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NREL study finds Class 8 hybrid trucks delivered 13.7% higher fuel economy than conventional diesels in comparable usage

Laboratory and in-use fuel economy comparison. Source: NREL. Click to enlarge.

A performance evaluation of Class 8 hybrid electric tractor trailers (Kenworth T370 hybrid tractors) compared with similar conventional vehicles (Freightliner M2106 standard diesel tractors) by the US Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) found that hybrid tractors demonstrated 13.7% higher fuel economy than the conventional tractors, resulting in a 12% reduction in fuel costs for the hybrids.

The study also highlighted the importance of route selection and vehicle placement to optimize hybrid advantage—i.e., the hybrid advantage is optimized in routes which include high start-stop density.

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 parallel hybrid-electric diesel tractor trailers and five conventional Class 8 tractor trailers operated by Coca-Cola Refreshments (CCR) for delivery service in the Miami, Fla., area.

Eaton hybrid system schematic. Click to enlarge.

A random dispatch system ensured the vehicles are used in a similar manner. Route and drive cycle analysis showed that both study groups drove similar duty cycles with similar kinetic intensity (KI) (0.95 vs. 0.69), average speed (20.6 vs. 24.3 mph), and stops per mile (1.9 vs. 1.5).

The hybrid group accumulated 27% fewer miles than the diesel group during the study, although the hybrids were driving a comparable number of miles per operational day. The discrepancy primarily stems from non-hybrid-related down- time experienced by two hybrid trucks during the first six months of the study.

The team found that hybrid fuel costs per mile were 12% less than for the diesels. Hybrid vehicle total cost of operation per mile was 24% less than the cost of operation for the diesel group ($0.74 vs. $0.97 per mile)—i.e.,. the customer is realizing real savings with the hybrid.

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.

—NREL Senior Project Leader Michael Lammert

The hybrid tractors featured the Eaton parallel hybrid propulsion systems with 44 kW peak, 26 kW continuous electric motors working with a Fuller medium-duty automated manual 6-speed transmission. The systems included regenerative braking and a 1.8 kWh, 340 VDC Li-ion battery pack 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 pounds.

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 US Department of Energy’s Advanced Vehicle Testing Activity, which conducts comprehensive, unbiased evaluations of advanced technology vehicles operating in fleets across the nation.

NREL is the US Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for DOE by The Alliance for Sustainable Energy, LLC.




This may not be enough to fully offset the higher initial cost.

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