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Water injection, also known as anti-detonation 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. Unless you know
exactly what you're doing, try making a kit yourself. But let me warn
you; Just one solid drop of water hitting an extremely
hot piston will cause catastrophic engine failure! Your piston (or pistons)
will do more than just crack, you'll be picking up little pieces out of every
crack and crevice in your block!
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-detonation injection.
Effects:
In a piston engine, 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). This 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, but at a much
slower rate than gasoline. The net result is that
the combustion process happens slower, preventing the destructive supersonic
shockwave characteristic 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
automobiles:
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, particularly those used for drag racing and illegal(?) street racing. I all but condone it as should always take your racing
to the track. It's all but impossible to race on the streets because all it
takes is a minor misjudgment and you can kill a family or somebody's little
kid, could you live with that, think about it! I know just the
'thought of it' can turn your stomach. I'll admit, I did it and did it often
but more places you may feel is safe is for someone to get in the way or even
step out in front of you at 130 and there's nowhere to go.
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 overheating, 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.
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