Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Modeling of advanced heat pump cycles and aerodynamic design of a small-scale centrifugal compressor for electric vehicles(2023) mei, zhenyuan; Radermacher, Reinhard; Hwang, Yunho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unlike conventional vehicles powered by internal combustion engines, electric vehicles do not have enough waste heat to provide sufficient heating to the cabin. Thus, an additional heating system, such as a heat pump, is needed. However, its performance decreases significantly when the ambient temperature is low. The new kangaroo heat pump cycle (KC) is proposed to increase the heating capacity in low-temperature climates. It is an enhanced flash tank-based vapor injection heat pump cycle (FT-VIC). A sub-cycle is added to the system to increase the refrigerant inlet quality entering the flash tank, which leads to a higher refrigerant mass flow rate and heating capacity. Because KC has a higher heating capacity, the heating needed from the electric heater can be reduced, thus reducing energy consumption and increasing the driving distance. In this study, thermodynamic models were developed for the basic heat pump cycle (BC), FT-VIC, and KC. And a new method evaluating the life cycle climate performance (LCCP) of electric vehicle heat pumps based on the SAE J2766 standard was proposed. Results show that KC is effective at low ambient temperatures. At -15°C, KC can save 13.8% energy compared to BC, and save 2.7% energy compared to FT-VIC. However, due to the additional weight, KC has a higher LCCP than other cycles. If the pressure ratio limit is removed and the compressor efficiencies are constant, KC can have a lower LCCP than other cycles in cold climates. Transient models were also developed to assess their performance in urban driving conditions. Results show that at the end of the simulation, the cabin room temperature of KC is 3.6°C and 7.0°C higher than that of FT-VIC and BC, respectively. However, due to the high pressure ratio and refrigerant mass flow rate, the accumulated power consumption of KC is 32.4% higher than FT-VIC and 64.4% higher than BC. Despite its high energy consumption, it is more efficient than adding heat from an electric heater. In addition, a small-scale centrifugal compressor was designed for an electric vehicle to reduce the compressor’s size and weight. Results show that its COP is 6.6% higher than that of the scroll compressor at the design point. However, its efficiency at the off-design point quickly drops. Future studies are needed to improve its off-design point performance.Item Distributed Control for Formula SAE-Type Electric Vehicle(2022) Falco, Samantha Rose; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The recent trend in transportation electrification creates an enormous increase in demand for electric vehicles (EVs). Increasingly, electric cars have novel features like autonomous driving and fault tolerance, all of which require additional hardware and computation power. Changes to the electronic control unit (ECU) structure will be needed to make these advances scalable. This thesis examines the driving economic, technical, and societal factors behind needed changes to the existing control structures. It proposes a control platform design to address issues of complexity and scalability. A generic, modular control board structure using the TMS320F2837xS digital signal processor (DSP) is described with several input/output functionalities including a wide range of analog inputs, multiple logic levels for digital pins, CAN communication, and wireless communication capabilities. A distributed control network is built by interconnecting multiple implementations of the control board, each of which has distinct responsibilities dictated by software instead of hardware. A prototype electric vehicle control structure for a Formula SAE electric vehicle was built utilizing a network of three control boards and tested to prove the viability of the proposed concept. Results of these tests and future steps for the project are discussed.Item High-Frequency Bidirectional DC-DC Converters for Electric Vehicle Applications(2018) He, Peiwen; Khaligh, Alireza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As a part of an electric vehicle (EV) onboard charger, a highly efficient, highly compact, lightweight and isolated DC-DC converter is required to enable battery charging through voltage/current regulation. In addition, a bidirectional on-board charger requires the DC-DC converter to achieve bidirectional power flow: grid-to-vehicle (G2V) and vehicle-to-grid (V2G). In this work, performance characteristics of two popular DC-DC topologies, CLLC and dual active bridge (DAB), are analyzed and compared for EV charging applications. The CLLC topology is selected due to its wide gain range, soft-switching capability over the full load range, and potential for a smaller and more compact size. This dissertation outlines the feasibility, analyses, and performance of a CLLC converter investigated and designed to operate at 1 MHz and 3.3 kW for EV onboard chargers. The proposed design utilizes the emerging wide bandgap (WBG) gallium nitride (GaN) based MOSFETs to enable high-frequency switching without sacrificing the conversion efficiency. One of the major challenges in MHz-level power converter design is to reduce the parasitic components of printed circuit boards (PCBs), which can cause faulty triggering of switches leading to circuit failure. An innovative gate driver is designed and optimized to minimize the effect of parasitic components, which includes a +6/-3 V driving logic enhancing the noise immunity of the system. Another challenge is the efficient design of magnetic components, which requires minimizing the impacts of skin and proximity effects on the transformer winding to reduce the conduction loss at high frequencies. A novel MHz-level planar transformer with adjustable leakage inductance is modeled, designed, and developed for the proposed converter. A comprehensive system level power loss analysis is completed and confirmed with the help of experimental results. This is the first prototype of a 3.3 kW power bidirectional CLLC converter operating at 1 MHz operating frequency with 400-450 V input voltage range, 250-420 V output voltage range. The experiment results have successfully validated the feasibility of the proposed converter conforming to the analysis carried out during the design phase. With an appropriate design of driving circuit and control signal, the prototype achieves a peak efficiency of 97.2% with 9.22 W/cm3 (151.1 W/in3) power density which is twice more power dense than other state-of-the-art isolated DC-DC converters.Item Institutional Logics, Collective Actions and Development of New Technologies(2011) Jin, Byungchae; Kirsch, David A; Business and Management: Management & Organization; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Technology development is an outcome of collective social processes among actors in different institutional fields. In the literature on technology development, there have been long debates regarding whether technology shapes social structure and order, or whether social forces determine the developmental trajectories of technology. From a series of studies, I seek to understand the social dynamics of technology development in order to address theoretical tensions, both theoretically and empirically. Three separate yet related studies together provide a theoretical model and relevant empirical evidence for the linkages among actors, institutional logics and technologies. In Chapter 1 I first attempt to theorize about how actors, including scientists, engineers and technology users, collectively shape technological evolution in the general technology context. Combining the two perspectives--institutional logics and collective actions, I develop a theoretical model that addresses how scientists and engineers, faced with multiple institutional logics, strategically respond to the multiple institutional logics, and how the different formation of institutional logics can systematically lead to different types of technology development. In the theoretical model, I discuss four distinctive social mechanisms of framing institutional logics--replacing, patching, sequencing, and reinforcing, and the relationships between the social mechanisms and the types of technology development. In Chapter 2, building upon the theoretical model proposed in Chapter 1, I empirically investigate the emergence and decline of electric and hybrid drives in the community of electric vehicle researchers from 1969-2009. Combining the perspectives of institutional logics and social movements, I argue that an institutional logic is a product of collective social processes among actors in different institutional fields, and that established logics play an integral role in shaping the differential development of new technology. Empirical findings suggest that environmental protests and economic recessions systematically influence technologists' incorporation of two institutional logics (environmentalism vs. industrialism), and that social cohesion among actors within each institutional logic tends to shape differential developmental trajectories of electric and hybrid drives in the community of electric vehicle researchers. In Chapter 3 I further explore the process through which actors respond to multiple and conflicting institutional logics, suggesting that actors can purposefully create new concepts and meanings, modify meanings of institutional logics, or reinforce existing meanings. While existing institutional work has suggested and empirically demonstrated that institutional logics shape cognitive and behavioral patterns of actors, it still remains unanswered as to how actors can mobilize existing and new logics--differential decoupling processes. To trace the processes of constructing meanings of institutional logics, I conducted an inductive study by employing keyword-based, computer-aided text analysis of research proceedings published by the international Electric Vehicle Symposium in 1969 and in 1994. From the analysis, I identify four social mechanisms of logic construction: clarifying, patching, expanding and reinforcing. Moreover, empirical findings suggest that social mechanisms of patching, expanding and reinforcing are closely related to the emergence of hybrid drive.