The Environmental Protection Agency has updated its guidance for certification of truck engines using selective catalytic reduction to reduce emissions, calling on SCR engine makers to develop warning systems that alert drivers when the truck’s diesel exhaust fluid tank is nearly empty or filled with a liquid other than DEF.

The new guidance, mostly in response to claims made by Navistar Inc. that SCR technology can be circumvented, also urged OEMs using SCR to research methods that would inhibit tampering with SCR system operation and incorporate further inducements for drivers to comply.

Concerns about SCR’s environmental compliance were brought to EPA’s attention by Navistar, which uses a competitive technology, exhaust gas recirculation, to meet EPA 2010 regulations. Navistar had sued both EPA and the California Air Resources Board over their acceptance of SCR technology without stronger measures to prevent engine operation without DEF or an operational SCR system. The truckmaker last year settled both lawsuits by garnering a commitment for further review.

Navistar representatives contended that independent test findings showed new commercial vehicles that must contain liquid urea to meet federal NOx emissions standards continued to operate effectively when urea was not present. At such times, Navistar said, the vehicles threw off levels of NOx as much as 10 times higher than when urea was 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 used SCR. 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. Navistar said one truck tested appeared to operate indefinitely with water and as a result without any functioning SCR NOx control, and that the truck had accumulated more than 13,000 miles with its SCR NOx emissions control turned off.

However, EPA said testing of SCR systems has yielded mostly positive results. While the agency said Navistar’s findings arose from intentional attempts to circumvent the SCR system and that the majority of operations using SCR were in compliance, it encouraged SCR manufacturers to look for methods to impede tampering.