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Encyclopedia of Marine and Energy Technology

Direct Water Injection (DWI)


The method used for reduction of NOx emissions by the injection of water directly into the combustion chamber via a separate nozzle: 50-60% NOx reduction with some fuel consumption penalty. The key element in the design concept is the combined injection valve through which both fuel and water are injected. One needle in the combined nozzle is used for water injection, and the other one for fuel injection. Water injection starts before fuel injection in order to cool down the combustion space to ensure low NOxformation before fuel ignition. A high pressure water pump is used to generate water pressure of 200 – 400 bar. After filtration and dampening of pressure pulses, the water is fed to the injectors via a pressure regulating valve to provide the correct injection pressure. Water injection timing is electronically controlled and can easily be adjusted from a keyboard. The amount of water injected, i.e. the water/fuel ratio, is controlled by duration of injection. The DWI was an intermediary step used to comply with the Class 1 requirements. Other technologies such as NOx Reducer, exhaust gas recirculation (EGR) and Natural Gas as a marine fuel will need to be utilized in the future to meet the tighter emissions regulatory requirements.