How it works: Water injection

Here’s something to think about next time you’re at the pumps: even advanced petrol engines waste roughly a fifth of their fuel. This mostly occurs at high engine speeds, when some of the fuel is used for cooling instead of propulsion.

Thankfully, there is a solution. It may sound crazy, but injecting water into the system makes it possible to reduce fuel consumption by up to 13%, particularly when accelerating quickly or driving on the motorway. Bosch, which has co-developed this technology with BMW (who put it centre stage by integrating it into the BMW M4 MotoGP safety car), also claims that it offers up to a 5% uplift in engine performance and up to a 4% decrease in carbon monoxide emissions.

The fuel economy offered by this technology really comes to the fore in three and four-cylinder downsized engines, as found in most average midsize cars. At present, the most common use of water injection today is in vehicles with high-performance forced induction systems (either turbocharged or supercharged). These engines are commonly tuned with a narrower margin of safety from detonation and hence benefit greatly from the cooling effects of water injection.

The basis of this technology is a simple fact: an engine must not be allowed to overheat. To stop this happening, additional fuel is injected into nearly every spark ignition engine on the road. This fuel evaporates, cooling different parts of the engine block.

With water injection, the engineers at Bosch have exploited the same physical principle. Before the fuel ignites, a fine mist of water is injected into the intake duct. Water’s high vaporisation point means that it provides effective cooling.

For every 100km driven, only a few hundred millilitres of water are necessary. As a result, the compact water tank that supplies the injection system with distilled water will only have to be refilled every few thousand kilometres. If the tank is empty, the engine will still run smoothly, albeit without the higher torque and lower fuel consumption provided by water injection. Plans are also afoot to get cars capturing water from the air so drivers never have to fill the tank.

Admittedly, this isn’t a wholly new idea. A number of manufacturers have trialled forms of water injection in the past. The 1962 Oldsmobile F85, for example, came with the Fluid-Injection Jetfire engine. Oldsmobile even referred to the water/alcohol mixture as “Turbo-Rocket Fluid”. Saab also offered water injection on the Saab 99 Turbo.

With the introduction of the intercooler, interest in water injection waned. However, it now seems to be making a comeback because of its potential role in cutting harmful carbon monoxide emissions from the exhaust.

Industry expert Tom Denton has penned numerous automotive books, including Electric and Hybrid Vehicles. Visit tomdenton.org for more details.

This is an edited extract from IMI's new MotorPro magazine, received free as part of IMI membership