Anna G. Stefanopoulou

University of Michigan | U-M · Department of Mechanical Engineering

Voltage-based battery metrics are ubiquitous and essential in battery manufacturing diagnostics. They enable electrochemical “fingerprinting” of batteries at the end of the manufacturing line and are naturally scalable, since voltage data is already collected as part of the formation process which is the last step in battery manufacturing. Yet, des...

Lithium-ion batteries degrade over time, based on their usage and these changes can be observed in the terminal voltage and cell expansion curves with respect to SOC. Physics-based battery models augmented with degradation modes can describe the changes in battery electrical and mechanical response as the battery ages and can be tuned to predict th...

Lithium-ion batteries can last many years but sometimes exhibit rapid, nonlinear degradation that severely limits battery lifetime. Here, we review prior work on “knees” in lithium-ion battery aging trajectories. We first review definitions for knees and three classes of “internal state trajectories” (termed snowball, hidden, and threshold trajecto...

Increasing the speed of battery formation can significantly lower lithium-ion battery manufacturing costs. However, adopting faster formation protocols in practical manufacturing settings is challenging due to a lack of inexpensive, rapid diagnostic signals that can inform possible impacts to long-term battery lifetime. This work identifies the cel...

In recent years, as engine control strategies have grown increasingly sophisticated in a continued drive for increasing efficiency and reducing emissions, engine operation has been pushed into regimes that are limited by combustion instabilities. These instabilities produce undesirable abnormal combustion events, which pose barriers to further impr...

We propose algorithms to speed up physics-based battery lifetime simulations by one to two orders of magnitude compared to the state-of-the-art. First, we propose a reformulation of the Single Particle Model with side reactions to remove algebraic equations and hence reduce stiffness, with 3x speed-up in simulation time (intra-cycle reformulation)....

Diesel engines equipped with ignition assist devices such as glow plugs may improve combustion behavior at low temperatures and with low cetane fuels found in remote fields. The coordination of injection timing and the energy input of the ignition assist needs to continuously adjust to maintain the best combustion phasing at all conditions. However...

Repurposing automotive batteries to second-use battery energy storage systems (2-BESS) may have environmental and economic benefits. The challenge with second-use batteries is the uncertainty and diversity of the expected packs in terms of their chemistry, capacity and remaining useful life. This paper introduces a new strategy to optimize 2-BESS p...

While electric vehicle battery packs are designed to address safety issues, battery failures can still occur. Most prominently, electric vehicle fires can occur after accidents or from internal shorts due to defects, abuse, or accelerated aging. Battery fires can be difficult to manage due to the active lithium materials and the presence of both th...

The formation process is the last step in the lithium ion battery manufacturing process but plays an outsize role in determining cell costs [1], [2] . A typical formation protocol consists of multiple low-rate charge-discharge cycles which can take many days to complete. During formation, the solid-electrolyte interface (SEI) is created through irr...

Li-ion battery thermal runaway is a critical safety issue for large-scale battery packs. Upon battery failure, gas venting can occur when the cell's internal pressure exceeds a critical value and causes cell rupture and electrolyte leakage. The gas venting is usually categorized into first venting events and thermal runaway gas venting events (seco...

The rapid growth of electric vehicles (EVs) will include electric grid stress from EV chargers and produce a large number of diminished EV batteries. EV batteries are expected to retain about 80 % of their original capacity at the end of vehicle life. Employing these in second-use battery energy storage systems (2-BESS) as energy buffers for EV cha...

This paper presents a rate-based model predictive control (MPC) design to track a fuel optimal reference intake manifold pressure (Pim) without violating tailpipe oxides of Nitrogen (NOx) constraints utilizing dynamic linear air path and NOx model for one-step ahead prediction of a heavy-duty diesel engine. Hardware-in-the-loop experimental validat...

Fuel cell control systems are designed to regulate temperature and reactant flow rates to ensure efficient electrochemical reactions and to avoid degradation and membrane failure from stack overheating. In air-cooled systems, the temperature is controlled by regulating the cathode airflow. In this study, a pseudo-spectral optimization method is app...

This work investigates an innovative low-voltage (<60 V) hybrid device that enables engine boosting and downsizing in addition to mild hybrid functionalities such as regenerative braking, start-stop, and torque assist. A planetary gear set and a brake permit the power split supercharger (PSS) to share a 9 kW motor between supercharging the engine a...

Plug-in electric vehicles (PEVs) have the highest promise for dramatically reducing transportation emissions. No other option has comparable emission reduction potential or as a promising pathway. Still, PEVs can offer more than green transportation. In particular, their onboard storage can further serve the society by providing an energy buffer to...

Li-ion battery thermal runaway is a critical safety issue for Electric Vehicles. The proposed global technical regulation No. 20 by the United Nations on Electric Vehicle Safety requires an advanced warning five minutes prior to the evolution of hazardous conditions caused by thermal runaway. To achieve this five-minute advanced warning, a robust a...

This work studies a novel low voltage (<60 V) hybrid system that supports engine boosting and downsizing in addition to start-stop, regenerative braking, and constrained torque assist/regeneration. The hybrid power split supercharger (PSS) shares a 9 kW motor between supercharging the engine or providing hybrid functionalities through a planetary g...

This paper proposes a predictive equivalent consumption minimization strategy (P-ECMS) for a plug-in hybrid electric vehicle (PHEV), assuming the availability of two levels of traffic information. The two levels of traffic information include 1) segmented traffic information available in mobile mapping applications, and 2) detailed velocity informa...

Differential voltage analysis (DVA) is a conventional approach for estimating capacity degradation in batteries. During charging, a graphite electrode goes through several phase transitions observed as plateaus in the voltage response. The transitions between these plateaus emerge as observable peaks in the differential voltage. The DVA method util...

A novel hybrid system enabling both flexible supercharging and limited torque assist is studied to mitigate transient response challenges of a turbocharged spark ignition engine equipped with low pressure exhaust gas recirculation. The hybrid system, called a power split supercharger (SC), is configured with a planetary gear set that splits the sup...

Estimation of electrode state of health (eSOH) is essential to understanding battery degradation status in detail. This is accomplished by considering electrode capacity and a utilization range as eSOH parameters. In this paper, we propose a novel combination of two eSOH estimation approaches (i.e. voltage fitting and differential voltage analysis)...

Advanced battery management system, which leverages an in-depth understanding of the battery state of health, can improve efficiently and safely. To this end, we introduce the electrode-level battery state of health (eSOH) estimation problem with open-circuit voltage (OCV) data. In real-world applications, collecting the full-range OCV data is diff...

Eco-driving and hybridization both save fuel by reducing the energy loss during braking and using the engine efficiently. Considering this similarity, the hybridization requirements are different for the future connected and automated vehicles that can precisely optimize the vehicle velocity profile. This work investigates the necessary hybridizati...

Publisher’s Note: This paper was selected for publication in ASME Letters in Dynamic Systems and Control. https://www.asmedigitalcollection.asme.org/lettersdynsys/article/doi/10.1115/1.4046450/1074858/Control-Oriented-Physics-Based-NOX-Emission-Model

This study uses full drive cycle simulation to compare the fuel consumption of a vehicle with a turbocharged (TC) engine to the same vehicle with an alternative boosting technology, namely, a hybrid supercharger, in which a planetary gear mechanism governs the power split to the supercharger between the crankshaft and a 48 V 5 kW electric motor. Co...

Feedback control is employed to achieve optimal combustion phasing at high levels of exhaust gas recirculation (EGR) by adjusting the spark advance and the EGR valve position. The control development is based on a control-oriented model which is used to capture the effects of throttle position, EGR valve position, and spark timing on the combustion...

This work investigates the utilization of two low voltage hybrid mechanisms on fuel economy improvement of a vehicle with a downsized engine. The first mild hybrid system, configured with a motor, a planetary gear set, and a supercharger, permits switching between parallel hybrid operation and boosting but not both. This constrained flexibility off...

Eco-driving and hybridization both save fuel by reducing the energy loss during braking and using the engine efficiently. Considering this similarity, the hybridization requirements are different for the future connected and autonomous vehicles that can precisely optimize the vehicle velocity profile. This work investigates the necessary hybridizat...

A novel hybrid system enabling both power split supercharging and limited torque assist is studied in this work. The novel supercharger is powered by a planetary gear set that uses a small electric motor and the engine crankshaft. The controller relies on the coordination of four actuators in the engine side and three low-level actuators in the hyb...

Low voltage hybridization (<60 V) supports engine start/stop, regenerative braking, and constrained torque assist/regeneration at a lower cost. This work studies the potential benefits of a novel hybrid system, called a power split supercharger (PSS). A 9 kW motor is shared between boosting the engine or providing hybrid functionalities, hence it c...

This work investigates the utilization of two low voltage hybrid mechanisms on fuel economy improvement of a vehicle with a downsized engine. The first mild hybrid system, configured with a motor, a planetary gear set and a supercharger, permits switching between parallel hybrid operation and boosting but not both. This constrained flexibility offe...

The combustion process in spark-ignition (SI) engines exhibits cycle-to-cycle variability, which imposes limits on engine operation. When exhaust gas recirculation (EGR) is used to increase engine efficiency, the combustion variability (CV) increases and spark advance (SA) must be re-tuned to achieve maximum brake torque. In order to maximize EGR b...

Pressure difference across the exhaust and intake manifolds (ΔP) is a crucial variable to control the pumping loss and cylinder charge dilution through the exhaust gas recirculation in a diesel engine. This paper presents a novel architecture for controlling ΔP and the engine-out NOx emissions, which increases the controller tolerance to engine com...

This work presents the potential of fuel efficiency improvement of a plug-in hybrid electric vehicle (PHEV) by the sequential smoothing of a velocity profile given traffic constraints and the optimization of its charge depletion strategy. First, the vehicle velocity is smoothed by minimizing acceleration subject to the traffic-acceptable following...

A non-intuitive aspect of the transient management of a waste energy recovery (WER) system on a small turbocharged gasoline engine is presented in this work. The WER system is realized with a positive displacement expander and an electric motor, named exhaust turbo generating (ETG) system. The steady state simulations indicate that for a range of m...

This work investigates the utilization of a micro-hybrid mechanism on fuel economy improvement of a vehicle with a downsized engine. The micro-hybrid system, configured with a motor, a planetary gear set and a supercharger, permits switching between parallel hybrid operation and boosting but not both. This constrained flexibility offers additional...

A novel decentralized control architecture is developed based on a feedback from the pressure difference across the engine which is responsible for the pumping losses and the exhaust gas recirculation (EGR) flow in diesel engines. The controller is supplemented with another feedback loop based on NOx emissions measurement. Aiming for simple design...

Cycle-to-cycle variability in the combustion process of spark ignition engines imposes limits when operating at highly diluted conditions. When exhaust gas recirculation (EGR) is used to achieve higher fuel efficiency, the combustion variability (CV) increases due to the reduction in flame propagation speed. A tight control is required to operate c...

This paper details the fabrication and testing of a combined temperature and expansion sensor to improve state of charge (SOC) and state of health (SOH) estimation for Li-ion batteries. These sensors enable the characterization of periodic stress and strain changes in the electrode materials of Lithium-ion batteries during the charge and discharge...

This study uses full drive cycle simulation to compare the fuel consumption of a vehicle with a turbocharged engine to the same vehicle with an alternative boosting technology, namely, a hybrid supercharger, in which a planetary gear mechanism governs the power split to the supercharger between the crankshaft and a 48V 5kW electric motor. Conventio...

At a given speed and load, the spark advance (SA) is tuned to reach the maximum break torque (MBT) timing to maximize efficiency. The use of exhaust gas recirculation (EGR) can further improve fuel economy at the same speed and load. As EGR increases, MBT moves towards a more advanced timing that can be limited by the high variability in the combus...

The resistance of lithium-ion cells increases at subzero temperatures reducing the cells' power availability. One way to improve a cell's performance in these adverse operation conditions is to proactively heat them. This paper considers the scenario in which a cell is heated from both inside and outside; a current is drawn from the cell to power a...

A novel decentralized control architecture is developed based on a feedback from the pressure difference across the engine which is responsible for the pumping losses and the Exhaust Gas Recirculation (EGR) flow in diesel engines. The controller is supplemented with another feedback loop based on NOx emissions measurement. Aiming for simple design...

At a given speed and load, the spark advance (SA) is tuned to reach the maximum break torque (MBT) timing to maximize efficiency. The use of exhaust gas recirculation (EGR) can further improve fuel economy at the same speed and load. As EGR increases, MBT moves towards a more advanced timing that can be limited by the high variability in the combus...

Internal short circuit (ISCr) is one of the major safety issues of lithium batteries and would lead to thermal runaway of batteries. Repeating ISCr in laboratory requires to create small-scale short circuit inside integrated batteries, which is very hard for existed safety test methods. In this paper, a novel ISCr trigger method based on shape memo...

A method to detect the difference between two or more cells when there is only one voltage sensor across the various cells

Internal short circuit (ISCr) is one of the major reasons for lithium-ion battery thermal runaway. A new phenomenon, named as the Fusing Phenomenon, is observed during the ISCr experiments. During the Fusing Phenomenon, the ISCr current path will melt down due to the Joule heat of the short current and the ISCr process will be interrupted. The Fusi...

The papers in this special section were presented at the IEEE Vehicular Technology Society (VTS). The 12th VPPC was held in Montreal, QC, Canada, in October 2015.

Automatic Train (or Tram) Operation (ATO) systems have been widely studied in the last decade to guarantee energy-efficient operation. Speed profiles and timetables are derived by applying optimization methods geared towards finding the optimal trade-off between journey time, emissions and energy efficiency. An on-board controller is required to tr...

Exhaust gas recirculation (EGR) in spark-ignition (SI) internal combustion engines is a well known strategy to improve fuel economy. By lowering the temperature inside the cylinder, the combustion process becomes more resilient to knock and allows to advance spark timing towards maximum brake torque (MBT). Optimal combustion phasing (CA50) combined...

As high exhaust gas recirculation (EGR) is introduced for efficiency, the combustion duration and combustion delay is elongated due to slow fuel burn rates requiring flexible and robust management of both the combustion initiation and duration (what we call \emph{combustion shaping}). Combustion shaping through cylinder pressure sensing and feedbac...

A parameter adaptation method for a previously developed spark ignition (SI) to homogeneous charge compression ignition (HCCI) combustion mode transition control architecture is described. The goal of the adaptive method is to use transient SI-HCCI transition data gathered in online operation to tune the controller model parameters on a cylinder in...