全文获取类型
收费全文 | 1878篇 |
免费 | 9篇 |
专业分类
公路运输 | 664篇 |
综合类 | 56篇 |
水路运输 | 577篇 |
铁路运输 | 51篇 |
综合运输 | 539篇 |
出版年
2023年 | 11篇 |
2022年 | 36篇 |
2021年 | 15篇 |
2020年 | 11篇 |
2019年 | 17篇 |
2018年 | 60篇 |
2017年 | 46篇 |
2016年 | 73篇 |
2015年 | 16篇 |
2014年 | 76篇 |
2013年 | 293篇 |
2012年 | 96篇 |
2011年 | 97篇 |
2010年 | 91篇 |
2009年 | 90篇 |
2008年 | 97篇 |
2007年 | 46篇 |
2006年 | 36篇 |
2005年 | 36篇 |
2004年 | 28篇 |
2003年 | 29篇 |
2002年 | 23篇 |
2001年 | 24篇 |
2000年 | 34篇 |
1999年 | 20篇 |
1998年 | 29篇 |
1997年 | 35篇 |
1996年 | 31篇 |
1995年 | 44篇 |
1994年 | 20篇 |
1993年 | 20篇 |
1992年 | 14篇 |
1991年 | 11篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 14篇 |
1986年 | 11篇 |
1985年 | 19篇 |
1984年 | 17篇 |
1983年 | 14篇 |
1982年 | 22篇 |
1981年 | 27篇 |
1980年 | 18篇 |
1979年 | 33篇 |
1978年 | 12篇 |
1977年 | 18篇 |
1976年 | 8篇 |
1975年 | 10篇 |
1974年 | 8篇 |
排序方式: 共有1887条查询结果,搜索用时 15 毫秒
991.
This paper presents a system to identify road and non-road regions from monocular color images of paved and unpaved roads. Despite being a single object, the road in these images is subject to large changes in appearance due to environmental effects and track materials. This condition has challenged the practical application of road identification. The proposed system combines random forest with color correlogram to overcome such conditions and offers a classifier for road and non-road regions in traffic images. As a color feature, the color correlogram depicts the spatial correlation of color changes in an image. Using random forest, road identification is formulated as a learning paradigm. The combined effects of color correlograms and random forest create a robust system capable of identifying roads even in variable situations in real time. This combination is more effective than other combinations, such as a color histogram plus random forest, a color correlogram plus neural network, or a color histogram plus neural network. 相似文献
992.
The urban tram introduced recently has a low-floor structure for the convenience of passengers getting on and off. To adjust the low-floor level and improve performance on curves, most low-floor trams have IRWs (independently rotating wheels) with no central axle between the two wheels. Eliminating the central axle, however, creates several inherent problems, such as insufficient guiding force and excessive wear. To analyze these problems, a new analytical model is described in this paper to describe the dynamic characteristics of IRWs more precisely. This analytical model is developed to consider the effects of longitudinal creep in particular, which have been ignored in conventional analytical models of IRWs. In addition, a running stability analysis based on the newly developed analytical model is conducted to compare the critical speeds of IRW-axle vehicles and rigid-axle vehicles. The dynamic characteristics of an initial disturbance are compared to verify that the analytical model is effective in expressing the dynamic characteristics of IRWs. 相似文献
993.
In lean-DeNOX catalysis reactions, hydrogen is a good reducing agent in PGM catalysts as well as an effective promoter in
selective catalytic reduction reactions over base metal oxide catalysts. However, such a lean-DeNOX system, which uses hydrogen,
requires an on-board fuel reforming system applicable to internal combustion engines. In this study, catalytic partial oxidation
(CPOx) performance was tested in a laboratory for various reactants and hydrocarbon conditions. Volume concentrations of 5–10%
oxygen and 0-5% water vapor were used to simulate diesel exhaust, and n-C12H26 was used as the feedstock for the reforming reaction. In the CPOx of n-C12H26, the highest hydrogen selectivity was 64% and was achieved at 100,000 h-1 GHSV. Additionally, the C/O ratio was less than
unity in the absence of water vapor. However, as the water concentration was increased to 2.5 and 5.0 vol. % in the n-C12H26 CPOx reactions, the maximum hydrogen selectivity was increased from 64% in the absence of water to 70% and 75%, respectively.
This effect is a consequence of the water-gas shift reaction over the catalyst bed. Regarding oxygen concentration effects,
hydrogen selectivity slightly increased with increasing oxygen concentration from 10% to 15%. It was also found that the CPOx
reaction of n-C12H26 can be ignited at temperatures below 300 C. Accordingly, it can be concluded that CPOx is a useful and feasible device for
promoting diesel DeNOx catalysis in terms of hydrogen productivity and reaction initiation. 相似文献
994.
Q. Fan J. Bian H. Lu L. Li J. Deng 《International Journal of Automotive Technology》2012,13(4):523-531
The first firing cycle is very important during cold-start for all types of spark ignition engines. In addition, the combustion characteristics of the first firing cycle affect combustion and emissions in the following cycles. However, the first-cycle fuel-air mixing, combustion and emissions generation within the cylinder of a two-stage direct-injection (TSDI) engine during cold start is not completely understood. Based on the total stoichiometric air-fuel ratio and local richer mixture startup strategy, the first-cycle firing and combustion characteristic at cold start were investigated in a two-stage direct injection (TSDI) gasoline engine. In addition, the effects of the first injection timing, second injection timing, 1st and 2nd fuel injection proportion and total excess air ratio on the in-cylinder pressure, heat release rate and accumulated heat release were analyzed on the basis of a cycle-by-cycle analysis. It is shown that a larger 2nd fuel injection amount and later 2nd injection timing are more beneficial to the firing of the first cycle in the case of a total excess air ratio of 1.0. The optimum 1st and 2nd injection timing fuel injection proportions are 120°CA ATDC during the intake stroke, 60°CA BTDC during the compression stroke and 1:1. In addition, the firing boundary is a 2nd injection timing later than 90°CA BTDC during the compression stroke in the case of the 1st injection timing from 60°CA to 180°CA ATDC during an intake stroke and involves a 1st and 2nd fuel injection proportion of 1:1 and an excess air ratio of 1.0. The study provides a detailed understanding of cold-start combustion characteristics and a guide for optimizing the reliable first-cycle firing at cold start. 相似文献
995.
To solve the power steering (PS) hose oil leakage problem in automobile applications, a new geometry design of the metal fitting is required, and the swaging conditions applied to the PS hose should be optimized. In this study, a new metal fitting geometry for the PS hose was proposed. A CAE simulation for both the proposed and the conventional metal fittings was conducted, and both the stress and the strain in the PS hose were compared. An experimental verification was also performed with these two different metal fittings. Additionally, the swaging conditions were optimized by the durability cycle time, which was measured in an impulse test of the swaged PS hose. For a more economical production, practical working conditions were also recommended and experimentally verified. 相似文献
996.
Valve seats press-fitted in the cylinder head function to hold exhaust gas inside the ignition chamber and to transfer heat to the coolant moving in the water jacket of the head. The press-fitting of the valve seats to the head at ambient temperature has been widely spread out due to its many advantages over pressing with frozen valve seats or with a heated head. The benefits include lower equipment costs, lower running costs, and fewer installation faults during the press-fitting. Nevertheless, a systematic approach for pressing at ambient temperature (ATP; ambient temperature press-fitting) has not been studied and analyzed to date. A technique to check the reliability of the press-fitting by measuring hoop strain inside the valve seat and the FEM procedure to simulate ATP is developed in this study. The FEM procedure of ATP developed here exhibits a concurrence with experimental results. Utilizing the DOE (Design of Experiments) technique, we determined the effects of various geometric parameters and the optimal shapes of the valve seat and cylinder head. The optimal shapes have been successfully applied in an actual engine and varified in a running-engine test. 相似文献
997.
A traction control system (TCS) is used to improve the acceleration performance on slippery roads by preventing excessive wheel slip. In this paper, a new traction control system using the integrated control of gear shifting and throttle actuation is developed for vehicles with automatic transmissions. In the design of the slip controller, by means of a differential manifold transformation, a slip control system with nonlinearities and uncertainties is transformed into a linear system, and a sliding mode controller is applied for the purpose of increasing the robustness of the system. Next, to achieve the required driving torque, the optimal throttle and gear position, maps are constructed based on dynamic programming. The simulation results indicate that the present traction control system can improve the acceleration performance of an automatic transmission vehicle for various types of road conditions. 相似文献
998.
S. W. Lee Y. S. Cho W. C. Choi J. H. Lee Y. J. Park 《International Journal of Automotive Technology》2012,13(7):1149-1157
This study aims to investigate the combustion characteristics of mixed fuel of liquefied propane gas (LPG) and biodiesel under compression ignition (CI) in an effort to develop highly efficient and environmentally friendly mixed fuelbased CI engines. Although LPG fuel is known to be eco-friendly due to its low CO2 emission, LPG has not yet been widely applied for highly efficient CI engines because of its low cetane number and is usually mixed with other types of CI-friendly fuels. In this study, a number of experiments were prepared with a constant volume chamber (CVC) setup to understand the fundamental combustion characteristics of mixed fuel with LPG and biodiesel in two weight-based ratios and exhaust gas recirculation (EGR) conditions. The results from the current investigations verify the applicability of mixed fuel of LPG and biodiesel in CI engines with a carefully designed combustion control strategy that maximizes the benefits of the mixed fuel. Based on the results of this study, ignition is improved by increasing the cetane value by using higher blending ratios of biodiesel. As the blending ratios of biodiesel increased, CO and HC decreased and CO2 and NOx increases. 相似文献
999.
H. S. Heo S. J. Bae H. K. Lee K. S. Park 《International Journal of Automotive Technology》2012,13(2):167-173
In a conventional MPI engine, a pulsation damper is usually mounted on the fuel rail to diminish undesirable noise in the
vehicle cabin room; however, pulsation dampers are quite expensive. Therefore, several studies have focused on reducing fuel
pressure pulsation by increasing the self-damping characteristics of the fuel rail. This paper details the development of
a fuel rail that reduces pulsation using a self-damping effect. Using an oil hammer simulation technique, pressure pulsation
characteristics were investigated with respect to the aspect ratio of the cross-section, wall thickness, and fuel rail material.
Increasing the aspect ratio and decreasing the wall thickness efficiently reduced the pressure pulsation. In addition, the
pressure pulsation characteristics were investigated with respect to the resonant engine speed and injection period. These
simulated data can be used to reduce the pressure pulsation peak and to avoid the resonant point in the design stage during
the development of a fuel rail. 相似文献
1000.
K. Jeon H. Hwang S. Choi J. Kim K. Jang K. Yi 《International Journal of Automotive Technology》2012,13(2):247-253
Cornering maneuvers with reduced body roll and without loss in comfort are leading requirements for car manufacturers. An
electric active roll control (ARC) system controls body roll angle with motor-driven actuators installed in the centers of
the front and rear stabilizer bars. A vehicle analysis model developed using a CarSim S/W was validated using vehicle test
data. Two ARC algorithms for a sports utility vehicle (SUV) were designed using a sliding-mode control algorithm based on
a nonlinear roll model and an estimated lateral acceleration based on a linearized roll model. Co-simulation with the Matlab
simulink controller model and the CarSim vehicle model were conducted to evaluate the performance of two ARC control algorithms.
To validate the ARC performance in a real vehicle, vehicle tests were conducted at KATECH proving ground using a small SUV
equipped with two ARC actuators, upper and lower controllers and a few subsystems. From the simulation and vehicle validation
test results, the proposed ARC control algorithm for the developed ARC actuator prototypes improves the vehicle’s dynamic
performance. 相似文献