Water injection (engine)

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 combustion chamber to cool certain parts of the induction system where "hot points" could produce premature ignition. In jet engines—particularly early turbojets or engines in which it is not practical or desirable to have an afterburner—water injection may be used to increase engine thrust, particularly at low-altitudes and at takeoff.

Water injection was used historically to increase the power output of military aviation engines for short durations, such as during aerial combat or takeoff. However it has also been used in motor sports and notably in drag racing. In Otto cycle engines, the cooling effect of water injection also enables greater compression ratios by reducing engine knocking (detonation). Alternatively, 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.

Many water injection systems use a mixture of water and alcohol (often close to 50/50), with trace amounts of water-soluble oil. The water provides the primary cooling effect due to its great density and high heat absorption properties. The alcohol is combustible, and also serves as an antifreeze for the water. The main purpose of the oil is to prevent corrosion of water injection and fuel system components.

Water injection has been used in both reciprocating and turbine aircraft engines.

In a reciprocating engine, the use of water injection, also called anti-detonation injection or ADI, is used to prevent engine knocking also known as "detonation". Commonly found on large radial engines with pressure carburetors, it is a mixture of water and alcohol injected into the carburetor at high power settings. When using a rich mixture, the engine runs cooler, but cannot reach maximum power, and a leaner mixture means detonation is likely. With the use of ADI, the injected water and alcohol (which is mixed with the water to prevent it from becoming ice) absorbs the excess heat to prevent detonation while still allowing for a leaner and more powerful mixture. Notable engine with water fuel injection: BMW 801, Daimler-Benz DB 605, Junkers Jumo 213, Pratt & Whitney Double Wasp.

When used in a turbine engine, the effects are similar, except that preventing detonation is normally not the primary goal. Water is normally injected either at the compressor inlet or in the diffuser just before the combustion chambers. Adding water increases the mass being accelerated out of the engine, increasing thrust; and it also serves to cool the turbines. Since temperature is normally the limiting factor in turbine engine performance at low altitudes, the cooling effect lets the engine run at higher RPM with more fuel injected and more thrust created without overheating.

Prior to the widespread adoption of afterburning engines, some first-generation jet fighters used water injection to provide a moderate boost in performance. For example, the late-model variant of the Lockheed F-80 Shooting Star, the F-80C, used water injection on its Allison J33-A-35 engine. Water injection increased thrust from 20.5 to 24.0 kN (4,600 to 5,400 lbf), a 17% thrust increase (at sea level).