BMW Continues to Experiment On Water-Injected Turbo Engines

BMW has dropped a water-injected turbocharged petrol engine into a 1-Series Prototype as part of the BMW Group Innovation Days 2015. The water-injection technology finds its way into a relatively small 1.5-litre 3-cylinder unit that produces 218 PS, previously seeing some action in the M4 MotoGP Safety Car, where it drew its supply of water from a large water tank on board the car. In this 1-Series though, no refilling of water is required, as it’s drawn from the condensed water from the air-conditioning system. 

Water-injection sounds a little strange doesn’t it? But it has been proven to increase power figures by up to 10% and lower emissions. The performance achievable by a combustion engine is limited by various factors, including the process temperature in the combustion chamber. If this temperature is exceeded, the result is uncontrolled combustion (knocking) and thus a loss in performance and, in the worst case, expensive damage to the engine.

This is particularly important when the engine is charged, as the intake air is already heated intensely in the turbocharger’s supercharger. An intercooler does ensure that the temperature drops as necessary, but even that has ist physical limits. Depending on design and dimensions of the cooling system and the car’s aerodynamics, the intake air reaches temperatures that are just below the maximum permitted temperatures.

Increasing the supercharging pressure would exceed the knocking limit and is therefore not a viable means for increasing performance. BMW M Division has a solution: injecting a fine spray of water into the collector once more significantly reduces the temperature of the combustion air.

The cooler supercharged air reduces the engine’s tendency to knock, making it possible to bring the point of ignition forward and thus closer to the optimum value. This makes the combustion process more effective, whilst at the same time reducing the combustion temperature. On the other hand, cool air has a higher density which increases the oxygen content in the combustion chamber. This results in a higher mean pressure during the combustion process and in turn optimises performance and torque. Finally, the effective internal cooling of the combustion chamber reduces the thermal strain on numerous performance-related components. This not only prevents damage to pistons, exhaust valves and catalytic converters, but also reduces the strain on the turbocharger, which is subjected to lower exhaust temperatures.

The lower process temperatures also reduce the formation of hazardous substances, in particular nitrogen oxide (NOX). Water injection consequently dramatically improves the effectiveness of the engine.

Using water injection to increase the knocking limit also helps to largely resolve a familiar conflict in objectives when designing powerful engines. Performance and consumption are not least determined by the compression ratio. This also applies, in particular, to highly charged turbo engines like the BMW M TwinPower Turbo, inline six-cylinder. Thanks to a high compression ratio, this engine is highly efficient and boasts low consumption figures, especially in the partial load range. However, the maximum compression ratio is limited by the knocking tendency when fully loaded. Water injection is also hugely beneficial here, as it reduces the tendency for the engine to knock, whilst at the same time increasing the compression ratio. This way, the turbo engine can achieve optimal performance across a wide range of operating points