A performance evaluation of Class 8 hybrid electric tractor-trailers compared with similar conventional vehicles by the U.S. Dept. of Energy's National Renewable Energy Laboratory shows significant improvements in fuel economy.

“During our 13-month study, the hybrid tractors demonstrated 13.7% higher fuel economy than the conventional tractors, resulting in a 12% reduction in fuel costs for the hybrids,” said NREL Senior Project Leader Michael Lammert.

For the “Coca-Cola Refreshments Class 8 Diesel Electric Hybrid Tractor Evaluation: 13-Month Final Report,” the NREL team collected and analyzed fuel economy, maintenance and other vehicle performance data on five hybrid and five conventional Class 8 tractor trailers operated by Coca-Cola Refreshments (CCR) for delivery service in the Miami, Fla., area.

NREL has been tracking and evaluating new propulsion systems in transit buses and trucks for more than 10 years. These evaluations are part of the DOE’s Advanced Vehicle Testing Activity, which conducts comprehensive, unbiased evaluations of advanced technology vehicles operating in fleets across the nation.

In addition to field-testing, the NREL team performed dynamometer testing at its Renewable Fuels and Lubricants (ReFUEL) Research Laboratory. During dynamometer testing, the hybrids demonstrated up to a 30% improvement in fuel economy, depending on drive cycle, and up to a 32.1% improvement in ton-miles-per-gallon. The comparable conventional tractors were approximately the same age and operated in similar conditions in the same facility.

“Our analysis identified key variables on trucking routes-—such as idle time, kinetic intensity and average speed-—that, if taken into consideration, can help Coca-Cola Refreshments optimize the use of its hybrid vehicles on routes where they offer the greatest fuel economy benefits,” Lammert said.

The hybrid tractors featured hybrid propulsion systems with 44kW electric motors. The systems included regenerative braking and lithium-ion batteries that provided energy storage. Energy that is normally lost during braking is captured, stored in the batteries and used to power the electric motor, which is helpful on a truck that can weigh as much as 55,000 lbs.

“We see cost as the number one barrier to companies using advanced technologies,” Lammert said. “Testing likes this helps companies understand whether these vehicles are going to save them money over the long run.”