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### Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2011;133(10):101201-101201-10. doi:10.1115/1.4002914.

This paper presents observations and simulations of the impact of several technologies on modifying the flow-field and acoustic emissions from supersonic jets from nozzles typical of those used on military aircraft. The flow-field is measured experimentally by shadowgraph and particle image velocimetry. The acoustics are characterized by near- and far-field microphone measurements. The flow- and near-field pressures are simulated by a monotonically integrated large eddy simulation. Use of unstructured grids allows accurate modeling of the nozzle geometry. The emphasis of the work is on “off-design” or nonideally expanded flow conditions. The technologies applied to these nozzles include chevrons, fluidic injection, and fluidically enhanced chevrons. The fluidic injection geometry and the fluidic enhancement geometry follow the approach found successful for subsonic jets by employing jets pitched 60 deg into the flow, impinging on the shear layer just past the tips of the chevrons or in the same axial position when injection is without chevrons.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2011;133(10):101501-101501-10. doi:10.1115/1.4003164.

A frequently employed method for aerodynamic flame stabilization in modern premixed low emission combustors is the breakdown of swirling flows; with carefully optimized tailoring of the swirler, a sudden transition in the flow field in the combustor can be achieved. A central recirculation zone evolves at the cross-sectional area change located at the entrance of the combustion chamber and anchors the flame in a fixed position. In general, premixed combustion in swirling flows can lead to flame flashback that is caused by combustion induced vortex breakdown near the centerline of the flow. In this case, the recirculation zone suddenly moves upstream and stabilizes in the premix zone (Kröner, 2007, “Flame Propagation in Swirling Flows—Effect of Local Extinction on the Combustion Induced Vortex Breakdown,” Combust. Sci. Technol., 179, pp. 1385–1416). This type of flame flashback is caused by a strong interaction between the flame chemistry and vortex dynamics. The analysis of the vorticity transport equation shows that the axial gradient of the azimuthal vorticity is of particular importance for flame stability. A negative azimuthal vorticity gradient decelerates the core flow and finally causes vortex breakdown. Based on fundamental fluid mechanics, guidelines for a proper aerodynamic design of gas turbine combustors are given. These guidelines summarize the experience from several previous aerodynamic and combustion studies of the authors.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):101502-101502-7. doi:10.1115/1.4002946.

Due to stringent emission restrictions, modern gas turbines mostly rely on lean premixed combustion. Since this combustion mode is susceptible to thermoacoustic instabilities, there is a need for modeling tools with predictive capabilities. Linear network models are able to predict the occurrence of thermoacoustic instabilities but yield no information on the oscillation amplitude. The prediction of the pulsation levels and hence an estimation whether a certain operating condition has to be avoided is only possible if information on the nonlinear flame response is available. Typically, the flame response shows saturation at high forcing amplitudes. A newly constructed atmospheric test rig, specifically designed for the realization of high excitation amplitudes over a broad frequency range, is used to generate extremely high acoustic forcing power with velocity fluctuations of up to 100% of the mean flow. The test rig consists of a generic combustor with a premixed swirl-stabilized natural gas flame, where the upstream part has a variable length to generate adaptive resonances of the acoustic field. The $OH∗$ chemiluminescence response, with respect to velocity fluctuations at the burner, is measured for various excitation frequencies and amplitudes. From these measurements, an amplitude dependent flame transfer function is obtained. Phase-averaged $OH∗$ pictures are used to identify changes in the flame shape related to saturation mechanisms. For different frequency regimes, different saturation mechanisms are identified.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):101503-101503-11. doi:10.1115/1.4002808.

The mixing of fuel and air in combustion systems plays a key role in overall operability and emissions performance. Such systems are also being looked to for operation on a wide array of potential fuel types, including those derived from renewable sources such as biomass or agricultural waste. The optimization of premixers for such systems is greatly enhanced if efficient design tools can be utilized. The increased capability of computational systems has allowed tools such as computational fluid dynamics to be regularly used for such purpose. However, to be applied with confidence, validation is required. In the present work, a systematic evaluation of fuel mixing in a specific geometry, which entails cross flow fuel injection into axial nonswirling air streams has been carried out for methane and hydrogen. Fuel concentration is measured at different planes downstream of the point of injection. In parallel, different computational fluid dynamics approaches are used to predict the concentration fields resulting from the mixing of fuel and air. Different steady turbulence models including variants of Reynolds averaged Navier–Stokes (RANS) have been applied. In addition, unsteady RANS and large eddy simulation are used. To accomplish mass transport with any of the RANS approaches, the concept of the turbulent Schmidt number is generally used. As a result, the sensitivity of the RANS simulations to different turbulent Schmidt number values is also examined. In general, the results show that the Reynolds stress model, with use of an appropriate turbulent Schmidt number for the fuel used, provides the best agreement with the measured values of the variation in fuel distribution over a given plane in a relatively time efficient manner. It is also found that, for a fixed momentum flux ratio, both hydrogen and methane penetrate and disperse in a similar manner for the flow field studied despite their significant differences in density and diffusivity.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

J. Eng. Gas Turbines Power. 2011;133(10):101601-101601-8. doi:10.1115/1.4002812.

The CF-18 (CF denotes Canadian Forces) aircraft is a complex system for which a variety of data are systematically being recorded: flight data from sensors, built-in test equipment data, and maintenance data. Without proper analytical and statistical tools, these data resources are of limited use to the operating organization. Focusing on data mining-based modeling, this paper investigates the use of readily available CF-18 data to support the development of prognostics and health management systems. A generic data mining methodology has been developed to build prognostic models from operational and maintenance data. This paper introduces the methodology and elaborates on challenges specific to the use of CF-18 data from the Canadian Forces. A number of key data mining tasks are examined including data gathering, information fusion, data preprocessing, model building, and model evaluation. The solutions developed to address these tasks are described. A software tool developed to automate the model development process is also presented. Finally, this paper discusses preliminary results on the creation of models to predict F404 no. 4 bearing and main fuel control failures on the CF-18.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Industrial & Cogeneration

J. Eng. Gas Turbines Power. 2011;133(10):102001-102001-8. doi:10.1115/1.4002947.

At present, inlet fogging and wet compression are two of the most widely used approaches to enhance gas turbine performance, especially during hot seasons. However, potentially negative effects of these practices on long-term operational integrity of gas turbines should be evaluated carefully; in particular, wet compression may lead to the erosion of first compressor stages due to the impact of water droplets within the flow at compressor intake. This issue is still controversial in technical literature since only limited historical field operating data and information are available. Therefore, a test facility was specifically set up in the laboratories of the University of Ferrara, to evaluate the effects of wet compression on a small-size compressor. This paper presents the experimental facility developed for wet compression investigation and some preliminary results.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

J. Eng. Gas Turbines Power. 2011;133(10):102301-102301-10. doi:10.1115/1.4002826.

A compact, high power density turbo-generator system was conceived, designed, and experimentally tested. The air-to-power (A2P) device with a nominal design point of 50 W electric power output operates on high pressure air such as from a plant pneumatic system or from a portable bottle of pressurized air. A concept design study was first carried out to explore the design space for a range of output power at cost efficiency levels specified in collaboration with industry. The cost efficiency is defined as the cost of electrical power over the cost of pressurized air. The key challenge in the design is the relatively low power demand of 50 W while operating at high supply pressures of nominally 5–6 bars. To meet the cost efficiency goal under these conditions, a high-speed turbine and generator $(∼450,000 rpm)$ are required with small blade span $(∼200 μm)$, minimizing the mass flow while achieving the highest possible turbine performance. Since turbines with such small turbomachinery blading are not commercially available, a silicon-based micro electro mechanical systems (MEMS) turbine was designed using 2D and 3D computational fluid dynamics (CFD) computations. To reduce the development time, existing and previously demonstrated custom-made generator and ceramic ball bearing technology were used, resulting in a compact A2P proof-of-concept demonstration. The cylindrical device of 35 mm diameter resembles a tube fitting with a standard M24 adapter. Without load, a top turbine speed of 475,000 rpm was demonstrated, exceeding the design specification. Using load resistors, the proof-of-concept A2P device achieved 30 W of electrical power at 360,000 rpm and a turbine efficiency of 47%, meeting the cost efficiency goal. Higher speeds under load could not be achieved due to thrust load limitations of the off-shelf ball bearings. The demonstrated performance is in good agreement with the projected CFD based predictions. To the authors’ knowledge, this is the first successful demonstration of a self-contained, 50 W class turbo-generator of hybrid architecture where a MEMS turbine disk is joined with a precision machined titanium shaft and aluminum housing.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):102302-102302-6. doi:10.1115/1.4002827.

A corrosion- and creep-resistant austenitic stainless steel has been developed for advanced recuperator applications. By optimizing the Al and Cr contents, the alloy is fully austenitic for creep strength while allowing the formation of a chemically stable external alumina scale at temperatures up to $900°C$. An alumina scale eliminates long-term problems with the formation of volatile Cr oxy-hydroxides in the presence of water vapor in exhaust gas. As a first step in producing foil for primary surface recuperators, three commercially cast heats have been rolled to $∼100 μm$ thick foil in the laboratory to evaluate performance in creep and oxidation testing. Results from initial creep testing are presented at $675°C$ and $750°C$, showing excellent creep strength compared with other candidate foil materials. Laboratory exposures in humid air at $650–800°C$ have shown acceptable oxidation resistance. A similar oxidation behavior was observed for sheet specimens of these alloys exposed in a modified 65 kW microturbine for 2871 h. One composition that showed superior creep and oxidation resistance has been selected for the preparation of a commercial batch of foil.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2011;133(10):102501-102501-9. doi:10.1115/1.4002908.

This paper addresses self-diagnostic properties of active magnetic bearing (AMB) supported rotors for online detection of the transverse crack on a rotating shaft. In addition to pure levitation, the rotor supporting bearing also serves as an actuator that transforms current signals additionally injected into the control loop into the superimposed specially selected excitation forces into the suspended rotor. These additional excitations induce combination frequencies in the rotor response, providing unique signatures for the presence of crack. The background of theoretical modeling, experimental, and computer simulation results for the AMB supported cracked rotor with self-diagnostic excitation forces are presented and discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):102502-102502-8. doi:10.1115/1.4003021.

It is known that small differences between nominally identical turbomachine blades, known as mistuning, can lead to significant variation in their vibration response levels. A commonly used term in mistuning studies called the “amplification factor” is clearly defined in this paper, and the high sensitivity of high-cycle-fatigue-related fatigue life to the level of vibration response levels is presented. Computer simulations are run to study the distribution of the amplification factor in three situations, namely, (i) bladed disks with damping mistuning, (ii) EO excitation of bladed disk modes in the veering region, and (iii) apparently tuned bladed disks. In addition to running simulations, the upper bound of the adjusted amplification factor in damping-mistuned bladed disks is derived theoretically.

Topics: Damping , Vibration , Disks , Blades
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):102503-102503-10. doi:10.1115/1.4002810.

A highly accurate and computationally efficient method is proposed for reduced modeling of jointed structures in the frequency domain analysis of nonlinear steady-state forced response. The method has significant advantages comparing with the popular variety of mode synthesis methods or forced response matrix methods and can be easily implemented in the nonlinear forced response analysis using standard finite element codes. The superior qualities of the new method are demonstrated on a set of major problems of nonlinear forced response analysis of bladed disks with contact interfaces: (i) at blade roots, (ii) between interlock shrouds, and (iii) at underplatform dampers. The numerical properties of the method are thoroughly studied on a number of special test cases.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):102504-102504-9. doi:10.1115/1.4002865.

The use of gas bearings has increased over the past several decades to include microturbines, air cycle machines, and hermetically sealed compressors and turbines. Gas bearings have many advantages over traditional bearings, such as rolling element or oil lubricated fluid film bearings, including longer life, ability to use the process fluid, no contamination of the process with lubricants, accommodating high shaft speeds, and operation over a wide range of temperatures. Unlike fluid film bearings that utilize oil, gas lubricated bearings generate very little damping from the gas itself. Therefore, successful bearing designs such as foil bearings utilize damping features on the bearing to improve the damping generated. Similar to oil bearings, gas bearing designers strive to develop gas bearings with good rotordynamic stability. Gas bearings are challenging to design, requiring a fully coupled thermo-elastic, hydrodynamic analysis including complex nonlinear mechanisms such as Coulomb friction. There is a surprisingly low amount of rotordynamic force coefficient measurement in the literature despite the need to verify the model predictions and the stability of the bearing. This paper describes the development and testing of a 60,000 rpm gas bearing test rig and presents measured stiffness and damping coefficients for a 57 mm foil type bearing. The design of the rig overcomes many challenges in making this measurement by developing a patented, high-frequency, high-amplitude shaker system, resulting in excitation over most of the subsynchronous range.

Commentary by Dr. Valentin Fuster

### Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2011;133(10):102801-102801-13. doi:10.1115/1.4003048.

A new multizone premixed-diffusive combustion model has been developed, assessed, and applied to diagnose the burning process and emission formation in a conventional and in a premixed charge compression ignition (PCCI) diesel engine. The model is based on the Dec conceptual scheme, which considers combustion as a two-stage quasi-steady process: All fuel particles undergo a first rich premixed combustion phase, and the products complete their oxidation in close-to-stoichiometric conditions at the jet periphery through a diffusion flame. The combustion chamber contents have been divided into several homogeneous zones to which the energy and mass conservation principles were applied. The computed thermodynamic and thermochemical properties in the burned gas zones allowed a post-processing analysis to be made of the nitric oxides (NO), particulate matter (PM), and carbon monoxide (CO) formation. The model requires the in-cylinder pressure trace and other experimental engine quantities as input data and calculates the premixed and diffusive heat release rates along with the temperature and mass evolutions of the different zones. Thus, the model is not predictive but diagnostic: The objective is to interpret measured engine data in order to obtain insight into the in-chamber combustion and pollutant formation processes. The model has been tested on EGR-sweeps and under full-load conditions on the conventional engine and under a high EGR operating condition on the PCCI engine. With reference to NO emissions, the model results showed an excellent agreement with the experimental data for all the tests even when the main model parameters were kept constant for different test conditions. Good results were also obtained for the prediction of the CO and PM emission levels. Finally, for the premixed combustion zone, it was ascertained that higher local A/F ratios were required in the PCCI combustion mode than in the conventional mode as a consequence of the increase in the degree of premixing.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):102802-102802-5. doi:10.1115/1.4003165.

Many marine and stationary engines operate on fuels that contain corrosive elements, with the result that some highly loaded combustion chamber components must be replaced frequently. Märkisches Werk, GmbH (MWH) has pioneered the development of mineral-metal, multiphase coatings to protect valves and other highly loaded engine components against hot-gas corrosion. Mineral-metal, multiphase coatings are a unique and innovative approach to improving hot-gas corrosion resistance in a cost-effective manner. In general, these coatings combine the beneficial chemical and thermal attributes of ceramic coatings with the mechanical properties and substrate adhesion characteristics of a metal. Extensive laboratory and field trials have proven that MWH CrystalCoat protects heavy fuel oil (HFO) engine exhaust valves against hot-gas corrosion. It is projected that the newest coating formulation (CrystalCoat HT) will protect four-stroke HFO exhaust valves against hot-gas corrosion over their entire service life.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):102803-102803-7. doi:10.1115/1.4002916.

This paper documents the exhaust emission test results from a Tier 2 General Electric ES44DC line-haul locomotive with 3280 kW rated traction power and the impact of biodiesel fuel blends on regulated exhaust emissions. Baseline exhaust emission testing was performed with a test fuel containing a sulfur concentration of approximately 400 ppm and was followed by testing of fuel blends containing 2%, 10%, 20%, and 100% soybean derived biodiesel (B2, B10, B20, and B100). Gaseous and particulate emissions were sampled per Title 40 of the United States Code of Federal Regulations, Part 92. Test results indicate particulate matter (PM) reductions occurred over the Environmental Protection Agency (EPA) locomotive line-haul and switch duty cycles for each biodiesel blend tested, as compared with the base fuel. The bulk of the PM reduction benefit was present with the 10% biodiesel blend, with comparatively small additional amounts of PM reductions found with increased amounts of biodiesel. PM reduction associated with biodiesel was greater over the switch duty cycle than for the line-haul duty cycle. The change in cycle weighted oxides of nitrogen $(NOx)$ for B2, B10, and B20 was not greater than the expected test measurement variation; however, B100 increased $NOx$ by nearly 15% over the line-haul cycle. Changes in hydrocarbon (HC) emissions over the duty cycles were within normal test measurement variation except for neat biodiesel, where HC was reduced by 21% and 24% over the line-haul and switch cycles, respectively. Carbon monoxide reductions of 17% and 24% over the line-haul cycle were measured for B20 and B100, respectively, as compared with the base fuel. Volumetric fuel consumption increased to about 1% for both B2 and B10 blends. Just over 2% increase in volumetric fuel consumption was observed at B20 and nearly 7% increase in volumetric fuel consumption at B100.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):102804-102804-6. doi:10.1115/1.4002918.

Partially premixed low temperature combustion (LTC) is an established advanced engine strategy that enables the simultaneous reduction of soot and $NOx$ emissions in diesel engines. Measuring extremely low levels of soot emissions achievable with LTC modes using a filter smoke meter requires large sample volumes and repeated measurements to achieve the desired data precision and accuracy. Even taking such measures, doubt exists as to whether filter smoke number (FSN) accurately represents the actual smoke emissions emitted from such low soot conditions. The use of alternative fuels such as biodiesel also compounds efforts to accurately report soot emissions since the reflectivity of high levels of organic matter found on the particulate matter collected may result in erroneous readings from the optical detector. Using FSN, it is desired to report mass emissions of soot using empirical correlations derived for use with petroleum diesel fuels and conventional modes of combustion. The work presented in this paper compares the experimental results of well known formulas for calculating the mass of soot using FSN and the elemental carbon mass using thermal optical analysis (TOA) over a range of operating conditions and fuels from a four-cylinder direct-injection passenger car diesel engine. The data show that the mass of soot emitted by the engine can be accurately predicted with the smoke meter method utilizing a 3000 ml sample volume over a range of FSN from 0.02 to 1.5. Soot mass exhaust concentration calculated from FSN using the best of the literature expressions and that from TOA taken over all conditions correlated linearly with a slope of 0.99 and $R2$ value of 0.94. A primary implication of the work is that the level of confidence in reporting the soot mass based on FSN for low soot formation regimes such as LTC is improved for both petroleum diesel and biodiesel fuels.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):102805-102805-10. doi:10.1115/1.4002948.

In this work, a multimode combustion model that combines a comprehensive kinetics scheme for volumetric heat release and a level-set-based model for turbulent flame propagation is applied over the range of engine combustion regimes from non-premixed to premixed conditions. The model predictions of the ignition processes and flame structures are compared with the measurements from the literature of naturally occurring luminous emission and OH planar laser induced fluorescence. Comparisons are performed over a range of conditions from a conventional diesel operation (i.e., short ignition delay, high oxygen concentration) to a low temperature combustion mode (i.e., long ignition delay, low oxygen concentration). The multimode combustion model shows an excellent prediction of the bulk thermodynamic properties (e.g., rate of heat release), as well as local phenomena (i.e., ignition location, fuel and combustion intermediate species distributions, and flame structure). The results of this study show that, even in the limit of mixing controlled combustion, the flame structure is captured extremely well without considering subgrid scale turbulence-chemistry interactions. The combustion process is dominated by volumetric heat release in a thin zone around the periphery of the jet. The rate of combustion is controlled by the transport of a reactive mixture to the reaction zone, and the dominant mixing processes are well described by the large scale mixing and diffusion. As the ignition delay is increased past the end of injection (i.e., positive ignition dwell), both the simulations and optical engine experiments show that the reaction zone spans the entire jet cross section. In this combustion mode, the combustion rate is no longer limited by the transport to the reaction zone, but rather by the kinetic time scales. Although comparisons of results with and without consideration of flame propagation show very similar flame structures and combustion characteristics, the addition of the flame propagation model reveals details of the edge or triple-flame structure in the region surrounding the diffusion flame at the lift-off location. These details are not captured by the purely kinetics based combustion model, but are well represented by the present multimode model.

Commentary by Dr. Valentin Fuster

### Research Papers: Power Engineering

J. Eng. Gas Turbines Power. 2011;133(10):103001-103001-10. doi:10.1115/1.4003069.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):103002-103002-11. doi:10.1115/1.4003195.

Positive displacement expanders are quickly gaining popularity in the fields of micropower generation and refrigeration engineering. Unlike turbomachines, expanders can handle two-phase flow applications at low speed and flow rate levels. This paper is concerned with a simple-design positive displacement expander based on the limaçon of Pascal. The paper offers an insight into the thermodynamic workings of the limaçon gas expander and presents a mathematical model to describe the manner in which the port locations affect the expander performance. A stochastic optimization technique is adopted to find the locations, for the expander ports, which produce best expander performance for given chamber dimensions. The operating speed and other parameters will be held constant during the optimization procedure. A case study is offered in this paper to prove the validity of the presented approach, and comments are given on how various operating parameters affect system performance in the limaçon design.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Eng. Gas Turbines Power. 2011;133(10):104501-104501-4. doi:10.1115/1.4003453.

Gas turbine thermal performance is dependent on many external conditions, including fuel gas composition. Measured performance must be corrected to specified reference conditions prior to comparison against performance specifications. A performance correction for fuel composition is thus required. One current method of correction commonly used is to characterize fuel composition effects as a function of heating value and hydrogen to carbon ratio. This method has been used in the past within a limited range of fuel composition variation around the expected composition, yielding relatively small correction factors on the order of $±0.1%$. With industry trends suggesting continued exposure of gas turbines to a broader range of fuels such as liquefied natural gas and synthesized low BTU fuel, the corresponding performance effects will be much larger. As a result, a more comprehensive correction methodology is required to encompass a broader range of fuel constituents encountered. Analytical studies have been completed with the aid of thermodynamic models to identify the extent to which the Wobbe index can be used to correlate the response of gas turbine performance parameters to fuel gas composition. Results suggest that improved performance test accuracy can be achieved by using the Wobbe index compared with the aforementioned conventional fuel characteristics. This proposed method remains compliant with intent of internationally accepted test codes such as ASME PTC-22, ASME PTC-46, and ISO 2314.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(10):104502-104502-4. doi:10.1115/1.4003285.

Measured health signals incorporate significant details about any malfunction in a gas turbine. The attenuation of noise and removal of outliers from these health signals while preserving important features is an important problem in gas turbine diagnostics. The measured health signals are a time series of sensor measurements such as the low rotor speed, high rotor speed, fuel flow, and exhaust gas temperature in a gas turbine. In this article, a comparative study is done by varying the window length of acausal and unsymmetrical weighted recursive median filters and numerical results for error minimization are obtained. It is found that optimal filters exist, which can be used for engines where data are available slowly (three-point filter) and rapidly (seven-point filter). These smoothing filters are proposed as preprocessors of measurement delta signals before subjecting them to fault detection and isolation algorithms.

Commentary by Dr. Valentin Fuster