bluuebunny/crossref_metadata_2025 huggingface dataset:
Transient Control of Fully Variable Valve Actuation for Diesel Engine Using Dynamic Programming Method
Authors: Lu Yong , Li Jian , Hou Dongyan , Miao Lixian
Published: December 2019

Fully variable valve technology of diesel engine can change the movement parameters of valve flexibly, and then the performance of engine can be improved. But the actual valve movement can’t track the optimal valve profile due to the nonlinear characteristics of hydraulic system in transient work conditions, which make the engine performance be deteriorated. To solve the problem, the paper introduced the idea of dynamic programming (DP) into the control of FVVA system. Firstly, the paper presented a new electro-hydraulic FVVA system. To verify the FVVA system, the GT-suite models of FVVA system and engine were built. Secondly, for the purpose of achieving optimal performance, based on the NSGA-II genetic algorithm, we got the database of the optimal valve profile movement parameters using modeFRONTIER platform. In database, there are multiple feasible solutions for one work condition. To achieve the optimal engine performance in every cycle, according to the real-time valve movement profile, the controller will choose different solutions with DP method in one cycle based on the database obtained before. The DP controller can make the engine performance to be optimal according to the real-time valve profile in transient conditions. In steady conditions, optimal valve profiles can be reached by a PID controller. Thirdly, the DP controller and PID controller were designed with Simulink separately. The DP controller will adjust valve control parameters in one cycle and the PID controller will adjust the parameters cycle by cycle. At last, the DP-PID controller was compared with the single PID controller which adjust control parameters once in one cycle. The simulation results show that the performance of engine with DP-PID controller is improved compared to the PID controller especially in transient conditions. The average brake power can be improved by 3.3% to 4.7% compared to single PID controller.
Development of GDI Engine for Improving Brake Thermal Efficiency Over 44%
Authors: Jung Dongwon , Lee Byeongseok , Son Jinwook , Woo Soohyung , Kim Youngnam
Published: December 2019

This study demonstrates the effects of technologies applied for the development of a gasoline direct injection (GDI) engine for improving the brake thermal efficiency (BTE) over 44%. The GDI engine for the current study is an in-line four-cylinder engine with a displacement of 2156cm3, which has relatively high stroke to bore ratio of 1.4 (110mm stroke and 79mm bore). All experiments have been conducted using a gasoline having RON 92 for stoichiometric operation at 2000RPM.

First, since compression ratio is directly related to the thermal efficiency, four compression ratios (14.3, 15.2, 15.8 and 17.2) were explored for operation without exhaust gas recirculation (EGR). Then, for the same four compression ratios, EGR was used to suppress the knock occurrence at high loads with high compression ratio (CR), and its effect on initial and main combustion duration was compared. Second, the shape of intake port was revised to increase tumble flow of in-cylinder charge for reducing combustion duration at low and high load, and extending EGR-stability limit further eventually. Then, as an effective method to ensure stable, complete and fast combustion for EGR-diluted stoichiometric operation, the use of twin spark ignition system is examined by modifying both valve diameter of intake and exhaust, and its effect is compared against that of single spark ignition. In addition, the layout of twin spark ignition was also examined for the location of Front-Rear and Intake-Exhaust. To get the maximum BTE at high load, 12V electronic super charger (eSC) was applied. Under the condition of using 12V eSC, the effect of intake cam duration was identified by increasing from 260deg to 280deg. Finally, 48V eSC was applied with the longer intake camshaft duration of 280deg. As a result, the maximum BTE of 44% can be achieved for stoichiometric operation with EGR.
Numerical Simulation of a Large Bore Dual Fuel Marine Engine Using Tabulated Detailed Reaction Mechanisms
Authors: Andree Sascha , Goryntsev Dmitry , Theile Martin , Henke Björn , Schleef Karsten , Nocke Jürgen , Tap Ferry , Buchholz Bert , Hassel Egon
Published: December 2019

The simulation of a diesel natural gas dual fuel combustion process is the topic of this paper. Based on a detailed chemical reaction mechanism, which was applied for such a dual fuel combustion, the complete internal combustion engine process was simulated. Two single fuel combustion reaction mechanisms from literature were merged, to consider the simultaneous reaction paths of diesel and natural gas. N-heptane was chosen as a surrogate for diesel. The chemical reaction mechanisms are solved by applying a tabulation method using the software tool AVL Tabkin™. In combination with a Flamelet Generated Manifold (FGM) combustion model, this leads to a reduction of computational effort compared to a direct solving of the reaction mechanism, because of a decoupling of chemistry and flow calculations. Turbulence was modelled using an unsteady Reynolds-Averaged Navier Stokes (URANS) model. In comparison to conventional combustion models, this approach allows for detailed investigations of the complex ignition process of the dual fuel combustion process. The unexpected inversely proportional relationship between start of injection (SOI) and start of combustion (SOC), a later start of injection makes for an earlier combustion of the main load, is only one of these interesting combustion phenomena, which can now be analyzed in detail. Further investigations are done for different engine load points and multiple pilot injection strategies. The simulation results are confirmed by experimental measurements at a medium speed dual fuel single cylinder research engine.
Sensitivity Analysis of Fuel Physical Property Effects on Spark Ignition Engine Performance
Authors: Van Dam Noah , Kalvakala R. Krishna , Boink Frederik , Yue Zongyu , Som Sibendu
Published: December 2019

Alternative fuels are of interest to automakers and regulators due to their potential to reduce net greenhouse gas emissions from transportation sources. Alternative fuels also have fuel properties which may enable advanced combustion modes with higher engine thermal efficiencies. There has been previous work to identify the relationship between various fuel properties and engine performance, but most of this work has been experiments or simulations where the change in properties was obtained through changing the fuel composition, making isolating the effects of individual fuel properties difficult. In this study, numerical simulations have been used to investigate the effects of individual fuel physical properties such as viscosity or heat of vaporization (HoV) on engine performance.

Simulations have been performed of two different engine platforms, the first an optical, single-cylinder research engine and the second a multi-cylinder production engine. Both engines are direct-injection spark-ignition engines with pent-roof heads and are designed for automotive applications. Each engine was run at a different operating condition, one stable and one knock-limited. Different base fuels provided a variety of simulated conditions. Up to six different fuel properties were varied as part of Global Sensitivity Analyses performed for each of the engines with multiple performance targets including thermal efficiency, combustion efficiency and combustion phasing.

Results show trends that are largely consistent with previous experimental findings using multiple fuels. The engine thermal efficiency was primarily sensitive to the fuel’s HoV, with other fuel physical properties having smaller effects. For optical engine results, the magnitude of the effect was greater in this study than expected based on previous experimental results were many fuel physical and chemical properties were varied simultaneously. However, for the multi-cylinder production engine, the relationship between thermal efficiency and HoV was slightly smaller.
A Study on a Rotational Force Acting on the Piston Ring of a Gasoline Engine
Authors: Tomizawa Kenta , Ito Akemi
Published: December 2019

Oil consumption of an engine causes particulate matter, poisoning catalysts and sometimes abnormal combustion like pre-ignition. One of the factors of oil consumption is oil transport via a piston ring-gap. Coincident of ring-gaps at a same position may cause an increase in oil consumption.

In this research, the effect of coincident the ring gaps on oil consumption was measured using with/without the a stopper pin for the ring rotation by sulfur tracer method. A lot of spikes was found in the wave form of sulfur concentrate for the rings without the stopper pin, and higher value of oil consumption was simultaneously measured. Then the force which caused ring rotation (hereafter ‘rotational force’) was measured by a newly developed method. A cantilever was installed in the ring gap, and the strain gauges were pasted on the cantilever. Therefore, the rotational force was measured as the bending stress of the cantilever. It was found that the rotational force showed a periodic wave form against the crank angle. Furthermore, it was also found that the amplitude of the rotational force was strong affected by the engine operating conditions. The rotational force was also affected by the gap position. It was assumed that not only the piston lateral motion but also the cylinder bore shape affected the rotational force. The mechanism of generating the rotational force is the future subject.
Phenomenological Modeling of Combustion in Pre-Chamber and the Pilot Flame for Natural Gas Engines
Authors: Liu Long , Wen Xia , Xiong Qian , Ma Xiuzhen
Published: December 2019

With energy shortages and increasing environmental problems, natural gas, as a clean energy, has the advantages of cheap price and large reserves and has become one of the main alternative fuels for marine diesel engines. For large bore natural gas engines, pre-chamber spark plug ignition can be used to increase engine efficiency. The engine mainly relies on the flame ejected from the pre-chamber to ignite the mixture of natural gas and air in the main combustion chamber. The ignition flame in the main combustion chamber is the main factor affecting the combustion process. Although the pre-chamber natural gas engines have been extensively studied, the characteristics of combustion in the pre-chamber and the development of ignition flame in the main combustion chamber have not been fully understood. In this study, a two-zone phenomenological combustion model of pre-chamber spark-ignition natural gas engines is established based on the exchange of mass and energy of the gas flow process in the pre-chamber and the main combustion chamber. The basic characteristics of the developed model are: a spherical flame surface is used to describe the combustion state in the pre-chamber, and according to the turbulent jet theory, the influence of turbulence on the state of the pilot flame is considered based on the Reynolds number. According to the phenomenological model, the time when the flame starts to be injected from the pre-chamber to the main combustion chamber, and the parameters such as the length of the pilot flame are analyzed. The model was verified by experimental data, and the results showed that the calculated values were in good agreement with the experimental values. It provides an effective tool for mastering the law of flame development and supporting the optimization of combustion efficiency.
A Novel Active Optimization Approach for Rapid and Efficient Design Space Exploration Using Ensemble Machine Learning
Authors: Owoyele Opeoluwa , Pal Pinaki
Published: December 2019

In this work, a novel design optimization technique based on active learning, which involves dynamic exploration and exploitation of the design space of interest using an ensemble of machine learning algorithms, is presented. In this approach, a hybrid methodology incorporating an explorative weak learner (regularized basis function model) which fits high-level information about the response surface, and an exploitative strong learner (based on committee machine) that fits finer details around promising regions identified by the weak learner, is employed. For each design iteration, an aristocratic approach is used to select a set of nominees, where points that meet a threshold merit value as predicted by the weak learner are selected to be evaluated using expensive function evaluation. In addition to these points, the global optimum as predicted by the strong learner is also evaluated to enable rapid convergence to the actual global optimum once the most promising region has been identified by the optimizer. This methodology is first tested by applying it to the optimization of a two-dimensional multi-modal surface. The performance of the new active learning approach is compared with traditional global optimization methods, namely micro-genetic algorithm (μGA) and particle swarm optimization (PSO). It is demonstrated that the new optimizer is able to reach the global optimum much faster, with a significantly fewer number of function evaluations. Subsequently, the new optimizer is also applied to a complex internal combustion (IC) engine combustion optimization case with nine control parameters related to fuel injection, initial thermodynamic conditions, and in-cylinder flow. It is again found that the new approach significantly lowers the number of function evaluations that are needed to reach the optimum design configuration (by up to 80%) when compared to particle swarm and genetic algorithm-based optimization techniques.
Thermochemical and Sensible Energy Recuperation Using Thermally-Integrated Reactor and Diesel-Ammonia Dual Fueling Strategy
Authors: Kane Seamus P. , Zarling Darrick , Northrop William F.
Published: December 2019

Anhydrous ammonia produced using wind power on farms can be a renewable alternative to conventional fertilizers and to fossil fuels used in engine-powered equipment. Although it has been shown that ammonia can be used in dual fuel modes in diesel engines, its inherently low flame speed results in poor combustion efficiency and thus reduces allowable diesel fuel replacement ratios. In this work, a novel method using a thermochemical recuperation (TCR) reactor system to partially decompose ammonia into hydrogen and nitrogen over a catalyst was demonstrated in diesel engine powered tractor. In the experiments, a John Deere 6400 agricultural tractor powered by a non-EPA tier-certified 4045TL diesel engine was operated in dual-fuel mode using anhydrous ammonia as the secondary fuel. Liquid ammonia from a tank was vaporized and heated using a series of heat exchangers and partially decomposed to hydrogen gas before being fumigated into the intake manifold. The catalytic TCR reactor utilized both exhaust waste heat and unburned hydrocarbon heating value to drive the ammonia decomposition process. Engine emissions and performance data were collected across a standard 8-mode test. The engine was operated using diesel only and in dual fuel mode with up to 42% replacement of diesel with ammonia on a lower heating value basis. Engine loading was accomplished using a power takeoff (PTO) dynamometer. Measured brake thermal efficiency was improved by up to 5.0% using thermochemical recuperation, and brake specific CO2 emissions were reduced by up to 44% over diesel-only rates.
Characteristics of Cycle-to-Cycle Combustion Variability at Partial-Burn Limited and Misfire Limited Spark Timing Under Highly Diluted Conditions
Authors: Maldonado Bryan , Stefanopoulou Anna , Scarcelli Riccardo , Som Sibendu
Published: December 2019

Diluted combustion with exhaust gas recirculation (EGR) has been widely employed to improve the fuel economy of spark ignition engines. The combustion kinetics, however, are affected and the flame propagation speed is decreased. In order to compensate for this adverse effect, the spark timing needs to be recalibrated to achieve maximum brake torque (MBT). At high levels of EGR dilution, the spark timing is constrained by two ignition limits: 1) the partial-burn limit where the spark timing is retarded from MBT and 2) the misfire limit where the spark timing is too advanced. This work uses a probabilistic framework to capture the differences between both ignition limits. In particular, it introduces the concept of a nominal indicated mean effective pressure (IMEP) distribution based on the stochastic properties of the cycle-to-cycle variability (CCV) at nominal stable conditions. By defining a nominal band where fully burned cycles occur with high probability, we introduce a cycle classification method that can be used to 1) determine the level of randomness of misfire and partial-burn events, and 2) measure CCV. The new CCV metric based on the density of the nominal band is compared with the traditional coefficient of variation of IMEP (CoVIMEP). It is shown that the nominal band concept, together with the CoVIMEP, can help to discern between partial-burn limited and misfire limited conditions. Furthermore, the Kullback-Leibler divergence is used to demonstrate that the IMEP distribution is significantly different between nominal and partial-burn/misfire limited conditions. Experiments are carried at various EGR levels and spark timings while recoding in-cylinder pressure at steady state. Although the emphasis of this work is to characterize the differences of both ignition limits from a probabilistic point of view, similarities between partial-burn cycles at either limiting conditions are also discussed.
CFD Simulation of a Premixed Spark Injection Hydrogen Engine
Authors: Lee Benjamin , Ge Haiwen , Parameswaran Siva , Zhao Peng
Published: December 2019

Hydrogen has great potential as an alternative fuel for internal combustion engines (ICE) because of its high energy density by mass, high specific heat ratio, free of CO2 and PM emission, and ultra-low NOx emission. In the present paper, a premixed spark-ignition engine fueled by hydrogen was simulated. 3D combustion CFD was employed to simulate and optimize the engine combustion. Parametric studies of equivalence ratio were conducted to identify the appropriate range for practical operation. It was found that higher equivalence ratio in H2ICE is not feasible since it exhibits very high pressure rise rate and strong pressure oscillations. It is due to its much fast flame speed and detonabiltiy. Frequency analysis was conducted on the predicted pressure trace. The dominant frequencies were compared with the first four drum mode. It was found that the dominant resonance frequency corresponds to the drum mode of (0,1). This is due to the fact that the pressure wave originates from the center of combustion chamber. With an equivalence ratio of 0.4, the engine produces similar pressure trace as the baseline gasoline engine, while its close-cycle work is less than half of the gasoline engine, due to low equivalence ratio and hydrogen’s low energy density per unit volume. NOx emission is two order of magnitude lower than the stoichiometric gasoline engine. However, for lean burn PFI H2ICE, N2O emission is much higher than stoichiometric gasoline engine, which needs special attention in the future. Spark timing was swept for the equivalence ratio 0.4 case to further optimize the engine. The mechanisms of NO and N2O formation at different operating conditions are compared and discussed. It was found that NO emission is sensitive to spark timing, while N2O emission is not. The engine-out N2O emission is about one order of magnitude lower than the engine-out NO emission.
Assessment of Turbulent Combustion Models for Simulating Pre-Chamber Ignition in a Natural Gas Engine
Authors: Kim Joohan , Scarcelli Riccardo , Som Sibendu , Shah Ashish , Biruduganti Munidhar S. , Longman Douglas E.
Published: December 2019

Lean combustion in an internal combustion engine is a promising strategy to increase thermal efficiency by leveraging a more favorable specific heat ratio of the fresh mixture and simultaneously suppressing the heat losses to the cylinder wall. However, unstable ignition events and slow flame propagation at fuel-lean condition lead to high cycle-to-cycle variability and hence limit the high-efficiency engine operating range. Pre-chamber ignition is considered an effective concept to extend the lean operating limit, by providing spatially distributed ignition with multiple turbulent flame-jets and enabling faster combustion rate compared to the conventional spark ignition approach. From a numerical modeling perspective, to date, still the science base and available simulation tools are inadequate for understanding and predicting the combustion processes in pre-chamber ignited engines. In this paper, conceptually different RANS combustion models widely adopted in the engine modeling community were used to simulate the ignition and combustion processes in a medium-duty natural gas engine with a pre-chamber spark-ignition system. A flamelet-based turbulent combustion model, i.e., G-equation, and a multi-zone well-stirred reactor model were employed for the multi-dimensional study. Simulation results were compared with experimental data in terms of in-cylinder pressure and heat release rate. Finally, the analysis of the performance of the two models is carried out to highlight the strengths and limitations of the two formulations respectively.
Numerical Study of Advanced Compression Ignition and Combustion in a Gasoline Direct Injection Engine
Authors: Kim Sayop , Kim Joohan , Shah Ashish , Pal Pinaki , Scarcelli Riccardo , Rockstroh Toby , Som Sibendu , Wu Yunchao , Lu Tianfeng
Published: December 2019

Multi-mode combustion strategies may provide a considerable thermal efficiency improvement targeted at part-load operating conditions for light-duty spark-ignition (SI) engines. The extension from boosted SI mode at high loads to advanced compression ignition (ACI) mode at low loads can be achieved by increasing compression ratio and utilizing intake air heating. In order to enable an accurate control of intake charge condition for ACI control and rapid mode-switches, it is essential to gain fundamental insight into the autoignition process with regard to thermal and fuel-air mixture stratification in the combustion chamber. In this work, a computational fluid dynamics (CFD) study is carried out to reveal some degrees of correlation between the mixture and thermal stratifications induced by the cylinder wall temperature and combustion-phasing across varying engine load conditions.

The computational analysis begins with a calibrated simulation setup best matching the engine experiments, and subsequently evaluates the baseline setup for an extended range of engine load conditions for two excess-air ratio cases. The present study emphasizes the dominance of thermal stratifications for autoignition process due to wall temperature effects depending on the engine load conditions of interest. In addition, this study also aims to provide insight into the impact of mixture stratification on cyclic variability, especially at colder wall engine conditions (e.g., cold start). Observations herein provide highlight of the propensity of autoignition in correlation with mixture reactivity space.
Dynamic Control of Airflow Balance in Data Centers
Authors: Linder Stephen Paul , Van Gilder Jim , Zhang Yan , Barrett Enda
Published: December 2019

Efficient cooling of data center infrastructure is an important way to reduce total energy consumption. Containment, with separation of hot and cold airflows has allowed significant increase in efficiencies. However, balancing the airflow, so that IT equipment in an aisle only receives the cooling airflow that that aisle needs is still often not done. We propose a new architecture where IT racks are clustered together with shared hot aisles ducted to a common ceiling plenum. Each aisle has an actively controlled damper used to balance the airflow to the cooling infrastructure. Using a differential air pressure sensor in each aisle and an algorithm designed to balance the flow network, we minimize the cooling airflow and maximize cooling efficiency.
Evaporation Rate Measurement at Multiple Scales Using Temperature-Sensitive Fluorescence Dyes
Authors: Suh Youngjoon , Lin Cheng-Hui , Gowda Hamsa , Won Yoonjin
Published: December 2019

As modern electronics continuously exceed their performance limits, there is an urgent need to develop new cooling devices that balance the increasing power demands. To meet this need, cutting-edge cooling devices often utilize microscale structures that facilitate two-phase heat transfer. However, it has been difficult to understand how microstructures trigger enhanced evaporation performances through traditional experimental methods due to low spatial resolution. The previous methods can only provide coarse interpretations on how physical properties such as permeability, thermal conduction, and effective surface areas interact at the microscale to effectively dissipate heat. This motivates researchers to develop new methods to observe and analyze local evaporation phenomena at the microscale.

Herein, we present techniques to characterize submicron to macroscale evaporative phenomena of microscale structures using micro laser induced fluorescence (μLIF). We corroborate the use of unsealed temperature-sensitive dyes by systematically investigating their effects on temperature, concentration, and liquid thickness on the fluorescence intensity. Considering these factors, we analyze the evaporative performances of microstructures using two approaches. The first approach characterizes local or overall evaporation rates by measuring the solution drying time. The second method employs an intensity-to-temperature calibration curve to convert temperature-sensitive fluorescence signals to surface temperatures. Then, submicron-level evaporation rates are calculated by employing a species transport equation for vapor at the liquid-vapor interface. Using these methods, we reveal that capillary-assisted liquid feeding dominates evaporation phenomena on microstructured surfaces. This study will enable engineers to decompose the key thermofluidic parameters contributing to the evaporative performance of microscale structures.
Metallic PCM’s Microstructural Stability Under Repetitive Melting/Solidification Cycles
Authors: Báez Rafael , González Luis E. , Quintero Pedro O. , Boteler Lauren M.
Published: December 2019

Metallic phase change materials (mPCMs) have been demonstrated as potential passive cooling solution for pulse power applications. The possibility of integrating a metallic PCM directly on top of a heat source, reducing the thermal resistance between the device and the cooling system, could result in a significant improvement in thermal management for transient applications. However, many thermo-physical properties of these alloys are still unknown, furthermore their microstructural stability with repetitive melting/solidification cycles is not warrant. In this work we provide a series of potential mPCMs for thermal management of electronics operating on a wide range of application temperatures, followed by an experimental investigation of microstructural and thermo-physical stability of these materials under repetitive melting solidification cycles. Results of the effect of cyclic thermal loading of theses alloys on the melting behavior and latent heat of fusion is discussed.

Thermal stability of 51.0 wt.% In-32.5 wt.%Bi-16.5 wt.%Sn and 50wt.%Bi-26.7wt%Pb-13.3wt%Sn-10wt%Cd alloys, as potential mid temperature mPCM, have been evaluated. The results suggest that these mPCM maintain their thermo-physical stability over large periods of thermal cycles.
Thermoreflectance Imaging of Electromigration in Aluminum Interconnects at Different Ambient Temperatures
Authors: Alajlouni Sami , Maize Kerry , Bermel Peter , Shakouri Ali
Published: December 2019

Electromigration (EM) is the process of displacing atoms in metals due to current flow leading to interconnect failures in electronic circuits. As electronics feature sizes continue to shrink, EM is becoming an increasingly serious reliability concern. EM, in aluminum (Al) interconnects, has been studied previously, but typically on the device level using Black’s law [1], without emphasis on the localized heating and defect generation around the failure site. To better understand the local EM process, thermoreflectance (TR) thermal imaging technique is used to obtain temperature profiles with submicron resolution [2]. We show that a simple lifetime prediction using Black’s law is not possible for a micro Al wire having a patterned constriction. The wire fails at two distinct failure locations depending on the level of current excitation. Moreover, the lifetime dependence on ambient temperature was studied. Each failure location had its own extracted activation energy. Our findings suggest that Black’s law may be extended to local features. They also show the potential for the design of local features in extending the lifetime of metallic interconnects. In summary, the temperature profile with submicron spatial resolution offers a unique opportunity to better understand the different mechanisms contributing to EM failures which can be used to design highly reliable interconnects.
System-Level Thermal Management and Reliability of Automotive Electronics: Goals and Opportunities in the Next Generation of Electric and Hybrid Electric Vehicles
Authors: Nafis Bakhtiyar Mohammad , Iradukunda Ange-Christian , Al Razi Imam , Huitink David R. , Peng Yarui
Published: December 2019

The reliable operation of electronic equipment is strongly related to the thermal and mechanical conditions it is exposed to during operation. In order to ensure a long lifetime of components, it is imperative that any electronic packaging design takes into careful consideration the appropriateness of various thermal management schemes and the application-specific requirements in order to keep temperatures within certain limits. The exact requirement varies with the application, and electronic packaging designs for automotive applications are at particular risk of failure due to the naturally harsh conditions it is exposed to. Electronic devices in vehicles have to be able to operate and survive at much higher temperatures than their consumer counterparts. While that has always been an issue, the rise of electric and hybrid electric vehicles (EVs and HEVs), combined with a desire to fit as much as possible into the smallest form factor, the challenge of removing enough heat from electronic devices in automotive vehicles is constantly evolving. This paper closely examines the new challenges in thermal management in various driving environments and aims to classify each existing cooling methods in terms of their performance. Drive schedules used by the Environmental Protection Agency (EPA) for emission and fuel economy testing are taken as examples of different realistic driving scenarios and their predicted thermal profiles are evaluated against various cooling methods, both active, passive or a combination of the two (hybrid). Particular focus is placed upon emerging solutions regarded to hold great potential, such as phase change materials (PCMs). Phase change materials have been regarded for some time as a means of transferring heat quickly away from the region with the electronic components. Phase change materials are widely regarded as a possible means of carrying out cooling in large scales from small areas, considering their advantages such as high latent heat of fusion, high specific heat, controllable temperature stability and small volume change during phase change, etc. They have already been utilized as a method of passive cooling in electronics in various ways, such as in heat spreaders and finned heat sinks. The applications, however, have been mostly for system-on-chip handheld devices, and their adoption in automotive power electronics, such as those used in traction inverters, has been much slower. A brief discussion is made on some of the potential areas of application and challenges relating to more widespread adoption of PCMs. Merits of some of the existing PCM based solutions for automotive electronics applications are also discussed, as are their drawbacks and modifications.
Integrated Optical Probing of the Thermal Dynamics of Wide Bandgap Power Electronics
Authors: Lundh James Spencer , Song Yiwen , Chatterjee Bikram , Baca Albert G. , Kaplar Robert J. , Armstrong Andrew M. , Allerman Andrew A. , Kim Hyungtak , Choi Sukwon
Published: December 2019

Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power (SWaP) of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current (DC) measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the thermal dynamics of these systems in practical applications. In this work, the transient thermal dynamics of an AlGaN/GaN high electron mobility transistor (HEMT) were studied using thermoreflectance thermal imaging and Raman thermometry. Also, the proper use of iterative pulsed measurement schemes such as thermoreflectance thermal imaging to determine the steady-state operating temperature of devices is discussed. These studies are followed with subsequent transient thermal characterization to accurately probe the self-heating from steady-state down to sub-microsecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.
An Experimental Investigation on the Fluid Distribution in a Two-Phase Cooled Rack Under Steady and Transient IT Load
Authors: Khalili Sadegh , Rangarajan Srikanth , Sammakia Bahgat , Gektin Vadim
Published: December 2019

Increasing power densities in data centers due to the rise of Artificial Intelligence (AI), high-performance computing (HPC) and machine learning compel engineers to develop new cooling strategies and designs for high-density data centers. Two-phase cooling is one of the promising technologies which exploits the latent heat of the fluid. This technology is much more effective in removing high heat fluxes than when using the sensible heat of fluid and requires lower coolant flow rates. The latent heat also implies more uniformity in the temperature of a heated surface. Despite the benefits of two-phase cooling, the phase change adds complexities to a system when multiple evaporators (exposed to different heat fluxes potentially) are connected to one coolant distribution unit (CDU). In this paper, a commercial pumped two-phase cooling system is investigated in a rack level. Seventeen 2-rack unit (RU) servers from two distinct models are retrofitted and deployed in the rack. The flow rate and pressure distribution across the rack are studied in various filling ratios. Also, investigated is the transient behavior of the cooling system due to a step change in the information technology (IT) load.
Fatigue Life of Sn3.0Ag0.5Cu Solder Alloys Under Combined Shear and Compressive Loads
Authors: Dale Travis , Singh Yuvraj , Bernander Ian , Subbarayan Ganesh , Handwerker Carol , Su Peng , Glasauer Bernard
Published: December 2019

Solder joints in electronic assemblies experience damage due to cyclic thermomechanical loading that eventually leads to fatigue fracture and electrical failure. While solder joints in smaller, die-sized area-array packages largely experience shear fatigue due to thermal expansion mismatch between the component and the substrate, larger area-array packages experience a combination of cyclic shear and axial tensile/compressive loads due to flexure of the substrate. Additionally, on larger processor packages, the attachment of heatsinks further exacerbates the imposed axial loads, as does package warpage. With the increase in size of packages due to 2.5D Heterogeneous integration, the above additional axial loads can be significant. Thus, there exists a critical need to understand the impact on fatigue life of solder joints with superposed compressive/tensile loads on the cyclic shear loads. In this paper, we describe a carefully constructed microscale mechanical tester as well as fatigue test results on Sn3.0Ag0.5Cu (SAC305) solder joints subjected to controlled cyclic shear and constant compressive loads. The tester design allows one to apply cyclic shear loads up to 200 N while maintaining a constant axial load of up to 38 N in tension or compression. The tester is capable of maintaining the compressive load to within a tolerance of +/− 0.5 N during the entirety of fatigue experiment. Carefully constructed samples of Sn3.0Ag0.5Cu solder joints were isothermally fatigued under systematically increased compressive load imposed on the sample subject to repeated loading (R = 0) under lap-shear. In general, the imposition of the superposed compressive load increases the fatigue life of the solder joint compared to application of pure cyclic shear load. At larger compressive loads, friction between fractured surfaces is responsible for significant energy dissipation during the cyclic load-unload cycles.
Modeling of Underfilled PBGA Assemblies Using Both Viscoelastic and Elastic Material Properties
Authors: Chowdhury Promod R. , Suhling Jeffrey C. , Lall Pradeep
Published: December 2019

In microelectronics packaging industry, polymer based materials are used extensively. These polymer materials show viscoelastic behavior when subject to time dependent loads or deformations. The viscoelastic behavior highly depends on both temperature and time. In many cases, these viscoelastic properties are often neglected due to saving computational cost or unavailability of full characterization of the viscoelastic properties. To make accurate predictions of packaging mechanical behavior and reliability, it is important to accurately characterize the viscoelastic behavior of mold compounds, underfill encapsulants, adhesives and other polymers used in electronic assemblies. After characterization, these parameters can be used as input material property data for finite element analysis (FEA) simulations.

In this study, both frequency dependent dynamic mechanical analysis (DMA) measurements, and strain and temperature dependent stress relaxation experiments were performed on a typical underfill material in order to characterize its linear viscoelastic behavior. In both cases, a master curve was determined using the assumption of time-temperature equivalence, and Prony series expansions were utilized to model the underfill material relaxation behavior. After that, these viscoelastic underfill material parameters were used in finite element models of underfilled ball grid array packages (Ultra CSP) subjected to thermal cycling from −40 to 125 °C. Separate simulations were also performed using temperature dependent elastic properties for the underfill material. In both cases, the solder joint fatigue life was estimated, and the effects of using viscoelastic properties for the underfill in solder joint fatigue life simulation were investigated.
Process Capability of Aerosol-Jet Additive Processes for Long-Runs up to 10-Hours
Authors: Lall Pradeep , Abrol Amrit , Kothari Nakul , Leever Ben , Miller Scott
Published: December 2019

Traditionally, the printed circuit assemblies have been fabricated through a combination of imaging and plating based subtractive processes involving use of photo-exposure followed by baths for plating and etching to form the needed circuitry on rigid and flexible laminates. Additive electronics is finding applications for fabrication of IoT sensors. The emergence of a number of additive technologies poses an opportunity for the development of processes for manufacture of flexible substrates using mainstream additive processes, which are now commercially available. Aerosol-Jet printing has shown the capability for printing lines and spaces below 10 μm in width. The Aerosol-Jet system supports a wide variety of materials, including nanoparticle inks and screen-printing pastes, conductive polymers, insulators, adhesives, and even biological matter. The adoption of additive manufacturing for high-volume commercial fabrication requires an understanding of the print consistency, electrical and mechanical properties. Little literature exists that addresses the effect of varying sintering time and temperature on the shear strength and resistivity of the printed lines. In this study, the effect of process parameters on the resultant line-consistency, mechanical and electrical properties has been studied. Print process parameters studied include the sheath rate, mass flow rate, nozzle size, substrate temperature and chiller temperature. Properties include resistance and shear load to failure of the printed electrical line as a function of varying sintering time and varying sintering temperature. Aerosol-Jet machine has been used to print interconnects. Printed samples have been exposed to different sintering times and temperatures. The resistance and shear load to failure of the printed lines has been measured. The underlying physics of the resultant trend was then investigated using elemental analysis and SEM. The effect of line-consistency driftover prolonged runtimes has been measured for up to 10-hours of runtime. Printing process efficiency has been gauged a function of process capability index (Cpk) and process capability ratio (Cp). Printed samples were studied offline using optical Profilometry to analyze the consistency within the line width, line height, line resistance and shear load to study the variance in the electrical and mechanical properties over time.
Health Monitoring of PCB’s Under Mechanical Shock Loads
Authors: Lall Pradeep , Thomas Tony , Suhling Jeff , Blecker Ken
Published: December 2019

Feature vectors for health monitoring of electronic assemblies under repetitive mechanical shock have been developed for assemblies subject to 3,000g acceleration levels. The resistance and strain measurements of the PCB are acquired during each drop to analyze the changes in the values during the experiment. Analysis on the progression of failure was carried out using frequency-based techniques on the strain signals from different locations of the board and failure of the package was identified from the increase in the resistance values of the package during the drop. Feature vectors selected were based on the time-frequency data as well as the logarithmic decrement of the strain signals during the different drops. Different statistical approaches on identifying the changes in the damping characteristics of the package during drop were also carried out. Statistical analysis on the changes in the resistance values were quantified in accordance with the changes in the strain and correlation of the both were attempted. The dependence on position of the strain gauge on the PCB were also studied by comparing the variation of the feature vectors of the corresponding strain signals. The before and after failure strain signals were compared on the frequency components and as well as the changes in the damping characteristics of the strain signals.
Folding-Reliability of Flexible Electronics in Wearable Applications
Authors: Lall Pradeep , Jang Hyesoo , Leever Ben , Miller Scott
Published: December 2019

There is a growing need for flexible hybrid electronics solutions for wearable applications, in which the user may often wear electronics on body, on fabric or on skin. Electronics in wearable application may be subjected to stresses of daily motion including bending, twisting and stretching. Thus, there is need for technologies capable of flexibility, robustness and small size while being lightweight. Existing standards for focus on rigid electronics and there is scarcity of guidance for test-levels needed to assure reliability of flexible electronics. There is need for studies focused on the development of accelerated test conditions representative of field applications and the identification of failure mechanisms for test levels. In this study, experimental analysis on fatigue life of the PCB in cyclical folding load is conducted. A folding test-stand capable of replicating the stresses of daily motion in a lab-environment has been developed for the test. For the better understanding of the failure mechanism, analysis of failure modes is carried out. Consequently, it is found that fatigue life of the PCB is related to the several conditions: folding direction, moving distance, folding diameter and strain rate.
Development of a Technique to Measure Deliquescent Relative Humidity of Particulate Contaminants and Determination of the Operating Relative Humidity of a Data Center
Authors: Shah Jimil M. , Anand Roshan , Saini Satyam , Cyriac Rawhan , Agonafer Dereje , Singh Prabjit , Kaler Mike
Published: December 2019

A remarkable amount of data center energy is consumed in eliminating the heat generated by the IT equipment to maintain and ensure safe operating conditions and optimum performance. The installation of Airside Economizers, while very energy efficient, bears the risk of particulate contamination in data centers, hence, deteriorating the reliability of IT equipment. When RH in data centers exceeds the deliquescent relative humidity (DRH) of salts or accumulated particulate matter, it absorbs moisture, becomes wet and subsequently leads to electrical short circuiting because of degraded surface insulation resistance between the closely spaced features on printed circuit boards. Another concern with this type of failure is the absence of evidence that hinders the process of evaluation and rectification. Therefore, it is imperative to develop a practical test method to determine the DRH value of the accumulated particulate matter found on PCBs (Printed Circuit Boards). This research is a first attempt to develop an experimental technique to measure the DRH of dust particles by logging the leakage current versus RH% (Relative Humidity percentage) for the particulate matter dispensed on an interdigitated comb coupon. To validate this methodology, the DRH of pure salts like MgCl2, NH4NO3 and NaCl is determined and their results are then compared with their published values. This methodology was therefore implemented to help lay a modus operandi of establishing the limiting value or an effective relative humidity envelope to be maintained at a real-world data center facility situated in Dallas industrial area for its continuous and reliable operation.