Introducing a small and controlled quantity of water into an engine's intake system, however, can result in significant benefits - primarily by reducing the chance of detonation. This process, also called 'knocking', occurs once combustion has started and when temperatures and pressures in the cylinder get too high. This can cause the remaining fuel to self-ignite and burn in a rapid, uncontrolled fashion that results in a pronounced rise in pressure.
Detonation will reduce power, as it's not an efficient means of burning the mixture, but it can also cause significant damage to the engine's components if left unchecked. Feeding water into the engine's intake manifold, however, cools the incoming charge - which lowers the temperature in the cylinder, reducing the chance of detonation. When combustion starts, the water is converted to steam - a process that absorbs energy - and this further reduces peak temperatures and the occurrence of detonation.
The concept is by no means a recent development, with references to 'internal cooling' by water injection dating back to around 1900. It was then exploited during World War II, granting higher boost levels and reducing the chance of terminal detonation in force-fed aviation engines. It even outlived the introduction of the jet engine, as it was discovered that its use could cut temperatures and boost thrust - by increasing the mass moving through the turbine - when required.
Water injection is also particularly useful in turbocharged cars, where cylinder pressures and temperatures can easily reach excessive levels. The high-performance, twin-turbocharged BMW M4 GTS, unveiled in October 2015, was one such car that benefitted from a water injection system. BMW stated: 'It relaxes the thermal constraints on power and torque, and has already proved itself on race tracks around the world'. BMW also claimed that the M4 GTS was the 'first production road car fitted with such innovative technology'.
The upgrade pack even included a subtle 'S' badge for the rear of your Saab, with the fitting instructions requiring you to 'loosen the 900 turbo emblem and move it a bit to the left' to create the necessary space. Some cars were even fitted with water injection as standard, including the rare 1980 Saab 900 Enduro - an Australian market special edition.
The Swedish solution also dispensed its water in front of the turbocharger and used the compressor wheel to atomise the water before it was ingested by the engine. This caused the compressor to erode over time. BMW avoided this problem by using a series of precise high-pressure injectors that spray a fine mist into the intake after the turbocharger.
The system was much needed, too, as the boosted V8 - destined to later be reinvented in naturally aspirated form as the Rover V8 - packed a then turbo-unfriendly compression ratio of 10.25:1. The 'Turbo-Rocket Fluid' used was effectively a 50/50 mix of distilled water and methanol; the water served primarily to cut temperatures and detonation, while the methanol increased the cooling effect of the mixture, served as additional fuel and protected against freezing.
All of this additional complexity and cost, and myriad engine-related developments, have resulted in water injection remaining relatively uncommon in production automotive applications. That said, aftermarket systems are popular in performance turbocharged petrol and diesel applications - with myriad companies, including Snow Performance, AEM, Aquamist and Nitrous Express offering various water and methanol injection set-ups.