Ducted Propeller Flow Analysis by Means of a Generalized Actuator Disk Model

The paper presents a generalized semi-analytical actuator disk model as applied to the analysis of the flow around ducted propellers at different operating conditions. The model strongly couples the non-linear actuator disk method of Conway[1] (J. Fluid Mech. 1998; 365: 235-267) and the vortex element method of Martensen[2] (Arch. Rat. Mech. 1959; 3: 235-270) and it returns the exact solution, although in an implicit formulation, for incompressible, axisymmetric and inviscid flows. The solution is made explicit through a semi-analytical procedure developed and validated by Bontempo and Manna[3] (J. Fluid Mech. 2013;728:163-195). Moreover the method duly accounts for non-uniform load radial distribution, slipstream contraction, mutual non-linear interaction between duct and propeller, wake rotation, and ducts of general shape. Thanks to its extremely reduced computational cost it can easily be integrated into design systems based on the repeated analysis scheme of hierarchical type. A comparison between open and ducted rotors is carried out in order to quantify the effects of the duct on the overall performance of the device. Emphasis is given to the appropriate matching between the duct geometry and the propeller load to exploit the benefits that could arise ducting the propeller

Highly Flexible Hot Gas Generation System for Turbocharger Testing

AbstractThe paper presents the design and prototyping of a gas generator system for turbocharger experimental testing capable of delivering a wide range of flow rates with adequate thermodynamic characteristics. The system feeds the turbocharger test section with an hot gas stream of prescribed mean and time varying pressure and temperature in order to fully span the operating domain of the device with controlled accuracy. Compared with the more conventional gas combustor system, it allows for a safer rig operation ensured by the structural robustness of a four stroke diesel engine and an easier turbine inlet flow control. The latter is achieved by means of an external supercharging station and a modern ICE electronic control unit so that mass flow rate, pressure and temperature values can be set independently.The steady compressor and turbine performance maps can be obtained operating the rig according to a conventional procedure, i.e. collecting a set of flow rate pressure ratio data points (mg, π) for given hot gas properties. Alternatively using more advanced operating modes unsteady testing is possible to reproduce the complexities characterizing the driving cycles required by the latest European regulations

Mass Benefit Analysis of 4-Stroke and Wankel Range Extenders in an Electric Vehicle over a Defined Drive Cycle with Respect to Vehicle Range and Fuel Consumption

The gradual push towards electric vehicles (EV) as aprimary mode of transport has resulted in an increasedfocus on electric and hybrid powertrain research. Oneanswer to the consumers’ concern over EV range is the implementationof small combustion engines as generators tosupplement the energy stored in the vehicle battery. Sincethese range extender generators have the opportunity to runin a small operating window, some engine types that havehistorically struggled in an automotive setting have the potentialto be competitive.The relative merits of two different engine options forrange extended electric vehicles are simulated in vehicle acrossthe WLTP drive cycle. The baseline electric vehicle chosenwas the BMW i3 owing to its availability as an EV with andwithout a range extender gasoline engine.Two different range extenders were considered; a singlerotor Wankel rotary and a 4-stroke reciprocating engine, withthe baseline vehicle electric glider mass fixed for all options.Fuel tank capacity was fixed at 9 litres. Baseline EV performancewas evaluated on simulated European drive cycles withmass sensitivity conducted before the implementation of eachrange extender.Potential options for the optimisation of the rangeextender operation were considered with respect to theirimpact on vehicle performance. Total combined fuel efficiencywas compared and an assessment of maximum range andvehicle performance was also conducted

Large-Eddy Simulation of a Wankel Rotary Engine for Range Extender Applications

The Wankel rotary engine offers unrivalled power density as a consequence of having a combustion event every revolution, as well as lightness, compactness and vibrationless operation due to its perfect balance. These attributes have led to its success as a powerplant for unmanned aerial vehicles, and which should also make it an attractive proposition for range-extended electric vehicles. However, it is not currently in production for automotive applications having historically struggled with poor combustion efficiency, and high fuel consumption and hydrocarbon emissions. The purpose of this work is to study the in-chamber flow motion in order to better understand how such limitations may be overcome in future. A 225cc 30 kW Wankel rotary engine is modelled using Large-Eddy Simulation (LES) to ensure turbulent flow features are faithfully recreated, since Reynolds-averaged Navier-Stokes-based approaches are insufficient in this regard. The LES-predicted peak chamber pressure lies within 3.7% of the engine test data, demonstrating good model validation. Combustion simulation parameters are calibrated to match the measured heat release profile, for high-load engine operation at 4000 rpm. Simulation results provide insight into the generation of turbulent structures as the incoming flow interacts with the throttle, intake port, rotor and housing surfaces; how the turbulence breaks down as the combustion chamber is compressed; and how the flame propagates following ignition, leaving a pocket of reactants unburned. Indeed, the computational approach described here allows detailed understanding of the impact of design parameters on the detailed in-chamber flow phenomena, and consequently engine performance and emissions. This will enable the optimization of Wankel rotary engine geometry, port and ignition timing for maximum combustion efficiency and low emissions, reasserting its potential as an effective and efficient prime mover for hybrid and range extended electric vehicles

Characterization of a supercharger as boosting & turbo-expansion device in sequential multi-stage systems

This paper proposes a detailed performance analysis and experimental characterization of a high-pressure supercharger in a multi-stage boosting system (turbo-super arrangement). Infact, besides the technical challenges associated with achieving adequate tuning, interoperability and driveability of multi-stage boosting systems, another challenge lies in their performance prediction during engine design. Indeed, performance maps of single boosting systems are usually provided by manufacturers and used as look-up tables in 1-D engine models. Tests are usually conducted in a standalone mode, with no information provided on the behaviour and performance of the combination of more than one boosting device. The supercharger was tested with varying inlet pressures and temperatures matching on-engine operating conditions and the results were then used to assess the effectiveness of 1-D engine models performance prediction when dealing with multi-stage boosting systems. An assessment on heat transfer in superchargers was also carried out together with the analysis on the nature of non-dimensional performance maps when dealing with a pressurized inlet. Finally, the analysis also looked into the opportunity to use the superchargers as expanders (‘expansion mode’) in order to cool the air charge entering the engine. The results showed that there is discrepancy between the efficiency values computed by 1-D engine models and those obtained experimentally under pressurized/heated inlet air conditions; the correction of the efficiency maps for heat transfer plays a significant role in the final measured efficiency and the correction of the maps for varying inlet temperatures must be carried out in order to avoid incurring in apparent efficiencies greater than unity. The experiments on the supercharger in ‘expansion mode’ showed that low isentropic efficiencies can be achieved; despite this, 1-D engine simulations showed that it is possible to achieve savings of a few percentage points in Brake Specific Fuel Consumption when the supercharger is used to recover some throttling energy by expanding the close-to-ambient pressure to the required intake pressure

Metodologie teorico-sperimentali per l'analisi delle prestazioni di turbosovralimentatori

Il presente lavoro di tesi ha riguardato lo sviluppo di metodologie teorico-sperimentali per il rilievo delle prestazioni delle turbomacchine ed in particolare dei turbo-sovralimentatori per motori a combustione interna. Lo scopo di tale lavoro è stato quello di mettere a punto un banco prova per turbocompressori, altamente flessibile e utile ad effettuare sia la ricerca di base che lo studio approfondito delle particolari problematiche di funzionamento di tali macchine. In particolare, il rig è stato progettato in modo tale da poter realizzare sia prove in regime stazionario, utili per la determinazione delle mappe di funzionamento di turbina e compressore, sia prove in regime instazionario per lo studio di particolari fenomeni quali il surge (pompaggio), il turbo-lag, etc. Dopo aver effettuato una attenta analisi bibliografica sulle sale prova realizzate per finalità simili, per la realizzazione dell’UNINA Turbocharger Test Rig si è scelto di utilizzare come generatore di gas caldi un motore a combustione interna a ciclo diesel, in modo da replicare quanto più fedelmente possibile le reali condizioni di funzionamento dei turbocompressori. Allo stesso tempo, scegliendo il motore endotermico alternativo, si è sfruttato appieno l’elevato grado di know-how, già presente all’interno del gruppo di ricerca, nei confronti di tali macchine. Il motore è stato poi privato del suo gruppo di sovralimentazione originale, in modo tale che i gas di scarico possano conservare completamente il loro contenuto energetico, da destinare successivamente al turbocompressore oggetto della sperimentazione. Successivamente alla rimozione del sovralimentatore originario, per garantire al motore il giusto grado di sovralimentazione e quindi evitare un degradamento delle prestazioni dello stesso, si è realizzato un complesso sistema di sovralimentazione esterna basato sull’utilizzo di due compressori a vite operanti in parallelo. Per preservare il funzionamento del motore, i fluidi elaborati da entrambi i compressori vengono preventivamente raffreddati all’interno di uno scambiatore di calore di tipo counterflow. Il sistema di sovralimentazione esterno e tutti i trasduttori di misura presenti all’interno della sala sono gestiti completamente attraverso un articolato sistema di controllo e acquisizione dati, appositamente realizzato e basato su hardware prodotto dalla National Instruments. Nella fattispecie, è stato utilizzato il sistema modulare denominato Compact FieldPoint e la performante scheda di acquisizione PCI6133, facente parte della famiglia di schede S Series Multifunctional DAQ, capace di effettuare campionamenti a 14 bit con una sampling frequency di 3 MS/s per canale. Per stabilire l’accuratezza dei sensori è stato condotto un approfondito studio analitico applicando la normativa internazionale “ISO Guide to the expression of uncertainly in measurement”, nota comunemente come ISO-GUM, valutando così le incertezze di misura di tutte le grandezze direttamente misurate. Per il sistema di controllo automatico della sala, l’acquisizione e la visualizzazione dei dati provenienti dalle schede di acquisizione National Instruments, si sono realizzati appositi Virtual Intrsuments attraverso il linguaggio di programmazione LabVIEW®. Si sono realizzati quindi i subVI per il controllo e l’acquisizione dei dati provenienti dal MCI, dal turbocompressore in prova e da tutti i servizi ausiliari presenti in sala. Di particolare rilievo è risultata la realizzazione dei subVI per il sistema di sovralimentazione automatica del MCI, dell’acceleratore elettronico e del sistema di controllo in remoto del freno dinamometrico: l’implementazione di tale parte del codice ha reso il banco prova motori equivalente ad uno di tipo dinamico, aumentandone notevolmente la flessibilità e permettendo la realizzazione degli innovativi test in regime instazionario sui turbocompressori. Dopo aver effettuato simulazioni e test preliminari per la validazione del Virtual Instrument realizzato, si è proceduto alla realizzazione delle prove in regime stazionario su un turbocompressore di taglia media prodotto dalla Garrett, siglato GT2052ELS. La mappa di funzionamento reale del compressore è stata rilevata attraverso una serie di test della macchina in regime stazionario. Utilizzando poi un codice scritto in MATLAB®, sono stati calcolati il rendimento isoentropico, il rendimento politropico e il lavoro reale della macchina. Sono stati calcolati inoltre i coefficienti adimensionali di portata e di lavoro φ e ψ che hanno permesso di stimare gli angoli di flusso reali β'_2 all’uscita della ruota del compressore e attraverso un procedimento grafico effettuato con un software CAD si è potuto stimare anche l’angolo β'_2∞ che permette il calcolo del lavoro ideale L_∞. E’ stato analizzato inoltre, per la macchina oggetto di studio, il fenomeno di slip, attraverso il confronto dei risultati di φ e ψ ricavati dai dati sperimentali con quelli ricavati dai modelli proposti da Stodola e da Wiesner, riscontrando un ottimo accordo proprio con il modello proposto da quest’ultimo Autore. I test in regime instazionario effettuati sul turbocompressore hanno rappresentato una vera e propria innovazione nell’ambito della sperimentazione sulle turbomacchine destinate alla sovralimentazione dei motori a combustione interna. Sfruttando appieno la flessibilità del banco prova realizzato e dei Virtual Instruments implementati per il controllo della sala, è stato possibile gestire in modo dinamico il motore, assegnandogli determinate leggi temporali di carico e velocità di rotazione. In questo modo si sono potute effettuare le acquisizioni dei dati durante le manovre effettuate dal generatore di gas caldi, determinando così le evoluzioni dinamiche compiute dal turbocompressore, soggetto ad una forzante esterna variabile nel tempo. Attraverso l’utilizzo di alcune manovre particolari da parte del motore, si è potuto approfondire lo studio del fenomeno di surge che rappresenta uno tra i fenomeni più critici in cui può incorrere il compressore. I rilievi sperimentali hanno mostrato che all’occorrenza di tale criticità si hanno forti oscillazioni delle pressioni e del regime di rotazione della macchina. La portata massica, in tali condizioni, addirittura inverte il verso, fluendo dalla mandata all’aspirazione, provocando notevoli sollecitazioni meccaniche alle palettature del compressore

Mass Benefit Analysis of 4-Stroke and Wankel Range Extenders in an Electric Vehicle over a Defined Drive Cycle with Respect to Vehicle Range and Fuel Consumption

The gradual push towards electric vehicles (EV) as aprimary mode of transport has resulted in an increasedfocus on electric and hybrid powertrain research. Oneanswer to the consumers’ concern over EV range is the implementationof small combustion engines as generators tosupplement the energy stored in the vehicle battery. Sincethese range extender generators have the opportunity to runin a small operating window, some engine types that havehistorically struggled in an automotive setting have the potentialto be competitive.The relative merits of two different engine options forrange extended electric vehicles are simulated in vehicle acrossthe WLTP drive cycle. The baseline electric vehicle chosenwas the BMW i3 owing to its availability as an EV with andwithout a range extender gasoline engine.Two different range extenders were considered; a singlerotor Wankel rotary and a 4-stroke reciprocating engine, withthe baseline vehicle electric glider mass fixed for all options.Fuel tank capacity was fixed at 9 litres. Baseline EV performancewas evaluated on simulated European drive cycles withmass sensitivity conducted before the implementation of eachrange extender.Potential options for the optimisation of the rangeextender operation were considered with respect to theirimpact on vehicle performance. Total combined fuel efficiencywas compared and an assessment of maximum range andvehicle performance was also conducted

Characterization of a supercharger as boosting & turbo-expansion device in sequential multi-stage systems

Engine downsizing is driving the automotive industry towards more performing, and less polluting drivetrains. However in order to achieve significant engine downsizing without sacrificing performance, these engines need to be heavily boosted. Different boosting configurations can be adopted on an automotive internal combustion engine, e.g. using turbocharger and supercharger either separately or in series or in parallel. However for highly downsized engines, multistage boosting systems configurations are becoming unavoidable due to the high charge pressure required on the engine intake. Besides the technical challenges associated with achieving adequate tuning, interoperability and driveability of multistage boosting systems, another challenge lies in their performance prediction during engine design. Indeed, performance maps of single boosting systems are usually provided by manufacturers and used as look-up tables in 1-D engine models. Tests are usually conducted in a standalone mode, with no information provided on the behaviour and performance of the combination of more than one system. Aim of this work is to provide an insight into the experimental characterization of a high-pressure supercharger in a sequential boosting arrangement.The UltraBoost project aims to achieve extreme engine downsizing by means of a turbo-supercharger arrangement for a 2.0L-4 cylinders gasoline capable to deliver the same output torque as an equivalent naturally aspirated 5.0L-V8 baseline (i.e. 60% engine downsizing). The aim of this paper is to assess the performance of the supercharger under different operating conditions. A dedicated test facility was set-up at Imperial College London to evaluate the supercharger performance under different inlet conditions: firstly an initial set of tests was performed with pressure and temperature at ambient conditions in order to calibrate and validate the test facility against the manufacturer performance maps; secondly the supercharger was tested with varying inlet pressures and temperatures matching on-engine operating conditions and the results were then used to assess the effectiveness of 1-D engine models performance prediction when dealing with multistage boosting systems. In addition to this, an assessment on heat transfer in superchargers was also carried out together with the analysis on the nature of non-dimensional performance maps when dealing with a pressurized inlet. Finally, the analysis also looked into the opportunity to use the superchargers as expanders (‘expansion mode’) in order to cool the air charge entering the engine; experiments were carried out. The results showed that there is discrepancy between the efficiency values computed by 1-D engine models and those obtained experimentally under pressurized/heated inlet air conditions; the correction of the efficiency maps for heat transfer plays a significant role in the final measured efficiency and the correction of the maps for varying inlet temperatures must be carried out in order to avoid incurring in apparent efficiencies greater than unity. The experiments on the supercharger in ‘expansion mode’ showed that low isentropic efficiencies can be achieved; despite this, preliminary simulations on the UltraBoost engine showed that it is possible to achieve savings of few percentage points in BSFC when the supercharger is used to recover some throttling energy by expanding the close-to-ambient pressure to the required intake pressure. <br/