Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2018;140(7):071201-071201-13. doi:10.1115/1.4038814.

This paper describes a methodology used for propeller performance estimation, which was implemented in an in-house modular program for gas turbine performance prediction. A model based on subsonic generic propeller maps and corrected for compressibility effects, under high subsonic speeds, was proposed and implemented. Considering this methodology, it is possible to simulate conventional turboprop architectures and counter-rotating open rotor (CROR) engines in both steady-state and transient operating conditions. Two simulation scenarios are available: variable pitch angle propeller with constant speed; or variable speed propeller with constant pitch angle. The simulations results were compared with test bench data and two gas turbine performance commercial software packages were used to fulfill the model validation for conventional turboprop configurations. Furthermore, a direct drive CROR engine was simulated using a variable inlet guide vanes (VIGV) control strategy during transient operation. The model has shown to be able to provide several information about propeller-based engine performance using few input data, and a comprehensive understanding on steady-state and transient performance behavior was achieved in the obtained results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):071202-071202-13. doi:10.1115/1.4038838.

This paper documents the setup and validation of nonlinear autoregressive network with exogenous inputs (NARX) models of a heavy-duty single-shaft gas turbine (GT). The data used for model training are time series datasets of several different maneuvers taken experimentally on a GT General Electric PG 9351FA during the start-up procedure and refer to cold, warm, and hot start-up. The trained NARX models are used to predict other experimental datasets, and comparisons are made among the outputs of the models and the corresponding measured data. Therefore, this paper addresses the challenge of setting up robust and reliable NARX models, by means of a sound selection of training datasets and a sensitivity analysis on the number of neurons. Moreover, a new performance function for the training process is defined to weigh more the most rapid transients. The final aim of this paper is the setup of a powerful, easy-to-build and very accurate simulation tool, which can be used for both control logic tuning and GT diagnostics, characterized by good generalization capability.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):071203-071203-10. doi:10.1115/1.4038837.

Conventional propulsion systems are typically represented as uninstalled systems to suit the simple separation between airframe and engine in a podded configuration. However, boundary layer ingesting systems are inherently integrated, and require a different perspective for performance analysis. Simulations of boundary layer ingesting propulsions systems must represent the change in inlet flow characteristics, which result from different local flow conditions. In addition, a suitable accounting system is required to split the airframe forces from the propulsion system forces. The research assesses the performance of a conceptual vehicle, which applies a boundary layer ingesting propulsion system—NASA's N3-X blended wing body aircraft—as a case study. The performance of the aircraft's distributed propulsor array is assessed using a performance method, which accounts for installation terms resulting from the boundary layer ingesting nature of the system. A “thrust split” option is considered, which splits the source of thrust between the aircraft's main turbojet engines and the distributed propulsor array. An optimum thrust split (TS) for a specific fuel consumption at design point (DP) is found to occur for a TS value of 94.1%. In comparison, the optimum TS with respect to fuel consumption for the design 7500 nmi mission is found to be 93.6%, leading to a 1.5% fuel saving for the configuration considered.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

J. Eng. Gas Turbines Power. 2018;140(7):071501-071501-13. doi:10.1115/1.4038460.

It has been recognized in recent years that high altitude atmospheric ice crystals pose a threat to aircraft engines. Instances of damage, surge, and shutdown have been recorded at altitudes significantly greater than those associated with supercooled water icing. It is believed that solid ice particles can accrete inside the core compressor, although the exact mechanism by which this occurs remains poorly understood. Development of analytical and empirical models of the ice crystal icing phenomenon is necessary for both future engine design and this-generation engine certification. A comprehensive model will require the integration of a number of aerodynamic, thermodynamic, and mechanical components. This paper studies one such component, specifically the thermodynamic and mechanical processes experienced by ice particles impinging on a warm surface. Results are presented from an experimental campaign using a heated and instrumented flat plate. The plate was installed in the Altitude Icing Wind Tunnel (AIWT) at the National Research Council of Canada (NRC). This facility is capable of replicating ice crystal conditions at altitudes up to 9 km and Mach numbers up to 0.55. The heated plate is designed to measure the heat flux from a surface at temperatures representative of the early core compressor, under varying convective and icing heat loads. Heat transfer enhancement was observed to rise approximately linearly with both total water content (TWC) and particle diameter over the ranges tested. A Stokes number greater than unity proved to be a useful parameter in determining whether heat transfer enhancement would occur. A particle energy parameter was used to estimate the likelihood of fragmentation. Results showed that when particles were both ballistic and likely to fragment, heat transfer enhancement was independent of both Mach and Reynolds numbers over the ranges tested.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):071502-071502-9. doi:10.1115/1.4038523.

To understand the physics of volcanic ash impact on gas turbine hot-components and develop much-needed tools for engine design and fleet management, the behaviors of volcanic ash in a gas turbine combustor and nozzle guide vanes (NGV) have been numerically investigated. High-fidelity numerical models are generated, and volcanic ash sample, physical, and thermal properties are identified. A simple critical particle viscosity—critical wall temperature model is proposed and implemented in all simulations to account for ash particles bouncing off or sticking on metal walls. The results indicate that due to the particle inertia and combustor geometry, the volcanic ash concentration in the NGV cooling passage generally increases with ash size and density, and is less sensitive to inlet velocity. It can reach three times as high as that at the air inlet for the engine conditions and ash properties investigated. More importantly, a large number of the ash particles entering the NGV cooling chamber are trapped in the cooling flow passage for all four turbine inlet temperature conditions. This may reveal another volcanic ash damage mechanism originated from engine cooling flow passage. Finally, some suggestions are recommended for further research and development in this challenging field. To the best of our knowledge, it is the first study on detailed ash behaviors inside practical gas turbine hot-components in the open literature.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):071503-071503-9. doi:10.1115/1.4038322.

This paper describes experimental measurements of forced ignition of prevaporized liquid fuels in a well-controlled facility that incorporates nonuniform flow conditions similar to those of gas turbine engine combustors. The goal here is to elucidate the processes by which the initially unfueled kernel evolves into a self-sustained flame. Three fuels are examined: a conventional Jet-A and two synthesized fuels that are used to explore fuel composition effects. A commercial, high-energy recessed cavity discharge igniter located at the test section wall ejects kernels at 15 Hz into a preheated, striated crossflow. Next to the igniter wall is an unfueled air flow; above this is a premixed, prevaporized, fuel–air flow, with a matched velocity and an equivalence ratio near 0.75. The fuels are prevaporized in order to isolate chemical effects. Differences in early ignition kernel development are explored using three synchronized, high-speed imaging diagnostics: schlieren, emission/chemiluminescence, and OH planar laser-induced fluorescence (PLIF). The schlieren images reveal rapid entrainment of crossflow fluid into the kernel. The PLIF and emission images suggest chemical reactions between the hot kernel and the entrained fuel–air mixture start within tens of microseconds after the kernel begins entraining fuel, with some heat release possibly occurring. Initially, dilution cooling of the kernel appears to outweigh whatever heat release occurs; so whether the kernel leads to successful ignition or not, the reaction rate and the spatial extent of the reacting region decrease significantly with time. During a successful ignition event, small regions of the reacting kernel survive this dilution and are able to transition into a self-sustained flame after ∼1–2 ms. The low-aromatic/low-cetane-number fuel, which also has the lowest ignition probability, takes much longer for the reaction zone to grow after the initial decay. The high-aromatic, more easily ignited fuel, shows the largest reaction region at early times.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2018;140(7):071701-071701-10. doi:10.1115/1.4038362.

This work presents an exergy analysis and performance assessment of three recuperative thermodynamic cycles for gas turbine applications. The first configuration is the conventional recuperative (CR) cycle in which a heat exchanger is placed after the power turbine (PT). In the second configuration, referred as alternative recuperative (AR) cycle, a heat exchanger is placed between the high pressure and the PT, while in the third configuration, referred as staged heat recovery (SHR) cycle, two heat exchangers are employed, the primary one between the high and PTs and the secondary at the exhaust, downstream the PT. The first part of this work is focused on a detailed exergetic analysis on conceptual gas turbine cycles for a wide range of heat exchanger performance parameters. The second part focuses on the implementation of recuperative cycles in aero engines, focused on the MTU-developed intercooled recuperative aero (IRA) engine concept, which is based on a conventional recuperation approach. Exergy analysis is applied on specifically developed IRA engine derivatives using both alternative and SHR recuperation concepts to quantify energy exploitation and exergy destruction per cycle and component, showing the amount of exergy that is left unexploited, which should be targeted in future optimization actions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):071702-071702-8. doi:10.1115/1.4038476.

For the concentrating solar power (CSP) applications, the supercritical carbon dioxide (s-CO2) power cycle is beneficial in many aspects, including high cycle efficiencies, reduced component sizing, and potential for the dry cooling option. More research is involved in improving this technology to realize the s-CO2 cycle as a candidate to replace the conventional power conversion systems for CSP applications. In this study, an isothermal compressor, a turbomachine which undergoes the compression process at constant temperature to minimize compression work, is applied to the s-CO2 power cycle layout. To investigate the cycle performance changes of adopting the novel technology, a framework for defining the efficiency of the isothermal compressor is revised and suggested. This study demonstrates how the compression work for the isothermal compressor is reduced, up to 50%, compared to that of the conventional compressor under varying compressor inlet conditions. Furthermore, the simple recuperated and recompression Brayton cycle layouts using s-CO2 as a working fluid are evaluated for the CSP applications. Results show that for compressor inlet temperatures (CIT) near the critical point, the recompression Brayton cycle using an isothermal compressor has 0.2–1.0% point higher cycle thermal efficiency compared to its reference cycle. For higher CIT values, the recompression cycle using an isothermal compressor can perform above 50% in thermal efficiency for a wider range of CIT than the reference cycle. Adopting an isothermal compressor in the s-CO2 layout can imply larger heat exchange area for the compressor which requires further development.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2018;140(7):071901-071901-12. doi:10.1115/1.4038618.

This paper deals with a numerical study aimed at the validation of a computational procedure for the aerothermal characterization of preswirl systems employed in axial gas turbines. The numerical campaign focused on an experimental facility which models the flow field inside a direct-flow preswirl system. Steady and unsteady simulation techniques were adopted in conjunction with both a standard two-equation Reynolds-averaged Navier–Stokes (RANS)/unsteady RANS (URANS) modeling and more advanced approaches such as the scale-adaptive-simulation (SAS) principle, the stress-blended eddy simulation (SBES), and large eddy simulation (LES). Overall, the steady-state computational fluid dynamics (CFD) predictions are in reasonable good agreement with the experimental evidences even though they are not able to confidently mimic the experimental swirl and pressure behavior in some regions. Scale-resolved approaches improve the computations accuracy significantly especially in terms of static pressure distribution and heat transfer on the rotating disk. Although the use of direct turbulence modeling would in principle increase the insight in the physical phenomenon, from a design perspective, the trade-off between accuracy and computational costs is not always favorable.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):071902-071902-10. doi:10.1115/1.4038756.

The buoyancy-induced flow and heat transfer inside the compressor rotors of gas-turbine engines affects the stresses and radial growth of the compressor disks, and it also causes a temperature rise in the axial throughflow of cooling air through the center of the disks. In turn, the radial growth of the disks affects the radial clearance between the rotating compressor blades and the surrounding stationary casing. The calculation of this clearance is extremely important, particularly in aeroengines where the increase in pressure ratios results in a decrease in the size of the blades. In this paper, a published theoretical model—based on buoyancy-induced laminar Ekman-layer flow on the rotating disks—is extended to include laminar free convection from the compressor shroud and forced convection between the bore of the disks and the axial throughflow. The predicted heat transfer from these three surfaces is then used to calculate the temperature rise of the throughflow. The predicted temperatures and Nusselt numbers are compared with measurements made in a multicavity compressor rig, and mainly good agreement is achieved for a range of Rossby, Reynolds, and Grashof numbers representative of those found in aeroengine compressors. Owing to compressibility effects in the fluid core between the disks—and as previously predicted—increasing rotational speed can result in an increase in the core temperature and a consequent decrease in the Nusselt numbers from the disks and shroud.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2018;140(7):072501-072501-13. doi:10.1115/1.4038542.

A finite element (FE) model of the rotor tester of an aero-engine, having a thin-walled casing structure, mounted with the way of an actual engine, is developed to simulate the intrinsic vibration characteristics under actual engine-mounting condition. First, a modal experiment of the rotor tester for the whole aero-engine is conducted, and the FE model is modified and validated based on the modal experimental results. Second, the first three orders of natural frequencies and the modal shapes are evaluated using the modified FE model under three different types of mounting stiffness, namely, a fixed mounting boundary, a free mounting boundary, and a flexible mounting boundary. Subsequently, the influences of the mounting stiffness on the coupling vibration of the rotor and stator are studied via a new rotor–stator coupling factor, which is proposed in this study. The results show that the higher the rotor–stator coupling degree of the modal shape, the greater the influence of the mounting condition on the modal shape. Moreover, the influence of the mounting stiffness on the rotor–stator coupling degree is nonlinear. The coupling phenomena of the rotor and stator exist in many modal shapes of actual large turbofan engines, and the effect of mounting stiffness on the rotor–stator coupling cannot be ignored. Hence, the mounting stiffness needs to be considered carefully while modeling the whole aero-engine and simulating the dynamic characteristics of the whole aero-engine.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072502-072502-10. doi:10.1115/1.4038549.

Achieving an optimal design of journal bearings is a very challenging effort due to the many input and output variables involved, including rotordynamic and tribological responses. This paper demonstrates the use of a multivariate response modeling approach based on response surface design of experiments (RSDOE) to design tilting pad bearings. It is shown that an optimal configuration can be achieved in the early stages of the design process while substantially reducing the amount of calculations. To refine the multivariate response model, statistical significance of the factors was assessed by examining the test's p-value. The effect coefficient calculation complemented the statistical hypothesis testing as an overall quantitative measure of the strength of factors, namely; main effects, quadratic effects, and interactions between variables. This provided insight into the potential nonlinearity of the phenomena. Once arriving at an optimized design, a sensitivity analysis was performed to identify the input variables whose variabilities have the greatest influence on the mean of a given response. Finally, an analysis of percent contribution of each input variable standard deviation to the actual response standard deviation was performed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072503-072503-10. doi:10.1115/1.4038603.

High-speed foil bearings are currently used in increasingly demanding, high performance applications. The application under consideration is a 120 krpm natural gas turboexpander-compressor, which requires 38 mm (1.5 in.) foil journal bearings with high stiffness and load capacity to help enhance rotordynamic stability. This paper describes the development of the foil bearing for this application and includes measured stiffness and damping coefficients recorded on a high-speed dynamic bearing test rig. The dynamic test data were taken for several different foil bearing configurations with varying spring-element foil thicknesses, number of spring-element foils, and bearing shim thickness. All three parameters have a direct impact on bearing clearance. The influence of these different parameters on measured stiffness and damping coefficients and thermal performance of the bearings are presented and discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072504-072504-9. doi:10.1115/1.4038458.

Engine designers require accurate predictions of ingestion (or ingress) principally caused by circumferential pressure asymmetry in the mainstream annulus. Cooling air systems provide purge flow designed to limit metal temperatures and protect vulnerable components from the hot gases which would otherwise be entrained into disk cavities through clearances between rotating and static disks. Rim seals are fitted at the periphery of these disks to minimize purge. The mixing between the efflux of purge (or egress) and the mainstream gases near the hub end-wall results in a deterioration of aerodynamic performance. This paper presents experimental results using a turbine test rig with wheel-spaces upstream and downstream of a rotor disk. Ingress and egress was quantified using a CO2 concentration probe, with seeding injected into the upstream and downstream sealing flows. The probe measurements have identified an outer region in the wheel-space and confirmed the expected flow structure. For the first time, asymmetric variations of concentration have been shown to penetrate through the seal clearance and the outer portion of the wheel-space between the disks. For a given flow coefficient in the annulus, the concentration profiles were invariant with rotational Reynolds number. The measurements also reveal that the egress provides a film-cooling benefit on the vane and rotor platforms. Further, these measurements provide unprecedented insight into the flow interaction and provide quantitative data for computational fluid dynamics (CFD) validation, which should help to reduce the use of purge and improve engine efficiency.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072505-072505-10. doi:10.1115/1.4038613.

Current efforts to model multistage turbomachinery systems rely on calculating independent constraint modes for each degree-of-freedom (DOF) on the boundary between stages. While this approach works, it is computationally expensive to calculate all the required constraint modes. This paper presents a new way to calculate a reduced set of constraint modes referred to as Fourier constraint modes (FCMs). These FCMs greatly reduce the number of computations required to construct a multistage reduced order model (ROM). The FCM method can also be integrated readily with the component mode mistuning (CMM) method to handle small mistuning and the pristine rogue interface modal expansion (PRIME) method to handle large and/or geometric mistuning. These methods all use sector-level models and calculations, which make them very efficient. This paper demonstrates the efficiency of the FCM method on a multistage system that is tuned and, for the first time, creates a multistage ROM with large mistuning using only sector-level quantities and calculations. The results of the multistage ROM for the tuned and large mistuning cases are compared with full finite element results and are found in good agreement.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072506-072506-8. doi:10.1115/1.4038880.

Fracture of blades is usually catastrophic and creates serious damages in the turbomachines. Blades are subjected to high centrifugal force, oscillating stresses, and high temperature which makes their life limited. Therefore, blades should be checked and replaced at specified intervals or utilize a health monitoring method for them. Crack detection by nondestructive tests can only be performed during machine overhaul which is not suitable for monitoring purposes. Blade tip timing (BTT) method as a noncontact monitoring technique is spreading for health monitoring of the turbine blades. One of the main challenges of BTT method is identification of vibration parameters from one per revolution samples which is quite below Nyquist sampling rate. In this study, a new method for derivation of blade asynchronous vibration parameters from BTT data is proposed. The proposed method requires only two BTT sensors and applies least mean square algorithm to identify frequency and amplitude of blade vibration. These parameters can be further used as blade health indicators to predict defect growth in the blades. Robustness of the proposed method against measurement noise which is an important factor has been examined by numerical simulation. An experimental test was conducted on a bladed disk to show efficiency of the proposed method.

Topics: Vibration , Blades , Sensors
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072507-072507-10. doi:10.1115/1.4038361.

In gas turbines, rim seals are fitted at the periphery of stator and rotor discs to minimize the purge flow required to seal the wheel-space between the discs. Ingestion (or ingress) of hot mainstream gases through rim seals is a threat to the operating life and integrity of highly stressed components, particularly in the first-stage turbine. Egress of sealing flow from the first-stage can be re-ingested in downstream stages. This paper presents experimental results using a 1.5-stage test facility designed to investigate ingress into the wheel-spaces upstream and downstream of a rotor disk. Re-ingestion was quantified using measurements of CO2 concentration, with seeding injected into the upstream and downstream sealing flows. Here, a theoretical mixing model has been developed from first principles and validated by the experimental measurements. For the first time, a method to quantify the mass fraction of the fluid carried over from upstream egress into downstream ingress has been presented and measured; it was shown that this fraction increased as the downstream sealing flow rate increased. The upstream purge was shown to not significantly disturb the fluid dynamics but only partially mixes with the annulus flow near the downstream seal, with the ingested fluid emanating from the boundary layer on the blade platform. From the analogy between heat and mass transfer, the measured mass-concentration flux is equivalent to an enthalpy flux, and this re-ingestion could significantly reduce the adverse effect of ingress in the downstream wheel-space. Radial traverses using a concentration probe in and around the rim seal clearances provide insight into the complex interaction between the egress, ingress and mainstream flows.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2018;140(7):072601-072601-8. doi:10.1115/1.4038544.

The steam consumption in a turbine within an operating pressure range determines the effectiveness of thermal energy conversion to electric power generation in a turbo-alternator. The low pressure (LP) stage of the steam turbine produces largest amount of steam to shaft-power in comparison to other stages of turbine although susceptible to various additional losses due to condensation of wet steam near penultimate and ultimate stages. The surface deposition in blade is caused by inertial impaction and turbulent-diffusion. With increasing blade stagger angle along the larger diameter of blading, the fractional deposition of wet steam is largely influenced by blade shape. From this background, the aim of this work is to predict the effect of mathematical models through computational fluid dynamics analysis on the characterization of thermodynamic and mechanical loss components based on unsaturated vapor water droplet size and pressure zones in LP stages of steam turbine and to investigate the influence of droplet size and rotor blade profile on cumulative energy losses due to condensation and provide an indication about the possible conceptual optimization of blade profile design to minimize moisture-induced energy losses.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072602-072602-14. doi:10.1115/1.4038856.

This paper describes studies completed using a quarter-scaled rig to assess the impact of turbine exit swirl angle and strut stagger on a turbine exhaust system consisting of an integral diffuser-collector. Advanced testing methods were applied to ascertain exhaust performance for a range of inlet conditions aerodynamically matched to flow exiting an industrial gas turbine. Flow visualization techniques along with complementary computational fluid dynamics (CFD) predictions were used to study flow behavior along the diffuser end walls. Complimentary CFD analysis was also completed with the aim to ascertain the performance prediction capability of modern day analytical tools for design phase and off-design analysis. The K-Epsilon model adequately captured the relevant flow features within both the diffuser and collector, and the model accurately predicted the recovery at design conditions. At off-design conditions, the recovery predictions were found to be pessimistic. The integral diffuser-collector exhaust accommodated a significant amount of inlet swirl without degradation in performance, so long as the inlet flow direction did not significantly deviate from the strut stagger angle. Strut incidence at the hub was directly correlated with reduction in overall performance, whereas the diffuser-collector performance was not significantly impacted by strut incidence at the shroud.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Vehicular and Small Turbomachines

J. Eng. Gas Turbines Power. 2018;140(7):072701-072701-10. doi:10.1115/1.4038323.

This paper presents the development approach, design, and evaluation of three turbocharger compressors with variable geometry for heavy duty engines. The main goal is the improvement of fuel economy without sacrifices regarding any other performance criteria. In a first step, a vaned diffuser parameter study shows that efficiency improvements in the relevant operating areas are possible at the cost of reduced map width. Concluding from the results, three variable geometries with varying complexity based on vaned diffusers are designed. Results from the hot gas test stand and engine test rig show that all systems are capable of increasing compressor efficiency and thus improving fuel economy in the main driving range of heavy duty engines. The most significant differences can be seen regarding the engine brake performance. Only one system meets all engine demands while improving fuel economy.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2018;140(7):072801-072801-12. doi:10.1115/1.4038543.

Modern diesel engines equip the exhaust gas recirculation (EGR) system because it can suppress NOx emissions effectively. However, since a large amount of exhaust gas might cause the degradation of drivability, the control strategy of EGR system is crucial. The conventional control structure of the EGR system uses the mass air flow (MAF) as a control indicator, and its set-point is determined from the well-calibrated look-up table (LUT). However, this control structure cannot guarantee the optimal engine performance during acceleration operating conditions because the MAF set-point is calibrated at steady operating conditions. In order to optimize the engine performance with regard to NOx emission and drivability, an optimization algorithm in a function of the intake oxygen fraction (IOF) is proposed because the IOF directly affects the combustion and engine emissions. Using the NOx and drivability models, the cost function for the performance optimization is designed and the optimal value of the IOF is determined. Then, the MAF set-point is adjusted to trace the optimal IOF under engine acceleration conditions. The proposed algorithm is validated through scheduled engine speeds and loads to simulate the extra-urban driving cycle of the European driving cycle. As validation results, the MAF is controlled to trace the optimal IOF from the optimization method. Consequently, the NOx emission is substantially reduced during acceleration operating conditions without the degradation of drivability.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072802-072802-9. doi:10.1115/1.4038818.

Measurement of film thickness between piston ring and cylinder bore has been a challenge for decades; laser-induced fluorescence (LIF) method was used by several groups, and promising results are obtained for the investigation of lubricant film transport. In this study, blue light generated by a laser source is transmitted to a beam splitter by means of a fiber optic cable and combined with another fiber optic line, then transmitted to the piston ring and cylinder bore conjunction. The light causes the fluorescence dye present in the lubricant to emit light in a longer wavelength, i.e., green. Reflected light is recollected; blue wavelength components are filtered out using a narrow band pass optical filter, and only components in the florescence wavelength is transmitted to a photomultiplier tube. The photomultiplier produces a voltage proportional to instantaneous lubricant film thickness. Then, the photomultiplier signal is calibrated for lubricant film thickness using a laser textured cylinder bore with known geometries. Additional marks were etched on the liner for calibration. The LIF system is adapted to a piston ring and cylinder bore friction test system simulating engine conditions. Static piston ring and reciprocating liner configuration of the bench test system allow the collection of continuous lubricant film thickness data as a function of crank angle position. The developed system has potential to evaluate new designs, materials, and surface properties in a controlled and repeatable environment.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;140(7):072803-072803-10. doi:10.1115/1.4038881.

Efforts to improve the range and endurance of group 2 (10–25 kg), internal combustion engine (ICE) powered unmanned aerial vehicles (UAVs) have been underway for several years at Air Force Research Laboratory (AFRL). To obtain the desired performance improvements, research into improving the overall efficiency of the ICE powerplants is of great interest. The high specific energy of hydrocarbon fuels (13,000 W h/kg for gasoline), but low fuel conversion efficiency for small ICEs means that relatively minor improvements in the fuel conversion efficiency of the engines can yield large improvements in range and endurance. Little information is available however for the efficiency of ICEs in the size range of interest (10–200 cm3 displacement volume) for group 2 UAVs. Most of the currently available efficiency data for 10–200 cm3 ICEs is for two-stroke engines. The goal of this study was to provide an in-depth probe of the efficiency and energy losses of a small displacement four-stroke engine which could potentially be used to power a group 2 UAV. Energy balances were performed on a Honda GX120 four-stroke engine using empirical research methods. The engine was a 118 cm3 displacement, single cylinder ICE. Energy pathways were characterized as a percentage of the total chemical energy available in the fuel. Energy pathways were characterized into four categories: brake power, cooling load, exhaust sensible enthalpy and incomplete combustion. The effect of five operating parameters was examined in the study. Fuel conversion efficiency ranged from 22.2% to 25.8% as engine speed was swept from 2000 to 3600 RPM, from 20.8% to 27.3% as equivalence ratio was swept from 0.85 to 1.25, and from 15.7% to 24.9% as throttle was swept from 28.5% to 100%. Combustion phasing and cylinder head temperature sweeps showed only minor changes in fuel conversion efficiency.

Commentary by Dr. Valentin Fuster

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