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New rules on replenishment of diesel exhaust fluid will allow vehicle manufacturers more leeway in equipping cars and trucks with DEF tanks without certifying them individually, according to the Environmental Protection Agency. Truck owners and drivers are not directly affected by the new rules, but must continue to maintain vehicles as before.

One practical result is that trucks might get smaller DEF tanks than they currently have. The special fluid is widely available today so not as much of it must be carried aboard the vehicle.

DEF is sprayed into diesel engines’ hot exhaust to reduce nitrogen oxide through a chemical reaction. It is the active part of selective catalytic reduction used by virtually all builders of on-road diesels, and many makers of off-road diesel engines. Dosing rates are about 2% of fuel use, builders have said, though this varies with the operation and engine size and type. 

The new rules reflect EPA’s greater confidence in availability of the special fluid, said a source within the agency who couldn’t be directly quoted. In 2001, when rules were first published, selective catalytic reduction was a new concept in North America and EPA wasn’t sure DEF would be readily available.

The new rules set tank-size “ratios” of DEF to fuel, which builders can use for any vehicle. This will give manufacturers “more flexibility” in choosing tank sizes than before, the EPA source said.

Vocational trucks that return home daily for fueling can have DEF tanks that need filling whenever the fuel tanks do, which the rules call a 1 to 1 ratio. This might allow trucks to carry a smaller DEF tank, which could save some weight and frame space.

For long-haul trucks the ratio is 2 to 1, meaning their DEF tanks could be filled every other time the fuel tanks are. This might also allow a smaller DEF tank, though truck owners might prefer larger tanks so the fluid refilling task could be done less often.

Truck builders size tanks based on estimated fluid-dosing rates, and have established several tank sizes for various truck classes. These may or may not change, but under the new rules, manufacturers could follow the 1 to 1 or 2 to 1 ratio and be legal.

Truck owners and drivers are not affected, except that they must continue properly maintaining their vehicles so the anti-pollution equipment, including SCR gear, works properly, EPA says. Like now, failure to keep enough fluid in DEF tanks results in warning lights and eventual cutting of power and torque until tanks are replenished.

However, the rules might have more effect on diesel-powered light-duty vehicles, especially cars, because EPA wants replenishing schedules to be shorter than they are becoming, said Allen Schaeffer, executive director at the Diesel Technology Forum. Manufacturers have sized DEF tanks so top-offs could be done at engine oil-change times, when dealers and service shops could do both, and motorists need not be concerned with DEF.

But those intervals are becoming longer, Schaeffer noted, and EPA worries that fluid fills could be delayed. Motorists who drive diesel-powered cars might have to learn about DEF and know when to top off the tanks.

The rules continue exemptions for emergency vehicles, like fire trucks and ambulances, so their diesels can keep operating at full power even if DEF tanks run low. Emergency exemptions for off-road engines, like diesel-powered generators and pumps, also remain in effect.

By Tom Berg