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Water injection (engines)

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Water injection is a method for cooling the combustion chambers of engines by adding water to the incoming fuel-air mixture, allowing for greater compression ratios and largely eliminating the problem of engine knocking. This effectively increases the octane rating of the fuel, meaning that performance gains can be obtained when used in conjunction with a supercharger or turbocharger, altered spark ignition timing, and other modifications. Many water injection systems use a mixture of water and alcohol (usually 50/50), partly because the alcohol is combustible, while water is not; in addition, the alcohol serves as an antifreeze for the water. Hence, water injection is also often known as methanol-water injection, owing to the fact that the alcohol mixed into the injection solution is often methanol, CH₃OH.

Due to its corrosive nature, the alcohol in water injection systems cannot be ethanol. Only methanol and isopropanol are used, methanol being the more commonly-utilised form of alcohol for this purpose due to its higher availability and lower cost than isopropanol.

Water injection is often necessary when adding forced induction to an engine that was not designed for it; the compression ratio of a normally-aspirated engine is too high to cope with more than a very modest boost unless this technique is used.

The system was first used extensively on World War II fighter aircraft to increase power upon takeoff and bring up the service ceiling. A limited number of road vehicles with large-displacement engines from manufacturers such as Chrysler have also included water injection. Saab offered water injection for the Saab 99 Turbo. With the introduction of the intercooler the interest in water injection disappeared, but today, water injection is also of interest because it can potentially decrease nitrogen oxide (NO_x) emissions in exhaust. Water injection is primarily used with piston-powered internal combustion engines, but it has also seen use with turbines.

The initial injection of water cools the fuel-air mixture somewhat, which increases its density and hence the amount of mixture that enters the cylinder. But the greater effect comes later during combustion when the water takes in significant amounts of heat energy as it converts from liquid to gas (steam), increasing piston pressure (torque) and reducing the peak temperature with its resultant NO_x formation as well as the amount of energy absorbed into the cylinder walls. The duration of combustion is said to be longer. An interesting side effect that has been reported by some is that water injection effectively "steam cleans" the engine interior, resulting in less carbon residue buildup. Glowing hot carbon deposits are a known cause of knocking.

Fuel economy can be improved with water injection, although the effect on most engines with no other modification, like leaning out the mixture, appears to be rather limited or even negligible in some cases.

Some degree of control over the water injection is important. It needs to be injected only when the engine is heavily loaded and the throttle is wide open; as with other systems which need to monitor engine load, this can be determined by manifold vacuum, which is low when the engine is loaded and high when it is unloaded; however, provision must be made for starting, when the manifold vacuum is also low but water injection is undesirable.

Reports of more rapid corrosion of the steel and cast iron components of engines to which water injection has been added suggest that more frequent oil changes, particularly when the engine does not experience sustained high temperature operation to evaporate any water from the oil, are prudent.