water injection to gas engines 7

[SBBlue]

Glueontop discussed the ideal gas laws and Otto cycle thermodynamics, and concluded:

"These laws have been proven time after time for at least a hundred year. No one has ever prove them wrong, and no one ever will."

What you say is true. . . .but, as Paul Harvey would say, there's the "rest of the story" that needs to be told.

First, what you said about the ideal gas law and the Otto cycle could also be said of the open Brayton cycle, i.e., the Gas Turbine. Yet there is no question at all that water injection into the compressor of a gas turbine increases turbine efficiency. Here are some websites of companies that specialize in gas turbine "inlet fogging":

http://www.amco.com/power_generation.htm

http://www.air-process.com/inlet_fogging.htm

http://www.realfog.com/turbine/gtps-ifbc.html

Second, your comments about the ideal gas law and the Otto cycle only apply to single substance systems.

Consider the following;

Take one pound of air at an atmospheric pressure of 14.7 psi and a temperature of 1000 deg F. Mix this with one pound of steam at a pressure of 14.7 psi and a temperature of 212 deg F. What do you get?

Clearly heat will be transferred from the hot air to the steam, and the temperature of the steam will obviously be equal to the temperature of the hot air. If you work out the problem with the steam tables and ideal gas tables, you will discover that the resulting mixture has a temperature of about 490 deg F and a pressure of about 15.3 psi.

Note that the temperature of the hot gas dropped after we added the steam to it, yet the pressure of the mixture increased. Obviously, if we couldn't get any work from either the steam or the hot air prior to mixture because they were already at atmospheric pressure, and there wouldn't be pressure difference. But we could get some work from the mixture, because the pressure of the mixture is above atmospheric pressure.

The ideal gas laws and otto cycle thermodynamic laws are certainly quite solid. However, if you start mixing different substances, some of the implications will be different.

Third ---

You must consider that there are other implications to efficiency then just pressure and temperature. You must consider the efficiency of the compressor and expander devices. If adding water to the compressor makes it more efficient, it may more than offset any peak temperature considerations. If you can make the compression isothermal by adding water, than the efficiency of the overall system may improve.

 

[turboice]

glueontop:

Yes the laws are poven and hold in the controlled conditions in which they describe. I don't have a NASA budget so my cars have to operate in the real world where the conditions that predicate the laws rarely exist. In that case I trust empirical results and then seek explanations. The presence of water at the wrong time is destructive (i.e. in the gas tank at the pickup) at the right time in the right quantities the presence of water mixtures have been shown time and again to be beneficial to intercooling and combustion - without negating any laws.

Studies have shown that while water injection will beneficially reduce maximum pressure it will increase BMEP and it is BMEP that matters in an Otto engine - not the max pressure - max pressure at the wrong point destroys engines.

 

[RobWard]

Being pedantic here...
water actually contains an awful lot or energy.

e=mc^2

The trick is liberating that energy without the unpleasant side effects of nuclear bombs.

But, I'll grant you that water doesn't burn very well.

Sorry for the interuption everyone.
Please do continue, I'm finding this very interesting, (even if a lot of it is over my head)....

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[bvicknair]

Hint:
The water droplet size at time of fuel ignition is my area of study. If in a pure vapor or large droplet form the effect will not occur. Power loss and emissions go up per the SAE tests. Possibly a water in oil emulsion with droplet size corrected is key. I am banking on getting it right in the intake plenum.

 

[patprimmer]

glueontop

The flaw in the combined gas laws and otto cycle theories as applied here, are not flaws in the laws, but rather the interpretation that they apply absolutely to the caes of water injection.

The flaw is that you are interpreting this as gas only. The water is injected in a liquid phase, and changes to the gas phase. Latent heat of vaporisation and expansion from liquid to gas are both involved, and not covered by the gas laws nor the otto cycle.

Also, no one is argueing that the water should be added to the tank, as this will cause corrosion, will be erratic in it's supply unless a very stable emulsion is formed, and will lean off the mixture by the amount of fuel it displaces, unless the fuel calibrating system is ajusted to compensate.

The water injection systems being discussed are the type that adds water to the airstream, independantly of the fuel supply system.

As already stated (by SBBlue I think), the water does not increase the peak preassure, but it tncreases the mean effective pressure. If the peak pressure is to high it destroys engines. As I keep saying, the main benifit of water injection, is that it helps to stop preignition and detonation (the results of excessive cylinder cylinder pressure and or temperatures). It can help to control the pressure build up rate to better match volume increase or piston speed as the piston travels over TDC and down the stroke.

bvicknair

I expect that water partical size, airspeed (shear), relative humidity, compression ratio, temperature changes within the manifold, and time between water discharge point and ignition will all impact somewhat on how much water evaporates and where it happens.

Regards
pat

 

[glueontop]

obviously the ideal-gas law doesn't apply on the compression stroke, where we have a liquid/gas mixture but after the explosion on the power stroke.

SBBlue.. I don't have my tables at hand, but I can see a mistake in your calculation. You consider the water to be in the steam phase at the start. You don't take into account the heat needed to bring your water from about 50C to the boiling point and the heat of vaporisation wich will bring your mixture much below the temperature you have calculated. That needed energy will be taken from your air and will probably lower its temperature a couple hundred degrees. Then you can use your table to make your calculations. If you remove a margin for the lost of temperature trough the engine block, you will end up with less energy then you had at the beginning.

It is not true that the otto engine can be compared to a gas turbine. There is a major difference that makes it possible in a turbine but not in a Otto engine. The gas turbine is a continuous process where the water as the required time to expand before it reaches the expander. In the Otto cycle, the power cycle appends so fast that the fuel/air mixture barely as time to finish to burn, in fact it continues to burn in the exhaust manifold. Any water you add won't have time to vaporise and expand before the begining of the exhaust cycle, it will then create back pressure and reduce power.

 

[SBBlue]

Glueontop. . . .

I purposely did the calculations that way to disprove your hypothesis about the ideal gas laws and otto cycle and show that it is possible to decrease temperature but improve efficiency.

I think if you do your calculations comparing equivalent power Otto or Diesel cycle engines to Brayton cycle engines you will find that air moves much faster through the Diesel and Brayton engines then it does a same power output Otto engine.

The proof is simple; the Otto cycle engine is essentially stoichometric; all the oxygen is used to combust the fuel. The diesel and Brayton cycle engines both run quite lean; there is much more oxygen than is needed for combustion. Hence, much more air is pumped through a diesel or Brayton cycle engine than a corresponding Otto cycle engine.

One other thing you are neglecting in your calculations is the effect of raising the temperature of the air during the compression stroke. Water is always steam above 700 deg F. Air reaches that temperature during a 7:1 compression stroke (I think -- I need to check the tables) and has a significantly higher temperature with higher compression ratios.

You may argue that there isn't time for water droplets to change to vapor, but I don't think you have any actual studies to support that assertation. There is certainly time for all sorts of chemical reactions to occur during the combustion stroke, so I don't believe that there isn't enough time for the water liquid phase to be changed to the vapor phase.

 

[bvicknair]

I have a book written in 1952 by a German engineer that used "exploding water" to hydroform precision pieces during WW2.

A more current publication dealing with the subject is 'Newtonian Electrodynamics' Peter and Neal Graneau. See Ch. 6-7.

Pressures routinely reach 20,000 ATM in water explosions.(290,000psi) The source or the anomalous pressure is currently under debate.

bv

 

[turbododge]

I just got referred to this thread by a friend and being a long time user of water injection thought you all may be interested in what I have personally seen over the years. I run a twin turbo, now EFI (was carb), twin turbo 340 Mopar in a 70 Cahllenger, strictly street, 14 psi boost, intercooled.

I use an old Edelbrock Varijection system that has been altered for instant on under boost conditions at 7 psi. It varies the amount of water based on rpm and absolute manifold pressure. It does not have adequate pressure to spray directly into a pressurized intake manifold, so I spray directly on the turbine blades. The turbos are lower than the inlet ducting to the throttle body, so even if I have a valve failure, I cannot water slug the engine. I have run this system for over 15 years on carbed and EFI turbo engines, and would not go without water injection. I can run 8 to 1 compression at 14 psi on 92 octane all day without worrying about detonation.

I find the talk about atomization, hummidty, where the water evaporates, etc to be very interesting, as I have watched my system extensively over the years. When I spray on the turbine blades, the water (with 50% isopropyl) vaporizes instantly, before it even touches the blades. I have 50K mile turbos that show no erosion at all, and even the intercooler does not get any significant amount of moisture in it. The ductwork all stays dry. The low pressure area at the turbine inlet easliy evaporates the water spray. I found no difference with EFI or blow thru carb in this respect.

When I was running a suck through carb setup, I was spraying directly into the carb throat with the water, and found that the carb venturi would vaporize the water right along with the fuel from the venturi. Subsequent running through the turbo also further atomized both elements. All was good unless the inlet ducting was cold enough to condense out the fuel and slug the engine.

In an N/A engine, you would spray straight into the carb with most systems like a Spearco. An Aquamist you could go into the manifold below the carb as they atomize better, but most folks don't do that. Unless you are dumping very large amounts of water into the carb, it will atomize, if it did not, it would build up in the intake and give a big slug to the engine when you turned, etc, and probably thermal shock or hydraulic the engine to distruction. I think it would be a very risky business to try to get, or count on getting unvaporized water into the cylinder.

As far as the power is concerned. As I mentioned before, my setup is instant on, off a pressure switch, or an override switch in the cockpit. I have done lots of testing at boost levels to 12 psi (max without detonation without water) with the water on and off. I give people rides and turn the water on and off, then off and on to see if they can feel the accelleration rate change. I can tell you that no one has ever not been able to tell that the water came on and the accelleration rate increased. It is very significant. I would be surprised, however, if you would be able to readily feel the difference in an N/A engine, as when I turn the water on and off while holding the boost down with the throttle, I cannot feel a difference at all, in my low compression engine.

My net conclusion is that on a boosted car above 10 psi, water injection is necessary if you want to be able to run decent compression, timing, and mixtures, plus you will get more power do to charge cooling, even if you are intercooled. On an N/A car, I would use water only if you need it to kill detonation because of high compression, bad gas, etc. I don't believe it would give you enough more power to notice.

 

[turboice]

At 12:1 CR water injection I found makes a big difference on with high compression N/A on load.

From a direct causal effect - water injection is solely for suppression of knock.

The following is predicated on an properly implemented water injection system.

The great part of WI is the freedom you have tuning once the knock is effectively suppressed. You are now free to run maximum power AFR on performance engines and maximum economy AFR on commuter engines. Additionally whereas even when ineffecient fuel dumping was being used to suppress knock it was still difficult advance timing to MBT prior to overcoming the excess fuel and getting knock anyway, with water injection at optimal AFR levels you are able to always advance to MBT.

I am aware of only one other person who has injected prior to the turbo and he found that the wheel was pitted as a result - though he admits his jets likely were not sufficiently atomizing the water.

There is a discussion at

http://www.turobice.net/forum/

(a noncommercial forum) and this very question is being asked - can water be safely injected prior to a turbo and it would seem your experience would indicate yes. If you don't mind passing your story along there, it would be of benefit to those discussing that topic.

Ed.

 

[patprimmer]

Thank you Turbododge
It's good to see another contributor who actually knows what he is talking about

Regards
pat

 

[aviat]

Pat!

What you said about the water injection system used on aircraft engines is true. It is called an anti detonation injection system (ADI) and that is exactly what it is. At take-off power settings, particularly in hot weather, a richer than best power A/F mixture must be used to prevent detonation, with the excess fuel used as a coolant. With ADI the water and alcohol mixture is injected into the air stream and act as a coolant. The alcohol is added to prevent freezing. As the mixture is injected a de-enrichment valve opens and leans the mixture to best power setting.

This system is mostly used on larger radial engines - 2000 BHP and greater. On one chart I have there has to be a reduction of 30 BMEP and 5”Hg of manifold pressure with system off, so it makes quite a difference. Overall as much as 15% may be gained by this system on these engines, but they are air cooled with aluminium cylinders.

Using water injection does give other benefits to engines, particularly from a cleansing point. Even the compressors on turbines are sometimes washed by spraying water into them while they are turning. As far as using this system in a street car I think you would might end up with a net loss in power because you have no means to adjust the mixture and detonation is not that much of a problem.
.
As to the question of which is heavier, dry vs humid air, you are correct in saying that you get more power in dry air. It shows up on piston engine power charts (up to 3%) and must be corrected for, although for an engine in a vehicle being driven you get lower drag and will cancel part of engine power loss. The reason is that water vapour is a gas, which has more hydrogen molecules (which are lighter) than air does, and these hydrogen molecules replace some of the molecules in the air. By Dalton’s law of partial pressures the measured total pressure is the sum of the pressure of dry alone and the pressure of the water vapour alone, and because the average density of air with water vapour is less than that of dry air, at the same pressure and pressure. Hot air can hold more water vapour and therefore a hot humid day has the most effect. I have a formula to determine the reduction in density with increase in humidity it you need it.

Cheers

 

[turbododge]

Turboice: You are absolutely correct in that one big factor with water injection is killing the detonation to allow you optimize other conditions. It is very common in the turbo crowd to try to drown the detonation with extra fuel, even down to 10 to 1 A/F. All it does is cost power and give minimal results. The is a very good chart in the Hugh MacInnis book Turbochargers on the affects of water on detonation, boost level and air fuel ratio. One look at the chart and you can become a water booster for life.

Concerning the non erosion of my blades, it may have to do with the fact that my water only comes on when I really need it, so it is on a very small % of the time, but I am also very carefull to send the nozzle stream (it is not atomized) at the CENTER of the compressor shaft so that any impact is on a slower speed, less critical area, and the water flow moves out the blade rather than impacting it. Also, with Varijection, there is less water at lower rpms and boost levels and more at higher speeds and boost. A single output volume, set to deliver enough water at full load, like a Spearco, can easily overwhelm a turbo spinning at lower rpm and impact the blades harder. With the EFI controlling spark and A/F (12.5 A/F under boost)and careful tuning of the water we normally will only use a gallon of water per 1 to 2K miles of street driving, as we are good to 10 psi very safely without any water, (and 12 if the gas is good). So putting is enough water to get to 14 psi is pretty easy.

 

[turboice]

Aviat - the benefits in tuning performance automotive engines is very similar. Knock is a very large problem for forced induction and high compression normally aspirated engines. The OEM solution is usually to throw fuel at the problem - which first is a very inefficient coolant low specific heat and secondly as Glassman proved excessive fuel (rich settings) inhibits the oxidation of CO which releases a large amount of the energy released in hydrocarbon combustion. Now on your average commuter car you are generally running leaner than optimum power anyway - but here water injection can be used to run at optimum consumption instead. There are many methods available to automobile tuners to modify the OEM fuel and timing settings to take advantage of water injection - for my car alone there are no less than a half dozen tools I can use to adjust fueling and timing.

turbododge - I need to get the MacInnis book since it is a new reference and I have collected a dozen or so other books for even the smallest reference to water injection. Sir Ricardo also has a great chart that is very telling as to why water injection is the optimal tuning tool as opposed to what Corky might throw out there in pop literature with no basis and no peer review. Sir Ricardo's chart is in my paper on water injection which I referenced above on October 1st (

http://www.turboice.net/?documents/turbowhitepaper/title.htm)

and also linked in the FAQ I have written on WI.

 

[turboice]

Here is the result of Sir Ricardo's research on WI:

http://www.turboice.net/?images/RicardoResults.gif

Ed.

http://www.turboice.net

 

[patprimmer]

In the 70's and 80's, I saw a number of suck through turbo combo's that injected before the turbo.

I have seen a system where the boost pressure was fed back to the water bottle, to boost the pump pressure under boost.

The idea of water damaging the blades was not considered in wet or suck through systems, as the fuel also impacted the blades, so the turbo was robust enough and slow enough to not have a problem. In those days hotrodders adapted turbos originally designed and built fot diesel truck engines. These were very HD with no real consideration for lag.

I was advised against useing a before turbo injection system on a turbo system which was OEM on modern EFI, high performance sports coupe (Toyota 7M-GTE). The advice given by some turbo tuneing specialists was that in their experience the water did impact on the blades and as the Turbo was a modern high speed lightweight high performance design, the blades would be lightweight and not robust enough to withstand the constant water impacts.

I took the advice, but also discovered that knock was not the limiting factor re boost levels with the stock engine, as fuel cut limited boost to about 9 to 10 lbs. It did not knock, even on normal unleaded at this boost.

As I took the advice, I cannot comment on it's accuracy.

Regards
pat

 

[turboice]

I have no evidence of this but my other concern besides the lower mass of today's blades is their balance. If you have ever seen a turbo balanced you know that at 20,000 rpm very small changes in mass can alter it. I would be concerned that any inconsistency in the spread of the liquid mass would throw the cartridge out of balance and cause undue stress on the bearings. With so much success injecting after the turbo, I have never felt compelled to test out the blade damage theories or my own dreamt up issue of balance - which I hope has at least some merit.

Ed.

http://www.turboice.net

 

[turbododge]

Turboice: I can't seem to get off the home page on the reference site to Sir Ricardo's information. Don't know why. It very well could be the same information as MacInnis, as many of these guys reference each others work, except Corky who just spouts unfounded and often untrue opinions (my opinion of his info).

The erosion of the blades discussion is very interesting, and may apply to others differently than me. I am running plain old T04, oil cooled turbos, and have been for many years. The newer turbos with the aforementioned lighter blades and perhaps more brittle alloys may be more of a problem. With those I have no experience. There was a post that mentioned someone who had a friend who did get blade erosion (post probably gotted whacked because it referenced another site). Was this person, by any chance, running a water/methanol mix in his system? From what I have seen and heard, methanol will take out turbine blades, throttle body plates and bearings, and can even corrode the tips of fuel injectors enough to cause pattern problems. That is why I use Isopropyl in mine.

 

[GregLocock]

That Ricardo graph is very interesting. If you look at the sfc trend it /seems/ to be decreasing at very high bmeps.

Does anyone have a good explanation for this? Is this the steam engine effect, where the expansion of the additional gas (steam) late in the stroke is thermodynamically better than raising a smaller mass of gas to a higher pressure, earlier in the stroke?

If this is a real and useable property then it could give a motor whose efficiency graph has two sweet spots, which might be very useful for an economy car.


Cheers

Greg Locock

 

[Azmio]

Wow, this is great, the discussion reminds me of the german WW2 aircraft that uses water mixed with alcohol to enable the engine to have higher boost during take off.

The water cools the charge and thus making the engine less likely to detonate. The alcohol on the other hand supplies more fuel (with higher RON rating) and maybe oxygen (correct me if I am wrong) for the combustion.