Disel Engine Nox Reduction

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It has long been known that NOx emissions from diesel engines can be reduced by reducing the peak temperatures of combustion. Dilution of the cylinder charge with inert gases is one method of lowering the peak cylinder temperatures. Nitrogen is an obvious diluent; however until recently its use was limited to stationary laboratory engines.

Compact, high productivity, air separation membranes have recently been developed. These membrane modules provide means for generating nitrogen-enriched air (NEA) at the point of use, for example, under the hood of a diesel truck. NEA offers an attractive, clean alternative to dilution with exhaust gases.
NEA is generated by feeding the cooled, turbocharged air to the bore side of a hollow fiber membrane device. A pressure differential across the wall of the hollow fiber causes oxygen to permeate preferentially through the polymeric wall.

Thus as air flows along the length of the hollow fibers, it becomes slightly depleted of oxygen and enriched in nitrogen. The resulting NEA is fed to the intake manifold of the engine at only slightly lower pressure than the turbocharged air. The oxygen-enriched co-product (OEA) is simply vented to the atmosphere. The effectiveness of the NEA for NOx emissions reduction is also related to the composition of the NEA. Only slight enrichment is needed and NEA compositions in the 80% to 82 % nitrogen range prove to be very effective in NOx reduction.

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