water injection

[mckpickle]

ANyone know at what stage the water is injected and the principle behind it? Thanks

 

[bafanguy]

Don't know about jet engines, but on the R2800, water injection, called ADI ( anti-detonation injection) was a 50-50 water methanol mix. The methanol kept the water from freezing at altitude. The water increased the mass of stuff in the cylinder, thereby increasing the power output at takeoff...I think.


My recollection may be lacking after 35 years, but Mar and A-Squared are current DC-6 guys who have the factual info.

My guess is that in a jet engine, the mass principle is the same as in a recip. There must be a KC135 guy here who flew the water wagons...

Great question. why do you ask ??

 

[mar]

Jet or Recip?

Here's a link to a short discussion about water injection on the R2800:

http://forums.flightinfo.com/showthread.php?t=30935&page=3&pp=15&highlight=water+injection

But maybe you were asking about water injection for jet engines???

Regardless, the principle is the same: Mixing water vapor into the same air that will be compressed and mixed with fuel increases the mass (volume) and thus increases power.

In a jet engine the water is injected into the compressor stage.

I hope the link works. If not, do a search for a thread called "I want my $50 back". It's a crazy rant by your's truly until page three when I get suckered into talking about airplanes. That's where you'll find the short discussion.

Good luck.

 

[avbug]

Water injection is generally not just water, and as mentioned previously Anti Detonation Injection fluid (ADI) wasn't to boost power, but to prevent detonation at higher boosted (turbocharged, supercharged, turbosupercharged-turbocompound, etc). While the use of the fluid in the cylinder does provide some increased expansion and expansive power, the true use of ADI is to prevent engine damage due to detonation as pressures increase under compression in the combustion chamber of a piston engine.

The intent is to prevent the fuel air mixture from detonating permaturely under pressure, as it is compressed during the compression stroke in each cylinder. A side benifit is the expansion afforded the vapors in the chamber as the increased humidity is vaporized; water to steam produces an expansion in the order of 240 times.

Fluids used for ADI include water, and water combinations. Common additives to form ADI include ethanol, methanol, and glycol. Sometimes straight alchahol or alchahol mixtures are used.

In a turbojet engine, the primary purpose of water injection is to increase the expansive value of exhaust gasses; thrust is increased as water expands to vapor exiting the engine. Water may also be used to modify flame behavior and to reduce internal temperatures in the engine. The use of water injection may also result in less efficient fuel burn, which results in black smoke or darker engine emissions. Usually such use is limited to brief periods of maximum power, such as takeoff.

 

[mar]

Good points

Avbug--Haven't seen (?) much of you lately--I'm glad you could 'expand' on the ADI topic.

And of course, that's the whole point isn't it? Anti-detonation, that is, which I failed to point out.

As you say, water injection doesn't "increase" power really, but it allows you higher manifold pressures without the detriments of detonation.

Those were great points that I should have thought of.

I hope you can finally take a breather from that fire season...

 

[RJones]

Liquid Injection

The sensitivity of gas turbine engines to compressor-inlet temperatures results in appreciable loss of thrust (or power, in the case of a turboprop) which is available for take-off on a hot day. It is frequently necessary, therefore, to provide some means of thrust augmentation for nonafterburing engines during take-off on warm or hot days. This is also the case on at least one afterburning engine. Ten to thirty percent additional thrust can be gained by injecting water, or a mixture of water and alcohol, into the engine, either at the compressor-air-inlet or at some other point, during power augmentation by means of water injection, the water acts primarily as a detonation suppressor and a cylinder-charge coolant. Induction air-cooling is secondary.



Higher take-off horsepower results chiefly because when water, or a mixture of water and alcohol, is added, the engine can operate at the fuel/air ratio that will produce “best power.” Sometimes a higher manifold pressure may be obtained than would otherwise be possible without experiencing detonation.



Gas turbine engines, however, have no detonation difficulties. When a liquid is added, thrust or power augmentation is obtained principally by cooling the air entering the engine, by means of vaporization of the liquid introduced into the air-stream. Cooling the air has the effect of reducing the compressor-inlet temperature. The reduction in temperature increases the air-density and the mass airflow. More and cooler air to the burners permits more fuel to be burned before limiting turbine-inlet temperatures are reached, which, in turn means more thrust.



Pratt & Whitney Aircraft



Hope this helps, RJ

 

[avbug]

Mar,

The fire season was a bust this year, and ended fairly abruptly for us at the first of the month. A few good ones, but not many. I think our aircraft made 60 hours all season.

 

[mckpickle]

You guys are smaaaaattttttt. Thanks!!!

 

[GravityHater]

Does this mean we should expect a minor increase in thrust from our engines in hi %RH environments vs drier ones?

 

[PHX767]

On the mighty J57

The water is (was) injected in the compressor as well as just prior to the burner cans.

As the dash one (flight manual) said, the air mass was cooled and condensed at the compression stage and the burner cans were cooled to allow more fuel to be added and thus thrust increased.

On the KC-135A, the engines were rated as in the 11,000 lb thrust class (dry) and in the 13,000 lb thrust class (wet) with water injection.

It used the 670 gallons of demineralized water in about 110 seconds.

And it made the most g*d awful noise.

That water running through the lines would lend a howling noise to the overall din. I remember folks saying it was about as loud as an F-106 in full AB.

 

[avbug]

Does this mean we should expect a minor increase in thrust from our engines in hi %RH environments vs drier ones?

No. High ambient relative humidity has the same effect as increased density altitude.

 

[rumpletumbler]

Don't cut the bleeds on until it has had time to burn itself up. It stank

 

[House_X]

I wish we had more threads like this one on flightinfo, learned a lot of great info...best thread I've seen in awhile.

 

[ArcticFlier]

In the mighty SA227-AC we had a 60-40 mix of water/meth. About 16 gallons worth if I remember right. The TPE 331-11U engines put out 1000 SHP without and 1100 SHP with. We were told to use it if, obviously when necessary, but also if it was borderline as to whether it was necessary or not. EGT's were cooler for the torque needed for takeoff, and the engines lasted longer. We always tried to brief the pax as to what was going on, because when the water was turned off, it got everyone's attention. Torque would drop a bunch (20-30% if I recall. Long time ago).


AF

 

[jergar999]

I'll second articfliers comments, and add that if you look at the front of a TPE-331-11U engine you can see the water/meth nozzles before the compressor section. The SA227 holds a 16 gallon water/meth tank in the nose, on the copilots side, which is absolutely no fun to fill whatsoever.

 

[Big Duke Six]

Yes, it was quite noticeable in the -227. Very effective! Like a whole new airplane. Too bad it wasn't permanent...

 

[FN FAL]

Although the water injection thread was pretty informative and interesting, I have to correlate it to the technology of the "Hot Beef Injection" which increased productivity in the end of the industry served by single engine PT-6's.

 

[TonyC]

From T.O. 1C-135(K)A-1 (more affectionately known as "The Dash One"):

WATER INJECTION SYSTEM

A water injection system (figure 1-12) provides thrust augmentation by allowing water to be sprayed into the air inlet and diffuser section of each engine. Water injected in this manner increases the density of inlet and combustion air allowing increased thrust.


Figure 1-12, located 7 pages back, naturally, shows water injection just forward of the first N1 compressor stage, and just aft of the last N2 compressor stage.


That was Page 1-12, Change 25 (10 Aug 73)


Page Count anyone?


I can't believe I actually found this thing.

Anyway, that's all I can find about the mechanics or the why's and wherefore's of it - - I couldn't find what PHX767 said about "the air mass was cooled and condensed at the compression stage and the burner cans were cooled to allow more fuel to be added and thus thrust increased." Not sayin' it's not there, I just didn't see it.

And about the thrust:

ENGINE THRUST

The primary purpose of the flat-rated engine is to provide constant thrust throughout an ambient temperature range. At sea level the engine, as intalled on the airplane, will develop 12,845 pounds of thrust at all ambient temperatures from -65° F to approximately +5° F without use of water augmentation, and 12,925 pounds of thrust from +20° F to 100° F with water augmentation. Additional thrust is available for emergencies.

Page 1-6 Change 42 (10 Apr 79)

 

[EagleRJ]

AFAIK, water/alcohol injection works in the same way for both reciprocating and jet engines. When injected into the intake air, the evaporation of the water lowers the temperature of the air by a large amount.

That allows higher power settings than would normally be possible. In jets, it keeps ITT within limits at the higher thrust, and in recips it delays the onset of detonation.
The manuals that say it works by creating a denser intake charge are correct- colder air = denser.

I've seen videos of KC-135s doing augmented takeoffs, and they made a huge amount of smoke. Why is that? Could be that the fuel/air mixture is much richer than stoich with water injection, or it could just be that the earlier models smoked a lot!

 

[ArcticFlier]

The water injection was how I used to get the F/O's attention on the ground while he/she was helping with passengers and not properly paying attention to me.

Let's see: SRL off
Water on
Power lever (or whatever it was called) forward

Copious amounts of water/meth splashing around the left inlet and some onto said F/O.

AF

 

[Smoking Man]

RR dart 532-7r

Wather Methonal is injected into the first stage of the compressor, The mix was about 64/36 mix of water to methonal. The reason for the Methonal was to keep the water from freezing at altitude and doing some damage to the system.

The way that it was explained to me was it made the air more dense/ sea level pressure for better engine performance.

The water meth was usually armed and used during conditions that increased the density altitude, and the F-27's weight approached the MAX takeoff.

Regards

Smoking Man

 

[Wang Chung]

We used ADI on for most takeoffs in the Convair 240 (P&W R-2800 radial engines) when I was flying cargo. Using ADI allowed for us to pull about 3" of extra MP over a "dry" takeoff, increasing our allowable MTOW by over 2000 pounds and increasing horsepower about 300hp per engine.

Using ADI de-riches the mixture and decreases the fuel flow about 200pph to the engine at takeoff. I was never 100% clear on the complete explanation, but my understanding was that the ADI somehow displaces some of the fuel being delivered to the engine thru the pressure carb, thereby automatically leaning the mixture to a "best power" setting. Such a setting would normally cause detonation, but the cooling effect of the ADI counteracted this.

Any round engine guys able to confirm or deny this theory??

 

[transpac]

We used ADI on for most takeoffs in the Convair 240 (P&W R-2800 radial engines) when I was flying cargo. Using ADI allowed for us to pull about 3" of extra MP over a "dry" takeoff, increasing our allowable MTOW by over 2000 pounds and increasing horsepower about 300hp per engine.

Using ADI de-riches the mixture and decreases the fuel flow about 200pph to the engine at takeoff. I was never 100% clear on the complete explanation, but my understanding was that the ADI somehow displaces some of the fuel being delivered to the engine thru the pressure carb, thereby automatically leaning the mixture to a "best power" setting. Such a setting would normally cause detonation, but the cooling effect of the ADI counteracted this.

Any round engine guys able to confirm or deny this theory??

You got it right, Wang Chung. There's no density increase induced by introducing water vapor in the mixture, in fact the opposite is true: Dry air is more dense than moist air.

On a recip, ADI automatically resets the mixture to best power vice full rich. A best power fuel-air mixture always produces the most power for a given throttle setting, but contributes to excess combustion chamber heating at high power settings. To compensate, recips employ an overly rich fuel-air mixture for cooling at high power settings. During ADI use, the excess fuel used for cooling is replaced by ADI fluid. ADI is efficient at combustion chamber cooling so most engines will withstand a higher manifold pressure without detonation when ADI is used. Increased power from ADI results from using the best power (chemically correct) fuel-air mixture and the higher MAP permitted.

 

[enigma]

My only water/meth experience was in the J31 with Garrett TPE331's. In that engine, the water/meth served a dual purpose. The mix increased mass flow throught the engine, the mix cooled the engine and the meth added power. As others have stated, the methanol also acted as anti-freeze.

Some will argue that the meth does not provide the power increase, so I'll give the reason now. In a turbine engine only about 30% of the air gets burned, the remaining air is used for cooling and shaping the flow in the combustion chambers. A turbine can always make more power by adding more fuel. You can keep adding fuel/making more power right up to the point where the engine melts. It would be possible to inject only pure water into the intake and achieve an increase in mass flow and cooling, but to increase power you'd still need to add fuel. If you do so (add fuel) by increasing the delivery of JetA, and then suffer a failure of the water delivery during water enhanced ops, you face the possibility of melting the engine before you can reduce the flow of JetA back to the normally scheduled amount.
If you include the fuel (methanol) along with the water, then the JetA delivery system can continue to deliver a normal amount of dead dinosaurs, an amount that allows the engine to revert back to normal power/temp following a failure of the water delivery system.

I had the misfortune of flying junk Junkstreams and experienced numerous water/meth failures, and can state from experience that the engine reverts to normally rated (for ambient conditions) take off power and normal EGT with no decrease in fuel flow after a water/meth failure.

And yes, water/meth makes a Junkstream31 think that it's a B1900D. Too bad that it only lasts a couple of minutes.

enigma

 

[Uncle Sparky]

The water injection was how I used to get the F/O's attention on the ground while he/she was helping with passengers and not properly paying attention to me.

Let's see: SRL off
Water on
Power lever (or whatever it was called) forward

Copious amounts of water/meth splashing around the left inlet and some onto said F/O.

AF

LMAO.......There have been many instances when I had wished airplanes had horns! I usually use igniters or at night, strobes.

OK.......Just to play devils advocate. I have learned not to argue with the engineers but.......Under the rationale of injecting water into a turbine engine, why don't you see an increase or change in power while flying in heavy rain? Or is that where the stage at which the water is injected comes into play?

 

[rumpletumbler]

In the 226 I flew there was almost no where we went where it wasn't required. I remember going out of SSI one day and there was luggage galore, unexpected pax, etc. and the Captain said......."If the nosewheel stays on the ground were going." Then when it did....it was my leg and he said shoot for "normal takeoff settings" but use whatever it takes...... It did fine on the Dash 7's and water. That was a fun a/c to fly.

 

[mar]

Not a change in power but EGT

Under the rationale of injecting water into a turbine engine, why don't you see an increase or change in power while flying in heavy rain?

When I operated the TPE331 in heavy rain or snow I observed a drop in *EGT* but the torque remained constant.

This would allow (as enigma said) more fuel to be added (temporarily) up to the max EGT limit and thus provide an increase in power.

But it's just temporary.

Hope that made sense.

 

[Lead Sled]

When I operated the TPE331 in heavy rain or snow I observed a drop in *EGT* but the torque remained constant.

This would allow (as enigma said) more fuel to be added (temporarily) up to the max EGT limit and thus provide an increase in power.

But it's just temporary.

Hope that made sense.

It's the nature of the beast...
I've got time in several different TPE331 powered aircraft - the MU-2, Conquest II, and Turbo Commander and you would always notice a drop in ITT/EGT when flying in clouds - not just heavy rain or snow. As I remember, though, there was also a warning in the AFM against increasing the power under those conditions.

Lead Sled

 

[Uncle Sparky]

I understand and agree with the dynamics of it. As far as turbine experience I have flown Lears with the CJ610 and the 731Garrett. On these engines, I can't recall any noticeable change in engine parameters. I'm thinking, on the Fan at least, the fact that the water enters the inlet and isn't injected means that perhaps it is "slung" into the bypass section before it can reach the turbine.

 

[Wankel7]

One day I would love to run water injection on my RX-7. Aquamist makes a great kit for 400 bucks.

This stuff works well for cars too. esp the turbo ones.