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In internal combustion engines, water injection, also known as anti-detonant injection (ADI), can spray water into the incoming air or fuel-air mixture, or directly into the cylinder, in order to cool certain parts of the induction system where "hot points" could produce premature ignition. In jet engines it is used to increase the thrust of the engine at low speeds and at takeoff. Water injection was used historically to increase the power output of military aviation engines for short durations such as dogfights or takeoff, however it has also been used in motor sport and notably in drag racing. In Otto cycle engines the cooling effects of water injection also allow for greater compression ratios by reducing the problem of engine knocking (detonation). Alternately this reduction in engine knocking in Otto cycle engines means that in some applications significant performance gains can be obtained when water injection is used in conjunction with a supercharger, turbocharger and/or other modifications such as a more aggressive ignition timing. Depending on the engine, improvements in power and fuel efficiency can also be obtained solely by injecting water.〔(Wilson, J. Parley., Effects of Water Injection and Increased Compression Ratio in a Gasoline Spark Ignition Engine. Wilson, Thesis, University of Idaho, 2011 )〕 Water injection may also be used to reduce NOx or carbon monoxide emissions.〔 Water injection is also used in some turbine engines and in some turboshaft engines, normally when a momentary high-thrust setting is needed to increase power and fuel efficiency. ==Theory== Water has a very high heat of vaporization. As the ambient temperature water is injected into the engine, heat is transferred from the hot cylinder head/ intake air into the water. This causes it to evaporate, cooling the intake charge. A cooler intake charge means it is more dense (higher volumetric efficiency) and also will have a lower tendency to knock. However the water will displace some air, negating the denser intake charge from the lower temperature at. Knocking is generally more of a problem in forced induction engines rather than naturally aspirated so this can be a useful aid in its prevention. On electronic ignition systems the ignition timing is generally retarded to prevent knock from occurring but with water injection it can be advanced closer to Maximum Brake Torque (MBT) timing for additional power. 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「Water injection (engine)」の詳細全文を読む スポンサード リンク
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