The M4 GTS system was developed by Bosch but is available to other manufacturers, and despite its use on the six-cylinder S55 M4 engine, Bosch says it’s ideal for three and four-cylinder engine applications. As things stand, virtually all turbocharged petrol cars exploit the cooling effect of injected petrol to help lower combustion chamber temperatures. As the fuel evaporates, the temperature drops (it’s called evaporative cooling).

Water is more effective as it has a ‘high heat of vapourisation’, which means vapourising it consumes a lot of heat. As a result, injecting small amounts of water into the engine moments before injecting the fuel does a good job of cooling the incoming air – better than an intercooler alone can do. And the amount of water is small: Bosch says a few hundred millilitres will last over 60 miles. It does mean that the distilled water must be stored in a tank and replenished periodically, though.

Bosch’s original figures show a 4% fuel consumption saving under Worldwide harmonized Light vehicles Test Cycles (WLTC) protocols and 13% in real driving conditions. To keep it simple, the water is injected into the intake manifold’s plenum chamber rather than directly into the combustion chamber, making it easier to adapt for a wide range of engines. When Bosch offered its system to the wider market in 2016, it didn’t exactly have customers beating a path to the door, but as the screws tighten further on CO2 and toxic emissions, water injection may have a wider role to play.

Why not use hydrogen?

Running existing piston engines on hydrogen instead of petrol has been tried by many manufacturers but none has adopted the idea. Running the ideal (stoichiometric) air/fuel ratio creates a lot of NOx; double the amount of air and the NOx falls to nearly zero, but power is significantly reduced, too. So it’s a case of too much NOx or not enough power.

READ MORE

Under the skin: Why you can always count on ABS

Under the skin: How Tesla is making cars think like humans

Under the skin: How paint is improving EV batteries and autonomous cars