Water injection (engines)

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 to cool certain parts of the induction system where "hot points" could produce premature ignition. In jet engines it increases engine thrust 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 enables greater compression ratios by reducing engine knocking (detonation). Alternately, this reduction in engine knocking in Otto cycle engines means that some applications gain significant performance when water injection is used in conjunction with a supercharger, turbocharger, or modifications such as aggressive ignition timing.

Depending on the engine, improvements in power and fuel efficiency can also be obtained solely by injecting water. 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.

Water has a very high heat of vaporization. As the ambient temperature water is injected into the engine, heat transfers from the hot cylinder head and intake air into the water. This makes it evaporate, cooling the intake charge. A cooler intake charge means it is more dense (higher volumetric efficiency) and also has a lower tendency to knock. However, the water vapor displaces some air, negating some of the denser intake charge benefit. Knocking is generally more of a problem in forced induction engines rather than naturally aspirated, so this can help prevent it. 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.

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Wikipedia