The U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) have presented preliminary proposals aimed at the alleged compliance loopholes found in current 2010 liquid-based selective catalytic reduction (SCR) systems on diesel engines. SCR technology doses engine exhaust with a urea-based diesel exhaust fluid (DEF). The resulting chemical reaction reduces emissions of oxides of nitrogen (NOx) pollutants to levels mandated by the EPA’s 2010 diesel emissions regulations.

Concerns about SCR’s environmental compliance were brought to CARB and EPA’s attention by truck and engine manufacturer Navistar International, which uses a competitive technology, exhaust gas recirculation (EGR) to meet EPA 2010 regulations. Navistar had sued both agencies over their acceptance of SCR technology without stronger measures to prevent engine operation without DEF or an operational SCR system. The truck maker settled both lawsuits by garnering a commitment for further review.

At an air resources workshop in, El Monte, CA, Navistar representatives contended that independent test findings show new commercial vehicles that must contain liquid urea to meet federal NOx emissions standards continue to operate effectively when urea is not present. At such times, Navistar said, the vehicles throw off levels of NOx as much as 10 times higher or more than when urea is present.

The research cited by Navistar was conducted by EnSIGHT, an independent environmental consulting firm, using two long-haul vehicles and one heavy-duty pickup, all of which use Selective Catalytic Reduction (SCR) technology that relies on liquid urea to clean up NOx emissions after they leave the engine.

According to Navistar, EnSIGHT’s research showed that when liquid urea was not present, there was little or no effect on the vehicles’ operations. This included long periods of time when the vehicles’ urea tanks were empty or were refilled with water instead of urea. One truck tested appears to operate indefinitely with water and as a result without any functioning SCR NOx control. That truck has accumulated more than 13,000 miles with its SCR NOx emission control turned off.

“Navistar first identified these loopholes to the agencies and also presented our concerns at today’s workshop,” said Jack Allen, president of Navistar’s North American truck group. “We will be working with the EPA and CARB to ensure full environmental compliance.”

“While others are saying there are ‘loopholes’ in the current 2010 regulations, we don’t agree with that terminology at all,” said Annette Hebert, Chief of Mobile Source Operations Division for CARB. “What we are doing is looking at tightening up SCR certification requirements and clarifying the guidelines so that manufacturers know exactly how to meet them.”