TORONTO, (Dec. 13, 2004) — The battle to reduce diesel engine emissions can be characterized as fighting two enemies at once, where a victory on one front produces a loss of equal magnitude on the other.

The villains in the emissions war are nitrogen oxides (NOx), and soot, or particulate matter (PM). Reducing one in the combustion process produces an increase in the other. The U.S. Environmental Protection Agency wants both reduced dramatically.

Soot is basically incompletely burned fuel along with ash from burned engine oil and sulfur residue from diesel fuel. Maximizing the efficiency of the burn by increasing injection and cylinder pressures will inhibit soot formation, but in doing so the temperature of the combustion event goes up. That increases NOx. To reduce NOx, the temperature of the burn must be lowered, but that produces soot.

The North American strategy is to minimize NOx formation in the combustion chamber. Exhaust soot is reduced by dumping more of it into the oil pan, and then using a filter in the exhaust system to trap the remaining particles.

The European plan of attack is to reduce the formation of soot by optimizing the combustion process, thus lowering NOx, and then treating the exhaust with a nitrogen compound called urea. When injected into the exhaust stream in measured doses, urea (the trade name of the product being distributed in Europe is Adblue) chemically turns NOx into harmless nitrogen gas and water vapor. The process is called Selective Catalytic Reduction, or SCR.

Both strategies achieve the objectives. Some say the European approach is more effective, resulting in less of a drop in fuel economy. They will ultimately need to add EGR (exhaust gas recirculation) to their arsenal (some European engine makers already follow more of a North American approach) to meet the Euro V standards, while we will need to address the NOx standards mandated for 2010 by adding another as-yet-to-be-decided exhaust aftertreatment method — possibly SCR.

The North American path to 2010 seems less clear at the moment. The technology EPA favours — NOx adsorbers — is far from perfect today, but the agency frowns upon SCR, concerned that engines could be run without urea, thus defeating the emissions benefits.

Another problem is the infrastructure to replenish the on-board SCR reagent. But Volvo Powertrain Vice-President of Engine Engineering Tony Greszler said recently that the need to have urea available at every fueling station might not be an issue by 2010. Urea will be needed in such low concentrations by then — likely about 1 per cent of fuel consumed, down from current European rates of 4 per cent — that it would be very easy to tote around a few gallons of urea solution in jugs.

While Volvo Trucks has announced that its North American division will go with cooled exhaust-gas recirculation (EGR) coupled with a diesel particulate filter (DPF) to meet EPA’s stringent 2007 diesel emissions, the company is one of the OEMs leading the charge for SCR adoption in the next round of emissions-cutting.

“We would have preferred to follow our European strategy of using selective catalytic reduction (SCR) using urea injection to reduce NOx while treating the PM challenge in the cylinder, and burning the soot in the combustion process,” Greszler said. “But EPA still has reservations about urea injection, stemming mostly from a distrust of the operator to keep the urea tank filled, and concerns about possible ammonia release in the exhaust gas (ammonia is a by product of the high-temperature injection of urea into the hot exhaust gas stream). SCR has been highly successful in meeting the Euro 4 standards.”

Somewhere between 2007 and 2010, a decision will have to be made regarding the final North American NOx reduction approach: low NOx adsorbers or SCR. But as Pierre Lecoq, senior vice president of global product development for Volvo Powertrain, said recently, “SCR is the only solution on earth today that will meet the new regulations.”