Injecting water into an engine?

[KLuuppo]

An even hello for everyone here! It's my first post and I hope not to make a complete fool of myself.

I spent yesterday reading about different designs and working principles of pulsejets, both valved and valveless. I came into an understanding that it's very typical for pulsejet to have too much heat going around without any mass it could propel. So I thought that would it be possible to use up this excess heat to vaporize water and inject into the exhaust tube? I made some searches, but I didn't notice that someone would have done this before.

I'm not a physicist, but I could imagine some benefits from this: the main reason is, of course, to get more thrust for free. Energy used would be waste energy, which otherwise would be released as heat directly from the pipe and as a noise and hindering force from the thermic shock (the thing which happens after the tube ends, where the excess heat of exhaust gases release chaotically into outside air). Now the heat would be used to vaporization and expansion of the water, thus increasing amount of propelled mass. Water would also cool down the pipe, which should decrease its wear. Also mounting of the engine would be a bit easier.

Of course, this installation would add into the complexity of engine, but I'm quite sure that whole water injection system could be made without a single moving part. One idea would be a pipe spiral going around the combustion chamber which would boil the water, thus increasing the pressure which could be used to inject vaporized or liquid water. Other con would be the need of carrying the extra water, but in surface vehicles it would be neglible.

Some thought should also be taken into the positioning of the injector, since it can't cool down all of the exhaust gases, since we want some hot gases to ignite the next pulse. So the injector should be positioned quite far into the exhaust tube. Also the injector's size should be consider, as well as the placement of heating 'coil' so the amount of injected water would be optimum and its pre-heat as high as possible. This installation would also decrease the temperature of burning chamber, and at least I would like to mount something less glowing into my vehicles or models.

[Mark]

Welcome KLuppo. I found a few tidbits about using water.

http://www.grc.nasa.gov/WWW/RT/2005/PB/PBA-snyder.html

"The Metro 23 (Garrett TPE331-12U turbo prop engines) uses a water/methyl alcohol mixture for high density altitude take-offs.
It's a mixture of 40% methyl alcohol & 60% distilled or demineralised water.
When the performance charts require it's use it's turned on just after you start rolling. It gives approximately a 30% increase in torque and is turned off at 500'.
The Metro has a 16 gallon tank for water/meth and you'll get approximately two take-off's out of one tank.
It's a bit of an eye opener for the passengers when you get to 500 feet, turn the water/meth off and suddenly get a 30% torque decrease!"

"Early model Boeing 707-100's used water injection with the JT3C-6 engine. Two pumps (one for left engines, one for right engines) fed demin water during takeoff for 2.5 minutes. When the water stopped, it felt as tho the power had been pulled back big-time, and in fact the thrust loss was 2,100 pounds per engine, and on a 10,600 pound/thrust engine, this was a LOT. These aircraft were affectionately known as "waterwagons". I flew these for awhile, and was pleased to move on to the B707-320 with fan engines."

"Not only does the water cool the turbine, but the water vapourises incresing the volume, consequently the pressure and thus the thrust. The design of the engine is important - if it is close to the surge margin, then obviouslty this is not a good idea."

"As an ex LAME of the era at Qantas of 'wet' JT9D engines, I must concur with "justavagander" entirely. The engine ground runs were just as spectacular as those described. Added to that was the exorbitant cost of the demineralised water supplied by fuel agents. Water Injection was a great idea but in practice, the bad far outweighed the good. However, I was able to pay off my house mortgage quickly on the overtime earned thru all those engine changes due to overtemps from malfunctioning water injection systems!!!!!!"

"The Harrier/AV8B has about 40 secs worth of water injection for hot/heavy hovering."

http://www.pprune.org/forums/archive/in ... 10124.html

" ...it’s doubtful that NASA is using Space Shuttle launches as an excuse for a giant wiener roast."
http://www.nasa.gov/returntoflight/laun ... sages.html

[Mike Everman]

nice bit of research, Mark. Very interesting.
Kluupo, Eric Beck here has done more pulse-jet water injection experiments than anyone I know. He's probably figured out how practical it is, too.

Any conclusions you can share, Eric?

[KLuuppo]

After reading Mark's post I suppose it really is possible to get more thrust with water. Though the advantage seems to be quite low if the water has to be carried airborne. I suppose that in surface crafts (especially boats) carrying the water around wouldn't be so bad. And with injection into pulse jet exhaust there wouldn't be need for the water to be pure. No need for massive purification costs.

[dynajetjerry]

Hi, Guys,
Read some of the comments on this thread and decided to inject (pun intended,) a coupla comments.
When Aeromarine developed the original Dyna-Fog and military smoke generators, they quickly found that additional fluids affected the jet's operation only a little, providing they were introduced well downstream. In fact, oil for fog/smoke could be introduced at 10-15 times the rate of fuel (gasoline.) I don't recall any investigations into resulting changes in thrust because such massive equipment (110 lbs for the DFG & 230 lbs. for the Navy's PJ 101) were completely immune to minor thrust changes.
Water was never used because such material is almost invisible and the goal was fogs that could be seen and would rise or settle, depending on droplet sizes.
Jerry

[heada]

There has been talk in some rocketry circles about using water injection as a thrust enhancer. It has mostly been shot down because you have to carry the water with you and rather than carry water you can just carry more fuel/oxidizer (LOX and LH2, which is just what water is anyway)

For a ground based system, it makes sense but anything that would require you to haul the water with you would add weight for a water tank, pump and the water itself. For the ~30% performance gain you can just carry more fuel.

Something that might prove interesting is to look at fuel injection against the combustion chamber walls. You would be running fuel rich (unless you cut back the normal fuel feed) but it would cool the CC walls and the fuel would be fully vaporized in the CC but not mixed as well.

-Aaron

[dynajetjerry]

Hi, Guys,
Another comment, if I may: Though the mass of ejcted exhaust would increase with water injection, its cooling effect might be enough to reduce expansion of the exhaust. Might this also reduce thrust? We must remember that "the hotter the better" in heat engines where efficiencies are paramount. Perhaps such a slight reduction would be more theoretical than real.
I am reminded of the "stealth" fighters and bombers. To reduce heat signatures, they expand and cool the exhausts before they are allowed to exit the plane. My question is this: how much (if any,) does this cool-down reduce thrust below that of a similar, uncooled jet engine? I sure don't know.
Jerry

[Johansson]

I think that Eric started a topic about water injection in the valveless forum a while ago, use the search function and you might find it.

[larry cottrill]

Hi, Guys,
Another comment, if I may: Though the mass of ejected exhaust would increase with water injection, its cooling effect might be enough to reduce expansion of the exhaust. Might this also reduce thrust? We must remember that "the hotter the better" in heat engines where efficiencies are paramount. Perhaps such a slight reduction would be more theoretical than real.
I am reminded of the "stealth" fighters and bombers. To reduce heat signatures, they expand and cool the exhausts before they are allowed to exit the plane. My question is this: how much (if any) does this cool-down reduce thrust below that of a similar, uncooled jet engine?

Jerry, I think the only issue is whether the heat expansion has already done its job. Where does the thrust force in, say, a Dynajet, really get applied to the engine? Though crudely done and cut short by material destruction, my attempts at measuring the "pressure equivalent of momentum" in the Dynajet suggested that my theory was basically right: the thrust is all developed by the time the gas gets through the nozzle, and the tailpipe has nothing to do with it except to establish resonance (and re-build the tail "piston" mass, of course).

You would think that would mean that once the gas has departed from the engine, there's nothing more you could do to boost thrust -- but the usefulness of augmentors proves that this is not the case. What we get there is a different kind of "expansion", i.e. an exchange of heat between the ejected jet exhaust stream and newly entrained air mass. That's a lot like the "passive ramjet" effect that Bruno talked about in relation to the P-51 Mustang radiator exhaust. I have no idea what they do in the stealth machines, but it could be something like that. It has been said that an augmentor behind a turbo is useless, basically because "dilution air" has already been applied, ahead of each chamber nozzle. Useless for thrust, perhaps - but maybe not for further heat dilution.

There are probably several ways this could be done. The trick is to make sure you do it where any tiny "thrust gain" is enough to overcome any additional drag. Or, at worst, the net added drag is some tiny percentage of the overall thrust. An "augmentor" design is one way of assuring this.

So, my guess is that doing water injection correctly is largely a matter of exactly where and how you get it into the stream; I would think that it would need to be somewhere downstream from the chamber, though, in some kind of "quasi augmentor" arrangement.

Of course, I'm probably ignorant of many possible ways this could work. But, that's what creative experimentation is for.

L Cottrill

[Irvine.J]

What about injection near the megaphone -transition- on a valveless, the idea being that it wouldn't need to pass directly through the chamber, but would still absorb enough heat to effectively increase exhaust mass. However, and a big however, water is quite heavy... even using 1/2 the volume of water to liquid fuel would substantially make any flight platform a great deal heavier, a significant increase in thrust within 10% water to the delivered fuel volume it might just warrant it. I would estimate if you used 10% of the fuel volume in a water injection system, i'd want atleast 20% greater thrust out of it.

[Irvine.J]

If anyone knows alot about fuels, then this perhaps is for you...

A lot of talk goes on about water injection, seems as if we know it but are a little un-motivated to try it, all that extra setup required to work out the flow rate needed etc to provide increased performance....

So, this is where fuel gurus may be able to bring us up to scratch on something thats been nagging at me....

When you introduce water to gasoline, i know a little will mix with it, even more with a touch of detergent. Methanol will actively absorb water quickly straight out the air....
BUT
In doing so, is the waters assimilation actually responsible for a chemical change in the fuel at a molecular level, or is it just suspended in the hydrocarbon soup? Heres why i'm interested.

If direct injection of water into the intake does infact generate extra thrust, a % water constituent could be introduced in the fuel itself. It would make things harder to start, I am aware of this, but starting could be done on an axillary tank then switched over very easily, rather then use the same direct "WEP" (War emergency power solution.) IE, direct injection of an external source into the intake.

....In normal service, the P-51H Mustang was rated at 1,380 hp, but WEP would deliver up to 2,218 hp.[3] The Vought F4U Corsair, not originally equipped for WEP, later boasted a power increase of up to 410 hp (17%) when WEP was engaged.[2] Several methods were used to boost engine power by manufacturers, including water injection and methanol-water injection. Some earlier engines simply allowed the throttle to open wider than normal, allowing more air to flow through the intake. All WEP methods result in greater-than-usual stresses on the engine, and correspond to a reduced engine lifetime. For some airplanes, such as the P-51, use of WEP required the plane to be grounded after landing and the engine torn down and inspected for damage before returning to the air.

If, we assumed that mixing a % of water with the fuel would provide extra power our flow rate would indeed increase, but, heres the beauty of it...
If one was carrying for instance 1 liter of fuel, 100mls of water in a seperate tank would be very little in terms of weight. A small open / close valve could easily in a WEP situation, allow a desired flow rate for the maximum throttle of the engine, be introduced to the fuel line leading to the pump. Very much like your shower plumbing, ensuring a pre-mixed water/fuel injection before reaching the engine. If the normal fuel has a 10 to 15% methanol content, i would assume that the mixing would happen very fast.

(THIS IS ALL ASSUMING WATER DOES NOT EFFECT THE MOLECULAR COMPOSITION OF THE FUEL MOLECULES)

Pulse-WEP ??? Remember you heard it on PJ.com first!

Comments?

[Jim Berquist]

James , In a application like say a boat? Water is free, no weight to carry. Do you think you could put a ventury in the cone that would have enough vacuum to take in the water? Would it over pressure or take some in? Use some sort of Ram set up to get the water in?

[Brian Davis]

Someone asked for a fuel specialist opinion on mixing water directly with the fuel. I am a Quality Control Specialist for a Jet-A Fuel Storage Facility. I have seven years experience. Before that I put 21 years in the USAF as a Fuel System Specialist with most of my experience on the F-16 but alos worked on F-15, A-7, and T-38. There are a number of surfacants or surface acting agents (detergents) that get into Jet-A mostly from pipeline contamination. These detergents are used in Gasoline and diesel to keep engines and injectors clean. Ethanol and Methanol will also cause Jet Fuel to hold water.

There are two types of water that occur in Jet Fuel, free water and disolved water. Free water is small droplets that are suspended in fuel with the aid of a surfacant and will usually settle to the bottom of the tank if given enough time.

The limits of free water allowed in Jet-A is 15part per milion. Not enough to make a difference in thrust or burning characteristics. I think you could use surfactants to get Jet or kerosne based fuels to burn upto 1000parts per million water but it would be harder to get it to light off. This is less than .15% by weight and not enough to make a difference in thrust.

Dissolved water is individual molecules of water mixed in the fuel. If the fuel gets cooled or sets for a very long time the molecules of water will begin to glob together into droplets and fall to the bottom. Jet-A will burn just fine with large amounts of dissolved water in it. I doubt you could ever get more than 1% water to fuel ratio though without it begining to glob and settle. With the addition of methanol you might get 5% water by weight to stay disolved. This might be enough to make a difference you would have to experiment.

First you would have to get your engine running on a liquid fuel. Then start adding water and see what happens. Would love to know what you find if you do some experimentation.

[Mike Everman]

Welcome to the forum, Brian, and thanks for chiming in!
Eric Beck here has done quite a bit of water injection experiments, though I believe aft of the combustion chamber(?) I hope he can shed some light on that. It's a great idea, but I'd not thought of adding it to the fuel as James suggests. Seems like the quickest way to shut a pulse-jet down in any meaningful quantity. A momentary boost using all of this waste heat is a very compelling idea to say the least, though I've not tried this simple thing ever. So many ideas, so little time. That too is a common thread around here!

[Viv]

Hi Brian

Welcome to the forum as well, My question would be about dissolved oxygen rather than water in these type of heaver fuels, any answers you have on that would be interesting.

Interesting points you mention on the surfactants (and lubricants) as liquid propane can suffer similar contamination, that was a problem on the heat exchanger coils of the pressure jet engines, break down of the contaminants leading to oxidization of the fuel in the coils with subsequent clogging with carbon or worse carbide precipitation of the stainless steel of the coil.

Viv