Navistar has started customer shipments of its International DuraStar and WorkStar trucks with the company's 9.3-liter diesels fitted with selective catalytic reduction emissions-aftertreatment equipment.

Internal testing shows the products are delivering up to 8% better fuel economy than the previous engines without SCR, the company said in an announcement.

The engines are the inline six-cylinder N9 and N10, also known as MaxxForce 9 and MaxxForce 10, which share a 9.3-liter (570-cubic-inch) displacement but are split in name for lower and higher ratings.

The first Navistar engine with SCR was the 12.4-liter MaxxForce 13, released in early 2013 with SCR gear and called N13. Navistar is equipping all its diesels with SCR gear from Cummins Emissions Solutions. (Internally, the company designates its SCR diesels with an N, but they're also labeled that way on the InternationalTrucks.com website.)

"Our field test customers are seeing positive results – with our new trucks registering significant fuel economy improvements," said Bill Kozek, Navistar’s president, North America Truck and Parts.

"Our vocational trucks with 9- and 10-liter engines are also experiencing fewer active regenerations while still delivering the durability and power that customers expect from International trucks."

The company says it conducted field tests in extreme environments such as Alaska, Colorado and Arizona. Tests included a variety of vocational duty cycles from garbage packing and utility trucks to dump trucks and sewer pumpers.

"We've run our DuraStar with the 9-liter for around 40,000 miles during the last two months," said Robbie Easley, owner, Easley Trucking, who is quoted in the announcement. "The truck runs between Phoenix and Yuma, Arizona.

"We haul mail and the 9-liter has performed well on the steep hills our trucks climb daily."

The International DuraStar is available with the N9 from 275 to 330 horsepower and 860 to 950 pounds-feet of torque. The WorkStar is available with N9 and N10 diesels from 275 to 350 horsepower and 860 to 1,150 pounds-feet.

Cummins’ ISB6.7 midrange diesel with SCR is also available in DuraStar trucks.