As noted in prior posts, diesel engines have had a well established reputation for producing particulates, oxides of nitrogen (an important ingredient in smog), and odor. With the recent requirement for the use of low-sulfur fuel in improved diesel engines for cars and trucks, the emissions have decreased.
Unfortunately, the diesel engines that power ships produce a lot of particulate matter and NOx. Researchers have estimated that the particulates and NOx from diesel ship engines contribute to 60 thousand deaths worldwide from heart and lung disease each year. Since significant progress has been made on controlling land-based sources of NOx, it is estimated that perhaps 30% of the world's emissions of NOx comes from seagoing cargo vessels.
For such sources, the problem is mostly incomplete combustion. Some of the carbon in the long-chain hydrocarbons that are diesel fuel thus end up as soot, rather than CO2, and NOx is produced as nitrogen from the air combines with oxygen in the thermal combustion environment of the engine. Ironically, adding water to the fuel helps to improve combustion. The heat of combustion breaks up the water molecules; the resultant hydrogen atoms help to split the long-chain hydrocarbon molecules, making them more combustible, and the oxygen combines with the carbon, ensuring that more of the carbon is burned. This improvement from the use of water has been known for some years. The difficulty is getting oil and water to mix properly.
Surfactants have been used for some time to maintain and oil-water mixture, but to date they have not worked perfectly. The oil and water still separate, but more slowly. Researchers think that they may have found the answer. Their magic brew is a combination of oleic acid (a fatty acid from vegetable oils) and amines, which are nitrogen containing compounds [see, for example, http://en.wikipedia.org/wiki/Amine]. The result, based on lab testing, is almost the elimination of soot, and an 80% reduction in NOx. The next step is to try it on ships and see if it is as effective as hoped.
The results of the work to date can be found at http://strey.pc.uni-koeln.de/fileadmin/user_upload/Download/ACS_2009_Preprint_Lada_Bemert.pdf.