Soon to be a no-smoking area

DIESEL engines, with their rough-and-ready, spark-plug-free method of fuel ignition, have a reputation for being smoky and smelly. These days, that is a bit unfair. Fussy consumers and even fussier regulations mean the sophisticated diesels used to propel modern cars are pretty much as clean as their petrol-powered, sparked-up equivalents. But the heavy-duty diesels employed in ships still have a long way to go.

And that matters. Research by James Corbett of the University of Delaware estimates that soot from ships' diesels contributes to 60,000 deaths from heart and lung disease every year. Dirty ships' diesels also produce oxides of nitrogen—the main ingredient of smog. Now that land-based nitrogen-oxide pollution is being cleaned up, about 30% of the world's remaining emissions of the stuff are reckoned to come out of the funnels of seagoing cargo vessels.

The problem is incomplete combustion. The long-chain hydrocarbons used as diesel fuel do not burn as easily as the lighter ones found in petrol. Some of the carbon therefore ends up as soot, rather than carbon dioxide, and oxygen that should be combining with carbon combines with nitrogen from the air instead.

But there may be a way out. Though it sounds bizarre, mixing water into the fuel helps it to burn better. The heat of combustion breaks water molecules up. The resulting hydrogen atoms help to split hydrocarbon molecules, making them more combustible, while the oxygen released goes on to combine with the carbon, ensuring that more of it burns.

All this has, in truth, been known for years. But previous attempts to turn it into practical technology have failed. Reinhard Strey of the University of Cologne is, however, having another go. He thinks he has cracked the main (and obvious) difficulty—that oil and water do not easily mix.

His answer is to use a surfactant. This is a molecule or molecular combination that has different properties on different parts of its surface: in particular, one part prefers to dissolve in water whereas another part prefers oil. Adding a surfactant to a mixture of oil and water therefore binds the mixture together and allows it to form an emulsion of water droplets dispersed in oil. (The dispersants being used against the Gulf of Mexico oil spill work similarly, to create droplets of oil in water.)

That is effective, and has been tried in the past. But it is only part of the trick. Previous attempts to mix water and diesel fuel this way have resulted in emulsions that still separate, albeit more slowly than a surfactant-free mixture would. In practice, fuel can hang around in a tank for a long time, so for the process to be useful, the emulsion has to be stable for ever.

Dr Strey was not put off by these previous failures. He searched for years and eventually lighted on a mixture of oleic acid (a fatty acid found in various vegetable oils) and nitrogen-containing compounds called amines. This mixture dissolves readily in diesel fuel and binds water to it without any need for stirring. The water droplets themselves can be as small as a nanometre (a billionth of a metre) across. That they are so small helps stabilise the emulsion. The result is, in effect, a liquid sponge, and means the mixture can be stored indefinitely, like ordinary diesel, without risk of separation.

The result, when it is burned, is the near-complete abolition of soot, and a reduction of up to 80% in nitrogen-oxide emissions. The surfactant itself also burns without creating emissions beyond water, carbon dioxide and nitrogen.

The next stage is to test the mixture in the real world. MTU, a German engine-builder, is now looking into the matter. If all goes well, though, the days when a smoking funnel was an icon of every child's drawing of a ship on the horizon may be numbered.