Howdy folks,
I just recently took a long trip from DFW to McAllen, Tx. Each way was over 560 miles and I did it solo. Needless to say, once the "road trip thrill" wore off, I had a lot of time to myself to think about things. I spent the majority of it ruminating over technical stuff and especially the whole AI thing. I'm not interested in making a "water vs. alcohol" thing as I feel that both fluids are powerful in as much as being helpful to the engine for reliability reasons.
I've said it a zillion times both on these forums and in public: The determining factor in making engine power, hardware aside, is the quality of the fuel used. Period.
Even after my experiment w/ the hot-air setup on my Turbo II, I've still got questions about the modes of action and the real efficacy AI has in our engines. It's my contention that both due to the unique nature of the rotary engine as well as its tendency at creating massive amounts of heat under heavy loads compared to piston engines that alcohol and water injection are very desirable. The longer I continue tuning the cars the more I'm leaning towards the notion that every one of our turbo cars need, at the very least, a basic water injection setup, if not a more complex all alcohol setup. I also believe that, due to the experiments that several of us have done up to this point over the last few years, there's an explosive amount of power with reliability awaiting us once this stuff becomes more technically understood and "standardized". It wouldn't surprise me if the somewhat ambiguous "1 bar on pump gas" deal becomes a thing of the past for us.
So far, we've got these basic setups available to us:
-) Pre-turbo water injection, pumpless (pneumatic)
-) Pre-turbo water injection, pumped (electronic)
-) Post-turbo water injection, aft of the throttle body, pressurized system
-) Post-turbo alcohol injection, pre throttle body
-) Post-turbo alcohol injection, hot-air setup (ala Stylemon and myself)
-) Post-Turbo water/alcohol combination
The properties of both fluids that are important in my opinion:
-) Specific Heat Index
-) Latent Heat of Evapouration
-) Autoignition Temperature (Alcohols)
-) Flash Temperature (Alcohols)
Also, I'm not forgetting that both are proven to combat carbon buildup in an engine. I've personally seen what heavy alcohol injection does to the innards - literally zero carbon on the rotors and little carbon in both intake manifolds all the way to the back of the throttle body (via reversion). Amazing stuff. The lingering thorn in my side I've had about carbon build-up is that not only does it theoretically raise compression ratios artificially but it also can produce hot spots that can act as "glow plugs" and intermittently ignite a compressing charge prior to a timed spark event.
As Howard Coleman's written about in his 3rd-gen section thread, the big one here is the latent heat factor in as much as the cooling BTU's that are delivered when the water or alcohol makes a change of state from fluid to vapour:
"The input of energy required by a change of state from liquid to vapor at constant temperature is called the latent heat of vaporization. When a liquid undergoes a change to vapor state at normal boiling point the temperature of the liquid will not rise beyond the temperature of the boiling point.
The specific latent heat of vaporization is the amount of heat required to convert unit mass of a liquid into the vapor without a change in temperature."
http://www.engineeringtoolbox.com/fl...eat-d_147.html
Some latent heat, autoignition, and flash numbers:
-) Gasoline, 150btu/lb, ~500*F, -45*F
-) Methyl Alcohol, 506btu/lb, ~870*F, 52*F
-) Ethyl Alcohol, 400btu/lb, ~790*F, 57*F
-) Water, ~1000 btu/lb, n/a, n/a
http://www.afdc.energy.gov/afdc/pdfs/fueltable.pdf
The main differences between the two fluids as I see them: even though water has twice the latent heat of methyl alcohol, it's inert, therefore its only real benefit is strictly cooling. It strikes me that given its high latent heat it makes for a stellar combustion chamber coolant (to more importantly work during the compression stroke when pressures and temperatures are going up to help prevent fuel autoignition) as well as efficacious as a turbocharger's compressor coolant when done pre-turbo. Of course, the question of wheel erosion comes into play here with respect to the atomization of water: Can and will it destroy a compressor wheel over time? Water is extremely tough and I can't see a wheel not being chewed up over time if the atomization is extremely fine. But, that's just me. It might be that it's a moot point in terms of the trade-off of being able to extend the operating efficiency of the compressor by drawing heat out of its job of being an air compressor. I read a lengthy article once about how a gentleman, due to pre-turbo water injection, was able to run higher boosts (and prevent overheating) on his diesel truck when it was challenged by heat and high altitude/steep angle towing on the highway from LA to Las Vegas.
The other thing that comes to mind about pre-turbo water injection is this: Even though from a fault tolerance perspective (of not having to run an electric pump that can fail, etc.), if the latent heat factor is the big benefit of water injection, how much of that desirable property is lost with heat being drawn out of the water into the compressor? In other words, how much would one have to inject to be both beneficial to the compressor as well as ultimately the engine's combustion chamber? How much water here is being converted to steam in the compressor and therefore how much is left over on the compressor discharge to be of benefit in the combustion chamber? That's the meaning behind the thread's title, "Modes of Action and Efficacy". While I'm sure some (like rx72c) may disagree here, and that could just simply be due to my ignorance, it seems to me that a water injection setup belongs on an already tuned, 100% gasoline car where a measure of reliability is desired or when a small margin of that gasoline is to be replaced with water (where gasoline is commonly used as the assumed, de facto chamber coolant with air/fuel mixtures richer than 12:1). Perhaps the disagreement I would have with them is because of the massive amounts (upwards of 700cc/min!) folks like rx72c or Rice Racing use in their pre-turbo setups. Perhaps that amount of water is doing two things - moderate charge temp reduction and massive combustion chamber temp cooling. However, I still can't help but believe it's not the best approach (attempting to fight ever increasingly higher chamber temps by injecting more and more water even though the setup is still using the same, base fuel that's only efficacious in a colder and less brutal environment). From a certain perspective, it seems to make more sense instead to do two things: Create the same chamber cooling effects while bringing the base fuel's stability and "heat range" up to better match the hotter and more ferocious chamber environment we're trying to create.
It's kind of like the difference in approach between bringing cold air to the air filter (ala the cold air box) vs. taking the air filter to the cold air. By the way, in my experimentation with taking an air filter outside of the engine bay to the outside showed a drop of 40-45*F in cruising IAT's and no doubt helped with boost IAT's. The type of material used in the ducting, heat wrapping, and all that other jazz did absolutely nothing to increase that difference in temps.
I also am curious what kind of ignition hardware is required for such large amounts of water injection like this and whether or not said requirements reach well past what's normally equipped on these cars.
Even as little as 100cc of water, assuming 100% of it hits the combustion chamber and isn't converted into steam prior, has the cooling BTUs (due to its unparalleled latent heat) equivalent to that of over 650cc/min of gasoline when being used strictly as a chamber coolant! It's 650% as efficient as gasoline as a chamber coolant! That's undeniably incredible. However, other than the possibility of it occurring on a pre-turbo setup, I can't see water being a big help with IAT's especially on an intercooled setup. To make use of its high latent heat, the charge temps would have to be very, very high; high enough past the point where, with a pump fuel setup, knock begins to occur. That's why I think it's better served at the throttle where more of it can reach the chamber in pure form and do its job there.
The argument I draw from the questions above are why I am philosophically-speaking a fan of post-turbo water injection even though I don't dismiss the obvious benefits of pre-turbo injection. In my view, when a powerplant is using an intercooler alongside an air filter setup whose air source is from the ambient atmosphere and
not the engine bay, I can see how a post-turbo water injection would be a powerful addition when running an all pump-gas setup. As shown, even a few hundred cc/min of injection, which is pretty easy to pull off post-turbo with a pump setup running high pressure, will do a gigantic amount of chamber cooling, will stave off heat-related knock, and will enable the ability to run somewhat leaner mixtures. It seems like the "cherry on top" of a typical, tuned pump gas setup when we want reliability, even being sprayed in a static amount. Water's not corrosive, cheap, and doesn't require extensive hardware to make safely work.
One side note.
The real reason why the tuning trend over the past many years for pump gas-only street cars being tuned to ridiculously rich air/fuel mixtures of mid-high 10's:1 is because the gasoline is being used as a chamber coolant (a piss-poor one at that) to try and help prevent blowing itself up. Yes, we run fatter what's necessary for power under acceleration (arguably mid-high 12's:1) because it's easy to manipulate an EFI system to inject more fuel under load as a marginal way to stave off engine knock (and even then it doesn't yield a large margin). Let me re-phrase this for those that may not have gotten it: We put more gasoline in because we're trying to keep gasoline from exploding early. I contend the problem is pre-ignition due to auto-ignition (when a fuel ignites on its own due to the hot environment around it) in the compression stroke. I think because of pump gasoline's lower auto-ignition temperature and instability it's making engines blow up at relatively low boost levels. This causes the power stroke to start too early, create wild chamber pressures in the successive strokes, and kaboom the motor. The same thing happens when a nimrod of a "tuner" manually fires the spark plug too early. It also happens when running too hot The other whammy is real detonation: the possibility of being able to cause a second flamefront behind the one initiated by a timed spark event from running too damn rich a mixture (when the mixture is so rich and the chamber being so hot that a second stage of combustion starts on its own). Running excessively richer gasoline mixtures in a denser and denser chamber environment also challenges the ignition system. It's difficult to get much burning past low-mid 10's:1 on a gasoline setup unless the ignition hardware behind it is really powerful.