Water injection system 

Water injection systems are only needed in turbo charged engines. The system has been around for a long time since it was already used in some World War II aircraft engines. A water injection system works similarly to a fuel injection system only it injects water instead of fuel. In high pressure turbo charged engines the air/fuel mixture that enters the cylinders can explode prematurely (before the spark plug ignites, effect also known as engine knock) due to the extreme engine environment conditions. This situation results in severe engine damage in the long run (piston piercing). To avoid damaging the engine, water is injected in the combustion chambers in order to provide a water/air/fuel mixture which not only burns more efficiently and avoids spontaneous detonation but also provides additional cylinder and piston cooling. There are mainly three variations of water injection devices. They are dependent on the location of the water injector(s). The first technique consists of injecting water at the entrance of the intake manifold. The second injects water at the exit pipe of the intercooler. The third technique injects water at the entry of the intercooler and  is only used in competition vehicles. In this latter variation most of the in-cylinder detonation prevention is done by injecting additional fuel which is used as coolant (i.e. not burned).

How water injection works

The system, usually, is made up of 3 elements. A water injector (similar to a fuel injector) a high pressure pump (capable of 3 to 4 bar pressure and sometimes even more) and a pressure sensor connected to the inlet manifold. The most advanced systems add 3D cartography similar to what is used in fuel injection systems.

When the pressure in the inlet manifold exceeds a certain, predefined, value the system starts injecting water. The cartographic based devices take into account many more parameters such as inlet air temperature, air/fuel ratio, ...