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Technical Paper

Failure Prediction and Design Optimization of Exhaust Manifold based on CFD and FEM Analysis

A thermo-mechanical fatigue analysis was conducted based on a coupled Finite Element Analysis (FEA) - Computational Fluid Dynamics (CFD) method on the crack failure of the exhaust manifold for an inline 4-cylinder turbo-charged diesel engine under the durability test. In the this analysis, the temperature-dependent material properties were obtained from measurements and the model was calibrated with comparison of the predicted exhaust manifold temperatures with the on-engine measurements under the same engine load condition. Temperature and stress/strain distributions in the exhaust manifold were predicted with the calibrated model. Analysis results showed that the cracks took place at locations with high plastic deformations, suggesting that the cause of the failure be thermo-mechanical fatigue (TMF). Using the equivalent plastic strain (PEEQ) as the indicator for thermal mechanical fatigue, three exhaust manifold design revisions were carried out by CAE analysis.
Technical Paper

Reducing Temperature Gradients in High-Power, Large-Capacity Lithium-Ion Cells through Ultra-High Thermal Conductivity Heat Spreaders Embedded in Cooling Plates for Battery Systems with Indirect Liquid Cooling

For lithium-ion battery systems assembled with high-capacity, high-power pouch cells, the cells are commonly cooled with thin aluminum cooling plates in contact with the cells. The cooling plates extract the cell heat and dissipate it to a cooling medium (air or liquid). During the pack utilizations with high-pulse currents, large temperature gradients along the cell surfaces can be encountered as a result of non-uniform distributions of the ohmic heat generated in the cells. The non-uniform cell temperature distributions can be significant for large-size cells. Maximum cell temperatures typically occur near the cell terminal tabs as a result of the ohmic heat of the terminal tabs and connecting busbars and the high local current densities. In this study, a new cooling plate is proposed for improving the uniformity in temperature distributions for the cells with large capacities.
Technical Paper

Characterizing Thermal Behavior of an Air-Cooled Lithium-Ion Battery System for Hybrid Electrical Vehicle Applications Using Finite Element Analysis Approach

Thermal behavior of a Lithium-ion (Li-ion) battery module under a user-defined cycle corresponding to hybrid electrical vehicle (HEV) applications is analyzed. The module is stacked with 12 high-power 8Ah pouch Li-ion battery cells connected in series electrically. The cells are cooled indirectly with air through aluminum cooling plate sandwiched between each pair of cells. The cooling plate has extended cooling surfaces exposed in the cooling air flow channel. Thermal behavior of the battery system under a user specified electrical-load cycle for the target hybrid vehicle is characterized with the equivalent continuous load profile using a 3D finite element analysis (FEA) model for battery cooling. Analysis results are compared with measurements. Good agreement is observed between the simulated and measured cell temperatures. Improvement of the cooling system design is also studied with assistance of the battery cooling analyses.
Journal Article

Characterizing Thermal Runaway of Lithium-ion Cells in a Battery System Using Finite Element Analysis Approach

In this study, thermal runaway of a 3-cell Li-ion battery module is analyzed using a 3D finite-element-analysis (FEA) method. The module is stacked with three 70Ah lithium-nickel-manganese-cobalt (NMC) pouch cells and indirectly cooled with a liquid-cooled cold plate. Thermal runaway of the module is assumed to be triggered by the instantaneous increase of the middle cell temperature due to an abusive condition. The self-heating rate for the runaway cell is modeled on the basis of Accelerating Rate Calorimetry (ARC) test data. Thermal runaway of the battery module is simulated with and without cooling from the cold plate; with the latter representing a failed cooling system. Simulation results reveal that a minimum of 165°C for the middle cell is needed to trigger thermal runaway of the 3-cell module for cases with and without cold plate cooling.
Journal Article

Design of Direct and Indirect Liquid Cooling Systems for High- Capacity, High-Power Lithium-Ion Battery Packs

Battery packs for plug-in hybrid electrical vehicle (PHEV) applications can be characterized as high-capacity and high-power packs. For PHEV battery packs, their power and electrical-energy capacities are determined by the range of the electrical-energy-driven operation and the required vehicle drive power. PHEV packs often employ high-power lithium-ion (Li-ion) pouch cells with large cell capacity in order to achieve high packing efficiency. Lithium-ion battery packs for PHEV applications generally have a 96SnP configuration, where S is for cells in series, P is for cells in parallel, and n = 1, 2 or 3. Two PHEV battery packs with 355V nominal voltage and 25-kWh nominal energy capacity are studied. The first pack is assembled with 96 70Ah high-power Li-ion pouch cells in 96S1P configuration. The second pack is assembled with 192 35Ah high-power Li-ion pouch cells in 96S2P configuration.
Technical Paper

Influence of Terminal Tabs/Busbar Ohmic Heat on Maximum Cell Temperature of a Li-ion Battery System for PHEV Applications

The battery packs for plug-in hybrid electrical vehicle (PHEV) applications are relatively small in the charge depleting (CD) mode but fairly large in the charge sustaining (CS) mode for their duties in comparison to the battery packs for hybrid electrical vehicle (HEV) applications. Thus, the heaviest battery thermal load for a PHEV pack is encountered at the end of the CD mode. Because the cells in PHEV battery packs are generally larger than those in the HEV packs in both capacity and size, control of the maximum cell temperature and the maximum differential cell temperature for the cells in a PHEV pack with high packing efficiency is a challenge for the cooling system design. The maximum cell temperatures locate in the areas near the terminal tabs where the current densities are highest.
Journal Article

Thermal Analysis of a High-Power Lithium-Ion Battery System with Indirect Air Cooling

Thermal behavior of a lithium-ion (Li-ion) battery module for hybrid electrical vehicle (HEV) applications is analyzed in this study. The module is stacked with 12 high-power pouch Li-ion battery cells. The cells are cooled indirectly with air through aluminum fins sandwiched between each two cells in the module, and each of the cooling fins has an extended cooling surface exposed in the cooling air flow channel. The cell temperatures are analyzed using a quasi-dimensional model under both the transient module load in a user-defined cycle for the battery system utilizations and an equivalent continuous load in the cycle. The cell thermal behavior is evaluated with the volume averaged cell temperature and the cell heat transfer is characterized with resistances for all thermal links in the heat transfer path from the cell to the cooling air. Simulations results are compared with measurements. Good agreement is observed between the simulated and measured cell temperatures.
Journal Article

Thermal Analysis of a Li-ion Battery System with Indirect Liquid Cooling Using Finite Element Analysis Approach

The performance and life of Li-ion battery packs for electric vehicle (EV), hybrid electrical vehicle (HEV), and plug-in hybrid electrical vehicle (PHEV) applications are influenced significantly by battery operation temperatures. Thermal management of a battery pack is one of the main factors to be considered in the pack design, especially for those with indirect air or indirect liquid cooling since the cooling medium is not in contact with the battery cells. In this paper, thermal behavior of Li-ion pouch cells in a battery system for PHEV applications is studied. The battery system is cooled indirectly with liquid through aluminum cooling fins in contact with each cell and a liquid cooled cold plate for each module in the battery pack. The aluminum cooling fins function as a thermal bridge between the cells and the cold plate. Cell temperature distributions are simulated using a finite element analysis approach under cell utilizations corresponding to PHEV applications.
Journal Article

Thermal Characterization of a Li-ion Battery Module Cooled through Aluminum Heat-Sink Plates

The temperature distribution is studied theoretically in a battery module stacked with 12 high-power Li-ion pouch cells. The module is cooled indirectly with ambient air through aluminum heat-sink plates or cooling plates sandwiched between each pair of cells in the module. Each of the cooling plates has an extended cooling fin exposed in the cooling air channel. The cell temperatures can be controlled by changing the air temperature and/or the heat transfer coefficient on the cooling fin surfaces by regulating the air flow rate. It is found that due to the high thermal conductivity and thermal diffusivity of the cooling plates, heat transfer of the cooling plate governs the cell temperature distribution by spreading the cell heat over the entire cell surface. Influence of thermal from the cooling fins is also simulated.
Journal Article

An Analysis of a Lithium-ion Battery System with Indirect Air Cooling and Warm-Up

Ideal operation temperatures for Li-ion batteries fall in a narrow range from 20°C to 40°C. If the cell operation temperatures are too high, active materials in the cells may become thermally unstable. If the temperatures are too low, the resistance to lithium-ion transport in the cells may become very high, limiting the electrochemical reactions. Good battery thermal management is crucial to both the battery performance and life. Characteristics of various battery thermal management systems are reviewed. Analyses show that the advantages of direct and indirect air cooling systems are their simplicity and capability of cooling the cells in a battery pack at ambient temperatures up to 40°C. However, the disadvantages are their poor control of the cell-to-cell differential temperatures in the pack and their capability to dissipate high cell generations.
Technical Paper

Comparative Study of Thermal Characteristics of Lithium-ion Batteries for Vehicle Applications

Lithium ion batteries can be developed for vehicle applications from high power specification to high energy specification. Thermal response of a battery cell is the main factor to be considered for battery selection in the design of an electrified vehicle because some materials in the cells have low thermal stability and they may become thermally unstable when their working temperature becomes higher than the upper limit of allowed operating range. In this paper the thermal characteristics of different sizes and forms of commercially available batteries is investigated through electro-thermal analysis. The relation between cell capacity and cell internal resistance is also studied. The authors find that certain criteria can be defined for battery selection for electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles. These criteria can be served as design guidelines for battery development for vehicle applications.
Journal Article

Electro-Thermal Modeling of a Lithium-ion Battery System

Lithium-ion (Li-ion) batteries are becoming widely used high-energy sources and a replacement of the Nickel Metal Hydride batteries in electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). Because of their light weight and high energy density, Li-ion cells can significantly reduce the weight and volume of the battery packs for EVs, HEVs and PHEVs. Some materials in the Li-ion cells have low thermal stabilities and they may become thermally unstable when their working temperature becomes higher than the upper limit of allowed operating temperature range. Thus, the cell working temperature has a significant impact on the life of Li-ion batteries. A proper control of the cell working temperature is crucial to the safety of the battery system and improving the battery life. This paper outlines an approach for the thermal analysis of Li-ion battery cells and modules.
Technical Paper

Physicochemical Characteristics of Soot Deposits in EGR Coolers

Physicochemical characteristics of the soot deposits in a fouled EGR cooler are studied in this paper. It is found that a three-layer model for the soot deposited in the EGR cooler may well describe the behavior of the depositing process: a dense base layer with micro pores (≺ 5 nm), a randomly packed intermediate layer with meso pores (5-50 nm) and a loose surface layer with macro pores (≻ 50 nm). The surface layer is thick and highly porous, formed by mechanical interlocking of the agglomerated primary soot particles or soot clusters. The soot particles in the surface layer may be removed by a high shear EGR flow. Condensates in the deposit, especially water, can have a significant influence on the structure of the deposit. Capillary forces on the wetted soot particles could be comparable to the contact forces holding the particles together. It is found that the hydroscopicity of the soot particles vary with their content of soluble organic fraction (SOF).
Journal Article

Particulate Fouling in EGR Coolers

The physical process of particulate fouling in EGR coolers is analyzed in this paper. Various particulate-deposition mechanisms are discussed and an order of magnitude comparison suggests that thermophoresis is the dominant mechanism for the EGR cooler fouling. The EGR temperature at the cooler inlet, the soot particle concentration in EGR, and the EGR mass flow rate are found to be the parameters governing the EGR cooler fouling. The structure for the soot deposit buildup on the cooler wall is also discussed. It is found that the surface layer of the deposit governs the fouling factor. A comprehensive model for soot particle depositions is developed employing heat, mass, and momentum transfer theories for the particle-gas system. The fouling model developed in this study can predict the process of deterioration in the effectiveness. The predictions of EGR cooler fouling are compared with experimental data and good agreement is observed.
Technical Paper

Thermodynamic Properties of Dimethyl Ether - An Alternative Fuel for Compression-Ignition Engines

On the basis of the molecular thermodynamics for fluids, the thermodynamic properties of DME are developed for pressure p ≤ 500 bar and temperature T ≤ 200 °C, which covers pressures and temperatures that a DME fuel system for the CI-engine application would experience. The properties cover subcooled, two-phase, and superheated/supercritical regions, including p-v-T properties, enthalpy, entropy, latent heat, heat capacity, speed of sound in vapor, liquid and two-phase mixtures, bulk modulus, and surface tension. A volume-cubic equation of state for DME also is developed, which allows calculating the DME density at any given pressure and temperature analytically. All the properties are given in equations as well as in charts. For convenience in two-phase-flow applications, e.g., design of the fuel tank and cavitation analysis, the saturated properties are also given in tables, listed in both pressure and temperature up to the critical point.