Sometimes the easiest explanations are overlooked.
Water injection (engines)
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In internal combustion engines, water injection, also known as anti-detonant injection, is spraying water into the cylinder or incoming fuel-air mixture to cool the combustion chambers of the engine, 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, turbocharger, altered spark ignition timing, and other modifications. Increasing the octane rating allows for a higher compression ratio which increases the power output and efficiency of the engine. Depending on the engine, improvements in power and fuel efficiency can also be obtained solely by injecting water.[1] Water injection may also be used to reduce NOx or carbon monoxide emissions.[1]
Water injection is also used in some jet turbine engines and in some shaft turbine engines, when a momentary high-thrust setting is needed to increase power and fuel efficiency.
Contents
[hide] 1 Composition of fluid
2 Effects
3 Use in aircraft
4 Use in automobiles
5 See also
6 References
7 External links
[edit] 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. [2] 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.
[edit] 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. The water (if in small liquid droplets) may absorb heat (and lower the pressure) as the charge is compressed, thus reducing compression work.[1] An additional effect comes later during combustion when the water absorbs large amounts of heat as it vaporizes, reducing peak temperature and resultant NOx formation, and reducing the amount of heat energy absorbed into the cylinder walls. This also converts part of combustion energy from the form of heat to the form of pressure. As the water droplets vaporize by absorbing heat, it turns to high pressure steam (water vapor or steam mainly resulted from combustion chemical reaction). The alcohol in the mixture burns, but is also much more resistant to detonation than gasoline. The net result is a higher octane charge that will support very high compression ratios or significant forced induction pressures before onset of detonation.
Fuel economy can be improved with water injection. Depending on the engine, the effect of water injection, with no other modification, like leaning out the mixture, may be quite significant[1] or rather limited and in some cases negligible.
In some cases water may also reduce CO emissions, this might be attributable to the water-gas shift reaction, in which CO and H2O shift to form CO2 and H2.[1] However, water may also increase hydrocarbon emissions, possibly due to an increased quenching layer thickness.
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 cools the combustion process unnecessarily and reduces efficiency.
Direct injection of water is possible and is likely advantageous. In a piston engine, this can be done late in the power stroke or during the exhaust stroke.
[edit] Use in aircraft
Water injection has been used in both reciprocating and turbine aircraft engines. 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.[3] 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.
Piston engined petrol 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. Some fighter aircraft also used water injection to allow higher boost in short bursts during dogfights.
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 enriching 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 problem of taking off heavily loaded from short runways was solved by the availability of more powerful engines that had not been available previously.
The BAC One-Eleven airliner also used water injection for its Rolls-Royce Spey turbofan engines. Filling the tanks with jet fuel instead of water lead to the Paninternational Flight 112 crash.[4]
[edit] Use in automobiles
A limited number of road vehicles with large-displacement engines from manufacturers such as Chrysler have included water injection. The 1962 Oldsmobile F85 was delivered with the Fluid-Injection Jetfire[5] engine, which was incidentally the world's first turbocharged road car. Oldsmobile referred to the water/alcohol mixture as 'Turbo-Rocket Fluid'. 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 in vehicles with aftermarket forced induction systems, such as turbochargers or superchargers. Such engines are commonly tuned with a narrower margin of safety from detonation and hence benefit greatly from the cooling effects of vaporized water.[citation needed] ([/hide]
