Thunderchild

Water injection is an entirely different concept. What we are talking about here is induction icing. Simply put, when pressure drops (induction) so does temperature. This is what happens in the induction pipe attaching the carby to the engine. You may have even noticed water droplets accumulating on the outside of this pipe while the engine is running. This is because of the temperature drop of the charge mixture travelling from the carby to the combustion chamber inside the pipe ans is a good inication of high ambient water content.

When conditons are just right, the pressure drop allows moisture suspended in the air to come out of solution and form droplets. These droplets can then turn to freezing sludge and adhere to the inside of the carby - generally around the butterfly in large engine carbys, but any protruberance will offer a nice "grab hold" for ice to accumualte. If bad enough, the ice will eventually choke off the inlet and deprive the engine of fuel/air. We have carby heat selection in big aircraft where an alternate air source is provided by typically drawing it from a heater muff around the exhaust pipe. This preheats the air and prevents icing. The penalty if used in non-icing conditions is of course a power loss because you are now inducting hot air into the engine.

In big aeroplanes it is generally accepted that you are at risk when there is visible moisture in the air (rain or rain clouds) with the OAT at around plus 5 degrees celcius.