Detonation, also called 'knocking', can reduce power and cause engine damage - and engines that used forced induction are particularly prone to it.
Consequently, extremely high-performance turbocharged engines can be outfitted with water injection systems to reduce the likelihood of this occurring.
How does water injection work?
If the pressure and temperature in the cylinder rise too quickly, once the combustion stroke has started, detonation can occur. This is when the remaining fuel self-ignites and burns in a quick, uncontrolled fashion. This causes a pronounced rise in cylinder pressure, which can cause damage.
By delivering a controlled amount of water into the engine, however, the chance of detonation is greatly reduced. This is because the water cools the air - or air-fuel mixture - entering the cylinder, helping to bring the temperature down and thus reducing the possibility of detonation.
The water is also converted to steam during combustion, which again helps reduce the chance of detonation - because the process absorbs energy, cutting temperatures.
Water injection systems need not be complicated and usually consist of a small header tank, an electric pump and a power circuit. They can be triggered manually, or by a boost-sensitive switch, but function automatically in most modern set-ups.
The system is most often used in turbocharged petrol cars because these are far more likely to experience excessive cylinder temperatures. It does not increase power on its own, however, but the additional safety margin it grants can be used to run more aggressive boost, ignition or timing settings that then result in higher power outputs. Some modern set-ups may also be used to help control exhaust emissions and consumption.
Can diesels benefit from water injection?
Yes - and it's particularly common in tuned turbocharged diesels, as it can greatly reduce exhaust gas temperatures which helps protect the turbocharger's turbine against damage.
Most, however, use water-methanol injection; the methanol serves as additional fuel to boost or maintain power, while the water helps control the temperatures in the cylinder and exhaust.
A brief history of water injection
Water injection has been used to cool the combustion chambers of engines since the 1900s. It became increasingly prominent during World War II, during which many a turbocharged or supercharged engine benefitted from its detonation-reducing capabilities - although, in many cases, a mixture of water and methanol was used.
Later, water injection was used in jet engines to cut temperatures and increase thrust when required. The first production water injection system for cars appeared in the early 1980s when Saab offered an official upgrade kit for the 99 and 900 Turbo. An earlier anti-detonant injection system was used by Oldsmobile, in 1962, but this used a mixture of water and methanol.
The system has since remained a relatively rare feature, due to the extra cost, complexity and user care required; advances in engine control systems have also made it less necessary. That said, the technology has recently been used in BMW's high-performance M4 GTS.