Water is injected as a fine spray into the intake manifold plenum chamber where it evaporates, significantly lowering the temperature of the intake air. This reduces the final compression temperature in the combustion chamber, which also reduces the risk of knock, allowing the turbocharged engine to operate with higher boost pressure and earlier spark timing. The result is increased power and torque, and a substantial improvement in efficiency. Despite the extra power output, thermal stress on all performance-related components is reduced. All of these features ultimately help to diminish wear and prolong engine life. The benefits of water injection can be utilised in various ways, depending on engine and vehicle type. In particular the engineers have considerable latitude when deciding how to balance their priorities between increased power and fuel efficiency. If water injection is taken into account in the design of a high-performance engine right from the start, it is possible to use turbochargers with a higher boost ratio and compression ratio.

If the turbocharger produces peak power at peak rpm, it is possible to increase engine power by approximately eight per cent. At the same time, power losses due to an increase in ambient temperature (> 20° C) can be compensated by increasing the amount of water injected. The output of an internal combustion engine is physically limited not least by the operating temperature in the combustion chamber. If a given operating temperature is exceeded, this will result in uncontrolled combustion (knock), leading to power losses and, in the worst case, to severe engine damage. This is particularly relevant in the case of turbocharged engines, where the intake air is heated in the turbocharger compressor to as much as 160° C. Although intercooling can be used to cool the boost air, the capacity of intercooling systems is physically limited. Depending on the design and size of the system, and the aerodynamics of the vehicle, it is possible to cool the intake air to a temperature below 70° C before it enters the plenum chamber. To raise engine power by increasing boost pressure is not an option as it would mean exceeding the knock threshold.

This is where the BMW M division’s solution comes in: if water is injected in a fine spray into the intake plenum chamber, it is possible to reduce the temperature of the intake air by around 25° C. This further cooling of the boost air makes it possible to advance the spark timing closer to the optimal value. This results in a more efficient combustion process, while at the same time reducing the final combustion temperature. At the same time cool air is more dense, which increases the proportion of oxygen in the combustion mixture and results in a higher mean combustion pressure, leading to optimised power and torque development. Finally, this efficient in-cylinder cooling system also reduces the thermal stress on a range of basic components including not only the pistons, exhaust valves and catalytic converter but also, due to the lower exhaust gas temperatures, the turbocharger. Using water injection to raise the knock threshold also goes a long way towards resolving a fundamental conflict in the design of high-performance engines, caused by the fact that power output and fuel consumption are closely dependent on compression ratio. This is particularly true in the case of highly turbocharged engines like the M TwinPower Turbo six-cylinder in-line engine. Here, a high compression ratio provides high efficiency and low fuel consumption at low and medium throttle. In the full-throttle range, however, the compression ratio is limited by the knock threshold.

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.

On safety grounds, the BMW M water injection system is equipped with a sophisticated self-diagnosis system. If the water tank runs dry, or in the event of a system malfunction, appropriate measures are taken to protect the engine. Boost pressure is reduced and the spark timing is retarded, allowing the engine to continue to operate safely at reduced power. Even when things are working normally, a variety of precautions are taken to keep the system fully functional. Every time the engine is switched off, all the water in the hose system is drained into the tank to prevent system components from icing up in sub-zero temperatures. The water tank is likewise frost-proof.

Further information about the official fuel consumption and the official specific CO2 emissions for new passenger automobiles can be found in the 'New Passenger Vehicle Fuel Consumption and CO2 Emission Guidelines', which are available free of charge at all sales outlets, from Deutsche Automobil Treuhand GmbH (DAT), Hellmuth-Hirth-Str. 1, 73760 Ostfildern, Germany, or under http://www.dat.de/en/offers/publications/guideline-for-fuel-consumption.html. The figures are not based on an individual vehicle and do not constitute part of the product offer; they are provided solely for the purposes of comparison between different vehicle types. CO2 emissions caused by the production and provision of fuel or other energy sources are not taken into account in the determination of CO2 emissions pursuant to Directive 1999/94/EC.