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   7811篇
  免费   104篇
公路运输   1663篇
综合类   581篇
水路运输   2519篇
铁路运输   764篇
综合运输   2388篇
  2023年   43篇
  2022年   112篇
  2021年   56篇
  2019年   80篇
  2018年   190篇
  2017年   105篇
  2016年   126篇
  2015年   90篇
  2014年   240篇
  2013年   1220篇
  2012年   316篇
  2011年   372篇
  2010年   224篇
  2009年   366篇
  2008年   298篇
  2007年   266篇
  2006年   230篇
  2005年   270篇
  2004年   288篇
  2003年   180篇
  2002年   146篇
  2001年   132篇
  2000年   138篇
  1999年   98篇
  1998年   129篇
  1997年   111篇
  1996年   138篇
  1995年   149篇
  1994年   84篇
  1993年   189篇
  1992年   160篇
  1991年   77篇
  1990年   83篇
  1989年   57篇
  1988年   74篇
  1987年   65篇
  1986年   59篇
  1985年   77篇
  1984年   73篇
  1983年   73篇
  1982年   73篇
  1981年   95篇
  1980年   68篇
  1979年   91篇
  1978年   56篇
  1977年   69篇
  1976年   53篇
  1975年   65篇
  1974年   50篇
  1973年   45篇
排序方式: 共有7915条查询结果,搜索用时 15 毫秒
671.
Extensive usage of automobiles has certain disadvantages and one of them is its negative effect on environment. Carbon dioxide (CO2), carbon monoxide (CO), hydrocarbons (HC), oxides of nitrogen (NOx), sulphur dioxide (SO2) and particulate matter (PM) come out as harmful products during incomplete combustion from internal combustion (IC) engines. As these substances affect human health, regulatory bodies impose increasingly stringent restrictions on the level of emissions coming out from IC engines. This trend suggests the urgent need for the investigation of all aspects relevant to emissions. It is required to modify existing engine technologies and to develop a better after-treatment system to achieve the upcoming emission norms. Diesel engines are generally preferred over gasoline engines due to their undisputed benefit of fuel economy and higher torque output. However, diesel engines produce higher emissions, particularly NOx and PM. Aftertreatment systems are costly and occupy more space, hence, in-cylinder solutions are preferred in reducing emissions. Exhaust gas recirculation (EGR) technology has been utilized previously to reduce NOx. Though it is quite successful for small engines, problem persists with large bore engines and with high rate of EGR. EGR helps in reducing NOx, but increases particulate emissions and fuel consumption. Many in-cylinder solutions such as lower compression ratios, modified injection characteristics, improved air intake system etc. are required along with EGR to accomplish the future emission norms. Modern combustion techniques such as low temperature combustion (LTC), homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI) etc. would be helpful for reducing the exhaust emissions and improving the engine performance. However, controlling of autoignition timing and achieving wider operating range are the major challenges with these techniques. A comprehensive review of diesel engine performance and emission characteristics is given in this paper.  相似文献   
672.
This paper gives a literature review from experimental, epidemiological and naturalistic studies, having tried to measure on the one hand, the impact of mobile phone use on driving and, on the other hand, the risk of accident associated with this practice. The overview of these studies allowed first of all to show that the driving task is altered during the period of mobile phone use, and to quantify the disturbance (experimental studies), then to verify that this disturbance can be at the origin of accidents and quantify this risk (epidemiological studies). The naturalistic studies, consisting in following-up fleet vehicles, give some more explanations and tend to confirm the results. Advantages and limits of each approach are also discussed.  相似文献   
673.
Based on the Lagrange classical stability concept, the approaches and corresponding equations for determining conditions for providing a static and tip-over stability of a two-chain suspension arrangement have been developed. Specifically, it has been proven that the arrangement is in the stable position of equilibrium if the cargo centre of gravity is placed within the isosceles triangle (named as static stability triangle). The base of this triangle is the base of the secondary suspension, and its height is a function of the geometric peculiarities of the arrangement and ratio between mass of spreader and cargo unit. Comparative tip-over stability analysis of the arrangements with different geometric peculiarities and configurations has been also performed.  相似文献   
674.
Many French and European cities have developed a bike sharing system since 2005. This policy may be explained in a great part by the success of the Velo??v in Lyon, which contributed to improve the image of cycling in the city. If bicycles, however, appear to find a new place in the city, one can wonder whether this is really due to bike sharing systems? What are the real uses of bicycles in contemporary cities, with or without such a system? To answer these questions, the paper starts with a literature review on bicycle in the city, to draw lessons from recent experiences of bike sharing systems. It then discusses the two cases of Lyon and Lille, which never had a bike sharing system on its own. The evolution of bicycle use is measured with a comparison of household transportation surveys in Lyon (1995 and 1998) and Lille (1998 and 2006). In both cities, the analysis reveals a rise in the proportion of healthiest citizens among urban bikers, as well as a return of bikes in dense areas. Bike sharing systems are part of such tendencies. They support them, but should not become a panacea for policies supporting bike use in the city.  相似文献   
675.
In this study, a method for vehicle tracking through video analysis based on Markov chain Monte Carlo (MCMC) particle filtering with metropolis sampling is proposed. The method handles multiple targets with low computational requirements and is, therefore, ideally suited for advanced-driver assistance systems that involve real-time operation. The method exploits the removed perspective domain given by inverse perspective mapping (IPM) to define a fast and efficient likelihood model. Additionally, the method encompasses an interaction model using Markov Random Fields (MRF) that allows treatment of dependencies between the motions of targets. The proposed method is tested in highway sequences and compared to state-of-the-art methods for vehicle tracking, i.e., independent target tracking with Kalman filtering (KF) and joint tracking with particle filtering. The results showed fewer tracking failures using the proposed method.  相似文献   
676.
This article addresses interdisciplinary sustainable aspects of fisheries as part of ocean management. Human-caused impacts and their role as modifiers of living marine resources is discussed. The research note also theorizes about contemporary global change and its prospective biological consequences, especially when coupled with human-induced factors in coastal marine waters. Also addressed is the management and ecological aspects of fish stock populations as part of a large marine ecosystem (LME) in the Northeast United States continental shelf of the Atlantic Ocean with suggestions for an interdisciplinary policy orientation paradigm to foster the sustainability of marine life in the sea.  相似文献   
677.
This work estimates new regionalized empirical parameterizations for preformed alkalinity (ATo) and the CO2 air–sea disequilibrium (?Cdis). Both are key terms for the computation of anthropogenic CO2 in the back-calculation methods. Data from the subsurface layer (75–180 m depth range) covering an area from North to South and from 19°E to 67.5°W (Pacific and Indian oceans) were taken from GLODAP (The Global Ocean Data Analysis Project) database. The subsurface layer is proved as a reliable reference for representing the main characteristics of the different water masses of the oceans. Besides, handing data from the two ocean basins altogether makes the new parameterizations of ATo and ?Cdis to be more globally consistent. Nevertheless, each ocean basin, at least in some regions, has different oceanographic characteristics based on its proper dynamical processes and water masses formation. In order to maintain each ocean basin ‘identity’ the whole domain was divided in six different regions (two of them sharing waters from Pacific and Indian oceans) and parameterizations in each region for both terms were obtained. Previously, data were transformed into a grid of 4°lat. × 5°lon. and the results obtained from the parameterizations were visualized and compare with pCO2 climatologies. From the comparisons with previous ?Cdis estimations good results are obtained showing the reliability and robustness of the new regionalized empiric parameterizations.  相似文献   
678.
This paper discusses the European Union's (EU) proposals to improve the efficiency and infrastructure of European ports, and investigates the extent to which these proposals are welcomed by the ports industry. On the one hand, the analysis of the recent EU policy developments makes apparent that the EU is increasingly impinging upon the port industry's life. On the other hand, the findings of the semi-structured interviews with the industry's representatives suggest that although port authorities and port operators remain in principle against a coherent common EU ports policy, the industry believes that certain EU initiatives can enhance the competitiveness of the maritime transport system.  相似文献   
679.
The problems associated with hydrocarbon extraction in the EEZ are discussed under the headings of resources and environment, technology and the industrial structure, government management, decision-making, and policy formulation. Policy areas identified include boundary delimitation relative to resources, environment and technology, co-ordination and decision-making machinery, relationships with UK and EEC energy policies, and regional planning of the sea and coastal areas used by the off-shore industry.  相似文献   
680.
Port information management is considered a critical instrument towards enabling international transport and trade; thus, various forms of Port Information Systems (PORTIS) have been developed today, namely Port Community Systems, Terminal Operating Systems and Single Window systems. In Europe, the nautical information system SafeSeaNet is viewed as an important e-infrastructure. PORTIS systems are expected to evolve into next-generation technological platforms in order to offer a fully integrated digital environment to a multitude of maritime business actors and public authorities towards more efficient, safe and environment-aware transport and trade infrastructures and services. We propose service-oriented computing and model-driven development techniques as a robust PORTIS modelling and development approach. We present a reference model of PORTIS and a particular enabling methodological and technological framework. The proposed approach has been tested in a maritime single-window case.  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