Vapour reduces NOx

A system which can reduced NOx by over 70 per cent without resorting to Selective Catalytic Reduction (SCR) is being developed by Swedish- based Munters AB. Called the Humid Air Motor (HAM), the system replaces the charge-air cooler with a device which cools and adds water vapour to the inlet air, taking the humidity up to several times that in the atmosphere.

By adding water vapour the amount of free oxygen in the cylinder is reduced, `hot spots` are prevented and there is a slight reduction in flame temperature – all of which contribute to the overall reduction. With direct injection or emulsified fuel, water is added to the cylinder which removes a greater amount of heat and may slightly impair efficiency.

On smaller engines, heat from the cooling water is used to create the vapour, while with larger engines the exhaust heat will be utilised. Turbo matching and engine timing may require minor adjustments.

Ferry operator Viking Line is to undertake a trial of the system on one of its vessels fitted with Pielstick engines. In preparation, Pielstick undertook testbed trials on a 2MW (PC2-6B) which showed an 80 per cent reduction in NOx while there was no real change in CO, HC or fuel consumption.