Could anybody let me know if water injection can help to reduce the intake air temperature significantly in NA engines? I heard that it is mostly used in Turbocharged engines.

And if it is useful Practically, is there any way to make a DIY version on a low budget?/filter

Engine: 1.6L, 112 BhP, compression ratio: 10.5.

There is a way to demonstrate to yourself the effects of water vapor on the normal small gasoline engines , used on todays midsized sedans or trucks .
    You will have some positive results, but remember these results will not happen all the time . Like many have mentioned,, Winter months can freeze water and you will not get things going again  until the system  thaws  out .
    Simply install a small Ultrasonic  water mist generator upstream of the intake manifold , behind the sensors and air filter .. They vary in size and this will take some calculations , and/or engineering data to determine the best Mist generator to use . In other words engineering data along with experiamentation will determine how much Distilled Water can be used without hurting the engine . 
     Ultrasonic Miss Generators are Pizeo Electric Transducers ,, typically Steiner and Martin type , (Out of Florida) They have a long life of 3000 or more hours. They atomize Water ,, and it can be installed in such a way to be sucked into the intake manifold .. No re-condensing occurs in the Manifold , due to it is too hot ,a Baff plate or  Mistermat can be used to be sure not to put too much Water in the system ..  
     The idea is to keep the manifold hot enough to super heat the atomized water  , this is easily done in summer months, but not so easy in the extreme colds of the north . Down here in the south ,or Texas  we use Atomized Distilled water, Mixed with ambient Air .
     We do not turn on the switch to atomize the water until temperatures are up in engines , We do not run it on rainy days ,, or Extreme cold conditions .
     A simple level control system Keeps the Ultrasonic Pizeo Electric Transducers covered with about 1.5 inches  of water ,, so this may not work to well in severe inclines .. The system automatically shuts down on a lack of water level .
     Results normally are from 10-20% increase in fuel economy, with a slight increase in Power .. Nothing is changed on the engine , except maybe making sure you have a 195 or better engine coolant thermostat .. A system like this will cost about $70-$100 to manufacture at home ,, and a few hours work .
     Results vary depending upon many factors ,, and we are not PHD engineers , but back yard  Novice inventors .. We simply try and find simple cost effective ways to improve Fuel economy While not destroying our expensive machines ..  Atomized Distilled water cost about 50 cents a gallon , consumption ranges from 1 liter-- 4 liters per hour, on the highway. I have a 2000 Buick Lesabre , and I get ruffly an average of 35 MPG on the highway using atomized Distilled water as an aid to Regular unleaded fuel . I combine the gasoline volume , and Water volume , in liquid gallons , to come up with the correct figure .  
     I am  employed  by Shell Oil, Shell will not give me a Patent release to Patent my device , due to it will hurt their bottom line . ( though Shell Oil has had nothing to do with this invention )  Ultrasonic Mist generators can work to atomize water and increase fuel economy without hurting the engine . But it must be installed at the  right location in the intake manifold . And used under the correct conditions.  WE ( myself and other professional Machinist/technicians ) have done extensive research , along with engineers and are convinced the technology exist to perfect this system . Hopefully real PHD engineers one day will accept Water injection as a simple means to increase Fuel economy . 
     The water droplets are 1 micron in size ,, and quickly split even smaller when heated. Expansion is 1600 times ..  Also we have experimented with fuels using Ultasonics for atomization  , also producing 1 micron or smaller size particals , small enough to pass through Fuel injectors , then the cavitation bubbles collapse ,and aid in mixing better with Oxygen .
    We are just stubborn Technicians , and are still looking to Improve our system .      
         

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