start/stop times Jerk Root Mean Square Acceleration Vibration Dose Value (VDV) Chapter 3: System Design Considerations Introduction Electrical System Evaluation Voltage Range Electrical Power and Current Wire and Fuse Sizing vii

9 Back EMF considerations Machine Performance Characteristics Mechanical Integration ISG vs. BAS Belt Coupling Thermal Considerations Chapter 4: Inverter Evaluation and Validation Infineon HybridKit LV System Validation HV System Validation Inverter Enclosure Design Inverter Control Rinehart PM100DX Inverter Calibration Performance Validation Thermal Validation BAS System Technical Specification Chapter 5: System Modeling BAS model viii

10 5.2. Engine model Crankshaft Kinematics Indicated Torque Inertial Torque Friction Torque Thermal Model Belt Transmission Model Assumptions Model Calibration and Validation Friction Parameter Calibration Chapter 6: Engine Start Control Control Problem Definition Engine Starts Engine Stops Exponential Speed Profiles Objective Function Open Loop Engine Start Controller Closed Loop Engine Start Controller Tuned Closed Loop Controller ix

11 Various Engine Starts Summary Chapter 7: Conclusions and Future Work Work Done Future Work References x

12 List of Tables Table 1: Vehicle Technical Specifications... 6 Table 2: HV Component Voltage Ratings Table 3: HV component Power and Current Table 4: BAS System Coupling Table 5: Infineon Hybrid Kit Specifications Table 6: Instrumentation used for HV testing Table 7: PM100DX Specification Table 8: BAS System Specification Table 9: BAS System Calibration Parameter Table 10: Minimum values for optimization function Table 11: Optimal reference engine start profile parameters Table 12: Open Loop controller parameters Table 13: Closed loop controller parameters Table 14: Tuned closed loop controller parameters Table 15: Various engine starts - parameter evaluation xi

13 List of Figures Figure 1: US Energy Consumption in the Transport Sector ( ) [2]... 1 Figure 2: Average MPG in the US ( ) [3] [4]... 2 Figure 3: EcoCAR 3 Vehicle Development Process (VDP)... 4 Figure 4: Systems Engineering V diagram [5]... 4 Figure 5: OSU EcoCAR 3 Vehicle Architecture... 5 Figure 6: Conceptual visualization of a Series configuration [9] Figure 7: Conceptual visualization of Parallel configuration [9] Figure 8: FOC for an AC machine [13] Figure 9: 35mm 2 wire specification [15] Figure 10: HV Component Layout showing BAS system wire sizes Figure 11: Normalized Back EMF vs. speed Figure 12: Motoring region of operation [16] Figure 13: Engine drive pulley Figure 14: BAS Engine coupling Figure 15: Electronics Cooling Loop Figure 16: Infineon HybridKit Figure 17 Inverter LV calibration setup Figure 18: Inverter packaging detailing HV connections xii

14 Figure 19: HV Test Setup Figure 20: Packaged inverter fully integrated in-vehicle Figure 21: Control implemented for BAS inverter Figure 22: Controller performance at a steady state condition of 5Nm at 1300 rpm Figure 23: Controller performance during transient operation (5Nm, 1300 rpm to 10Nm, 1300 rpm) Figure 24: Higher Torque region of operation Figure 25: Analysis of inverter performance Figure 26: Rinehart PM100DX Figure 27: Experimental data torque-speed curve Figure 28: Steady state BAS testing at 30Nm, 3600 rpm Figure 29: BAS Peak region testing from 30Nm at 3000 rpm to 46Nm at 5500 rpm Figure 30: BAS system model overview Figure 31: BAS torque-speed curve Figure 32: Crank slider mechanism [18] Figure 33: Belt Transmission Figure 34: Comparison of BAS and Engine Speeds Figure 35: Engine start showing engine speed overshoot Figure 36: Engine stop showing engine speed undershoot Figure 37: Exponential engine start profiles Figure 38: Exponential engine stop profiles Figure 39: Variation of metrics for various time constants Figure 40: Cost vs. τ [ms] xiii

15 Figure 41: Optimal reference engine start profile Figure 42: Open loop controller response Figure 43: Closed loop controller structure Figure 44: Closed loop controller response Figure 45: Closed Loop controller tuned response Figure 46: Engine starts for various engine temperatures xiv

16 List of Acronyms AC Alternating Current AVTC Advanced Vehicle Technology Competition BAS Belted Alternator Starter CAN Controller Area Network DC Direct Current ECM Engine Control Module ESS Energy Storage System EV Electric Vehicle HEV Hybrid Electric Vehicle HSC Hybrid Supervisory Controller HV High Voltage ICE Internal Combustion Engine NVH Noise, Vibration and Harshness PHEV Plug-in Hybrid Electric Vehicle VTS Vehicle Technical Specifications xv

17 Chapter 1: Introduction 1.1. Energy Trends Analysis and Motivation The transportation sector is the second largest contributor to Green House Gas (GHG) emissions totaling about 27% of the total US emissions in The transportation sector accounted for 27.5% of the US energy use by sector in 2014 [1]. Figure 1: US Energy Consumption in the Transport Sector ( ) [2] 1

18 As shown in Figure 1, the energy used by the transportation sector has increased over the years and is projected to continue along a similar trend. The stringent regulations imposed on fuel economy and emissions have prompted automotive OEMs to re-engineer the powertrain to meet these regulations; which correlates to increasing MPG over the years as shown in Figure 2. Figure 2: Average MPG in the US ( ) [3] [4] The ever-increasing consumer demands, and stringent regulations on emissions and fuel consumption drove the automotive OEMs to re-engineer the drivetrain of the vehicle. This has ranged from engine downsizing, and innovative solutions that improve the efficiency of ICEs to various levels of drivetrain electrification/hybridization. The need for alternative 2

19 energy vehicles has been driven by the necessity to make a shift from petroleum to cap dependency on a finite resource and shift to alternative sources of energy for propulsion power. As an intervention, the U.S. DOE has over the years organized various series of AVTCs in collaboration with industry sponsors and ANL to train the next generation of automotive engineers. They are tasked with applying innovative solutions to re-engineer the vehicle powertrain with an end goal of meeting the ever-changing consumer demands and stringent regulations governing the automotive industry. The latest in this series of AVTCs is the EcoCAR 3 competition AVTCs & The EcoCAR 3 Competition EcoCAR 3 competition is the latest in the series of AVTCs that challenges 16 North American universities to re-engineer a 2016 Chevrolet Camaro into a performance hybrid electric vehicle. The competition is managed by ANL and the headline sponsors are General Motors and the U.S. DOE. The competition goals are: Increasing the fuel economy of the vehicle Minimizing GHG and criteria emissions Petroleum use reduction Maintaining and/or improving performance and consumer acceptability The EcoCAR 3 competition is a 4-year competition that implements the Vehicle Development Process (VDP) shown in Figure 3. 3

20 Figure 3: EcoCAR 3 Vehicle Development Process (VDP) To achieve the goals set forth by the competition, the OSU EcoCAR team employed a development process that followed both the EcoCAR VDP and a V diagram based systems engineering approach in Figure 4. The goal of this was to mirror standard VDP practices employed in the automotive industry. Figure 4: Systems Engineering V diagram [5] 4

21 1.3. Vehicle Architecture With the end goal of meeting the team defined Vehicle Technical Specifications (VTS) targets, the OSU EcoCAR team is developing a Parallel-Series Plug-in Hybrid Electric Vehicle (PHEV) with the architecture shown in Figure 5. Figure 5: OSU EcoCAR 3 Vehicle Architecture The OSU team vehicle architecture consists of a 119kW 2.0L GDI in-line 4 engine which is belt coupled via the engine accessory drive pulley to a 32kW electrical machine; colloquially referred to as the Belted Alternator Starter (BAS). The engine flywheel is then coupled to a 5- speed automated manual transmission via a clutch. The clutch is controlled by a CAN controlled hydraulic clutch actuator. The AMT is a manual transmission that is automated by the team to have better control over shift points and efficiency. The transmission output shaft connects to a team designed Power Transfer Unit (PTU) which is also connected to a 112kW 5

22 Rear Electric Machine (REM). The output of the PTU is connected to the rear wheels via a differential. The powertrain maintains the RWD aspect of the stock vehicle. The HV electrical system is powered by an 18.9 kwh Li-ion phosphate Energy Storage System (ESS) which supplies power to the BAS, REM, DC/DC converter and HV (Heating Ventilation and Air Conditioning) HVAC compressor module. The team also incorporated an onboard charger, a 3.3kW BRUSA NLG513 charger that offers plug in charging capability Vehicle Technical Specifications The vehicle architecture adopted by the team can meet and/or even exceed the performance VTS are targets and requirements that represent the energy consumption, performance, utility and emissions metrics of the team designed vehicle. Emissions and energy consumption targets in initial modeling were consistent with the needs of the target customers. Table 1 compares the competition VTS targets with the team developed VTS. Table 1: Vehicle Technical Specifications Specification Units Targets Team VTS Acceleration, IVM-60 mph sec Acceleration, mph (passing) sec Braking, 60-0 mph ft Acceleration Events Torque Split (Front/Rear) % RWD RWD Lateral Acceleration, 300 ft. Skid Pad G Highway 20 min, 60 mph % 6 6 Total Vehicle Range* mi N/A CD Mode Range* mi N/A CD Mode Total Energy Consumption* Wh/km N/A CS Mode Fuel Consumption* Wh/km N/A UF-Weighted Total Energy Consumption* mpgge UF-Weighted WTW Petroleum Energy Use* Wh PE/km

23 The BAS system which is the subject of this thesis plays an instrumental role in achieving the numerous VTS targets for example 0 60 mph acceleration, total vehicle range, CS mode total energy consumption, etc Objectives The objectives of this thesis are: To detail the methodology adopted and metrics evaluated in the design of the BAS system while highlighting the rationale for the design decisions made Evaluate the two inverter choices adopted by the team over Years 2 and 3 of the competition and detail the design and validation work done on each of the inverters Develop a model of the BAS system to account for dynamics that occur during engine starts/stops. This model shall be calibrated and utilized in Year 4 for developing an engine start/stop controller. Evaluate the performance of the BAS system with open loop and closed loop engine start control strategies and quantify metrics of performance and NVH Lay the groundwork for optimization of engine start/stop that will be implemented on the vehicle in Year 4 of the competition Thesis Overview This thesis discusses the design metrics evaluated to design a BAS system for the OSU EcoCAR 3 vehicle with a goal of meeting the VTS targets set forth by the team. It then details the two inverter choices adopted by the team. This is followed by developing a model for the 7

24 BAS system that will be used in Year 4 for engine start/stop validation and an evaluation of the current engine start controller implemented in the vehicle. The outline is as shown below: Chapter 2 reviews hybridization trends in the automotive industry with keen attention to alternator starter systems and start/ stop systems. It also evaluates different methods used to quantitatively evaluate drivability and NVH, vehicle criteria that are even more important to realize synergy with hybrid powertrains Chapter 3 discusses the design metrics considered and evaluated in the design of the BAS system for the EcoCAR vehicle. These include performance, electrical, mechanical and thermal aspects of the BAS system. Chapter 4 details the inverter choices adopted by the team over Years 2 and 3 of the competition. This involves inverter selection, controls development and validation, packaging and the pros and cons of the two inverter choices. Chapter 5 describes the modeling activities to develop a higher fidelity BAS system model capable of approximating dominant engine dynamics occurring during engine starts and stops. This will be instrumental in engine start/stop controller implementation on the vehicle in Year 4 of the competition. Chapter 6 describes the control problem definition regarding engine starts/stops and describes the development work done to achieve engine start functionality in Year 3. The performance of this controller is evaluated. Chapter 7 summarizes the work done throughout the thesis and draws important conclusions and setbacks faced while pursuing this thesis. It also lays the groundwork for future work regarding the BAS system 8

25 Chapter 2: Literature Review 2.1. Introduction This chapter provides an overview to drivetrain electrification and the various Hybrid Electric Vehicle architectures. Further detail is paid to starter/alternator systems in various levels of hybridization. Relevant literature regarding inverter control techniques and an overview of the metrics to be evaluated with regards to drive quality, engine start times and NVH is also covered 2.2. Drivetrain Electrification Owing to significant strides in battery research, electric machine design, power electronics and the stringent regulations on emissions and fuel consumption, automotive OEMs have explored production of electric vehicles (EVs) which has led to increased production and registration of electric and hybrid cars [6]. EVs have an onboard battery pack, one or more electric machines and no ICE which implies that they are characterized by zero tailpipe emissions and high efficiency due to the high efficiencies of battery packs and electric machines relative to ICEs. Despite the increase in EVs over the years and associated advantages, there are still some factors hindering their wide adoption; high cost of batteries and electric machines hence high initial vehicle cost, low energy density of batteries relative to fuel used in ICEs, low total vehicle range, battery degradation, under developed charging network grid and long charging times. Some automotive OEMs are looking to break this 9

26 barrier e.g. Tesla through extensive research and lowering costs of production but on a general scale, wide adoption of EVs is still a few years away. A compromise that leverages the advantages of both EVs and conventional vehicles is a more viable option that is being adopted by automotive OEMS, hence drivetrain electrification. Drivetrain electrification refers to adding electronic propulsion components (batteries, electric machines, power electronics, ultra-capacitors, etc.) to the powertrain and/or driveline in a bid to leverage the advantages of electrified powertrain while maintaining core aspects of a conventional drivetrain. Per many automotive manufacturers, it will be impossible to achieve emission and fuel consumption targets without drivetrain electrification [7]. Drivetrain electrification ranges from start-stop technology, electric machines/modules used in various configurations as will be discussed later in this chapter, electric axles, in wheel motors, electric wheel drive systems, etc. [7]. Hybrid propulsion has emerged as the most viable option of drivetrain electrification and involves combining electric drive units with ICEs in a bid to improve fuel consumption and lower GHG and criteria emissions. Hybrid vehicles are already in production and have shown savings of up to 25% in fuel [8] Hybrid Electric Vehicles (HEVs) A HEV typically has 2 types of energy sources that are used to propel the vehicle; electricity and fuel. The fuel can be any type of fossil fuel (gasoline, diesel, ethanol, E85, E10, B10, etc.) stored in a fuel tank. The electricity is stored in a battery or sometimes both a battery and 10

27 ultra-capacitors. The most common energy form used for storing electrical energy is a battery pack and will be referred to as an Energy Storage System (ESS) from hereon. In HEVs, an ICE works together with one or more electric motors to provide propulsion force. Since there is at least one more source of propulsion force other than the engine, the engines in HEVs are typically downsized in a bid to improve overall vehicle fuel consumption while still having the capability of meeting the driver s torque request. HEVs are operated in charge sustaining mode with the goal of ensuring that the battery remains charged and thus don t require to be plugged in to charge the battery. The electric motors have the capability of recuperating some of the kinetic energy of the vehicle that would otherwise be lost through heat and convert this into electrical energy that is stored in the ESS; a phenomenon known as regenerative braking. The high efficiency of the electrical system, regenerative braking capability, engine downsizing and extra degree(s) of freedom of torque production make HEVs more appealing to automotive OEMs. On the downside, HEVs are more expensive than conventional vehicles due to use of a HV ESS and one or more electric motors [9] [10]. Some examples of HEVs currently on the market are the 2017 Buick Lacrosse HEV, Toyota Prius, and Ford Fusion Hybrid Plugin Hybrid Electric Vehicles (PHEVs) A PHEV is a HEV with a larger capacity battery pack that can be plugged into the electrical grid for charging. The larger capacity battery pack implies that PHEVs have all-electric range, a feature that HEVs lack. PHEVs can be operated in both Charge Depleting (CD) and Charge Sustaining (CS) modes. PHEVs can be operated first as EVs until the electric range is 11

28 exhausted and then the ICE can be used to propel the vehicle while sustaining the state of charge of the ESS. Another implementation can be to operate the vehicle as an HEV with a higher cost penalty imposed on using the ICE in a bid to use the more efficient electric powertrain to meet the driver s torque request as opposed to using the engine. PHEVs combine the all-electric range of EVs and the better fuel economy and lower emissions of HEVs. Due to having a larger capacity ESS, PHEVs are more expensive than HEVs and are hugely affected by issues of long charging times which poses a problem to their wide adoption by the public. Some examples of PHEVs currently on the market are the Chevrolet Volt, Audi A3 e- Tron, Ford C-Max Energi and the Toyota Prius Prime HEV & PHEV Configurations HEVs and PHEVs can be broadly categorized into three major configurations i.e. Series, Parallel and Parallel Series configurations which are explained in the following section. Series Configuration In a series architecture, the ICE is coupled to a generator and the ICE isn t directly coupled to the wheels. Traction force is provided by one or more electric motors that are coupled to the wheels. A conceptual visualization of a series architecture is shown in Figure 6. The advantages of this architecture are its ease of implementation from a packaging and controls perspective. The high efficiency of the electric motors across its entire torque-speed region dictate that a single speed or 2 speed gearbox can be implemented. Since the engine is completely decoupled form the wheels, the ICE can be operated in its most efficient region ensuring low fuel consumption. The disadvantage of a series powertrain architecture are the 12

29 high costs associated since the vehicle should have at least 2 electric motors with the traction motor largely sized to meet all the driver torque requests. Series powertrains are usually implemented in large vehicles that have enough space for the engine/generator set [10]. Figure 6: Conceptual visualization of a Series configuration [9] Parallel Configuration In a parallel configuration, the engine and electric motor can both supply torque to the wheels. A conceptual visualization of a parallel configuration is shown in Figure 7 with the electric motor located post transmission. Another parallel configuration exists where the electric motor can be located pre-transmission. With this configuration, the complexity of packaging and control increases but if implemented correctly, the advantages of this configuration can 13

30 outweigh the cons of having a series configuration. It is characterized by less energy losses since chemical energy from the engine is converted directly to mechanical energy. Due to compactness of this configuration, this has been adopted in passenger cars like the Honda Insight and Ford Escape [10]. Figure 7: Conceptual visualization of Parallel configuration [9] Parallel-Series Configuration This leverages the advantages of both the series and parallel configurations. It is also known as a multi-mode or power-spit configuration because it can perform as a series configuration when the engine is decoupled from the wheels through a planetary gearset or other mechanical coupling and ability to perform as a parallel configuration when the engine is mechanically coupled to the wheels. This configuration adds extra complexity in implementation and 14

31 control and extra cost compared to a parallel configuration. However, the benefits of this architecture have seen it prevail as the most prominent HEV architecture implemented in most production hybrids. The Toyota Prius employs this configuration [10]. The OSU EcoCAR vehicle architecture is also a parallel-series configuration Degrees of Hybridization PHEVs and HEVs can also be categorized according to the balance between electric and ICE power provided to the wheels i.e. Micro, Mild, or Full Hybrids. Micro Hybrids These typically utilizes start-stop technology and includes a 36V - 48V battery to power on board electrical systems. The motors in micro hybrids are typically not used to drive the wheels but instead reduce the burden on the ICE. Examples include the Chevrolet Malibu with stop-start and Mazda s i-eloop system. [11] Mild Hybrids Mild hybrids comprise of small sized HV battery packs capable of powering a motor that supplies torque to the wheels. The motor alone is usually not capable of providing all the propulsion torque and hence the vehicle is operated in a charge sustaining strategy. Engine start-stop technology can be implemented. Since the engine is on most of the time, mild hybrids don t return significantly greater improvements to EPA rated fuel consumption. The higher voltage batteries ensure less current, smaller size wire and less losses hence higher power motors can be used. Mild hybrids leverage the advantages of the higher voltage levels 15

32 while eliminating high costs associated with larger batteries and motors. Examples of vehicles with Mild hybrid systems are Honda CR-Z equipped with Honda Integrated Motor Assist (IMA) and the Chevrolet Malibu with eassist. Full Hybrid Systems These have large battery packs and motors capable of providing traction force. These tend to have an all-electric range and then transition to a charge sustaining strategy to maintain the charge on the battery between given set-points while relying largely on the ICE to meet the driver s torque request in CS mode. Examples of these include the Chevrolet Volt and the Prius Inverter Control The basis of AC motor control is Field Oriented Control (FOC) where the stator currents of an AC machine are converted into a time invariant orthogonal axis. This is based on reference frame theory which transforms a 3-phase time and speed based system (a, b, c reference frame) into a 2-coordinate time invariant system (d, q reference frame) [12]. This transformation transforms the complex AC machine control into a simpler form similar to that of DC machine control. This is achieved through Park and Clarke transformations [12]. The two input control parameters then become the torque component (which is aligned with the q axis coordinate) and the flux component (which is aligned with the d component). Figure 8 shows the general structure of a FOC controller which requires two reference inputs I ( and I ). 16

33 Figure 8: FOC for an AC machine [13] 2.4. Drive Quality and NVH According to [14], drive quality is a comprehensive term that encompasses vehicle responsiveness, and driving comfort. With regards to engine start/stop, drive quality can be quantified in terms of how fast the engine starts up and the frequency of engine starts and stops. The operating smoothness with regards to engine start/stop relates to NVH that may be experienced by a driver during a start and stop event. 17

34 Engine Start/Stop Times Despite the advantages presented by engine start/stop of reducing engine idle time. There are drawbacks to engine start/stop if not controlled appropriately. Frequent engine starts/stops can lead to more fuel consumption and worse emissions especially during cold start operation. Frequent engine starts can also pose a driveability concern and the number of engine starts/stops needs to be controlled. It is desirable to keep engine start/stops to a minimum to maintain a compromise between fuel consumption improvements and driveability concerns Jerk Jerk is the derivative of acceleration and represents changes in acceleration or deceleration in m/s + [14]. The most important quantity of measurement is the maximum Jerk as this is the maximum rate of change of acceleration that the driver is likely to experinece Root Mean Square Acceleration This is an objective measure of the average acceleration that occurs during a defined time period. When carried out and normalized over an 8 hour period of time, it is referred to as the A(8) method [14]. The root mean square acceleration is expressed by equation (1) a -./ = : <=>?@ a 1 dt 1 : =>=A=?@ (t t 565:578 ) The acceleration is filtered using a bandpass filter for the bandwidth ranging from 1 to 32 Hz since this is the frequency range for perception of vibration by humans [14] 18

35 Vibration Dose Value (VDV) Vibration Dose Value is a qualitative way of determining the vibration felt by being in contact with a vibrating surface over a defined period time. The VDV is defined by equation (2) VDV = G : <=>?@ a F t dt : =>=A=?@ 2 Similarly, the acceleration is filtered in the range of 1-32 Hz. 19

36 Chapter 3: System Design Considerations 3.1. Introduction This chapter details the various metrics considered while designing and integrating the BAS system. These range from electrical, performance, mechanical and thermal considerations Electrical System Evaluation This section details the various metrics evaluated regarding the electrical system. This involves the voltage ranges of the components, the wire and fuse sizing for the HV system with keen attention to the BAS system, continuous and peak power and current draws of the components, and back EMF considerations Voltage Range The initial choice was the voltage level adopted for the starter/alternator. A preliminary analysis of the pros and cons of the 12V, 36V - 48V and full hybrid starter/alternator systems revealed that the full hybrid system had the highest power output with a drawback of high cost. Since the team was developing a PHEV with an 18.9kWh battery pack available to the team, a HV starter/alternator was chosen over the other configurations. 20

37 The next step was ensuring that the voltage ranges of the selected components matched without having under-voltage and/or over-voltage fault out conditions occurring on any of the components. Table 2 shows the BAS system HV component voltage ratings. Table 2: HV Component Voltage Ratings HV Component Min. Voltage Max. Voltage Nominal Voltage ESS BAS As seen in Table 2, the BAS voltages are well within the minimum and maximum voltages of the ESS. The bottleneck in the system is the ESS which with HV contactors closed supplies a voltage ranging from VDC. This ensures that no under-voltage or over-voltage faults will be tripped on the system Electrical Power and Current The HV system was designed to ensure that the ESS can support continuous and peak operation of the BAS and the HV components. Table 3 outlines the continuous and peak power and currents for all the HV components integrated in the vehicle. Table 3: HV component Power and Current HV Nominal Peak Power Cont. Current Peak Current Component Power kw] [kw] ESS -21 Chg. / Chg. / Adc 612 Adc Dis. Dis. BAS Arms 150 Arms REM Arms 550 Arms REM inverter Arms 550 Arms DC/DC A 10A HVAC A 30A Charger A 9A 21

38 The major HV loads on the system are the BAS and the REM system. At peak conditions, assumed to be a 0 60 acceleration where the peak power is requested from both components, the BAS has a current draw of 120 Adc and the REM has a current draw of 350 Adc from simulation data. This is still lower than the maximum current that the ESS can provide thus this component sizing ensures that the ESS can support peak component functionality of the BAS. The peak power of the HV components is 160 kw which is still lower than the peak power that the ESS can support. It is worth mentioning that the ESS has discharge and charge buffers that determine how much current can be drawn from the ESS without tripping a fault and opening HV contactors. These buffers are monitored by the HSC and the component performance can be derated to ensure that the discharge and charge current limits set by the ESS are respected Wire and Fuse Sizing The wire size was selected based on continuous and peak currents expected from the BAS based on simulation data. 35 mm 2 wire was the chosen wire gauge as it can handle the peak currents on the BAS under a myriad of temperature conditions as shown in Figure 9. The selected 35 mm 2 wire is rated to handle 250Adc at 140 C. The wire is also rated to handle the voltage since it is rated up to a maximum of 600/900 V AC/DC which is well within the voltage limits expected on the BAS. 22

39 Figure 9: 35mm 2 wire specification [15] The BAS system is fused together with all components in the front HV junction box to ensure that both the BAS inverter and the wires from the rear HV junction box to the front junction box are fused. Figure 10 shows the layout of the HV components and a fuse size of 150A was selected for the BAS system to cater for peak loading conditions while avoiding nuisance fuse blows. 23

40 Figure 10: HV Component Layout showing BAS system wire sizes Back EMF Considerations The chosen electric machine is an interior permanent magnet (IPM) machine which is subject to back EMF since the permanent magnets induce a voltage in the windings as the magnets rotate. The maximum back EMF was determined to be 364 V /5.O V PQ from simulation data thus the chosen inverter should have IGBTs capable of handling this voltage in the event of a fault condition. Figure 11 shows the normalized back EMF of the BAS as a function of temperature and BAS speed. To ensure safety, the BAS inverter is not enabled unless the HV contactors have been closed thereby ensuring that the voltage on the HV DC bus is dependent on ESS voltage as opposed to the voltage that might be induced on the HV bus from spinning the BAS. Since the BAS is always coupled to the engine, the HV contactors are always closed when the engine is running to provide a voltage to counter the back EMF that would otherwise be present at the DC terminals of the inverter. 24

41 Figure 11: Normalized Back EMF vs. speed 3.3. Machine Performance Characteristics The torque-speed characteristics of the chosen electric machine are critical based on the application in which it is deployed. For engine start functionality, an electric machine with a high peak torque at low speeds is desirable to provide high torque for starting the engine. This is particularly paramount in cold cranking events and in an effort of achieving fast engine startup times. Figure 12 shows the regions of interest where region A is primarily for engine start and motoring while region B is used for launch assist and speed matching [16]. 25

42 Figure 12: Motoring region of operation [16] The chosen BAS has a maximum torque of 60Nm and a rated speed of 4000 rpm. This ensures that the BAS can crank the engine and provide torque to shift loading points for the engine. To further multiply the torque and match BAS and engine speeds, a gear multiplication was adopted which is discussed in the following section Mechanical Integration This highlights the considerations evaluated for the choice of the coupling that was implemented between the engine and the BAS ISG vs. BAS To couple the electric machine, there are various implementations that can be adopted. The most prominent are belt coupling the electric machine to the engine drive pulley hence a BAS 26

43 or coupling the electric machine between the engine flywheel and the transmission hence an Integrated Starter Generator (ISG). If considered in initial vehicle design, the ISG presents better packaging and space utilization and gets rid of belts that are subject to multiple dynamics, lower efficiencies and belt wear over time. The OSU vehicle however is based off a stock 2016 Camaro that wasn t designed to have an ISG. Fitting an ISG would necessitate multiple modifications to the engine, engine cradle and transmission to accommodate the electric machine in this location. This coupled with the fact that the EcoCAR competition has strict guidelines regarding modification to the vehicle chassis drove the team to opt for a belt coupling. The BAS option was adopted and modifications only needed to be made to the engine drive pulley. This was also boosted by the fact that there are no other accessories coupled to the engine drive pulley. The team adopted a HV DC/DC converter to take the place of a conventional alternator and a High Voltage Air Conditioning (HVAC) compressor. The vehicle features electric water pumps and power steering thus the only component coupled to the engine drive pulley is the BAS Belt Coupling The belt coupling adopted for the BAS system involved making modifications to both the engine and BAS pulleys. To match the BAS maximum operational speed to the engine maximum speed, a gear ratio was adopted that would ensure that both components would not over-speed. Table 4 shows the mapping of the engine and BAS to determine the appropriate gearing. 27

44 Table 4: BAS System Coupling Parameter Value Engine redline 6500 rpm BAS Max. Speed rpm Desired Gear Ratio Chosen Gear Ratio Based on availability of components, a gear ratio of was adopted. This comprised of using Helical Offset Tooth sprockets that were machined and fitted onto the engine and BAS. These sprockets were chosen because of their high efficiency, minimal slip, high power carrying capabilities and ease of matching belts based off the chosen sprockets. Figure 13: Engine drive pulley The engine pulley is a 60-tooth sprocket that is shrink fit onto the engine accessory pulley as shown in Figure 13. The BAS pulley is a 22-tooth sprocket that is locked and keyed to the BAS output shaft. 28

45 The BAS is rigidly mounted to the engine and a passive tensioning mechanism is adopted using a turn buckle to adjust the belt tension as desired. Figure 14 shows the coupling of the BAS and the engine. Figure 14: BAS Engine coupling 3.5. Thermal Considerations The BAS is a high power machine packaged in a form factor like that of a conventional alternator. With this high-power output in such a small form factor, the BAS has a small thermal mass. This implies that the BAS heats up much faster than an equally powered motor of larger size. It was expected that the BAS would heat up significantly during constant operation; the issue becoming more apparent during 0 60 acceleration runs where it is run at peak power or during series operation. The electronics coolant loop had to be designed with the capability of rejecting heat produced by the BAS at peak operating conditions. Due to the 29

46 small thermal mass, the BAS is placed closest to the radiator in the electronics cooling loop as shown in Figure 15. The BAS is rated for a maximum temperature of 150 C, the HSC code is structured to monitor the BAS temperature and derate performance of the BAS above temperatures of 120 C. Figure 15: Electronics Cooling Loop 30

47 Chapter 4: Inverter Evaluation and Validation The BAS is a sponsor donated component that wasn t available with an inverter solution. To that end, a custom inverter solution had to be developed by the team to achieve BAS functionality over the entire region of operation. Two inverter choices were evaluated by the team over Years 2 and 3 of the competition. This section details the two inverter choices that were adopted by the team and the pros and cons of each. It also shows the testing and validation work that was done to achieve BAS functionality with both inverters Infineon HybridKit 2 The Infineon Hybrid Kit 2 was chosen based on its specifications detailed in Table 5 and its modular structure. This inverter is a generic prototype kit developed by Infineon Technologies for rapid prototyping purposes of inverter control. Table 5: Infineon Hybrid Kit Specifications Parameter Specification Power 80 kw Peak Voltage 650V Motor Position Interface Encoder, Resolver, Hall sensor DC bus capacitance 500 μf Maximum Temperature 150 degrees C Cooling Liquid cooling 31

48 The inverter is comprised of the logic board, gate driver board, IGBT pack and heat sink. This modular construction of the inverter was ideal for making changes to the logic board while maintaining the other aspects of the inverter. The inverter shown in Figure 16 also includes a heat sink connected to the IGBT for liquid cooling. Figure 16: Infineon HybridKit 2 To systematically achieve BAS functionality, development phases were defined to achieve goals for Year 2. The development phases had corresponding test environments to validate system functionality. The following steps were performed to test the inverter under a myriad of conditions: - LV setup of the inverter and BAS - HV setup of the BAS & inverter under no load conditions 32

49 - HV setup of BAS & inverter with BAS coupled to the dynamometer LV System Validation Before the inverter could be supplied with high voltage to control the BAS, the inverter software had to be restructured to work with the motor and calibrated to work for motor specific parameters like motor pole pairs, motor torque constant, D and Q axis inductances, resolver pole pairs and CAN transceiver configuration. CAN Configuration and Software Modifications The CAN controller on the logic board was configured to run at a baud rate of 500 kbps and a CAN database file created to cater for the fix point number format used in the inverter. Figure 17 shows the setup used for LV calibration and flashing of the inverter. All changes to the inverter control software were made using Tasking VX Toolset and flashing the inverter was done using Infineon s MemTool. Diagnostics and real time parameter monitoring were done through a serial terminal and a serial based GUI. 33

50 Figure 17 Inverter LV calibration setup Analog inputs for the current sensors and thermistors were configured to accurately read currents and motor temperature. Circuitry Modifications The circuitry of the inverter was designed for compatibility with a resolver with a transformation ratio of 1. Since the resolver in the BAS has a transformation ratio of 0.2, the resolver excitation buffer circuitry on the inverter was adjusted to produce a gain of magnitude 5. This was achieved using three, 5 kω trim potentiometers soldered onto the logic board of the inverter and tuned to achieve the desired amplifier gain. A dual channel oscilloscope was used to verify the excitation, cosine and sine differential signals at the inverter and ensure that they were within the valid ranges for the inverter s resolver to digital converter; AD2S1200. Once the LV circuitry had been validated the next step was to validate the HV system 34

51 HV System Validation Circuitry Modifications The major circuitry modifications on the HV system were related to appropriate sizing of bus bars and integration of current transducers to be used in control of the motor. The current transducers and bus bars were chosen based on the maximum current expected in the BAS (approximately 150 A -./ ). Appropriate insulation was adopted using appropriately rated stands and insulating material to ensure safe integration. Figure 18 shows the final inverter packaging showing both LV and HV connections. Figure 18: Inverter packaging detailing HV connections 35

52 HV Testing HV tests were carried out using the setup shown in Figure 19. Figure 19: HV Test Setup The goal of the HV setup was to validate the inverter startup/shutdown process, motor enabling sequence, validate torque production, evaluate system efficiencies and validate the thermal performance. To achieve this, the instrumentation listed in Table 6 was used Table 6: Instrumentation used for HV testing Component Dynamometer Measured variables Speed, torque, mechanical power Inverter Speed, Torque, temperatures, Currents, I (, I ), V )W Current transducer I )W Belt coupling - dspace MABx HSC used to control the inverter Thermistor Motor temperature AV900 I )W, V )W & power 36

53 The tests carried out on the system and the results are detailed in the inverter control section Inverter Enclosure Design To package the inverter for safe integration into the vehicle, an enclosure had to be designed for the vehicle. This included designing for coolant connections, HV and LV electrical interface connections, servicing while meeting space claim constraints within the vehicle. Figure 20 shows the integrated inverter inside the vehicle Figure 20: Packaged inverter fully integrated in-vehicle Inverter Control A controller based on Field Oriented Control was implemented. The inverter was operated in Torque control mode. To achieve speed control of the BAS, a speed-based PI feedback 37

54 controller was wrapped around the Torque request. Control of the inverter is done over CAN with the BAS providing motor speed, position and temperature information to the inverter via resolver and thermistor connections. The torque control input parameters are the I ( and I ) current requests. Current map based lookup tables were implemented in the HSC to convert torque requests based on BAS speed to the appropriate I ( and I ) requests. The structure of the FOC implemented in the BAS is as shown in Figure 21 Figure 21: Control implemented for BAS inverter With the control structure above, the BAS was run in motoring operation to validate torque production under steady state and transient conditions. Steady State Conditions Torque production was validated at steady state conditions at various speeds and feedback I ( and I ) were compared against current maps to validate the performance of the inverter control. 38

55 Figure 22 shows the performance of the system at a steady state condition of 5 Nm, 1300 rpm. The commanded currents closely match the feedback currents which validates the inverter performance within this region. Figure 22: Controller performance at a steady state condition of 5Nm at 1300 rpm Transient Conditions Torque production was validated during transients to validate how the current controllers in the inverter functioned under transient conditions and feedback I ( and I ) were compared against current maps to validate the performance of the controller. Figure 23 shows the 39

56 performance of the system during transient from 5 Nm to 10 Nm at a speed of 1300 rpm. The commanded currents closely match the feedback currents which validates the inverter performance within this region Figure 23: Controller performance during transient operation (5Nm, 1300 rpm to 10Nm, 1300 rpm) Higher Region of Operation To realize faster engine startup times, a high torque output is desired so validation of the controller in the higher torque region of operation was explored. Figure 24 shows the response of the system in the higher torque region of operation at a torque of 20Nm. The current 40

57 controllers were characteristic of multiple ripples to meet the desired torque request. There were significant deviations between the commanded and feedback currents Figure 24: Higher Torque region of operation Analysis As shown in Figure 25, performance of the system in the continuous region of operation at low speeds validated the controller. However, the current controllers were not able to meet the required torque in the higher torque region of operation at the required currents. This 41

58 implied lower system efficiencies and unstable current controllers which posed risk of potential to the inverter IGBTs and gate driver board. Figure 25: Analysis of inverter performance The implication of this is that the BAS was validated in a limited region of operation i.e. up to 18Nm and 3000 rpm. To be able to meet Year 3 goals of full BAS system functionality, another inverter was evaluated which is detailed in the next section Rinehart PM100DX Unlike the Infineon HybridKit 2 which required extensive controls development, packaging and validation, a prepackaged inverter solution was adopted that required only controls 42

59 developments and calibration in collaboration with a third-party supplier. Figure 26 shows the second inverter that was adopted for BAS control. Figure 26: Rinehart PM100DX The technical specifications of the inverter shown in Table 7 ensured that the inverter can be used to control the BAS. Table 7: PM100DX Specification Parameter Power Peak Voltage Motor Position Interface DC bus capacitance Maximum Temperature Cooling Specification 100 kw 400V, 400V (over voltage) Encoder, Resolver, Hall sensor 440 μf 95 degrees C Liquid cooling 43