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Ymittos Athens, Greece

Rakopoulos D.C.,National Technical University of Athens | Rakopoulos C.D.,National Technical University of Athens | Papagiannakis R.G.,Hellenic Air Force Academy | Kyritsis D.C.,University of Illinois at Urbana - Champaign
Fuel | Year: 2011

An experimental study is conducted to evaluate the effects of using blends of diesel fuel with either ethanol in proportions of 5% and 10% or n-butanol in 8% and 16% (by vol.), on the combustion behavior of a fully-instrumented, six-cylinder, turbocharged and after-cooled, heavy duty, direct injection (DI), 'Mercedes-Benz' engine installed at the authors' laboratory. Combustion chamber and fuel injection pressure diagrams are obtained at two speeds and three loads using a developed, high-speed, data acquisition and processing system. A heat release analysis of the experimentally obtained cylinder pressure diagrams is developed and used. Plots of histories in the combustion chamber of the heat release rate and temperatures reveal some interesting features, which shed light into the combustion mechanism when using these promising bio-fuels that can be derived from biomass (bio-ethanol and bio-butanol). The key results are that with the use of these bio-fuels blends, fuel injection pressure diagrams are very slightly displaced (delayed), ignition delay is increased, maximum cylinder pressures are slightly reduced and cylinder temperatures are reduced during the first part of combustion. These results, combined with the differing physical and chemical properties of the ethanol and n-butanol against those for the diesel fuel, which constitutes the baseline fuel, aid the correct interpretation of the observed engine behavior performance- and emissions-wise. © 2010 Elsevier Ltd. All rights reserved. Source


Kliros G.S.,Hellenic Air Force Academy
Romanian Journal of Information Science and Technology | Year: 2010

Graphene nanostructures exhibit an intrinsic advantage in relation to the gate delay in three-terminal devices and provide additional benefits when operate in the quantum capacitance limit. In this paper, we developed a simple model that captures the Fermi energy and temperature dependence of the quantum capacitance for monolayer and bilayer graphene devices. Quantum capacitance is calculated from the broadened density of states taking into account electron-hole puddles and possible finite lifetime of electronic states through a Gaussian broadening distribution. The obtained results are in agreement with many features recently observed in quantum capacitance measurements on both gated monolayer and bilayer graphene devices. The temperature dependence of the minimum quantum capacitance around the charge neutrality point is also investigated. Source


Kehayas N.,Hellenic Air Force Academy
Journal of Aircraft | Year: 2011

It has been suggested that the basic configuration of subsonic civil transport aircraft is nearing its full evolutionary potential and a departure in the form of a new configuration or technology is needed. To accomplish this objective, a number of alternative concepts have been proposed, one of which is based on the jet flap. In this paper, the propulsion system of a jet-flapped subsonic civil transport aircraft design is evaluated. The jet engines of this design are embedded in the wings and exhaust through fishtail diffuser ducts, from high-aspect-ratio nozzles located at a small control flap at the trailing edge. The aim is to match jet engine, fishtail duct geometry and jet momentum coefficient requirements. It is found that it is possible to achieve the exceptional lift-to-drag ratios of the jet flap using very high bypass ratio geared turbofans operating at a lower temperature. The resulting jet-flapped design exhibits lift-to-drag ratios of over 60 without any significant effects on specific fuel consumption or weight. The jet-flapped design is then compared with other advanced technology designs and comes first on fuel consumption per seat . km, as well as in other areas of interest such as safety, emissions, and noise. © Copyright 2010. Source


Kliros G.S.,Hellenic Air Force Academy
Proceedings of the International Semiconductor Conference, CAS | Year: 2010

We present a simple phenomenological model for the quantum capacitance of bilayer graphene. Quantum capacitance is calculated from the broadened density of states taking into account electron-hole puddles and possible finite lifetime of electronic states through a Gaussian broadening distribution. The obtained results are in agreement with many features recently observed in quantum capacitance measurements on gated bilayer graphene. The temperature dependence of quantum capacitance is also investigated. © 2010 IEEE. Source


Kliros G.S.,Hellenic Air Force Academy
Proceedings of the International Conference on Microelectronics, ICM | Year: 2010

Gate voltage control of carrier density and quantum capacitance is an important step for understanding the device physics and assessing the performance of nanoscale transistors. In this paper, we present a simple phenomenological model for the carrier density and quantum capacitance of graphene nanoribbon field-effect transistors as functions of gate voltage, Fermi level position and temperature. Quantum capacitance is calculated from the broadened density of states incorporating the presence of electron-hole puddles and possible finite lifetime of electronic states through a Gaussian broadening distribution. Thin gate-insulators of high-κ dielectric constant are used in our calculations in order to approach the quantum capacitance limit. © 2009 IEEE. Source

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