Hard-Core BMW M4 GTS to Get Water-Injection System

BMW’s M4 MotoGP safety car has water injection, a far-from-new power-boosting technique

Despite Dorna’s hopes that BMW would field a MotoGP team after withdrawing from World Superbike, the Bavarian company has cannily continued to gain less costly worldwide exposure by supplying MotoGP’s safety car. It leads the field during the sighting lap, then follows the field after the start to guarantee prompt attention to any fallers on the tricky cold-tire opening lap, when riders are especially close to each other.

BMW has announced that the M4 used for this duty will now be equipped with a water-injection system that can allow more sustained yet safe (detonation-free) operation at high boost. The M4’s 3.0-liter inline-6 has twin turbochargers supplying a maximum of 18 psi of boost, which is good for a claimed 430 horsepower. Compressing air this much raises its temperature enough to make detonation more likely, so before being sent to the engine’s intakes, charge air is cooled by passing it through an air-to-engine-coolant heat exchanger (folks like to call this an “intercooler,” which annoys engineers, who prefer “charge air cooler”)..

Even so, prolonged use of high boost (chasing the MotoGP bikes on lap one calls for very high speed) pushes up engine temperature, allowing engine intake air temperature to rise. To prevent this rise in the safety car’s engine, BMW injects water through three nozzles into the intake air plenum. Heat in the air causes the water to evaporate, bringing intake temperature down to a safe level—and maybe also allowing more boost to be used. Car and Driver’s website attributes “production intent” to the system.
Hmm, the R-4360 aircraft piston engines I had in my shop until recently were the -20WA variant, equipped with water injection pumps. Normally, those big engines were enriched 20 percent during take-off, the evaporation of the added extra fuel acting to keep high-boost air temperature down to safe levels. But water, whose evaporation sucks up a lot more heat than can gasoline, did an even better job, pulling intake temp down enough to allow safe use of increased supercharger boost. When such systems were used on fighter aircraft to boost combat power, the water was mixed 50/50 with methyl alcohol to prevent freezing. Either way, evaporative intake cooling could allow more boost to be used without provoking the detonation (engine knock) that would otherwise occur.

In the case of the BMW M4 safety car, 1.3 gallons of water are carried in a trunk-mounted tank.
Water injection has been applied to auto engines before now, but today’s control electronics allow it to be used only when evaporation will be complete, preventing any liquid water from possibly being spattered onto cylinder walls whence it could be swept by the piston rings into the crankcase and oil.

BMW M4 safety car
BMW M4 safety car
BMW M4 safety car
BMW M4 safety car
Engine performance cutaway illustration.

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