Water mist is injected into an engine's supercharger along with the fuel/air mixture from the carburetor. This water/fuel/air cocktail is homogenized as it is pressurized in the supercharger. The mixture becomes heated as it is pressurized and by subsequently passing through hot engine components. This elevated temperature of the mixture eventually causes the water component to change phase from liquid to gas. You may recall from high school chemistry that the water must absorb a tremendous amount of heat energy (latent heat of vaporization) to complete the phase conversion. That heat energy is pulled from the surrounding fuel, air, and engine parts…thus reducing their temperatures. Given that the burning fuel in the combustion chambers of a racing engine can reach about 4500 degrees Fahrenheit, shedding heat is mandatory to prevent total meltdown.


THIS HAND-DRAWN SCHEMATIC ILLUSTRATES THE BASIC LAYOUT OF LAW'S WATER INJECTION SYSTEMS. (IMAGE COURTESY OF PETE LAW AND DAN WHITNEY).
One source notes that when water injection was incorporated on the Pratt & Whitney R-2800 radial engine during WWII, the temperature of the fuel/air mixture entering the engine cylinders dropped from 350-degrees Fahrenheit to just 100-degrees. Such temperature reductions afforded by water injection allow engines to be run at much higher horsepower settings before detonation becomes a concern. Again, the R-2800 provides a convenient example. Although rated to produce 2,000 horsepower, Pratt & Whitney engineers were able to reliably coax 3,800 horsepower out of the engine when using water injection in lab conditions. Even seven decades later, pilots at Reno are pushing R-2800s somewhere north of 3,000 horsepower.

Piston airplane engines do not have a monopoly on water injection. Some jets also use it to reduce engine temperatures when under heavy load. There have even been a handful of production cars with factory-installed water injection systems. However, Law explained that water injection is usually of little value unless you intend to push an engine to its limits.

During wartime, pilots often had reason to push their engines. Water injection was used in those moments where a brief kick in the pants (i.e. War Emergency Power) could get a pilot out of a sticky situation. The tanks containing water injection fluid were usually only large enough for a few minutes of usage.


BY INJECTING A MIXTURE OF WATER AND ALCOHOL ALONG WITH AIR AND FUEL, RACING ENGINES CAN GENERATE SIGNIFICANTLY MORE HORSEPOWER BEFORE EXCESSIVE HEATING AND KNOCKING OCCURS. (PHOTO COURTESY OF MARK KALLIO, PYLON1.ORG)
Water injection systems are often called Anti-Detonation Injection (ADI). While water can be used "neat", it is most effective in airplane engines when cut with an equal volume of methyl alcohol (methanol). When ADI systems were first implemented in US warplanes during WWII, the necessary type of alcohol was not adequately communicated to front line units. Told only to mix alcohol with water, crew chiefs in the field made ADI fluid using alcohol that they had available, the isopropyl alcohol that was also used to de-ice airplanes. That communication oversight resulted in numerous destroyed engines and nearly caused the entire fleet of P-47 Thunderbolt fighters to be grounded. The water side of ADI fluid is not as critical. Law states that Reno tap water works just fine in this application.

All of the radials and V-12s that Law works with today had factory installed ADI systems when they were originally manufactured. Yet, parts for the V-12 ADI systems are tough to find these days. Law relates that this is because most post-WWII military and commercial piston-engined aircraft used radial engines, not V-12s. Consequently, most of the racing ADI systems that Law designs for unlimited racers, whether V-12 or radial-engined, utilize a water injection regulator pulled from an R-2800.


FUEL AND ENGINE OVERHAUL COSTS PREVENT NON-RACING WARBIRDS, SUCH AS THIS GRUMMAN F8F BEARCAT, FROM UTILIZING WATER INJECTION. (PHOTO COURTESY OF LEE RAY)
Many of the same types of aircaft that make up the unlimited racing class can also be seen in private hands as "warbirds". These versions are more historically accurate, but do not usually participate in air racing. Law conveyed that pilots of these airplanes rarely engage their water injection systems. He attributes that trend to the extreme fuel costs associated with running at high power settings, as well as a desire to baby the engines in order to extend their useful life between very expensive overhauls.