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Water Injection

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Water Injection

Post by LhYnxz on Tue Sep 23, 2008 7:48 pm

"Water lot of fun."

You've finished tuning up your engine but you find that at full
throttle the engine splutters or stalls. When you get the car on a
diagnostic you find that the knock sensor is kicking in to protect the
engine.


Engine knock
is a condition where the fuel inside the engine ignites prematurely and
occurs within an engine running at high compression. (It can also be
caused by other factors including low octane/poor quality fuel or hot
spots within the engine.)
For the purpose of this article we shall assume that the engine
knock is occurring because your engine is highly tuned and running big
compression figures or due to forced induction.
This can also be caused when adding or uprating the turbo in your
engine. Your options are to rebuild the engine with lower compression
pistons, reduce the airflow into the engine or use water injection.
Just as a fireman sprays water onto a fire to cool it down the water
injection in an engine helps to quell ignition. It will literally slow
up the burn of fuel and enable a more thorough combustion process. Some
advocates of water injection list the cleaning effect this has on the
engine and head but this is not a primary consideration.
Unless you are suffering from engine
knock water injection is pretty irrelevant but can literally save an
enthusiastic tuning project.


The question is where to inject the water spray. Your main choices
are after the compressor (turbo or supercharger or before it). Then we
have options around injecting into each intake or just prior to the
branch.
Your route depends a lot on the manifold and positioning of the
compressor. Ideally each intake manifold header will have a water
injection nozzle requiring a more complex injection controller. The
simplest systems go just downstream of the compressor before the branch
allowing the manifold flow to direct the water charge.
Your aim is to get the same amount of water to each cylinder
otherwise the engine will be out of dynamic balance and you will have
to tune for the weakest common denominator.
Some choose to run their water injection pre turbo but others assume
that this could have a detrimental effect on the long term life of the
impellers. Proponents of this method will state that this allows the
best atomisation of the water into the intake charge and also reduce
the intake temperature allowing the turbo to work more efficiently
(compressing air increases its temperature).


We would certainly agree that having a jet or large droplets of water hitting your impeller does not make sense.


The jet of water only really needs to start when you approach your
critical knock threshold so there is little point injecting the water
all of the time.


In an ideal world you need around 10 inches of intake length to mix the fuel/air and water in a uniform fashion.
Nozzle size is also vital and just like with a fuel injector you
want a mist rather than a jet. A mist size of around 50 microns is
acceptable with the droplets reducing in size as atomisation occurs.
The smaller 1mm nozzle sizes are much more effective at producing
a mist and allowing atomisation. The engine heat (and intake
compression heat) will finish off the atomisation process.
How does one control the quantity of the water injected into the
engine? The amount of water should vary according to engine load and
rpm. As an example lets look at manifold pressures.
Manifold pressures can reach 11psi at 3500rpm and also hit the same
11psi at 7000rpm. As the engine speed is 100% greater you will also
need to at least double the water injection. It really does pay to get
a good quality water injection controller which takes the RPM and
fuelling figures into the equation. Fuel delivery rate alone is a
reasonably good indicator of the amount of water so a crude controller
could take this into account.
Because water injection slows down the burn most common applications
will benefit from slightly advanced timing of a few degrees. Because
every engine is unique you need to carefully setup the advance. If you
record little or no power gain then chances are that you already have
the optimum timing. Advancing the timing further will just increase the
cylinder pressure.
The downsides of water injection include corrosion of metal clips
and joins within the intake. Rusty metal components going into the
engine is a very bad idea. So check the intake for bare metal clips
and replace these with corrosion resistant ones.
The next worst thing to happen is a sudden loss of water injection.
This can be caused by injector failure, the tank running dry or some
other component failure. A good water injection system will have a fail
safe built in which reduces the engine power for example by reducing
the boost pressure at the turbo. The more highly tuned the engine the
bigger this problem is.


On a fast road tuned street car the knock sensor should be able to
adjust the fuelling and it would be similar to using a low octane fuel.
As long as you keep the revs low you will be able to use the car. When
we are talking about a highly tuned drag machine the sudden loss of
water and subsequent knock can be catastrophic and requires some
serious thought. A backup system can be used or a twin injection system
will provide a 50% injection if one fails.


Daily checks on your water injection system are prudent, and the
more highly tuned the car the more vital this is. Always carry some
spare water with you, check all delivery lines for chaffing or rubbing.
Use filtered water and periodically check the intake for signs of
corrosion.


Water injection typically has a similar effect on an engine to
running higher octane fuel. As such it is irrelevant if you are not
suffering from knock. However on NASP engines with turbos added it can
make the difference between a drivable fun car and a lumpy dog of a car.

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