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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Propulsive efficiency ： ウィキペディア英語版 Propulsive efficiency In aircraft and rocket design, overall propulsive efficiency $\backslash eta$ is the efficiency, in percent, with which the energy contained in a vehicle's propellant is converted into useful energy, to replace losses due to aerodynamic drag, gravity, and acceleration. It can also be stated as the proportion of the mechanical energy actually used to propel the aircraft. It is always less than 100% because of kinetic energy loss to the exhaust, and less-than-ideal efficiency of the propulsive mechanism, whether a propeller, a jet exhaust, or a fan. In addition, propulsive efficiency is greatly dependent on air density and airspeed. Mathematically, it is represented as $\backslash eta\; =\; \backslash eta\_c\; \backslash eta\_p$〔,(ch10-3 )〕 where $\backslash eta\_c$ is the cycle efficiency and $\backslash eta\_p$ is the propulsive efficiency. The cycle efficiency, in percent, is the proportion of energy that can be derived from the energy source that is converted to mechanical energy by the engine. ==Cycle efficiency== (詳細はthermodynamics: ::$dW\; \backslash \; =\; \backslash \; dQ\_c\; \backslash \; -\; \backslash \; (-dQ\_h)$ :where ::$dW\; =\; -PdV$ is the work extracted from the engine. (It is negative since work is ''done by'' the engine.) ::$dQ\_h\; =\; T\_hdS\_h$ is the heat energy taken from the high temperature system. (It is negative since heat is extracted from the source, hence $(-dQ\_h)$ is positive.) ::$dQ\_c\; =\; T\_cdS\_c$ is the heat energy delivered to the cold temperature system. (It is positive since heat is added to the sink.) In other words, a heat engine absorbs heat energy from the high temperature heat source, converting part of it to useful work and delivering the rest to the cold temperature heat sink. In general, the efficiency of a given heat transfer process (whether it be a refrigerator, a heat pump or an engine) is defined informally by the ratio of "what you get out" to "what you put in". In the case of an engine, one desires to extract work and puts in a heat transfer. ::$\backslash eta\_c\; =\; \backslash frac\; =\; \backslash frac\; =\; 1\; -\; \backslash frac$ The ''theoretical'' maximum efficiency of any heat engine depends only on the temperatures it operates between. This efficiency is usually derived using an ideal imaginary heat engine such as the Carnot heat engine, although other engines using different cycles can also attain maximum efficiency. Mathematically, this is because in reversible processes, the change in entropy of the cold reservoir is the negative of that of the hot reservoir (i.e., $dS\_c\; =\; -dS\_h$), keeping the overall change of entropy zero. Thus: ::$\backslash eta\_\backslash text\; =\; 1\; -\; \backslash frac\; =\; 1\; -\; \backslash frac$ where $T\_h$ is the absolute temperature of the hot source and $T\_c$ that of the cold sink, usually measured in kelvin. Note that $dS\_c$ is positive while $dS\_h$ is negative; in any reversible work-extracting process, entropy is overall not increased, but rather is moved from a hot (high-entropy) system to a cold (low-entropy one), decreasing the entropy of the heat source and increasing that of the heat sink. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Propulsive efficiency」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.