The pollution control systems most engine manufacturers used to meet 2010 emission standards have been effective, but technical developments justify tougher guidelines, says the Environmental Protection Agency.

The proposed guidance on selective catalytic reduction systems specifies requirements for warning drivers when they are running low on diesel exhaust fluid. It also proposes slowing the truck down to 5 mph if the DEF tank runs down to empty or the SCR dosing is not working, and it would require the system to identify and respond to poor-quality DEF. And it proposes guidelines for better tamper resistance.

In its discussion, the agency sidelined Navistar International's contention that SCR systems are easily circumvented.

"EPA does not believe Navistar's finding reflect the overall efficacy of SCR systems on heavy-duty diesel engines currently in operation or the way they are actually used," the agency said in its June 7 proposal.

Navistar, the only truck manufacturer that does not use SCR technology to meet the 2010 standard, had challenged the standard on grounds that drivers could get around SCR systems. In a test conducted by a contractor Navistar hired, drivers operated SCR trucks for extended periods without DEF, ran engines with water rather than DEF and manipulated DEF warning systems.

But the agency said that these findings arose from deliberate attempts to circumvent the systems, and that most drivers make the effort to comply.

"While it is possible that drivers could intentionally take such actions … the manner of truck operations conducted in the Navistar study is clearly not representative of the vast majority of truck operations," the agency said.

EPA cited surveys by the California Air Resources Board, American Trucking Associations and Cummins Engine showing that the overwhelming majority of drivers do not wait for warning systems to go off before they refill their DEF tanks. If the warnings did go off, the drivers tended to refill before inducements such as engine slowdown took effect, the agency said.

The evidence shows that the inducements that are now required are working, the agency said. "It appears that manufacturers' past SCR designs and EPA's guidance have resulted in highly effective controls to protect the operation of SCR systems."

Revisions are needed, however, because of new developments in SCR technology, the agency said. New sensors are better able to detect poor quality DEF, for example, and engine manufacturers have been improving their inducement systems.

Specifically, the agency is suggesting that the DEF warning system should have a dashboard or message center light and possibly an audible signal when the level is low, and that the signals should increase in intensity as the DEF tank approaches empty.

The warning system needs to be backed up by an inducement system, the agency said. It is suggesting that when the DEF tank is empty or the SCR system is not dosing properly, the vehicle is slowed down as quickly as safely possible to 5 mph. An alternative would be to slow the engine speed to either shutdown or idle only.

The agency noted that some manufacturers prefer to trigger this inducement only when the truck is stopped. In that instance, the system should include a "severe inducement" such as an engine derate or a speed limitation that makes prolonged operation of the truck unacceptable to the driver.

The proposed guidance also would require a system for detecting poor quality DEF, or the use of some fluid other than DEF, within an hour of it being introduced. Detection should be followed by warnings and inducements.

And the guidance calls for manufacturers to devise ways to prevent tampering, such as disconnecting the fluid level sensor or the dosing valve.

The proposal was published in the June 7 Federal Register. Comments are due by July 7.