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In this study, we established a matching method for a twin variable geometry turbocharging (Twin-VGT) system under varying expansion ratios at high altitudes. In addition, we developed an equivalent thermodynamic model of a two-stage variable turbine with the effective flow areas of turbines obtained under velocity constraints for altitudes ranging from 0 to 5500 m. The effects of altitudes on the matching efficacy of a two-stage compressor under different operating conditions were evaluated. Additionally, an optimal distribution of the total boost pressure was determined to minimize the output power of the two-stage compressor during non-isothermal compression at various altitudes. Furthermore, the interplay between exhaust available energy (EAE), total expansion ratio (TER), exhaust temperature, and turbine power was investigated. Lastly, the peak efficiency of the turbocharging system was evaluated by analyzing the distribution of the TER and total enthalpy drop, aimed at optimizing EAE at different altitudes. • An optimized distribution method of total boost ratio at different altitudes. • Theoretical constraint equations for optimal distribution of available exhaust energy at different altitudes. • Matching principle for a two-stage compressor with engine under whole operating conditions at different altitudes. • Establishment of an equivalent thermodynamic model of Twin-VGT at various altitudes. [ABSTRACT FROM AUTHOR]

Abstract In this research, a comparative study of combustion characteristics, performance and emissions of a turbocharged diesel engine fueled with diesel and methanol-Fischer-Tropsch (F-T) diesel-biodiesel-diesel blends at various simulated altitudes (0 m, 3500 m and 5500 m) were conducted. The blends were composed of diesel, methanol, F-T diesel and biodiesel at volume fractions of 65%, 15.2%, 15.6% and 4.2%, respectively. Diesel was used as the baseline fuel, methanol, F-T diesel and biodiesel were used to improve the atomization characteristics, cetane number and stability of the blends, respectively. The results indicated that the cylinder pressure, pressure rise rate and heat release rate of the engine fueled with the blends were lower than those of the diesel at an altitude of 0 m, but higher at an altitude of 5500 m. Peak torque values for the test fuels both appeared at 1400 rpm near the sea level and the speeds corresponding to the peak torque values increased with altitudes. The use of the blends improved the engine power and economic performance at low-speed ranges at an altitude of 3500 m but a dramatically reduce was observed at 5500 m. Compared with the diesel, the particulate matter (2.5 (PM 2.5)) for the blends was markedly lower, while a slightly higher in NOx at various altitudes. [ABSTRACT FROM AUTHOR]

Abstract: The nanostructure, fractal dimension and size of in-cylinder soot during diesel combustion process have been investigated for a heavy-duty direct injection diesel engine, using a total cylinder sampling system followed by high-resolution transmission electron microscopy and Raman scattering spectrometry. Different structural organizations of in-cylinder soot are found depending upon the combustion phase. It is revealed that both the fringe tortuosity and separation distance decrease as combustion proceeds, while the mean fringe length increases distinctly from 1.00 to 2.13nm, indicating the soot evolution toward a more graphitic structure during the combustion process. The fractal dimensions of aggregates are in a range of 1.20–1.74 at various crank angles under the applied engine operating conditions. As temperature and pressure increase, the fractal dimension decreases significantly to a minimum at the early diffusion combustion stage. The soot particles become more compact again as the fractal dimension increases during the subsequent combustion period. Primary particle sizes start small, go through a maximum in the early diffusion combustion phase and decline again as combustion proceeds. [Copyright &y& Elsevier]

Abstract: The diesel soot was extracted from the chamber of a running diesel engine using a total cylinder sampling system. A structural description and size distribution of the soot spherules were performed by means of high-resolution transmission electron microscopy (HRTEM). HRTEM image analysis was carried out to achieve semi-quantitative information about the soot organization. Different nanostructures were found in dependence upon the engine test conditions. A better organization of the graphitic layers occurred for the in-cylinder soot as the expansion stroke developed. The oxygen concentrations at sampling angles and the cylinder pressure were measured. The oxidation rates of the in-cylinder soot during the late combustion phase were calculated according to the differences of spherules size distributions between two consecutive sampling angles. The soot reactivity in dependence on the nanostructure was shown after deducting the effect of oxygen concentration and temperature. The increase of resistance toward oxidation was associated to the increase of the structural order for the in-cylinder soot. The results are a useful database for testing the structure–property relationship between the nanostructure and the reactivity of the in-cylinder diesel soot. [ABSTRACT FROM AUTHOR]