首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2623篇
  免费   22篇
公路运输   416篇
综合类   886篇
水路运输   826篇
铁路运输   131篇
综合运输   386篇
  2024年   2篇
  2023年   7篇
  2022年   23篇
  2021年   32篇
  2020年   22篇
  2019年   18篇
  2018年   340篇
  2017年   296篇
  2016年   263篇
  2015年   23篇
  2014年   39篇
  2013年   41篇
  2012年   108篇
  2011年   268篇
  2010年   265篇
  2009年   104篇
  2008年   231篇
  2007年   184篇
  2006年   60篇
  2005年   89篇
  2004年   70篇
  2003年   76篇
  2002年   30篇
  2001年   9篇
  2000年   12篇
  1999年   6篇
  1998年   6篇
  1997年   2篇
  1996年   9篇
  1995年   3篇
  1994年   1篇
  1993年   1篇
  1991年   3篇
  1990年   2篇
排序方式: 共有2645条查询结果,搜索用时 15 毫秒
91.
This research attempts to characterize the injection of palm biodiesel blended with diesel in a Zuech’s chamber. Thailand conventional diesel (mandated blend of biodiesel at 5 % or B5), palm biodiesel (B100) and four other biodiesel blends ratios (B20, B40, B60 and B80) were investigated with single hole injector of 140 and 200 μm diameters, injection pressure of 40 MPa to 160 MPa, constant back pressure of 4.5 MPa and energize time of 2.5 ms. The results show that increasing biodiesel blending ratios leads to longer injection delay, larger injection pressure drop, smaller injection quantity discharge coefficient (Cd) and shorter injection duration. With increasing biodiesel blending ratio, high Cavitation number from biodiesel viscosity decreases Reynolds number. Increasing injector diameter from 140 μm to 200 μm has reduced injection delay, increased fuel injection quantity, discharge coefficient and remaining injection duration. The increasing of injection pressure were improve, injection delay, injection duration, injection quantity and discharge coefficient until injection pressure 120 MPa. In addition at injection pressure over 120 MPa are decrease injection quantity and discharge coefficient, it effect form the cavitation phenomena. Increasing of viscosity, density, Bulk modulus and sound velocity were effect to increase injection delay, with reduce injection quantity, injection duration and pressure drop during injection process.  相似文献   
92.
The passenger airbag (PAB) requires a large volume and fast deployment because of the large distance between the dashboard and the passenger. And various shapes and sizes of the PAB are required depending on the type of vehicle. However, since the PAB modeling process for each design change is complicated and time consuming, the design parameters of the PAB could not be well investigated. In this study, a unique feature-based CAD system has been proposed that easily constructs PAB CAD model and then generates PAB FE model for collision analysis. Main keypoints and widths of PAB that determine the shape and size have been extracted by analyzing the geometric-feature of airbag. The PAB CAD model can be easily constructed by inputting keypoints and widths information. Then, from the constructed PAB CAD model, the PAB FE model is automatically generated. Finally, the generated PAB FE model can be directly employed for collision analysis, thereby reducing the modeling time of the PAB and enabling efficient parametric studies on design.  相似文献   
93.
This paper suggests a real-time method for detecting a driver’s cognitive and visual distraction using lateral driving performance measures. The algorithm adopts radial basis probabilistic neural networks (RBPNNs) to construct classification models. In this study, combinations of two driving performance data measures, including the standard deviation of lane position (SDLP) and steering wheel reversal rate (SRR), were considered as measures of distraction. Data for training and testing the RBPNN models were collected under simulated conditions in which fifteen participants drove on a highway. While driving, they were asked to complete auditory recall tasks or arrow search tasks to create cognitively or visually distracted driving periods. As a result, the best performing model could detect distraction with an average accuracy of 78.0 %, which is a relatively high accuracy in the human factors domain. The results demonstrated that the RBPNN model using SDLP and SRR could be an effective distraction detector with easy-to-obtain and inexpensive inputs.  相似文献   
94.
In this study, the effect of hydrothermal aging over a commercial diesel oxidation catalyst (DOC) on deterioration in nitrogen dioxide (NO2) production activity has been experimentally investigated based on a micro-reactor DOC experiment. Through this experimental result, the NO2 to nitrogen oxides (NOx) ratio at DOC outlet has been mathematically expressed as a function of DOC temperature according to various aging conditions. The current study reveals that the catalyst aging temperature is a more dominant factor than the aging duration in terms of the decrease in NO2 production performance through DOC. The DOC sample hydrothermally aged for 25 h at 750 °C has displayed the lowest NO2 to NOx ratio compared to the samples aged for 25 ~ 100 h at 650 °C. Also, in this study, the impact of hydrothermal aging of a DOC on the selective catalytic reduction (SCR) efficiency in a ‘DOC + SCR’ aftertreatment system was predicted by using transient SCR simulations. To validate the SCR simulation, this study has conducted a dynamometer test of a non-road heavy-duty diesel engine with employing a commercial ‘DOC + SCR’ system on the exhaust line. The current study has quantitatively estimated the effect of the variation in NO2 to NOx ratio due to the hydrothermal aging of DOC on the NOx removal efficiency of SCR.  相似文献   
95.
Recently Plug-in hybrid electric vehicles (PHEVs) have gained increasing attention due to their ability to reduce the fuel consumption and emissions. In this paper a new efficient power management strategy is proposed for a series PHEV. According to the battery state of charge (SOC) and vehicle power requirement, a new rule-based optimal power controller with four different operating modes is designed to improve the fuel economy of the vehicle. Furthermore, the teaching-learning based optimization (TLBO) method is employed to find the optimal engine power and battery power under the specified driving cycle while the fuel consumption is considered as the fitness function. In order to demonstrate the effectiveness of the proposed method, four different driving cycles with various numbers of driving distances for each driving cycle are selected for the simulation study. The performance of the proposed optimal power management strategy is compared with the rule-based power management method. The results verify that the proposed power management method could significantly improve the fuel economy of the series PHEV for different driving conditions.  相似文献   
96.
The wind pressure distribution and wind-induced vibration responses of long-span spatial groined latticed vaults (SGLVs) were numerically simulated, which always are ones of the most important problems in the structural wind resistance design. Incompressible visco-fluid model was introduced, and the standard k-εtwo equation model and semi-implicit method for pressure linked equation (SIMPLE) were used to describe the flow turbulence. Furthermore, the structural dynamic equation was set up, which is solved by Newmark-β method. And several sort of wind-induced vibration coefficients such as the wind-induced vibration coefficient corresponding to the nodal displacement responses and wind loads were suggested. In the numerical simulation where the SGLV consisting of the cylindrical sectors with different curved surface was chosen as the example,the influence on the relative wind pressure distribution and structural wind-induced vibration responses of the closed or open SGLV caused by such parameters as the number of cylindrical sectors, structural curvature and the ratio of rise to span was investigated. Finally, some useful conclusions on the local wind pressure distribution on the structural surface and the wind-induced vibration coefficients of SGLV were developed.  相似文献   
97.
Image-guided computer aided surgery system (ICAS) contributes to safeness and success of surgery operations by means of displaying anatomical structures and showing correlative information to surgeons in the process of operation. Based on analysis of requirements for ICAS, a new concept of clinical knowledge-based ICAS was proposed. Designing a reasonable data structure model is essential for realizing this new concept. The traditional data structure is limited in expressing and reusing the clinical knowledge such as locating an anatomical object, topological relations of anatomical objects and correlative clinical attributes. A data structure model called mixed adjacency lists by octree-path-chain (MALOC) was outlined, which can combine patient's images with clinical knowledge, as well as efficiently locate the instrument and search the objects' information. The efficiency of data structures was analyzed and experimental results were given in comparison to other traditional data structures. The result of the nasal surgery experiment proves that MALOC is a proper model for clinical knowledge-based ICAS that has advantages in not only locating the operative instrument precisely but also proving surgeons with real-time operation-correlative information. It is shown that the clinical knowledge-based ICAS with MALOC model has advantages in terms of safety and success of surgical operations, and help in accurately locating the operative instrument and providing operation-correlative knowledge and information to surgeons in the process of operations.  相似文献   
98.
99.
A cybernetics model of manufacturing execution system(MES CM) was proposed and studied from the viewpoint of cybernetics.Combining with the features of manufacturing system, the MES CM was modeled by"generalized modeling"method that is discussed in large-scale system theory.The mathematical model of MES CM was constructed by the generalized operator model, and the main characteristics of MES CM were analyzed.  相似文献   
100.
螺旋桨负载模拟是舰船电力推进系统中的重要研究内容.文中利用双馈电机四象限运行以及通过转子侧控制能降低变频器容量的特性,提出了双馈电机能量反馈系统模拟螺旋桨负载特性的方案.采用定子磁链定向的矢量控制策略,利用MATLAB/SIMULINK建立双馈电机模型和控制系统模型,实现不同工况下的稳态和动态特性模拟.仿真结果表明,转矩响应迅速、准确、稳定,采用矢量控制的双馈电机能量回馈系统具有良好的四象限负载特性模拟性能.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号