The U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) recently wrapped up a major study that evaluated the performance of Class 8 hybrid-electric trucks against that of standard diesel models, and the results showed that heavy-duty hybrids yielded fuel-efficiency improvements of nearly 14% and total cost-per-mile reductions of 24%.
However, NREL noted that these improvements don't necessarily come cheap.
‘We see cost as the number one barrier to companies using advanced technologies,’ commented NREL senior project leader Michael Lammert, in a release.
The organization worked with Coca-Cola and its Miami-area fleet to conduct the research, which compared five Freightliner M2106 diesel tractors with five Kenworth T370 hybrid tractors over a 13-month period. NREL also performed dynamometer testing in a laboratory setting.
The Freightliner M2106s were single-axle tractors with Cummins ISC 8.3-liter diesel engines, Eaton Fuller seven-speed manual transmissions and 3.58:1 rear axle gearing. The five Kenworth hybrids used PACCAR PX-6 6.7-liter diesels, Eaton Fuller UltraShift transmissions and the Eaton Parallel Hybrid Electric System with 5.38:1 gearing.
The hybrid tractors' propulsion systems featured 44 kW electric motors and regenerative braking technology, which stores – in a lithium-ion battery pack – energy created during braking and then taps that stored energy to power the electric motor.
In the lab, the hybrid trucks showed fuel economy improvements of up to 30%. In the field, the figure was 13.7%.
In terms of actual cost, the hybrids' fuel cost per mile was 12% less than for comparable diesels. The disparity in overall operating costs for the hybrid vehicles vs. the diesel units was more pronounced: Total cost of operation per mile was 24% lower for the hybrid tractors ($0.74 vs. $0.97 per mile).
The two study groups drove similar duty cycles in the field, with similar kinetic intensity, average speed and stops per mile. The report notes that higher fuel economy results in the lab – when compared to those from the field – showed ‘similar trends along the range of kinetic intensity, average speed and stops per mile.’
‘This means the vehicles could achieve higher in-field fuel economy results if they were used in a more urban location,’ the report states.
The study results suggest that compelling cost savings await heavy-duty fleets that deploy hybrid-electric technologies. The study does not, however, outline the specific costs associated with equipping tractors with such technologies.
The cost differential between hybrid and diesel units depends on a number of factors, but fleet operators can generally expect hybridization to cost in the tens of thousands per unit. However, weighed against total cost-per-mile savings of nearly 25%, that level of investment in hybridization might be worth considering.