Nitrogen is separated from oxygen before combustion, thereby reducing the volume of exhaust gas and making the exhaust easier to treat by preventing the formation of polluting nitrogen oxides.

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The oxygen combines with hydrogen for combustion.

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The EPA pulled off the feat by sharply increasing the amount of oxygen-poor exhaust that is recirculated into the combustion chamber and increasing the pressure of the fuel being forced into the cylinder.

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Two other techniques, called gasification and oxy-combustion, work by reacting coal with pure oxygen rather than air, and thus produce exhausts that require little treatment before burial.

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The process takes place in the absence of oxygen, so many of the normal, noxious byproducts of combustion aren't produced.

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The combustion is slow and rich, using just enough oxygen to bust apart the diesel molecules into hydrogen and carbon monoxide without burning off the diesel entirely.

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These devices, which work by combining hydrogen with oxygen from the air to produce electricity, are popular candidates to replace internal-combustion engines in road vehicles.

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