NASHVILLE, TN – Variable engine-driven accessories offer “relatively modest” fuel economy gains and can be complex, making it difficult for fleets to justify the related investments, the North American Council for Freight Efficiency (NACFE) and Carbon War Room have found.

The findings are included in the research teams’ latest Confidence Report, one in a series of documents that explore existing and emerging fuel-saving technologies.

Engine-driven accessories use about 3-5% of the fuel consumed by a Class 8 tractor-trailer, NACFE says. About 40% of the fuel’s energy is delivered to the powertrain, with the remainder lost in the form of heat through the radiator, exhaust and charge air cooler.

“We’re not talking a ton of fuel here, but it is fuel,” says Michael Roeth, operation lead – trucking efficiency. That said, the group didn’t find enough payback to support the broad adoption of several advanced engine-driven accessories. Not yet. That could change if the manufacturers begin to shift to 48-volt systems or waste heat recovery. The higher voltages, for example, can also lead to smaller wires and smaller motors, and introduce new approaches for everything from air conditioning to steering.

Studied options for the latest report included:

• Variable-speed water pumps that don’t operate at full power all the time
• Cooling fans that run at multiple or variable speeds
• Clutched air compressors that don’t draw on engine power when air tanks are at required levels
• Dual-displacement power steering pumps that account for the fact that linehaul tractors spend most of their time traveling in a straight line
• High-efficiency alternators used to recharge batteries and power electrical loads when the engine is running
• Electrically driven air conditioning compressors
• Smart air dryers
• Electrically driven accessories

Most of these components are still seen as emerging technologies, tested through programs such as the SuperTruck initiative.

Fleets should continue to review the accessories and manufacturers should continue to develop the solutions, he said. And since some will help to reduce Greenhouse Gas emissions, they may be added as standard features and help to improve the payback periods. “However, the savings enabled by improved accessories are unlikely to be a big driving force behind increasing vehicle system voltages,” the report says.

“Uptime is mandatory for fleets, and they have reliability concerns with variable engine-driven accessories,” Roeth added.

NACFE has released 15 confidence reports overall. Fleets that use 30-40 of the 70 technologies that have been studied so far are achieving around 9-9.5 miles per gallon (24 liters per 100 kilometers), he said.

The group is scheduled to update its research into downspeeding on March 22, and will revisit 6×2 liftable axles on May 4. Its annual fleet fuel study will come in August.