The car uses what BMW calls “an innovative water injection system,” which sprays a fine mist of H2O into the engine’s collector before it enters the combustion chamber.
According to the automaker, the process lowers the temperature of combustion air, reducing the risk of pre-ignition while increasing performance and efficiency. Technology like this is generally used on turbocharged or supercharged engines because of the high temperatures involved.
While the announcement might set off a WATER + ENGINE = BAD alarm off in your brain, remember that similar systems have been used for years in drag racing, military aviation, and production vehicles like the Saab 99 Turbo and Ford Escort Cosworth RS.
“The cooler supercharged air reduces the engine’s tendency to knock, making it possible to bring the point of ignition forward and thus closer to the optimum value,” BMW says. “This makes the combustion process more effective, whilst at the same time reducing the combustion temperature. On the other hand, cool air has a higher density, which increases the oxygen content in the combustion chamber.”
Aside from enhancing performance, H2O injection can improve the engine’s longevity by lessening thermal strain on the pistons, exhaust valves, catalytic converters, and the turbo itself.
The water is stored in the trunk of the M4 in a five-liter (1.3-gallon), frost-proof tank that’s been fitted with an array of sensors and valves.
How much does the system use? During high-stress racing operation, the water tank must be refilled at the same rate as the fuel tank. When driving normally on the motorway, though, the container will only need replenishing once in every five fuel stops. Make sure to use pure, distilled water though, because tap water could bring some microscopic friends along for the ride.
According to BMW, this technology will eventually make it into a production vehicle. Watch the video below for a computer simulation of how water injection works.