Urea could be the key element in cleaning up diesel exhausts in Europe as new regulations come into effect.

The EU requires that emissions of nitrogen oxides be reduced by 30% for trucks and 50% for diesel cars. By 2008, the regulations will tightened even more in Europe, and in the USA.

Nitrogen oxide emissions from heavy trucks account for 40% of this pollutant from traffic and represent a major environmental concern. Three-way catalytic converters cleaned-up gasoline-powered vehicles considerably, but diesel has until now remained problematic. But not any more. Lund University researchers have turned to urea, the end product of protein metabolism found in urine, and demonstrated that injecting the compound into the exhaust stream as it passes through a catalytic converter initiates a clean-up reaction.

"Urea is converted into ammonia," explains Lund's Ingemar Odenbrand, "this reduces the nitrogen oxide to innocuous nitrogen gas and also reduces the amount of smaller, but harmful, residual particles such as diesel soot, carbon, and hydrocarbons."

The Lund team has also been working with Volvo, Scania, and the catalytic converter manufacturer Johnson Matthey, as well as the Chalmers Institute of Technology in Göteborg to develop a nitrogen oxides storage method that can reduce emissions still further. The method is based on storing nitrogen oxides, NOx, in the catalytic converter and successively reducing them by repeated injections of hydrocarbons, often diesel fuel, every 60 or 90 seconds. The hydrocarbons are broken down into smaller hydrocarbons and carbon monoxide, which then reduces NOx to molecular nitrogen. "In our latest experiments we have managed to reduce emissions from 7 g of NOx per kWh to 3 g," adds Odenbrand.

Using the full-scale exhaust and engine system constructed at Lund, the researchers are fine-tuning the variations in temperature, flow, and consistency that arise in real exhaust fumes with the aim of meeting the US environmental requirements in 2008. These will entail reducing NOx levels to 94% of today's levels. The EU will simultaneously ratchet up its requirements to 2 g, a further reduction from the 3.5 g that comes into effect in the fall of 2005.