Water injection explained

Bartman
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Water injection explained

Postby Bartman » Sun Sep 16, 2012 12:28 am

Thanks to John MacKenzie
The issue you'll strike is that the current build will almost certainly have a borderline too high compression ratio to handle much boost. It's not the strength of the pistons 'as such' - it's that the compression ratio is high enough that with the addition of boost, it will lead to the air/fuel detonating.

When an engine burns fuel, it's a quick burn but still very controlled, and it's called deflagration (same goes for a bullet fired out of a gun!) - not that the name is that important. but if it is compressed too highly or sees a really high temp spike just before igniting, then it can go into a violent much more rapid and uncontrolled combustion - detonation. And that can break stuff in a big hurry.

now where does this heat come from? well obviously the turbo or supercharger compressing the air will do it. So an intercooler helps. But the main culprit is actually the engine itself. That's right - as the piston moves up the bore compressing the intake charge, it heats it up too, and far far far more than the turbo heats it up. the issue with a turbo or blower is it heats it up 'somewhat' then it starts much hotter, so an engine with a compression ratio safe for a given fuel and 'regular' air intake temps, will be over the edge, once the starting temp, before the piston 'has it's way with it' compressing it further is hotter - hotter starting temp, hotter final temp, and so it goes beyond the safe realm.

An intercooler will help, but it's still warmer, and it can't do anything to combat how much the piston will heat things up on the compression stroke (and even if the intake temp after the cooler was close to 'regular' outside temps, there's more air and fuel in there, so it will end up hotter when the piston squeezes it into the same small space above the piston in the combustion chamber of the head.

This is when water injection comes in big time. Water acts as a coolant (as does methanol injection for that matter). But whlist it cools things a little as it is injected into the intake stream, it takes X amount of heat energy to heat up water to about 99 degrees C. But to get it that extra 1 degree - to 100 - and to make it change state into steam - relatively speaking that takes MUCH MUCH more heat energy. And if it is 'taking' the heat energy from around it to do so, then the 'stuff' around it won't be as hot as a result.

This is important, because when the water is introduced, it will cool a little bit, but once it is inside the cylinder, and the piston is rising in the bore, compressing the charge, it will heat things up to beyond 100C. And then it will be hot enough to try and change the water into steam. So the water won't cool things 'exactly' but rather it will 'prevent' them from ever heating up as much. The higher the air pressure, the higher the boiling point of water (which is why water in the cooling system can be over boiling point, but still stay a liquid as the radiator cap is holding the pressure in).

So now if you can picture it, as the piston is rising up in teh bore, increasing pressure, the boiling point of the water rises too, and so it sort of 'sails on the edge' of turning the water into steam and it is 'here' that the water is pulling a lot of heat out of the air. It probably doesn't ever fully turn to steam until after the burn is initiated, then ironically it's expansion rate would actually give some push on the piston (to make up for some of the power it took to heat it up in the first place.)

So the point of all this jibberish from me - if you happen to be running a compression ratio that is really on the edge for the fuel you are using and then add boost, water injection can be your saviour. In a big big way.

Incidentally - this is why a richer mixture 'works' to help protect a turbo or supercharged engine - that extra fuel (only under full boost/throttle conditions of course) isn't actually burned (well very little of it, some does see what you might callpartial combustion, which is why the CO goes up a bit) but instead that extra fuel works essentially as a coolant. There's plenty of data from ww2 fighter planes and other similar tests, that show that the same power can be made by turning the mixture leaner (closer but not all the way to stoich) and replacing that extra fuel with water to act as the coolant instead. Meaning it proves the fuel wasn't being burned (the additional enrichment fuel I mean, not the whole fuel load being metered in there!) and was acting as a coolant.

TO that end, methanol injection combined with water works similarly, except that the methanol starts cooling significantly a lot earlier on - in the intake tract - so it results in a slightly denser air/feul mixture getting into the cylinder, so there's a little more air in there and that will permit a little more fuel to be burned, and so water/methanol combo will produce slightly more power than straight water injection, having said that, methanol isn't cheap in the long run and not all hosing/pumps handle it really well, so water tends to be much cheaper and therefore just the ticket for a setup where you don't necessarily want to spend megabucks (which is fair enough too!)



Eventually you can push enough boost that even with the right compression ratio, the sheer amount of heat from the burning charge can overwhelm the piston crown itself. Once you get to that point it can damage stuff (and then the piston becomes a hot spot so it can lead to detonation anyway, or if not, just general piston failure) - and so for those sort of high boost applications, you need custom pistons, designed with enough of a dish to have the compression ratio where it needs to be, but also a thicker crown (the top area of the piston) to be able to handle far more heat soak, and lastly the piston to be of a stronger/denser material - which is where you move from cast pistons to forged pistons. You also look at stronger top rings too, for the same reason. You also tend to want to run a little wider top ring gaps for higher boost, since the higher boost means more heat energy, more of it gets to the top ring and the top ring can expand enough to close up the gap and eventually seize.

BUt that takes some serious boost, and based on plenty of experiences here, you can happily run factory pistons up to and above 10psi. AS LONG AS you run something like water or water/methanol injection to prevent detonation.
They don't call me the man with the rubber arm for nothing you know.............

2 doors
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Re: Water injection explained

Postby 2 doors » Tue Apr 16, 2013 9:42 pm

:o , the most amazing thing about all of this is that , you actually explained it in a way most people will understand lol

Bartman
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Re: Water injection explained

Postby Bartman » Fri Apr 19, 2013 9:05 pm

I just copied and pasted Jmac's explanation.
If you want to see some clever words, go to 1200.com, and search Jmac. His posts are long, and very very informative.
They don't call me the man with the rubber arm for nothing you know.............

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M spec
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Re: Water injection explained

Postby M spec » Fri Apr 19, 2013 9:42 pm

A guy was just talking to me about this a work today....now I know he was talking shit :lol:
Thanks.


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