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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Experimental studies of performance and emissions in a 2/4-stroke engine

Ojapah, Mohammed January 2014 (has links)
Direct Injection (DI) gasoline engines are staging a come-back because of its potential for improved fuel economy through principally the engine down-sizing by boosting, stratified charge combustion and Controlled Auto Ignition (CAI) at part load operations. The problem with the Spark Ignition (SI) engine is its inherent low part-load efficiency. This arises due to the pumping loses that occur when the throttle closes or partially opens. One way of decreasing the pumping losses is to operate the engine lean or by adding residual gases. It is not possible to operate the engine unthrottled with a very lean or diluted mixture at low loads due to misfire. However, the load can also be controlled by changing the valve timing. This reduce the pumping loses and hence increase the efficiency. Due to the limited time available for complete fuel evaporation and the mixing of fuel and air mixture, locally fuel rich mixture or even liquid fuel can be present during the combustion process. This causes a significant increase in Particulate Matter (PM) emissions from direct injection gasoline engines compared to the conventional port fuel injection gasoline engines, which are of major concern because of its health implications. In the meantime, depleting reserves of fossil fuels and the increasing environmental pollution caused by burning of fossil fuels, have paved the way for fuel diversification. Cleaner and renewable fuel is being introduced worldwide. The use of ethanol as an alternative transportation fuel shows promise for several reasons. While ethanol can be produced from several types of biomass, it offers properties such as high octane number, higher oxygen content and high heat of evaporation, which make it a most attractive alternative fuel, in particular for the direct injection gasoline engine. In this research, a single cylinder camless engine equipped with an electro-hydraulic valve train system has been used to study and compare different engine operation modes in the SI and CAI combustion. The fuel consumption, gaseous and particulate emissions of gasoline and its mixture with ethanol (E15 and E85) were measured and analysed at the same engine operating condition. The heat release analysis and performance characteristics of CAI and SI combustion were carried out by the in-cylinder pressure measurement. The effect of load and valve timings on the gaseous and Particulate Matter (PM) emissions was investigated for both 4-stroke SI and CAI combustion. Within the achieved CAI operational ranges, particle emissions were found to be dominated by smaller particles (<50nm). Hotter charge and better mixing are the main parameters affecting the soot particles in the exhaust irrespective of the combustion modes and valve timings. At part-load conditions investigated, it was found that the CAI combustion produced the lowest NOx emissions of 0.4g/KWh in all fuel blends and lower fuel consumption 223g/KWh with improved combustion efficiency of 94.7% in ethanol fuel E15 and E85. The positive valve overlap was found to produce lowest fuel consumption of 222.8 g/KWh in all fuel blend and respond better to ethanol fuel in E15 and E85 with improved indicated efficiency of 40.5% compared to the other modes investigated. The early intake valve throttled SI operation led to a moderate improvement in the fuel consumption of 243.5g/KWh over the throttled SI operation but it was characterised by the slowest combustion and highest CO (33.5g/KWh) and HC (16.8g/KWh) emissions . Less and smaller particles numbers were detected for Early Intake Valve Closure (EIVC) from the combustion of E0 and E15 (4.0E+07#/cm3 less than 50nm in diameter) fuel blends. The particulate emission results showed that soot was the dominant particles in the exhaust, which could be reduced by leaner mixture combustion.

Enhanced vibration damping materials and structures for wind turbine blades inspired from auxetic configurations

Agnese, Fabio January 2014 (has links)
An extensive analysis of the current applications and possible employments of auxetic materials and configurations is presented. These novel materials show a negative Poisson's ratio and, potentially, enhanced mechanical properties. Despite a substantial amount of publications can be found in literature about auxetic material properties, not many of these consider practical applications for them. Objective and novelty of this project is therefore the application of auxetic material and/ or auxetic inspired configurations to existing structures and in particular to wind turbine blades to modify their dynamic characteristics. Wind turbine blades are complex systems manufactured using polymer matrix composite materials and at present made of a combination of glass and carbon fibre · reinforced plastic (GFRP-CFRP). Total damping in a blade is a combination of aerodynamic and structural loss factors, the latter being related to the inherent damping of the material. The two fundamental modes of vibration related to bending are of flapwise and edgewise type. The structural damping is material dependent, therefore the amount of structural damping available for these two vibration modes is the same. However, for the flapwise mode, the aerodynamic damping plays a very important role for the overall modal damping r.atio, whereas for the edgewise mode the only damping mechanism present is the str.uctural one. As a consequence, only a low value of loss factor can be achieved in the edgewise direction. The first aim of this project is then to demonstrate how auxetic inspired structure can be successfully applied to increase the loss factor of the blade in the edgewise direction of vibration. To this end several solutions have been investigated starting from the utilisation of 3D auxetic foams. They showed an effective increase in loss factor but limited by the fact that at present these foams present a low stiffness. Other solutions considered macro composites with shaped fibres and a novel damper design. Both these solutions have been analysed and characterised either by FE analysis and laboratory testing.

Evolution of stiffness and deformation of hostun sand under drained cyclic loading

Leiva, Daniella Escribano January 2014 (has links)
The study of long term cyclic loading has had increased interest in the last few years as many current and future projects for offshore wind turbines are taking place in the UK. Attention has been given to the changes in the foundation's stiffness as this may alter the natural frequency of the structure. As well, during the lifetime of the structure millions of cycles accumulate defoln1ations which may affect its serviceability. The condition of long term cyclic loading of soils can be idealized as a drained boundary problem, as wind and wave loads are applied at low frequencies. This quasi-static loading allows no development of excess pore water pressure in the soil. The main objective of this research was to simulate an element of soil subjected to quasi-static long term loading by means of drained cyclic triaxial tests and applying a simplified version of the stress paths imposed at in situ conditions. The testing program consisted of a set of drained and undrained monotonic triaxial tests, drained cyclic triaxial tests, and a small set of undrained cyclic triaxial tests. The experimental campaign studied,the elastic stiffness at isotropic and anisotropic stress states t6 quantify the inherent and stress induced anisotropy of Hostun sand. In addition, the evolving elastic properties of sand during progressive cyclic loading was investigated. Accumulated deformations during drained cyclic loading were obtained as well as the parameters that influenced their magnitude, intensity, and direction. Specifically, the influence of initial density, stress ratio, cyclic stress amplitude, state parameter, and confining pressure were explored. The relationship between creep deformations and cyclic deformations was analysed. Special attention was given to the limitations of triaxial testing and the effect of constrained boundaries applied to the specimen. Also, the effect of changing the sample preparation method on the results of accumulated volumetric strain. The results obtained from the testing program show the degree of inherent anisotropy of Hostun sand samples prepared by moist tamping method which is lower when compared with samples prepared by pluviation (Sadek, 2006; Sunyer, 2007). The stress-induced anisotropy of Hostun sand was studied by , means of constant mean' stress and constant radial stress paths. The results indicate the influence of vertical and horizontal effective stresses and the importance of quantifying these variables in the empirical expression for the elastic stiffness Go. The drained cyclic triaxial test results discuss about the influence of the stress ratio on the accumulated strains and that the direction of the deviatoric and volumetric strains depends' almost entirely in this parameter. The normalization of the data by stress ratio, cyclic stress amplitude, and the state parameter revealed that similar volumetric deformations are encountered for tests at different confining pressures. Comparison of the test results with simulations using the Severn Trent sand model (Gajo & Muir Wood, 1999) exposed the good agreement between the model and the tests under drained and undrained monotonic stress conditions. The limitations of the model for simulating drained cyclic loading conditions were revealed .

Experimental and simulation study of instabilities in a horizontal thermosyphon reboiler

Agunlejika, Ezekiel Oluwaseun January 2014 (has links)
Thermosyphon reboilers are widely used in the chemical industry because of their high heat transfer rates, limited space requirements, resistance to some types of fouling, and the absence of a pump. They are however susceptible to thermo-hydraulic instabilities, particularly at start-up and low system pressures. Flow instabilities are generally undesirable in process industries because flow oscillations may cause mechanical vibration, system control problems and adverse performance. This thesis details the investigation of two-phase flow instability in horizontal thermosyphon reboilers experimentally using a laboratory scale rig and via mathematical modelling.

Study of post-combustion carbon dioxide capture for coal-fired plant through modelling and simulation

Lawal, Adekola January 2010 (has links)
Fossil-fuel power plants are the largest single source of carbon dioxide (CO2) emissions. Post-combustion capture through monoethanolamine-based absorption is viewed as the most mature technology proposed for mitigating CO2 emissions from such power plants. Despite its advantages, several design and operational challenges arise in the application of this technology. The amount of flue gas to process is much greater than current applications. As a consequence, there will be a large thermal energy requirement for solvent regeneration. There are also concerns about how post-combustion CO2 capture would affect the flexibility and operability of coal-fired power plants. Though expensive pilot plant studies exist worldwide, they are still on a much smaller scale than what would be required commercially. This thesis provides useful insights for the design and operation of pilot and commercial plants through modelling and simulation. Cont/d.

Development of a self-tuned drive-train damper for utility-scale variable-speed wind turbines

Mathiopoulos, Athanasios January 2011 (has links)
This thesis describes the development of a procedure that tunes a wind turbine drivetrain damper (DTD) automatically. This procedure, when integrated into the controller of any utility-scale variable-speed wind turbine, will allow the turbine to autonomously and automatically tune its DTD on site. In practice this means that the effectiveness of the damper becomes independent on the accuracy of the model or the simulations used by the control engineers in order to tune the damper. This research is motivated by the fact that drive-train failures are still one of the biggest problems that stigmatises the wind turbine industry. The development of an automatically tuned DTD that alleviates the drive-train fatigue loads and thus increases the reliability and lifetime of the drive-train is thus considered very beneficial for the wind turbine industry. The procedure developed begins by running an experimental procedure to collect data that is then used to automatically system identify a linear model describing the drivetrain. Based on this model a single band-pass filter acting as a DTD is automatically tuned. This procedure is run for a number of times, and the resulting DTDs are compared in order to select the optimal one. The thesis demonstrates the effectiveness of the developed procedure and presents alternative procedures devised during research. Finally, insight into future work that could be performed is indicated in the last chapter of the thesis.

Automated on-line fault prognosis for wind turbine monitoring using SCADA data

Chen, Bindi January 2014 (has links)
Current wind turbine (WT) studies focus on improving their reliability and reducing the cost of energy, particularly when WTs are operated offshore. A Supervisory Control and Data Acquisition (SCADA) system is a standard installation on larger WTs, monitoring all major WT sub-assemblies and providing important information. Ideally, a WT’s health condition or state of the components can be deduced through rigorous analysis of SCADA data. Several programmes have been made for that purpose; however, the resulting cost savings are limited because of the data complexity and relatively low number of failures that can be easily detected in early stages. This thesis develops an automated on-line fault prognosis system for WT monitoring using SCADA data, concentrating particularly on WT pitch system, which is known to be fault significant. A number of preliminary activities were carried out in this research. They included building a dedicated server, developing a data visualisation tool, reviewing the existing WT monitoring techniques and investigating the possible AI techniques along with some examples detailing applications of how they can be utilised in this research. The a-priori knowledge-based Adaptive Neuro-Fuzzy Inference System (APK-ANFIS) was selected to research in further because it has been shown to be interpretable and allows domain knowledge to be incorporated. A fault prognosis system using APK-ANFIS based on four critical WT pitch system features is proposed. The proposed approach has been applied to the pitch data of two different designs of 26 Alstom and 22 Mitsubishi WTs, with two different types of SCADA system, demonstrating the adaptability of APK-ANFIS for application to variety of technologies. After that, the Alstom results were compared to a prior general alarm approach to show the advantage of prognostic horizon. In addition, both results are evaluated using Confusion Matrix analysis and a comparison study of the two tests to draw conclusions, demonstrating that the proposed approach is effective.

Thermo-mechanical fatigue of cast aluminium alloys for engine applications under severe conditions

Kliemt, Christian January 2012 (has links)
The increase in target performance of engines and hence the loading of their structural materials has dictated the need for more information about the behaviour of cast aluminium alloys under severe conditions up to 400°C. This study was therefore conducted in order to determine how different cooling rates, different pre-treatments and different alloying elements can improve the performance of cast aluminium (Al) alloys under thermo-mechanical fatigue (TMF) loading compared to a reference alloy and condition, AlSi6Cu4-T6. An existing TMF test rig was modified to allow an investigation of temperature gradients corresponding to those prevailing in real cylinder heads. The measured data were implemented in a Chaboche damage model and in an FEM tool in order to simulate low cycle fatigue (LCF) and TMF behaviour. These models provide a possibility to simulate LCF and TMF behaviour taking into account microstructural changes. On metallographic examination, a dependence of crack initiation on secondary dendrite arm spacing (SDAS) and on porosity was observed. Here, a smaller SDAS and a HIP modified microstructure led to a longer lifetime. Furthermore, clusters of brittle Si particles, decohesion or intermetallic phases were also found to initiate cracks. Stage I crack behaviour was seen at low strain amplitudes, where the crack propagates along the interface between the Al matrix and the intermetallic phases. Stage II behaviour was observed for higher strain amplitudes with crack propagation taking place along intermetallic phase boundaries such as Al2Cu, α-phase and β-phase or through pores. An increasing proportion of matrix cracks was observed for low strain amplitudes. Investigation of the decohesion behaviour revealed decohesion under high strain amplitudes and in areas with a high particle fracture volume attributable to high notch stresses. A refinement of the microstructure, particularly the particles, was found with increased Si content, associated with an increase in particle density. Following all TMF tests, an orientation of particles dependent on the loading conditions was observed, and this seemed to have had an influence on crack behaviour. A drift of stress was also found after long term high temperature exposure for strain amplitudes of 0.2%, attributed to creep.

Power electronic converters for motors with bifilar windings

Thong, Weng Kwai January 2005 (has links)
There is a high demand to improve performance and efficiency of motors used in domestic applications but there is constant downward pressure on manufacturing cost. This thesis identifies switched reluctance motors and flux switching motors as being low cost brushless motor technology and shows that bifilar windings can lead to lower cost electronic converters.;The thesis develops innovative power electronic converters for motor incorporating bifilar windings. Novel circuits for reliable capture, dissipation or recovery of energy associated with leakage inductance of the closely coupled bifilar windings are presented.;Two applications identified are dual voltage drives and bipolar excited motor drives. Several bifilar converter drives are presented in detail in this thesis. These include converters with dissipative snubbers, converters with energy recovery snubbers, converters with nondissipative snubbers and converters with inherent freewheel capability. Each of the presented converters offers advantages in cost and/or performance over existing converters.

Advanced algorithms for automatic wind turbine condition monitoring

Zappala, Donatella January 2014 (has links)
Reliable and efficient condition monitoring (CM) techniques play a crucial role in minimising wind turbine (WT) operations and maintenance (O&M) costs for a competitive development of wind energy, especially offshore. Although all new turbines are now fitted with some form of condition monitoring system (CMS), very few operators make use of the available monitoring information for maintenance purposes because of the volume and the complexity of the data. This Thesis is concerned with the development of advanced automatic fault detection techniques so that high on-line diagnostic accuracy for important WT drive train mechanical and electrical CM signals is achieved. Experimental work on small scale WT test rigs is described. Seeded fault tests were performed to investigate gear tooth damage, rotor electrical asymmetry and generator bearing failures. Test rig data were processed by using commercial WT CMSs. Based on the experimental evidence, three algorithms were proposed to aid in the automatic damage detection and diagnosis during WT non-stationary load and speed operating conditions. Uncertainty involved in analysing CM signals with field fitted equipment was reduced, and enhanced detection sensitivity was achieved, by identifying and collating characteristic fault frequencies in CM signals which could be tracked as the WT speed varies. The performance of the gearbox algorithm was validated against datasets of a full-size WT gearbox, that had sustained gear damage, from the National Renewable Energy Laboratory (NREL) WT Gearbox Condition Monitoring Round Robin project. The fault detection sensitivity of the proposed algorithms was assessed and quantified leading to conclusions about their applicability to operating WTs.

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