By: Dave Abrahams
Munich, Germany - The safety car for the 2015 MotoGP series, once again, is a BMW M4 Coupé - but this year it has a little extra something.
The standard M4's three-litre turbo straight six delivers an impressive 317kW and 550Nm, but even that's marginal when it comes to keeping up with the world's fastest motorcycles as they accelerate off the start-line of a Grand Prix.
So the petrolheads at BMW's M division went back to the company's roots as a manufacturer of aircraft engines to find a way to raise boost pressure and increase performance without pushing the engine beyond its knock limits - particularly as knock-resistant 98-octane fuel isn't available at every circuit where MotoGP races are run.
The solution was a mechanism used in, among others, the BMW 801 air-cooled 14-cylinder radial engine, which powered the Focke-Wulf 190 fighter aircraft during the Second World War: water injection.
Squirting water into the combustion chamber of a four-stroke engine just before ignition makes it possible to raise the performance limits without overheating. At ignition the water instantly becomes superheated steam, reducing detonation and allowing a significant increase in boost pressure without self-destruction.
COOL RUNNINGS
A fine spray of water is injected into the intake tract, cooling the exhaust gas as it vapourises, lowering the discharge temperature in the combustion chamber and thus reducing detonation - which means you can run the engine with higher boost pressure and more ignition advance.
Blowing cool, moist air into the combustion chamber also increases the density of the intake charge, increasing the oxygen content in the combustion chamber.
When the turbo is running at full capacity, water injection cools the engine's ambient temperature by about 20 degrees, reducing fuel consumption by as much as eight percent - and of course, you can safely use a higher-volume turbocharger to increase boost pressure and liberate a lot more power.
The M4 MotoGP Safety Car has a five-litre tank of water in the boot, supplying three injector nozzles in the plenum chamber (one for each pair of cylinders) at a pressure of about 10 bar, controlled by a suitably modified ECU according to engine revs, load and temperature.
Flat out on the track, say the M guys, the water tank will need to be topped up every time the car is refuelled. In less frenetic road use, they estimate it will need to be filled once every five tankfuls of petrol - and the system has been designed to be entirely maintenance-free, with an eye to using it in on future production models.
The extra boost, and the increased power the M4 Safety Car delivers, put extra strain on the car's cooling system, so the white-coat wizards at M fitted two extra radiators next to the standard unit to balance the temperature control of the high and low temperature circuits, as well as the gearbox and turbocharger.