WASHINGTON — In an effort to clear the air on its requirements for selective catalytic reduction (SCR) engines, the U.S. Environmental Protection Agency has issued new guidance on the miles and hours a truck can run after the DEF tank runs dry.

The original guidance from the EPA in February 2009, required engine performance to be degraded after a truck travelled a certain distance with an empty diesel exhaust fluid (DEF) tank.

As part of its lawsuit against the EPA, Navistar said that provision in the 2010 engine rules was essentially a “licence to pollute” and “pollution for convenience.”

Citing the petition for review filed in the U.S. Court of Appeals, the EPA decided to rewrite the provision and issued a letter to engine manufacturers on Dec. 30, 2009.

“Because some prescriptive language in CISD-09-04 may have led to confusion regarding our intent that the document be used as guidance, rather than setting forth binding requirements, I believe it is appropriate to provide a new document providing revised guidance regarding certification of heavy-duty diesel engines using SCR,” wrote Karl Simon, director of compliance and innovative strategies with the EPA’s Office of Transportation and Air Quality.

In the new guidance, any mention of miles or hours driven on an empty DEF tank is removed. Instead the EPA only suggests that it would likely take a 25 percent reduction in torque for a driver to notice decreased operation.

However, it will be up to the manufacturer to determine how long after a DEF tank runs dry that engine performance begins to decline:

“In determining strategies that are sufficiently onerous to cause the driver to replenish the DEF tank and minimize any adverse emission impact, manufacturers can consider strategies that begin to degrade performance prior to the DEF tank being empty and that progressively become more onerous as the DEF tank becomes empty.”