Characteristics of homogeneous charge compression ignition (HCCI) combustion and emissions of n-heptane

This paper reports the outcome from a systematic investigation carried out on HCCI (Homogeneous Charge Compression Ignition) combustion of a diesel type fuel. The n heptane was chosen in this study to study the premixed diesel HCCI combustion characteristics with port fuel injection. Measurements were carried out in a single-cylinder, 4-stroke and variable compression ratio engine. Premixed n-heptane/air/EGR mixture was introduced into the cylinder by a port fuel injector and an external EGR system. The operating regions with regard to Air/Fuel ratio and EGR rate were established for different compression ratios and intake temperatures. The effects of compression ratios, intake temperatures, Air/Fuel ratios and EGR rates on knock limit, auto-ignition timing, combustion rate, IMEP, and engine-out emissions, such as NOx, CO, and unburned HC, were analysed. The results have shown HCCI combustion of n-heptane could be implemented without intake charge heating with a typical diesel engine compression ratio. The attainable HCCI operating region was mainly limited by the knock limit, misfir, and low IMEP respectively. Higher intake temperature or compression ratio could extend the misfire limit of the HCCI operation at low load but they would reduce the maximum IMEP limit at higher load conditions. Compared with conventional diesel combustion, HCCI combustion lead to extremely low NOx emissions ( less than 5 ppm) and smoke free exhaust. But HCCI diesel combustion was found to produce higher HC and CO emissions. An increase in intake temperature or compression ratio helped to reduce HC and CO emissions.

Visualization of the homogeneous charge compression ignition/controlled autoignition combustion process using two-dimensional planar laser-induced fluorescence imaging of formaldehyde

The paper reports an investigation into the HCCI/CAI combustion process using the two-dimensional PLIF technique. The PLIF of formaldehyde formed during the low-temperature reactions of HCCI/CAI combustion was exciting by a tunable dye laser at 355nm wavelength and detected by a gated ICCD camera. Times and locations of the two-stage autoignition of HCCI/CAI combustion were observed in a single cylinder optical engine for several fuel blends mixed with n-heptane and iso-octane. The results show, when pure n-heptane was used, the initial formation of formaldehyde and its subsequent burning were closely related to the start of the low temperature heat release stage and the start of the main heat release stage of HCCI combustion respectively. Meanwhile, it was found that the formation of formaldehyde was more affected by the charge temperature than by the fuel concentration. But its subsequent burning or the start of main heat release combustion toke place at those areas where both the fuel concentration and the charge temperature were sufficient high. As a result, it was found that the presence of stratified residual gases affected both the spatial location and the temporal site of autoignition in a HCCI/CAI combustion engine. All studied fuels were found having similar formaldehyde formation timings with n-heptane. This means that the presence of iso-octane did not affect the start of low temperature reactions apparently. However, the heat release during low temperature reaction was significantly reduced with the presence of iso-octane in the studied fuels. In addition, the presence of iso-octane retarded the start of the main combustion stage

Many-body Green's function theory for electron-phonon interactions: ground state properties of the Holstein dimer

We study ground-state properties of a two-site, two-electron Holstein model describing two molecules coupled indirectly via electron-phonon interaction by using both exact diagonalization and self-consistent diagrammatic many-body perturbation theory. The Hartree and self-consistent Born approximations used in the present work are studied at different levels of self-consistency. The governing equations are shown to exhibit multiple solutions when the electron-phonon interaction is sufficiently strong whereas at smaller interactions only a single solution is found. The additional solutions at larger electron-phonon couplings correspond to symmetry-broken states with inhomogeneous electron densities. A comparison to exact results indicates that this symmetry breaking is strongly correlated with the formation of a bipolaron state in which the two electrons prefer to reside on the same molecule. The results further show that the Hartree and partially self-consistent Born solutions obtained by enforcing symmetry do not compare well with exact energetics, while the fully self-consistent Born approximation improves the qualitative and quantitative agreement with exact results in the same symmetric case. This together with a presented natural occupation number analysis supports the conclusion that the fully self-consistent approximation describes partially the bipolaron crossover. These results contribute to better understanding how these approximations cope with the strong localizing effect of the electron-phonon interaction.Comment: 9 figures, corrected typo

Ballistic transport at room temperature in micrometer size multigraphene

The intrinsic values of the carriers mobility and density of the graphene layers inside graphite, the well known structure built on these layers in the Bernal stacking configuration, are not well known mainly because most of the research was done in rather bulk samples where lattice defects hide their intrinsic values. By measuring the electrical resistance through microfabricated constrictions in micrometer small graphite flakes of a few tens of nanometers thickness we studied the ballistic behavior of the carriers. We found that the carriers' mean free path is micrometer large with a mobility $\mu \simeq 6 \times 10^6$cm$^2$/Vs and a carrier density $n \simeq 7 \times 10^8$cm$^{-2}$ per graphene layer at room temperature. These distinctive transport and ballistic properties have important implications for understanding the values obtained in single graphene and in graphite as well as for implementing this last in nanoelectronic devices.Comment: 6 pages, 6 figure

Strain Modulated Electronic Properties of Ge Nanowires - A First Principles Study

We used density-functional theory based first principles simulations to study the effects of uniaxial strain and quantum confinement on the electronic properties of germanium nanowires along the [110] direction, such as the energy gap and the effective masses of the electron and hole. The diameters of the nanowires being studied are up to 50 {\AA}. As shown in our calculations, the Ge [110] nanowires possess a direct band gap, in contrast to the nature of an indirect band gap in bulk. We discovered that the band gap and the effective masses of charge carries can be modulated by applying uniaxial strain to the nanowires. These strain modulations are size-dependent. For a smaller wire (~ 12 {\AA}), the band gap is almost a linear function of strain; compressive strain increases the gap while tensile strain reduces the gap. For a larger wire (20 {\AA} - 50 {\AA}), the variation of the band gap with respect to strain shows nearly parabolic behavior: compressive strain beyond -1% also reduces the gap. In addition, our studies showed that strain affects effective masses of the electron and hole very differently. The effective mass of the hole increases with a tensile strain while the effective mass of the electron increases with a compressive strain. Our results suggested both strain and size can be used to tune the band structures of nanowires, which may help in design of future nano-electronic devices. We also discussed our results by applying the tight-binding model.Comment: 1 table, 8 figure

Evaluating service marketing in airline industry and its influence on student passengers' purchasing behaviour-using Taipei-London route as an example.

This paper reports an empirical study into airline consumer purchase behaviour on a major global route, that between the United Kingdom and Taipei. It focuses on the factors influencing airline choice for a sample of 60 Taiwanese students based at UK Universities and travelling on five airlines: British Airways, Cathay Pacific Airways, China Airlines, EVA Airways and Royal Dutch Airways. Several stages of data-gathering were undertaken and a well-established analytical framework for services marketing was used to identify gaps between the desired and received service. The main findings suggest that service quality, including in-flight entertainment, cabin crew attitude and seating comfort, can be as important as, or more important than, price in student purchase decisions for long-haul flights. Other important factors in carrier choice included student discounts, baggage allowances and airline safety record. Key Words: Service Marketing, Airline Industry, Consumer Behaviour, Taiwan, Touris