On turbocharged cars water injection can also be used to cool the intake charge and reduce the possibility of detonation by spraying atomized water droplets into the induction system. While some engine tuners to frown upon this practice, it is something worth considering if you're running high boost or experiencing detonation. Water has a very high specific heat capacity, which means it can absorb a lot of heat energy when it changes from one state to another without causing an increase in temperature. The result is a reduction air temperature when water changes from liquid to vapor. For this reason, water injection systems have been around in one form or the other since the 1930s and were also used on a variety of turbocharged aircraft engines that were used during World War II. However, water injection is only required if you're running high boost of more than 12 psi, and should be used in conjunction with a good intercooler rather than as an alternative to an intercooler.
The water injection system basically consists of a plastic storage tank, a water injector, which is similar to a fuel injector and is responsible for atomizing the water droplets, a high pressure pump, a pressure sensor connected to the intake manifold, and an intake air temperature sensor. Calling it water injection is possibly inaccurate as it can either be pure water, preferably distilled water, or a mixture of water and methanol. Either way, atomized liquid is usually injected into the intake system when the intake air temperature is exceeding a certain value and the engine is on boost. It is usually injected into the intake runner three to six inches before the throttle body but can also be injected directly into the combustion chamber. Another option is to have the inject point downstream of the intercooler to assist in pre-cooling, but this can cause condensation and water damming up in the intercooler if the injector does not provide sufficient atomization.
How Water Injection Works
Atomized water rather, that vaporized water is injected into the air intake system as water only boils at 212° F at sea level while temperature of the intake charge would be much lower, having already passed through the intercooler. When water is injected into the air intake system it absorbs quite a bit a heat from the intake charge. When the atomized water, together with the intake charge, enters the combustion chamber, the high temperature of the combustion chamber causes the atomized water droplets to vaporize. During the process of vaporization a large amount of heat energy is absorbed, resulting in anther drop in intake charge temperature! Unfortunately, vaporized water also displaces a large volume that would have been filled with air molecules, but the lowered temperature in the combustion chamber more than makes up for this loss in volume. Indeed, the lowered temperature in the combustion chamber allows us to run higher boost pressures and, consequently, allows us to make more power!
Disadvantages of Water Injection
There are some disadvantages to using a water injection system, some of which can be catastrophic! Firstly, a failure in your water injection system would mean a complete lack of detonation suppression, which could quickly lead to engine failure! To prevent such a scenario, you need a failsafe system that will cut engine power when the intake charge temperature downstream from the water injector reaches a certain threshold. Secondly, the minerals in tap water will quickly clog up the water injector and will result in a failure to deliver the correct amount of water. For this reason you should use distilled water in your water injection system. As with any other system, you need to ensure that only quality hoses and clamps are used in order to ensure the reliability of you water injection system.
Finally, determining the correct amount of water that needs to be injected can be pretty tricky as you do not want high boost pressure with too little detonation suppression! The safest way of reaching the correct amount of water injection for a particular application is to start with a lower boost pressure and slowly increase boost pressure. If detonation occurs, back off immediately and increase the water injection if a greater boost pressure is required.
Water and Methanol Injection
As an alternative to straight water injection, some engine tuners use a mixture of water and methanol. Usually 50% methanol by weight is used. Water and methanol injection provides the same detonation suppression as just water while it also provides the potential for extra horsepower. Methanol is both hygroscopic, which means it absorbs water, and miscible, which means it mixes well with water. It is also much more volatile than water, which means it vaporizes much quicker than water. This means that the methanol will vaporize before the intake charge enters the combustion chamber unless it is injected directly into the combustion chamber. This vaporization reduces the temperature of the intake charge, and once the intake charge reaches the combustion chamber, the atomized water droplets will vaporize to further reduced the temperature of the intake charge in the combustion chamber. But methanol has another advantage; it is also a fuel that effectively increases the octane rating of gasoline, which helps reduce detonation and allows you to increase the compression pressure for an extra bit of horsepower as well.