全文获取类型
收费全文 | 912篇 |
免费 | 8篇 |
专业分类
公路运输 | 221篇 |
综合类 | 38篇 |
水路运输 | 339篇 |
铁路运输 | 32篇 |
综合运输 | 290篇 |
出版年
2022年 | 8篇 |
2021年 | 5篇 |
2020年 | 5篇 |
2019年 | 11篇 |
2018年 | 14篇 |
2017年 | 23篇 |
2016年 | 21篇 |
2015年 | 15篇 |
2014年 | 19篇 |
2013年 | 147篇 |
2012年 | 34篇 |
2011年 | 43篇 |
2010年 | 30篇 |
2009年 | 38篇 |
2008年 | 40篇 |
2007年 | 27篇 |
2006年 | 16篇 |
2005年 | 21篇 |
2004年 | 16篇 |
2003年 | 21篇 |
2002年 | 8篇 |
2001年 | 11篇 |
2000年 | 21篇 |
1999年 | 10篇 |
1998年 | 13篇 |
1997年 | 18篇 |
1996年 | 21篇 |
1995年 | 26篇 |
1994年 | 5篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 8篇 |
1990年 | 13篇 |
1989年 | 5篇 |
1988年 | 14篇 |
1987年 | 11篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 7篇 |
1983年 | 16篇 |
1982年 | 13篇 |
1981年 | 18篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 13篇 |
1974年 | 6篇 |
1973年 | 7篇 |
排序方式: 共有920条查询结果,搜索用时 15 毫秒
51.
Compression ratio (CR) is a design parameter with highest influence on efficiency, emission and engine characteristics. In conventional internal combustion (IC) engines, the compression ratio is fixed and their performance is, therefore, a compromise between conflicting requirements. One fundamental problem is that drive units in the vehicles must successfully operate at variable speed and loads and in different ambient conditions. If a diesel engine has a fixed CR, a minimal value must be chosen that can achieve a reliable self-ignition when starting the engine in cold start conditions. In diesel engines, variable compression ratio (VCR) provides control of peak cylinder pressure, improves cold start ability and low load operation, enabling the multi-fuel capability, increase of fuel economy and reduction of emissions. By application of VCR and other mechanisms, the optimal regime fields are extended to the prime requirements: consumption, power, emission, noise, etc., and/or the possibility of the engine to operate with different fuels is extended. An experimental Diesel engine has been developed at the Faculty of Engineering, University of Kragujevac. The changes of CR are realized by changing the piston chamber diameter. Detailed engine tests were performed at the Laboratory for IC engines. Special attention has been given to decrease of fuel consumption and exhaust emissions. An optimal field of CR variation has been determined depending on the given objectives: minimal fuel consumption, minimal nitric oxides, and particulate matter emissions, etc. 相似文献
52.
53.
54.
J. Zhou Jy Wong R. S. Sharp 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》1999,31(1):35-45
Based on the multi-spoke, single-plane, steady-state tyre model, a transient multi-spoke, three-plane tyre model is presented. This model involves updating the states of all the spokes under consideration to t+dt from the corresponding states at t. Also, a spoke base, which has lateral and longitudinal flexibilities relative to wheel hub, is included. By adding two extra planes of spokes to the original one, the effect of tyre width is built into the model. The three planes are equally spaced across the width of the tyre. The interaction with the ground of the spokes on these three planes is used to represent that of tyre elements at different locations across the width. Analytical results show good qualitative agreement with published experimental data. This model can be used to illustrate transient tyre behaviour and in simulations in which a generic tyre will suffice. 相似文献
55.
This paper presents the joint optimization of signal setting parameters and dynamic user equilibrium (DUE) traffic assignment for the congested urban road network. The simulation-based approach is employed to obtain the DUE condition for the case of multiple-origin multiple-destination traffic flows. The dynamic traffic assignment simulation program (DTASP), developed in C language is used to assign the traffic dynamically on the road network, whereas method of successive averages (MSA) is modified and used to arrive at the DUE condition. The artificial intelligence technique of genetic algorithms (GAs) is applied to obtain the optimal signal setting parameters and path flow distribution factor for DUE condition. The methodology developed in such a way that joint optimization of signal setting parameters with DUE is obtained. The proposed method is applied to the real network data of Fort Area of Mumbai city comprising of 17 nodes and 56 unidirectional links with 72 Origin–Destination pairs, where all the 17 nodes are signalized intersections. The traffic flow condition for the optimized signal setting parameters is considerably improved compared to the existing signal settings. The results prove that the GA is an effective technique to solve the joint optimization problem for the real network data. 相似文献
56.
In this paper, the dynamic response of a scale model of a jacket offshore structure is investigated both theoretically and experimentally. The experiments were conducted both in air and in water. The in-water experiments were done in the towing tank of Memorial University to simulate the realistic operating conditions. The model was subjected to random wave loads. Froude's law of modeling was used to obtain the dimensions of the scale model based on the dimensions of an existing structure. The effects of varying the structure's weight, and the characteristics of the wave loading were investigated. The structure's weight was changed by adding weights to the structure's deck. A finite element model was designed to determine the dynamic response of the model. Excellent agreement between the experimental and theoretical results was obtained.The reaction force at the foundation was estimated from strain measurements and compared with the finite element calculations. Fair agreement was obtained.This work is the first stage of a project whose objective is to develop a method for structural damage detection using the free vibration response of the structure. The free vibration response will be obtained from the stationary random response of the structure using the random decrement method. Having an accurate model to describe the dynamic response of the structure is the first step in this study. 相似文献
57.
This paper discusses the damage detection in offshore jacket platforms subjected to random loads using a combined method of random decrement signature and neural networks. The random decrement technique is used to extract the free decay of the structure from its online response while the structure is in service. The free decay and its time derivative are used as input for a neural network. The output of the neural network is used as an index for damage detection. It has been shown that function N is effective in damage detection in the members of an offshore structure. Experimental studies conducted on a reduced model for a real jacket structure with geometrical scale of 1:30 are used. The applied loads were random loads. Two different load spectra were used: White noise, and Pierson-Moskowitz. 相似文献
58.
J. Benajes S. Molina R. Novella M. Riesco 《International Journal of Automotive Technology》2008,9(3):257-265
A combined theoretical and experimental study has been carried out to determine the real potential of reducing pollutant emissions
in a HD diesel engine by means of retarding the intake valve closing. The effects produced by this alteration of the basic
operation cycle have been examined by a preliminary modelling study, and from the obtained results, a modified camshaft was
manufactured with a delayed intake valve closing of 60 crank angle degrees. Single-cylinder engine tests were carried out
with this modified camshaft, and the emissions and fuel consumption were recorded. The results showed that the retarded intake
valve closing can enhance the premixed combustion phase, and thus simultaneously reduce soot and NOx emissions. Moreover,
the combustion process attained is extremely tolerant to exhaust gas recirculation, and by adoption of this measure, Euro-5
emissions limits can be achieved at the tested conditions without after-treatment. 相似文献
59.
William R. Black 《Transportation》1991,18(3):199-206
The use of correlation coefficients to evaluate the accuracy of spatial interaction models is inappropriate unless such models have been fitted using least squares techniques. In other cases the correlation involves an implicit intercept value and a regression coefficient that may significantly modify the interaction model's estimates. Researchers have not acknowledged the role of these two parameters when the correlation is used. A generalized root mean square error is proposed as an alternative indicator of accuracy that may be used with any model. 相似文献
60.
This paper provides statistical evidence in support of the view, widely held in the tanker industry, that there are systematic differences in the degree of risk involved in investing in tankers of different sizes, and in operating tankers in spot and time charter markets. The industry view, broadly supported by the results of this paper, is that larger vessels are 'risker' assets than smaller vessels, and operating vessels in the time-charter market is less risky than employing them on a spot basis. The results are obtained by using a method derived from the financial economics literature, which models both the conditional mean and variance of a variable, known as GARCH modelling. Only one other paper has applied this method to the tanker market, and these results provide confirmatory support of those findings. 相似文献