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661.
汽车辅助制动装置发展综述 总被引:3,自引:0,他引:3
刘成晔 《中国安全科学学报》2008,18(1):105-111
平均行驶速度和载重量的增加,要求车辆具有更大的制动效能,为了有效分流制动负荷常采用辅助制动系统.介绍国内外制动法规的发展和现状,排气辅助制动装置、发动机缓速器、电涡流缓速器和液力缓速器的结构、工作原理及各辅助制动装置的制动性能;比较各辅助制动装置的优缺点,有利于汽车制动技术的发展,全面提高汽车行驶安全性能;展望未来汽车辅助制动装置的两大发展趋势,一是电子控制技术在汽车制动上的应用,二是多种辅助制动装置的联合制动. 相似文献
662.
Eutrophication and Sedimentation Patterns in Complete Exploitation of Water Resources Scenarios: An Example from Northwestern Semi-arid Mexico 总被引:1,自引:0,他引:1
Sánchez-Carrillo S Alatorre LC Sánchez-Andrés R Garatuza-Payán J 《Environmental monitoring and assessment》2007,132(1-3):377-393
Water requirements to supply human needs lead water stakeholders to store more water during surplus periods to fulfil the
demand during – not only – scarcity periods. At the reservoirs, mostly those in semi-arid regions, water level then fluctuates
extremely between rises and downward during one single year. Besides of water management implications, changes on physical,
chemical and biological dynamics of these drawdown and refilling are little known yet. This paper shows the results, throughout
a year, on solids, nutrients (N and P), chlorophyll-a, and sedimentation changes on the dynamics, when the former policy was applied in a reservoir from the semi-arid Northwestern
Mexico. Water level sinusoidal trend impinged changes on thermal stratification and mixing, modifying nutrient cycling and
primary producer responses. According to nitrogen and phosphorus concentration as well as chlorophyll-a, reservoir was mesotrophic, becoming hypertrophic during drawdown. Nutrient concentrations were high (1.22 ± 0.70 and 0.14 ± 0.12 mg
P l−1), increasing phosphorus and lowering N:P significantly throughout the study period, although no intensive agricultural, no
urban development, neither industrial activities take place in the watershed. This suggests nutrient recycling complex mechanisms,
including nutrient release from the sediment–water interface as the main nutrient pathway when shallowness, at the same time
as mineralization, increases. Outflows controlled nitrogen and phosphorus availability on the ecosystem while organic matter
depended on river inflows. As on other subtropical aquatic ecosystems, nitrogen limited primary productivity (Spearman correlation
R = 0.75) but chlorophyll-a seasonal pattern showed an irregular trend, prompting other no-nutrient related limitants. Shallowness induced a homogeneous
temporal pattern on water quality. This observed temporal variability was mainly explained statistically by changes on solids
(mineral and organic), chlorophyll-a and flows (62.3%). Annual sedimentation rates of total solids ranged from 11.73 to 16.29 kg m−2 year−1 with organic matter comprising around 30%. N:P ratio on sedimentation rates were as high as could be expected in a resuspension
dominated ecosystem, and spatially inverse related with N:P ratio on bottom sediments. Distance from river inlet into the
reservoir reveals a marked spatial heterogeneity on solid and nitrogen sedimentation, showing the system dependence on river
inflows and supporting resuspension as the main phosphorus pathway. Accretion rates (2.19 ± 0.40 cm year−1) were not related to hydrological variability but decreased with the distance to the river input. Total sediment accumulation
(9,895 tons km−2 year−1) denotes siltation as other serious environmental problem in reservoirs but possibly not related with operational procedures. 相似文献
663.
洪涝灾害灾情时间变化特性分析——以中国1736~1911年主要大江大河流域变化序列分析为例 总被引:1,自引:1,他引:0
利用已经建立的全国1736~1911年洪涝灾害时间序列,拟合了各个区域的洪涝灾害长期变化趋势,并通过功率谱分析了时间序列的波动规律;同时,分析了气温,降水等自然因素的变化和人口总量变化、人口空间迁移以及土地利用变化等人文因素变化与灾害增减之间的关系。结果表明,洪涝灾害的波动主要由气候变化等自然因素的波动所引起,而人类活动增强并不断向高风险区扩展是洪涝灾害持续增长的主要原因,最终使洪涝灾害表现为在波动中持续增长。 相似文献
664.
665.
Perfluorinated compounds (PFCs) were detected in sediment core samples collected in Tokyo Bay to reveal their time trends. The core sample deposited during 1950s-2004 was divided into two- to three-year intervals and the concentrations of 24 types of PFCs were determined. Perfluorooctane sulfonate (PFOS) decreased gradually from the early 1990s and its precursor decreased rapidly in the late 1990s, whereas perfluorooctanoic acid (PFOA) increased rapidly. The observed trends were regarded as a reflection of the shift from perfluorooctyl sulfonyl fluoride (PFOSF)-based products to telomer-based products after the phaseout of PFOSF-based products in 2001. The branched isomers of perfluoroundecanoic acid (PFUnDA) and perfluorotridecanoic acid (PFTrDA) were detected in the sample with its ratio of linear-isomer/branched-isomer concentrations decreasing. In this study, we revealed that the sediment core can serve as a tool for reconstructing the past pollution trend of PFCs and can provide interesting evidence concerning their environmental dynamics and time trend. 相似文献
666.
A methodology of characterizing status and trend of land changes in oases: a case study of Sangong River watershed, Xinjiang, China 总被引:11,自引:0,他引:11
Land change is often studied with Markov models to develop a probability transition matrix. The existing methods dependent on such matrixes cannot effectively characterize some important aspects associated with land change such as status, direction, trend and regional variations. This study presents mathematical models to quantify these elements, defining unbalanced, quasi-balanced and balanced status, one- and two-way transitions and the rising or falling trends. Using these models and remote-sensing imageries, the landscape was studied for a case area, the oasis of Sangong River in Xinjiang, Northwest China where typical arid conditions prevail. Land expansion and contraction among various land types and for the entire oasis were analyzed for the periods of 1978-1987, 1978-1998 and 1987-1998. The changes were closely related to a strong economic growth after the land-reform campaign and adoption of the market economy in China in the 1980s to early 1990s, a process not strictly Markovian that requires stationarity and randomness. Information on land-change status and trend is important for a better understanding of the underlying driving processes but also for land-use planning and decision-making. 相似文献
667.
Ecotechnological methods have been employed by developed countries like the United States of America, Europe, Australia and
Japan in the restoration of their degraded freshwater ecosystems. These methods according to literature sources, in many cases
have produced the desired results in these countries where they have been implemented. This article is aimed at providing
an overview on aspects of available ecotechnological methods employed in the reduction of eutrophication and acidification
and in Lake Ecosystem restoration in general. Ecotechnological methods offer great opportunities for developing countries
with tropical or subtropical climates due to their cost-effectiveness. The efficacy and potential limitations of the various
methods in their respective restorative approach is also reported in this article. 相似文献
668.
山东省耕地资源态势分析及可持续利用研究 总被引:8,自引:0,他引:8
利用山东省建国以来的耕地序列数据以及近10多年来的相关社会经济资料,宏观分析了山东省耕地资源的时空动态变化规律。用相关分析和主成分分析相结合,定量地研究耕地变化的驱动因子,揭示耕地变化的驱动机制;研究表明,社会经济发展、农业科技进步和人口压力是造成山东省耕地资源持续藏少的三类主要动因。在此基础上,运用多元线性回归分析构建耕地变化回归模型,同时通过人均粮食需求量预测法,分别预测出2020年的山东省的耕地数量和耕地最低需求量.结果表明山东省未来的耕地资源态势不容乐观,人地矛盾十分尖锐。最后提出了实现耕地资源可持续利用的几点建议。 相似文献
669.
670.