Water injection provides a particularly effective way of raising the knock threshold in this range, allowing the compression ratio to be increased. This makes it possible to optimise the power output of the turbocharged engine over a wide operating range. The lower the octane rating of the fuel, the greater the potential of this technology. In terms of practical implementation, the BMW M division’s engineers opted for an arrangement whereby three water injectors in the intake plenum chamber each supply water to two of the straight-six engine’s cylinders. This solution makes for uniform water distribution and a compact system design.
An underfloor stowage well in the boot of the vehicle houses a five-litre water tank, the water pump, sensors and valves. The pump and all the sensors and actuators are controlled by an expanded engine management system. The pump supplies water to the injectors at a pressure of approximately 10 bar. The injection quantity can be varied depending on load, engine speed and temperature, which helps to keep water consumption to a minimum.
Under hard driving on the track, the water tank has to be topped up every time the vehicle is refuelled. Under normal everyday operating conditions, on the other hand, the intervals are much longer, depending on driving style. Even in fast motorway driving, the water tank only needs topping up at every fifth refuelling stop. Otherwise the system is maintenance-free, for maximum everyday practicality.