<p>The introduction of hybrid powertrains has made it possible to utilise unconventional engines as primary power units in vehicles. The free-piston energy converter (FPEC) is such an engine. It is a combination of a free-piston combustion engine and a linear electrical machine. The main features of this configuration are high efficiency and a rapid transient response.</p><p>In this thesis the free-piston energy converter as part of a hybrid powertrain is studied. One issue of the FPEC is the generation of pulsating power due to the reciprocating motion of the translator. These pulsations affect the components in the powertrain. However, it is shown that these pulsations can be handled by a normal sized DC-link capacitor bank. In addition, two approaches to reduce these pulsations are suggested: the first approach is using generator force control and the second approach is based on phase-shifted operation of two FPEC units. The latter approach results in higher frequency and lower amplitude of the pulsations, which reduce the capacitor losses.</p><p>The FPEC start-up requirements are analysed and by choosing the correct amplitude of the generator force during start-up the energy consumption can be minimised.</p><p>The performance gain of utilising the FPEC in a medium sized series hybrid electric vehicle (SHEV) is also studied. An FPEC model suitable for vehicle simulation is developed and a series hybrid powertrain, with the same performance as the Toyota Prius, is dimensioned and modelled.</p><p>Optimisation is utilised to find a lower limit on the SHEV's fuel consumption for a given drivecycle. In addition, three power management control strategies for the FPEC system are investigated: two load-following strategies using one and two FPEC units respectively and one strategy based on the ideas of an equivalent consumption minimisation (ECM) proposed earlier in the literature.</p><p>The results show a significant decrease in fuel consumption, compared to a diesel-generator powered SHEV, just by replacing the diesel-generator with an FPEC. This result is improved even more by using two FPEC units to generate the propulsion power, as this increases the efficiency at low loads. The ECM control strategy does not reduce the fuel consumption compared to the load-following strategies but gives a better utilisation of the available power sources.</p>

The piston-driven oscillator is traditionally modeled by directly applying boundary conditions to the acoustic wave equations; with better models re-deriving the wave equations but retaining nonlinear and viscous effects. These better models are required as the acoustic solution exhibits singularity near the natural frequencies of the cavity, with an unbounded (and therefore unphysical) solution. Recently, a technique has been developed to model general pressure oscillations in propulsion systems and combustion devices. Here, it is shown that this technique applies equally well to the piston-driven gas-column oscillator; and that the piston experiment provides strong evidence for the validity of the general theory. Using a modified piston-tube apparatus, agreement between predicted and observed limit-cycle amplitudes is observed to be on the order of 1%.

Cette thèse est consacrée è la modélisation et a la simulation de l'ensemble PISTON /SEGMENT/ CHEMISE, d'un moteur thermique. Dans un moteur, le piston muni de segments est en mouvement relatif dans la chemise. Ce système est lubrifié par projection d'huile, les segments et la surface de la chemise "contrôlent" la quantité de lubrifiant restant sur la paroi. Pour modéliser ce phénomène, le modèle conservatif parabolique-hyperbolique d'Elrod-Adams (P − θ), prenant en compte la cavitation (présence de gaz dans le fluide, aspect multiphasique de l'écoulement), est couplé a un modèle de contact, celui de Greenwood-Tripp (basé sur une approche statistique de la surface). En se restreignant a l'écoulement et au contact, nous négligeons beaucoup de phénomènes physiques pouvant intervenir comme le grippage ou les effets thermiques. Dans une première partie, les phénomènes mécaniques sont étudiés, de la réalisation de la surface des chemises è la modélisation de la physique de la lubrification du contact segment/chemise. Cette étude expose les simplifications effectuées et le choix des conditions aux limites pour la prise en compte de la cavitation en tant que problème a frontière libre, notamment au niveau du débit d'entrée pour simuler un fonctionnement "normal". Dans une deuxième partie, la mise en oeuvre et la modification de l'algorithme "classique" pour des surfaces mesurées sont décrites en détail. Nous présentons les performances simulées de différentes surfaces, et nous les comparons è des essais effectués par Renault. Dans une troisième partie, des modifications sont introduites au modèle (P − θ) utilisé précédemment afin de rendre l'écoulement cohérent avec le modèle de Navier-Stokes. Bien que ce nouveau problème puisse être posé en dimension 2, l'étude mathématique ne concerne que la dimension 1. On se ramène a un système dynamique dont les inconnues sont les frontières libres. L'unicité de la solution impose l'introduction d'un paramètre artificiel. Le comportement local et/ou global en temps est déterminé par les paramètres géométriques et les conditions aux limites. L'introduction de deux paramètres supplémentaires permet de démontrer l'existence et l'unicité de la solution. Dans une dernière partie, nous comparons le modèle de cavitation proposé avec le modèle P − θ.

Different wave power technologies are in development around the world in different stages of prototype testing. So far only a few devices have been deployed offshore at full scale for extended periods of time. Little data is published about how these different devices perform. This thesis presents results from experiments with the full-scale offshore wave energy converters at the Lysekil research site on the Swedish west coast. The theories, experiments, measurements, performance evaluations and developments of the submerged transmission in the direct driven permanent magnet linear generator are in focus. The reciprocating submerged transmission fulfills the purpose of transmitting the absorbed mechanical wave energy through the capsule wall into the generator, while preventing the seawater from entering the capsule and reducing the life time of the converter. A measuring system with seven laser triangulation sensors has been developed to measure small relative displacements between piston rod and seal housing in the submerged transmission with excellent accuracy for the purpose of evaluating both functional behavior and successive wear in-situ. A method for calculating relative tilt angles, azimuth angles, differential tilt angles, and successive wear in the submerged transmission has been developed. Additional sensors systems have been installed in the converter enabling correlation and a thorough investigation into the operating conditions of the transmission and the converter. The thesis presents unique results from the measurements. A data acquisition system transmits the signals from the converter on the seabed to an onshore measuring station. Results are presented in time-, frequency- and the time-frequency domain. The results have given important information for further development of the submerged transmission, which is important to the survivability of the system. The thesis describes the status of research, and is a step that may influence future designs of wave energy devices for reaching survivability and a cost-effective renewable energy system. / <p>Published is a preprint version of the full text and should be combined by the errata.</p> / The Lysekil Wave Power Project

Made available in DSpace on 2014-06-11T19:23:39Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-11-27Bitstream added on 2014-06-13T18:10:04Z : No. of bitstreams: 1 ferreira_ad_me_ilha.pdf: 1079421 bytes, checksum: dcb82443da323ec46bf3bd207a17001c (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Devido à solubilidade mútua entre o fluido refrigerante e o fluido lubrificante usados em sistemas de refrigeração por compressão de vapor, eles formam uma mistura homogênea que influencia tanto os processos de transferência de calor no evaporador e no condensador, como os processos de lubrificação e de selagem de vazamentos no interior do compressor. O vazamento de refrigerante através da folga radial de compressores rotativos de pistão rolante é de particular importância para o bom desempenho do compressor, uma vez que ele influencia significativamente a eficiência volumétrica do compressor, chegando a somar cerca de 30% das perdas totais de refrigerante. No presente trabalho foram desenvolvidos modelos de escoamento de misturas óleo-refrigerante através desta folga, incluindo a mudança de fase do refrigerante devida à variação da sua solubilidade no lubrificante. A solução da equação da energia constitui uma evolução do processo de modelagem deste escoamento em relação aos modelos até então desenvolvidos. Quatro modelos diferentes foram usados para simular o escoamento: modelo de escoamento bifásico homogêneo isotérmico, modelo de escoamento bifásico isotérmico com formação de espuma, modelo de escoamento bifásico homogêneo não-isotérmico e modelo de escoamento bifásico homogêneo não-isotérmico com termo de força inercial. O estudo foi realizado para três misturas óleo-refrigerante: óleo éster Freol a10 e refrigerante R134a, óleo éster EMKARATE RL10H e refrigerante R134a e óleo mineral SUNISO 1 GS e refrigerante R12. Para todos os modelos e misturas, realizou-se um estudo paramétrico envolvendo as principais variáveis do problema: pressão de entrada, temperatura de entrada, vazão de mistura e valor da folga mínima... / Due to the mutual solubility between the refrigerant and lubricant of refrigeration systems using mechanical compression of vapor, they form a homogeneous mixture which influences the heat transfer processes in the evaporator and condenser as well as the compressor lubrication and refrigerant leakage. The refrigerant leakage through the radial clearance of rolling piston compressors plays an important role to the volumetric efficiency in this type of compressor, in which it represents about 30% of the total refrigerant loss. In the present work several models to predict the lubricant-refrigerant mixture flow through this clearance, including the refrigerant phase change due to the reduction of the refrigerant solubility in the lubricant, are developed. Four different models were developed to simulate the flow: isothermal homogeneous two-phase flow, isothermal two-phase flow with foam formation, non-isothermal homogeneous two-phase flow and non-isothermal homogeneous two-phase flow containing inertial force. The simulations were performed for three mixtures: ester oil Freol a10-refrigerant R134a, ester oil EMKARATE RL10H-refrigerant R134a, and mineral oil SUNISO 1 GS-refrigerant R12. The influences of the inlet pressure, inlet temperature, mixture mass flow rate, and minimal clearance were analyzed for all mixtures. The results showed that it is important to consider the foam formation, the inertial force, and the solution of the energy equation in the flow modeling. Concerning the volumetric efficiency of the compressor the ester oil Freol a10-R134a was the best mixture because it produced the lowest refrigerant leakage.

Au sein des parties statiques d’une turbine à gaz, l’étanchéité du système d’air derefroidissement est couramment assurée par un montage de segments. De par sa rigiditéélevée, ce type de joint s’adapte mal aux pièces avec lesquelles il entre en contact. Uneméthodologie, associant métrologie et calcul éléments finis, a effectivement montré quel’ouverture, à l’interface entre un segment et sa piste, est très supérieure à celle qui caractériseun contact simplement rugueux. Une analyse en ordres de grandeur a révélé que lesmécanismes d’écoulement du gaz, sur un champ d’ouverture typique de l’interface entre la pisteet le segment, sont conditionnés par deux nombres adimensionnels. Trois régimes peuvent êtredistingués en fonction des valeurs prises par ces nombres. L’un d’eux, le régime laminairecompressible inertiel, est particulièrement intéressant car il est présent dans les zones de laturbine où l’étanchéité est cruciale. Un modèle quasi analytique a été développé pour décrire cerégime. En l’associant à un modèle de déformation, on aboutit à un outil prédictif du débit degaz induit par un segment. Des essais sur banc partiel ont permis de le valider et de démontrersa précision. L’outil développé fournit le moyen de quantifier l’impact du champ de température,du champ de pression et des écarts géométriques, propres à un stator de turbine, sur lesperformances de l’étanchéité. Ce travail a débouché sur des règles de dimensionnement et deconception qui portent notamment sur l’obturation du jeu inter-extrémités, sur le choix dunombre de segments et du sens de serrage. / The sealing of gas turbine static parts is usually performed using piston rings.Because of its high stiffness, this type of gasket does not adapt itself to the parts with which it isdue to be in contact. A method involving metrology and a FEM actually revealed that the ringboreor ring-piston contact aperture is far larger than the one within a rough contact. Using ascale analysis, we showed that gas flow mechanisms, through the aperture field of the contactbetween the ring and the inner or outer part (i.e. the piston or the bore), depend on twodimensionless numbers. Three regimes can be distinguished depending on the values of thesenumbers. One of them, the compressible inertial laminar regime, is of particular interestbecause of its relevance in turbine areas where sealing is strongly critical. A quasi analyticalmodel was developed to describe this regime. By a coupling with a strain model, it wasupgraded to a predictive tool of leakage through a piston-ring or bore-ring contact. Acomparison with measurements carried out on a partial test rig allowed to prove its accuracy.This tool made it possible to quantify the influence of temperature field, pressure field andgeometrical defects, which are typical of a turbine stator, on sealing performance. This worklead us to propose design rules concerning, in particular, the ring gap type, number of rings tobe mounted and choice of tightening direction.

The constant pursuit in the automotive industry to increase the engines performance, new solutions are always developed and tested to reduce the friction and increase the efficiency in the engine. One component that contributes to friction losses is the piston ring pack where the top compression stands for up to 40 %. This master thesis collaborated with Scania’s material science department Basic engine and covers the friction and wear of four different materials on the cylinder liner surface against the top compression ring.The four tested materials were grey cast iron with different honing quality and three atmospheric plasma sprayed coatings with titanium oxide, chromium oxide and Metco’s mixture F2071 which is a stainless steel mixed with a ceramic. A martensitic steel piston ring with a chromium coated sliding surface was used for all the testing in the Cameron-Plint TE77 test-rig. This is a pin-on-disc test method and the parameters used for testing is set to replicate the environment the ring is exposed to at the top dead centre.The test-rig has been in Scania’s possession for a long time and has not always given a satisfying result. An uneven contact between the ring and liner has been a problem resulting in only worn edges of the liner specimen. The piston ring holder was therefore redesigned to be able to adjust the radius of the ring. This allowed a good conformability between the ring and liner to be obtained.The tested materials were evaluated according to friction and wear. Friction was measured with the test-rig and the wear was calculated with surface profiles that were measured before and after testing. Worn surfaces were studied in a SEM to verify which wear mechanism that was active. The changes of the surfaces was studied with the use of following surface parameters Ra, Rk, Rpk, Rvk and if there was a connection between these parameters and friction and wear coefficient.Independent of honing quality showed the grey cast iron lowest friction coefficient just under 0.13, the F2071 liner showed a friction coefficient just above 0.13. Both oxide layers showed similar friction where the chromium oxide had a friction just below 0.15 and the titanium oxide lay just above 0.15. Lowest wear coefficient had the chromium oxide followed by F2071, titanium oxide and the bad honed grey cast iron. These three liners showed almost the exact same wear coefficient. Worst wear coefficient had the grey cast iron with a good honing quality. A mild abrasive wear mechanism was active during the wear test and vague wear marks was found on the surface. There is no connection between wear coefficient and friction and the change in surface roughness during the test does not affect the friction.

La microgéométrie de la surface de la chemise joue un rôle très important dans les pertes par frottement et dans la consommation de l’huile dans un moteur à combustion interne. Une des texturations classiques de cette surface est celle créée par pierrage. Elle se compose de plateaux plus ou moins lisses et de profondes stries croisées. L’´épaisseur du film d’huile est influencée fortement par cette texturation. Un modèle simplifié du contact segment chemise a été créé en la présence de la microgéométrie. Puis, un code de calcul basé sur la méthode numérique multigrille a été développé. Ce code a été utilisé pour des études paramétriques avec des jeux de paramètres très variés. Les calculs quantifient l’influence de cette microgéométrie particulière sur la relation film d’huile - portance. Les résultats mettent en évidence deux mécanismes distincts de génération de portance selon le type du segment. Le segment parabolique porte par son convergent et les stries ne font que diminuer cette portance. Inversement, le segment plat ne porte pas et ce sont les stries qui génèrent la portance. Deux modèles de prédiction ont finalement été déduits, un pour les segments paraboliques et un pour les segments plats. Ces deux prédictions ont ensuite été validées par des calculs sur des surfaces mesurées; / The liner microgeometry influences the friction losses and the oil consumption in IC engines. The cross-hatched texturing, created by the honing process, plays an essential part in the load carrying capacity - film thickness relation. This work studies the influence of the crosshatched groove parameters on the piston ring load carrying capacity. First, a simplified model of the hydrodynamic contact has been created. Thus a Multigrid based code was developed. Calculations with different sets of microgeometric parameters have been performed. These calculations quantify the load carrying capacity for both parabolic and flat rings. The results show that the load carrying capacity is generated differently in both cases. For the parabolic case, the pressure is build up by the wedge in the inlet zone and the grooves decrease the global load carrying capacity. In the flat case the grooves generate the total load carrying capacity. Finally, two prediction models were deduced for the parabolic ring and for the flat ring. These predictions were validated by measured surface calculations.

The growing global demand for energy and environmental implications have created a need to further develop the current energy generation technologies (solar, wind, geothermal, etc.). Recovering energy from low grade energy sources such as waste heat is one of the methods for improving the performance of thermodynamic cycles. The objective of this work was to achieve long-term steady state operation of a Free-Piston Linear Expander (FPLE) and to compare the FPLE with the currently existing expander types for use in low temperature energy recovery systems. A previously designed FPLE with a single piston, two chambers, and linear alternator was studied and several modifications were applied on the sealing and over expansion. An experimental test bench was developed to measure the inlet and outlet temperatures, inlet and outlet pressures, flow rate, and voltage output. A method of thermodynamic analysis was developed by using the first and second law of thermodynamics with air as the working fluid. The experimental tests were designed to evaluate the performance of the FPLE with varying parameters of inlet air pressure, inlet air temperature, and electrical resistance. The initial and steady-state operation of the FPLE were successfully achieved. An uncertainty analysis was conducted on the measured values to determine the accuracies of the calculated parameters. The trends of several output parameters such as frequency, average root mean square (RMS) voltage, volumetric efficiency, electrical-mechanical conversion efficiency, isentropic efficiency, irreversibility, actual expander work, and electrical power were presented. Results showed that the maximum expander frequency was found to be 44.01 Hz and the frequency tended to increase as the inlet air pressure increased. The FPLE achieved the maximum isentropic efficiency of 21.5%, and produced maximum actual expander work and electrical work of 75.13 W and 3.302 W, respectively.

Residence time distributions have become an important analytical tool in the analysis of many types of flow systems. Residence time distributions have proven to be effective for analysing trickle bed reactors, as it allows determination of parameters under operating conditions allowing no interference of these conditions. By studying the residence time distribution a great amount of information can be obtained and therefore used to determine a number of hydrodynamic parameters. Due to recent findings that prewetting has a tremendous effect on a number of hydrodynamic parameters such as holdup, wetting efficiency and pressure drop, it is therefore the aim of this study to investigate the effect of trickle flow morphology or prewetting on a trickle bed reactor. The residence time distribution is obtained whereby hydrodynamic parameters are determined and therefore the effect the flow morphology has on various hydrodynamic parameters is highlighted. A number of methods were used to determine these parameters, namely that of the best-fit method, whereby the PDE model was used, and the method of moments. Operating conditions included varying gas and liquid flow rates for porous and non-porous catalyst particles at atmospheric pressure. The different prewetting procedures used during this work included the following: <ul><li>Non-wetted </li> <li>Levec-wetted </li> <li>Super-wetted</li></ul> From this investigation the following conclusions were made: <li>Prewetting has a great effect on the hydrodynamic parameters of trickle bed reactors</li> <li>The differences in prewetting can be attributed to differing flow morphologies for the different prewetted beds i.e. the dominant flow morphology for a non-wetted bed is that of rivulets and for prewetted beds that of film flow</li> <li>It was also found that at low liquid flow rates the flow morphology in prewetted beds changes from film flow to a combination of rivulet and film flow</li> <li>The different flow morphologies for prewetted and non prewetted beds was confirmed by the residence time distributions and various parameters obtained there from</li> <li>At low liquid flow rates the flow morphology becomes a more predominant factor in creating the tailing effect present in residence time distribution for prewetted beds</li> <li>The tailing effect in residence time distributions is a result of both internal diffusion and liquid flow morphology, where the liquid flow morphology is the more dominant factor</li> <li>The use of residence time distributions to determine a number of hydrodynamic parameters proved to be very useful and accurate by means of different methods, i.e. method of moments and best-fit method</li> <li>Differences in the liquid holdup determined from the method of moments and the weighing method confirmed that different flow morphologies exist for different prewetted beds</li> <li>An increase in the dispersion coefficient with prewetting was observed indicating that the amount of micro mixing is different for the different prewetted beds</li> <li>Differences in residence times and high values for the dynamic holdup, for the porous packing, confirmed that the PDE model does not model well the porous packing response curves due to the lack of internal diffusion and internal holdup in this model</li> <li>The dynamic-static mass transfer showed that film flow, as in prewetted beds, results in slower mass transfer as opposed to rivulet flow and therefore it is concluded that prewetting results in different flow morphologies.</li></ul> Following this study it is recommended that a residence time distribution model be used or developed that incorporates the effects of internal diffusion and internal holdup as present in porous catalyst particles. In addition, it was found that very few correlations could accurately predict hydrodynamic parameters due to the absence of the effect of prewetting and therefore it is recommended that correlations be developed that incorporate the effect of prewetting. / Dissertation (MEng)--University of Pretoria, 2008. / Chemical Engineering / unrestricted