Pre-turbo water injection (injecting water before the turbo compressor) is fundamentally different to port water injection in the intake manifold and it has some benefits which port injection does not.

Port Injection

Without going in to port water injection too deeply, this method works in part by cooling the intake charge but the primary reason why port injected water works, is because water plays an active role in the combustion process. If you want to read more about port injection and the chemistry behind it, this is an excellent post. And post #26 on that page digs into the chemistry of water in the combustion process. (I have also included the text at the bottom of this article)

Pre-Turbo Water Injection (Pre-Compressor)

Water injection pre-compressor is subtly different from port injection.

While the water from pre-compressor injection makes it into the combustion chamber and has the same effect (although not as much because we are probably going to be injecting less water due to the bends in our pipping) as port injection, the primary reason for pre-compressor water injection is to improve the effective efficiency of our turbocharger.

About Compressor Maps

The efficiency islands on a turbo compressor map shows where our compressor does it’s best work. Best work being turning ambient air into pressurised air while adding as little unnecessary heat to the air as possible.

If the peak compressor efficiency of our turbo is say 75%, that means the turbo is doing 75% of the theoretical perfect situation.

If for example.

Say increasing the pressure of 1 cubic meter of air (at 25C) to 15 PSI, increases the temperature of that air, in perfect conditions (100% efficiency) to 91C. And our turbo heats it to 125C, we can say it’s efficiency is 66%.

The efficiency percentage on a turbo compressor map, means how much hotter the turbo made the compressed air over perfect conditions.

You can use the MoviChip turbo compressor efficiency calculator here, to run the numbers for your turbo.

Okay, but why am I talking about compressor maps?

Because water injection, pre-compressor, allows the turbo to act more efficiently. With vaporised water in the air entering the turbo, the difference between the compressor inlet temp and outlet temp is less vs no water injection.

And because the air exiting the compressor is cooler, it’s more dense which means it has more oxygen which means we can use more fuel which means we can make more power.

Water injection thermodynamically enhances the turbo compressor.

And why do we care

Water injection pre-compressor allows us to run our turbo harder (or less hard for the same power).

Water injection pre-compressor enlarges the efficiency island of the turbo. It can run more boost while giving the same compressor outlet temps as non water injection at lower boost levels.

Maxxed out compressor

A turbo runs out of puff, on the compressor side, because there comes a point where the mass or air flowing through the compressor stops increasing and instead the air just gets hotter.

We could be running more boost with our small turbo but the power has stopped increasing because the temperature of the air leaving the turbo is increasing.

Boost is increasing but air mass is staying the same or maybe even decreasing.

And air mass entering the engine is the real indicator of power, not the pressure of the air entering the engine.

Colder air is more dense, the more the turbo heats up the air, the less dense the air becomes and at some point, the increasing temperature overcomes the increasing pressure. In other words, at some point the air mass entering the engine starts to decrease even though boost pressure is increasing.

There is an article here that details why a boost gauge is not giving us the full story.

Free Turbo Upgrade – Pre-Turbo Water Injection

Water injection gives us a free turbo upgrade, free in the sense we do not have to physically change the turbo.

Because the turbo can run at higher boost levels and at the same time provide more air mass, it behaves like a slightly bigger turbo would.

Hybrid Turbos & Turbines

But here there is another side to the turbo. It’s great making our compressor side more efficient but if we don’t do anything to our turbine side, the turbine side could become a limiting factor. How much of a factor and if it does, depends on the turbine we are running. If the turbine has a large A/R, say over 0.85, then it’s less of a factor.

And if we are running a hybrid turbo which has increased the capacity of the compressor but left the turbine untouched, then the hybrid is more likely to be a restriction sooner. Water injection pre-turbo on a hybrid turbo may be less beneficial.

Realising the Potential

So I think we’ve established why water injection pre-turbo has benefits, but how can we tap into this potential?

Ultimately, if we add water injection and the engine ECU doesn’t know about it, we probably aren’t going to get any power gains.

Engine Power

Engine power on a petrol engine is dictated by two things. Mass of air entering the engine and the ignition timing.

Because the lambda sensor is ultimately the controller of fuelling, and because water injection makes the AFR look slightly richer (the water displaces some air. How much richer depends on how much water injected) the engine ECU is more likely to pull fuel, so no gain here.

Ignition timing is based on normal air. When the engine is not tuned for water injection the engine ECU does not know that the air entering the engine contains vaporised water and is more knock resistant, so it won’t change ignition timing. So no gain here either.

Are there any benefits without a re-tune?

Knock

Well the air entering the engine is more knock resistant. If we have an engine that sometimes runs into knock, it can help the situation. And if the engine ECU dynamically alters ignition timing according to the knock it detects (for example some engine ECUs may be able to adapt to 100 Octane fuel over time) then over time, the engine may run more ignition timing and give more power.

Reliability

Reliability may also be improved. Because we are not having to run the turbo as hard to get the same air mass to flow through the compressor (because it’s more efficient). This means the wastegate is open more of the time which means less exhaust restriction in the turbine and less back pressure for the engine. Which also means lower temps.

In a nutshell

So while power may not increase with water injection, the engine is having to work less hard and it is less likely to knock which is a win.

If you just want to give your engine and turbo and easier life while maintaining the same power output, water injection without a re-tune has definite benefits.

I’m going to leave this article here, hopefully it has perked your interest in pre-compressor water injection. I’ll be doing more articles on water injection in the near future so if you want to read them, subscribe to the MoviChip newsletter!

Below are the super interesting words on pre-turbo water injection, that I referred to at the start of this article

Let us take a quick look at ignition. Those who have a Heywood can look it up
– mines on loan so going by memory. The first thing that happens is a plasma
cloud is formed by the arc consisting of super heated electron stripped atoms.
When this cloud “explodes” a ball of high energy particles is shot outward.

The highest energy particles are the hydrogen atoms – and they penetrate the
charge about 5 times as far as the rest of the particles. As they lose energy
and return to normal temps – about 5000 k – they begin to react chemically
with any surrounding fuel and oxygen particles. The effectiveness of spark
ignition is directly related to the availability of free hydrogen. Molecules
containing tightly bound hydrogen such as methanol, nitromethane, and methane
are far more difficult to ignite than those with less bonds.

During combustion – water – H2O ( present and formed ) is extremely active in
the oxidation of the hydrocarbon. The predominate reaction is the following:

OH + H ==> H2O
H2O + O ==> H2O2
H2O2 ==> OH + OH
Loop to top and repeat.

The OH radical is the most effective at stripping hydrogen from the HC
molecule in most ranges of combustion temperature.

Another predominate process is the HOO radical. It is more active at lower
temperatures and is competitive with the H2O2 at higher temps.

OO + H ==> HOO
HOO + H ==> H2O2
H2O2 ==> OH + OH

This mechanism is very active at both stripping hydrogen from the HC and for
getting O2 into usable combustion reactions.

Next consider the combustion of CO. Virtually no C ==> CO2. Its a two step
process. C+O ==> CO. CO virtually drops out of early mid combustion as the O
H reactions are significantly faster and effectively compete for the available
oxygen.

Then consider that pure CO and pure O2 burns very slowly if at all. Virtually
the only mechanism to complete the oxidization ( Glassman – Combustion Third
Edition ) of CO ==> CO2 is the “water method”.

CO + OH ==> CO2 + H
H + OH ==> H20
H2O + O ==> H2O2
H2O2 ==> OH + OH
goto to top and repeat.

This simple reaction accounts for 99% + of the conversion of CO to CO2. It is
important in that fully two thirds of the energy of carbon combustion is
released in the CO ==> CO2 process and that this process occurs slow and late
in the combustion of the fuel. Excess water can and does speed this
conversion – by actively entering into the conversion process thru the above
mechanism.

The peak flame temperature is determined by three factors alone – the energy
present and released, the total atomic mass, and the atomic ratio – commonly
called CHON for Carbon, Hydrogen, Oxygen, and Nitrogen. The chemical
reactions in combustion leading to peak temperature are supremely indifferent
to pressure. The temperatures and rates of normal IC combustion are
sufficient to cause most of the fuel and water present to be dissociated and
enter into the flame.

As can be seen above, water is most definitily not only not inert but is a
very active and important player in the combustion of hydrocarbon fuel.
Ricardo and others have documented that under certain conditions ( normally
supercharged ) water can replace fuel up to about 50% and develop the same
power output, or that the power output can be increased by up to 50% addition
of water. This conditions were investigated by NACA and others for piston
aircraft engines. It is important to note that these improvements came at the
upper end of the power range where sufficient fuel and air was available to
have an excess of energy that could not be converted to usable pressure in a
timely manner.

As a side note – Volvo recently released some SAE papers documenting the use
of cooled EGR to both reduce detonation and return to a stoic mixture under
boost in the 15 psi range – while maintaining approximately the same power
output. Notice – they reduced fuel and still get the same power output.

When you consider that EGR consists primarily of nitrogen, CO2, and water ( to
the tune of about two gallons formed from each gallon of water burned ), you
might draw the conclusion that it also was not “inert”. They peaked their
tests at about 18% cooled EGR – which would work out to about 36% water
injection and got about the same results under similar conditions that the
early NACA research got.

SOURCE: https://www.ausrotary.com/viewtopic.php?f=16&t=38929&sid=1196505b658a342ad57dbe793571f41f&start=15

And some info on water injection in general

#2 Post by bill shurvinton » Sun Jul 04, 2004 1:40 am

K Stage 1

Intro: This has been covered before, but just to set the scene. Water is NOT passive in combustion, it aids the

conversion of CO to CO2, which is where most of the energy of combustion is released. It also helps cool

things down( in the process reducing the work done in the compression stroke) and prevents detonation.

You can replace a significant amount of the fuel with water allowing you to run at the best power AFR rather

than running overly rich and losing power. Disclaimer: I give not a stuff what is said on WI equipment vendors websites, so if I disagree with aquamist so be it. They are a pump manufacturer.

Pre-turbo WI: Compression in a turbo is usually adiabatic. This means that the air is heated as it is

compressed, which takes power from the shaft and requires removing through an intercooler. If you inject

water before the turbo, then you can move compression to near isothermal, so very little heat is added, and

less power is taken from the turbine to compress the air for the same boost level. You can loose the

intercooler and therefore suffer less pressure drop between compressor and inlet. All of which gives you the possibility to get more power out the same turbo.

Water has a very high latent heat of vapourisation, but once you have reached the saturation pressure in air, then you gain no more cooling at that point. However Dalton’s law comes to the rescue at this point and if you bung in some other miscible fluids, say methanol and acetone, then each has its own saturation pressure, and you get an accumulative cooling effect. A little petrol would do the job if you added a second injector for it.

And here we hit the first problem. You need fine atomisation to avoid potential blade damage. For this you need an airshrouded injector, such as VW used in some of their CIS systems (hunting applications list for those). RX7s have an airpump that can be turned to this application. Other option is to weld a bung into the scroll as most of the compression happens here.

I need to run some numbers on exactly the effect of various fluids here, but imagine if you had cut the power needed to get the PSI wanted by a 1/3. You would get spool up at 2/3 of the gas flow before and could run at higher boost once you had spooled.

Post turbo WI: Here you are adding water to deal with combustion and detonation reduction. Ideally you would inject directly into the chambers to minimise any charge displacement. The OMP drilling in the housing would work nicely for this if you run premix. In terms of sizing you would be wanting to replace about 30% of the fuel with water, so size accordingly.

Quick note on A/R changes with WI: If you inject water directly into the exhaust, then it rapidly expands and turns to steam. In doing so it takes heat out of the exhaust, but the volume gain more than makes up for this. Tricky to implement, but can be used to change the effective A/R of the turbine, giving you a much wider operating range. MSD do a system for 2 strokes that uses this effect to change the effective tuned length of the exhaust to improve scavenging.

That’s me out of time for now, but have a think and ask some questions. Remember, if you say ‘but Mazda/aquamist/the lad with a supra on my block don’t do it this way’ I shall laugh maniacally and point you to the work of Sir Harry Ricardo on the rolls royce merlin engine in the late 30’s. He was a genius.

SOURCE: https://www.ausrotary.com/viewtopic.php?t=38929

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