Water_injection_(engines)

Water injection (engines)

Water injection, also known as anti-detonant 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 (detonation). 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.

Composition of fluid

Many water injection systems use a mixture of water and alcohol (approximately 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 purpose of the oil is to prevent corrosion of water injection and fuel system components. Because the alcohol mixed into the injection solution is often methanol (CH3OH), the system is known as methanol-water injection, or MW50. In the United States, the system is commonly referred to as anti-detonant injection, or ADI.

Effects

In a piston engine, the initial injection of water cools the fuel-air mixture significantly, which increases its density and hence the amount of mixture that enters the cylinder. An additional effect comes later during combustion when the water absorbs some combustion energy (remember high specific heat) further reducing peak cylinder temperatures and hot spots. The intake cooling increases the charge density which increases piston pressure (torque), reduces peak temperature and resultant NOx formation, and reduces the amount of heat energy absorbed into the cylinder walls. The alcohol in the mixture burns, contributing to combustion and fuel requirements, but is also much more resistant to detonation than gasoline. The net result is a cooler, higher octane charge that will support very high compression ratios and or significant forced induction pressures before onset of detonation.

When used in a turbine engine, the effects are similar, except that preventing detonation is 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, but it also serves to cool the turbines. Since temperature is normally the limiting factor in turbine engine performance at low altitudes, the cooling effect allows the engines to be run at a higher RPM with more fuel injected and more thrust created without overheating. The drawback of the system is that injecting water quenches the flame in the combustion chambers somewhat, as there is no way to cool the engine parts without cooling the flame accidentally. This leads to unburned fuel out the exhaust and a characteristic trail of black smoke.

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. Otherwise injecting water may simply drown the engine and cause it to quit.

Use in aircraft

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

Piston engines in military aircraft utilized water injection technology prior to World War II in order to increase takeoff power. This was used so that heavily-laden fighters could take off from shorter runways, climb faster, and quickly reach high altitudes to intercept enemy bomber formations.

As a general rule, the fuel mixture is set at full rich on an aircraft engine when running it at a high power settings (such as during takeoff). The extra fuel does not burn; its only purpose is to evaporate to absorb heat. This uses up more fuel, and it also decreases the efficiency of the combustion process. By using water injection, the cooling effect of the water allows the fuel mixture to be run leaner at its best-power setting. Many military aircraft engines of the 1940s utilized a pressure carburetor, a type of fuel metering system similar to a throttle body injection system. In a water-injected engine, the pressure carburetor features a mechanical derichment valve which makes the system nearly automatic. When the pilot turns on the water injection pump, water pressure moves the derichment valve to restrict fuel flow to lean the mixture while at the same time mixing the water/methanol fluid in to the system. When the system runs out of fluid the derichment valve shuts and cuts off the water injection system, while enrichening the fuel mixture to provide a cooling quench to prevent sudden detonation.

Due to the cooling effect of the water, aircraft engines can run at much higher manifold pressures without detonating, creating more power. This is the primary advantage of a water injection system when used on an aircraft engine.

The extra weight and complexity added by a water injection system was considered worthwhile for military purposes, while it is usually not considered worthwhile for civil use. The one exception is racing aircraft, which are focused on making a tremendous amount of power for a short time; in this case the disadvantages of a water injection system are less important.

The use of water injection in turbine engines has been limited, again, mostly to military aircraft. Many pictures are available of Boeing B-52 takeoffs which clearly show the black smoke emitted by turbine engines running with water injection. For early B-52s, water injection was seen as a vital part of take-off procedures. For later versions of the B-52 as well as later turbine-powered bombers, the solution to the problem of taking off heavily loaded from short runways was simply to build larger engines.

Use in automobiles

A limited number of road vehicles with large-displacement engines from manufacturers such as Chrysler have 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 (NOx) emissions in exhaust. The most common use of water injection today is vehicles with aftermarket forced induction systems such as turbochargers or superchargers which are commonly tuned with a narrower margin of safety from detonation and benefit greatly from the cooling effects of vaporized water.

See also

  • The Crower six stroke engine developed by Bruce Crower with steam stroke.
  • MW 50-the World War II German Luftwaffe's methanol-water engine power boosting system.

References

External links

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