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Water Injection: Water Injected In the Scavenge Air (Humid Air Motor)
Water injection methods are not as effective as WiFE in reducing emissions, and have been demonstrated to cause serious engine damage in some cases.
One water injection technique called Humid Air Motor (HAM) injects water in the scavenge air. Water injected into the combustion space will improve burning of the fuel because water will evaporate and transform into steam. With the Humid Air Motor method NOx is claimed to be reduced by as much as 35% and PM reduced by 20% with no increase in CO and HC emissions.
Water can be supplied by separate injectors, for example, mounted in the cylinder cover. This was tested on a 4-stroke medium speed engine, however the results were much less then favourable. The water could not be sprayed and atomized with sufficiently fine particles, resulting in extremely heavy corrosion and erosion in the cylinder covers and piston crowns. As a result, the method was abandoned.
Another technique is to inject water through the air intake of the turbocharger by simply installing a water mist nozzle in the turbocharger compressor side and injecting water continuously or at various intervals depending on the operational pattern. Water spray (mist) will have two effects: to improve the combustion and water mist on the compressor wheel, and will have a cleaning effect and ensure the compressor efficiency does not drop.
However, there is still the danger of engine erosion and damage to components if the water injected forms droplets at any time during the passage through the turbocharger, air manifold and cylinder liner into the combustion space. The compressor vanes will erode if exposed to water droplets over an extended period of time, and the cylinder liner wall and piston rings will wear excessively in cases where the water droplets interrupt the oil film.
Water mist injection through the combustion air is therefore only safe if the water mist is absolutely saturated and there is no chance the water will condense on its way through the scavenge air system including cooler, scavenge air receiver and scavenge air boxes, the so called "under piston space". On large 2-stroke slow speed engines this is virtually impossible.
In the case of 4-stroke medium speed engines the distance traveled from water injection to the combustion space is much less, and thus the risk of water droplets is reduced. However, serious damage from water droplets is still a well known phenomenon, especially on newer high-rated engine types where the scavenge air pressure has been increased to match the higher rating.
The most serious type of damage has been oil film damage in the cylinder liner followed by asymmetrical wear in patches that destroys the piston ring package. A series of damages in the mid 1990s led to a number of retrofits of water mist catchers in existing 4-stroke medium speed engines.
Since it is so critical that water mist does not form droplets it is extremely important that the water spray nozzles are kept absolutely clean and that no calcium forms in the spray holes because this will destroy the atomization. Also, fluctuation in the water pressure and flow rate can have a very negative effect on the quality of atomization. In addition, the water must be distilled or demineralized and thus usually not readily available on board. Industry leader MAN B&W is currently experimenting using Scavenge Air Moistening (SAM) or Humid Air Motor (HAM). The (SAM) systems have multiple high pressure injectors operating at a water pressure of 20 bar and are operating on steam. This system has been installed on one ship only and during SAM mode the high content of steam in the scavenge air resulted in an increased mass flow to the turbine causing a higher speed of the turbocharger, and it was therefore necessary to install a bypass from the exhaust receiver to the turbocharger outlet.
> The History of Water in Fuel Emulsion
> How is WiFE Different from Water Injection?
> Water in Fuel Emulsification (WiFE)
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