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Water Injection

 - Used to stop detonation and allow more aggressive ignition advance.
 - Used extensively in competition engines.
- Requires careful mapping.

Water injection was first used on tractors and industrial engines in the 1920s to avoid detonation. It received widespread use during WW2 in American war planes and has been employed ever since for engines which need to operate at the peak of power and efficiency. When the speed at which the fuel burns, or the speed at which the cylinder pressure rises, gets out of control the fuel mixture will explode uncontrollably causing detonation. Mild detonation will destroy head gasgets and wild detonation will quickly  destroy pistons, conrods and cylinder walls, bearings, cranks and everything else in the engine !!
However, in order to extract max power and economy from an engine, we must find a balance as close as possible to the point of detonation, without crossing the line. To eliminate detonation we must limit combustion speed and pressure rise in as gradual a way as possible so as to preserve max power and economy. One way to so this is to improve the knock resistance of the fuel but this is rarely possible for road use.   This leaves us with the option to either cut down on power and economy (no thanks) or to use some form of water injection.
Water injection isn’t about dumping water into the cylinders. If power is important then water flow and atomisation must be carefully controlled to provide the ideal quantity and delivery to stop detonation. Installing a cheap and crude water injection system into a car is the same as choosing carbs over fuel injection. You should never install any system based on substandard parts like windscreen washer pumps and nozzles operated by some kind of pressure switch. At best this kind of system will only prevent detonation.
It won’t promote more power or economy and will probably reduce power and contribute to premature cylinder wall ware by  contaminating the oil supply. An entry level water injection system should be electronically controlled with a variable pump controlled according to boost and rpm inputs as a minimum. The knock control should be altered or removed to stop spark retard or boost limiting. A flow meter for water and fuel is also helpful to maintain a ratio of 35% of water to fuel flow (by weight). In volume terms this is 28%.

The higher the intake temp and the lower the octane rating then the more water that will be required to maintain spark advance (and therefore power and economy).
On a normal road engine running 95octane fuel you should see a 5% increase in power and a 15deg drop in intake temps. Obviously if you have much higher intake temps than normal(eg.100deg) and worse fuel, you will experience even more gains by using water injection.   When we inject water it gets converted to steam, which pulls heat out of the combustion process, slowing the rate of pressure rise. These steam particles separate the fuel and oxygen molecules which further slows combustion and pressure rise. As this is happening, the piston is continuing to goto TDC, squeezing the gases tighter but with less heat in the chamber, detonation is avoided. As the piston descends a controlled burn occurs.

Proper Water Injection Systems.
 - Expensive and must be mapped carefully on the dyno.

Sophisticated water injection systems can be hard to justify because of the high cost involved. But for max horsepower and max engine protection from detonation then a top of the line water injection system is necessary. Such systems employ water nozzles in each inlet port and a high pressure pump delivering water at 90psi to maximize atomisation. A fully programmable ECU must also be used to map the system according to boost and rpm load, compressor discharge temp and engine temp, turbo temp and overboost.

Nozzle Location.
 - Equal water distribution and vaporisation is essential.

It is important to equally distribute the water haze across the chamber. This is why top end systems employ one nozzle per cylinder… but you can get close with one or two nozzles. If we are using an intercooler the nozzles are best located in the cooler outlet pipe. The turbulence here will help to disperse the water uniformly in the charge air. This also leaves plenty of time for proper mixing before the inlet. If the charge air is still hot after the intercooler it will help in vaporising the water (which will draw heat out ot the charge air early).

In non-intercooled systems, nozzle position varies depending on the type or blower or turbo being used. For centrifugal blowers and all turbos put the nozzle in the compressor outlet. With other types of blowers its better to inject into the blower intake (if excessive amounts of water are used then the blower will be damaged). However, the big gaps in roots blowers between the rotors and the case is reduced when water injection is used so we have an increase in boost and a drop in charge temperatures. Too much water will tighten the gap too much and put excessive pressure on the rotors.

Water and Alcohol Mix.
 - Used a lot in street-racing and sometimes in competition.. not good for the engine

Some people advocate the use of a 50/50 water/alcohol mix. The alcohol adds a fuel value to the mix and aids further in dispersal and atomisation. However, it leaves a powdery residue on aluminum and  causes corrosion of piston ring lands, the rings themselves and the bore walls by washing away oil.

Water and Methanol Mix.
 - Much better than alcohol, use between 30 and 50% mix of methanol.
 - Can be very poisonous, don’t allow skin exposure.

Another option is to use a 50/50 water/methanol mix. This has similar benefits as alcohol without the downsides. Better results can sometimes be got by using a 70/30 mix. But methanol is very poisonous and can cause blindness and cancer and eventually death if its not handled with respect. Never allow skin exposure, never inhale methanol fumes etc…

Toluol Mix.
 - Too expensive and rare.

Toluol can be used instead of water and has excellent resistance to detonation. But is mega expensive.