Due to sensitivity of engines to air entering the engine an appreciable loss of thrust available for t/o on a hot day or warm becomes readily apparent, so it is desirable to use some means of thrust augmentation,
Several different combustion variables are affected by water injection.
1st. charge cooling (denser charge, more oxygen content allows more fuel in the charge).
2nd. slowing of flame front speed (improving combustion completion - more bang per charge).
3rd. additive effect expansion (phase change from liquid to a gas ie., ratio of expansion of water from a liquid to a gas(steam) is about 600:1).
It used to be that a sure fire way of defining an aircraft using water injection on T/O is a stream of black smoke coming from the exhaust. While the CV-880 just had very smokey noisy CJ-805s, early 707's used water injection in their J-57's (same engine Buff uses), these early 707's were referred to as "Water Wagons". Early 747 aircraft with the JT-9D engines also had water injection, as did some of the old Metroliners (especially with Garrett TPE331-11 and -12 engines), although the Garret TPE331-10 did not. Many of the older 707's and DC8's with turbojets, didn't either. The heavier (intercontinental) 707 had JT4's instead of JT3's, and did not utilize water injection. A better indication of water-injection system is observation of fine water mist appearing under the belly approximatly in line with the engine intake. This is the exhaust from the turbine water pump.
The water injection system is actually a water-methanol/alcohol (60/40) mix that is sprayed at the compressor inlet or diffuser case, sometimes both are used, I'm not sure but I think on some a/c the pilot has Comp / diffuser individual selectors to control either. De-mineralised water having a total solid content not over 10 parts per million was required (any higher concentration of solids would leave deposits on the turbine blades - not a good thing to have on turbine blades).
Basically the water injection system allows the engine to operate as if it were flying at a lower density altitude. The lowered temperature allows the additional thrust to be created. The nozzels were located at the air inlet and would produse a fine mist to mix with the air so that the P2T2 tube that measures temp/pressure for the engine will now indicate a lower temp than actually exists. On Garret turboprops one would use a chart to determine the thrust available for take-off. For example, at 30 degrees celsius about 80% thrust was specified. By using the CAWI this could be increased to 100%. It was only used when you were pushing max gross weight because you are also weight restriced with the temp and runway limitations. CAWI could be used for 2 or 3 minutes before the water-alchohol was exhausted.
I knew it was a mixture of something and water. Very expensive. A number of years ago I flew the Gulfstream G-1 and with maybe 500 hours in the bird, I used it once or twice. Gave it a nice boost Those were Rolls Royce engines. Don't ask me the nomenclature.