Fuel consumption of new heavy-duty vehicles could be reduced by more than a third by 2025, according to the American Council for an Energy-Efficient Economy, if the next phase of greenhouse gas/fuel economy standards pushes improvements in areas such as trailer aerodynamics and waste-heat energy recovery.

Heavy-duty vehicles consume 2.9 million barrels per day of petroleum fuels in the United States today, ACEEE notes. In 2011, the National Highway Traffic Safety Administration and the Environmental Protection Agency adopted standards to reduce the fuel consumption and greenhouse gas emissions of heavy-duty vehicles in model years 2014–2018. These "phase 1" standards will reduce new heavy-duty vehicle fuel consumption by 15%, on average, producing savings of half a million barrels of oil per day by 2035.

The agencies are now working on the next phase of the standards, which will apply to vehicles in later model years. A proposed rule is not expected until late 2014, but ACEEE outlines its take on the opportunities in a new fact sheet, "Further Fuel Efficiency Gains for Heavy-Duty Vehicles."

The first phase, going into effect with the 2014 model year, took a component-by-component approach rather than a full-vehicle approach, and as a result missed major fuel efficiency opportunities, ACEEE contends. For instance, they did not capture potential gains from advanced transmissions and hybrid technologies, and excluded trailers. The standards for “vocational vehicles” (for example, delivery trucks, refuse trucks, and buses) reflected improvements in engines and tires only.

For the next phase, ACEEE says the agencies should treat vehicles as systems, rather than as collections of components, in setting the standards. That will help spur the development of better transmissions, driveline efficiency, powertrain integration, and hybridization. Aerodynamic trailers should be integrated with tractors. The standards should draw into the market advanced technologies such as the engine ’bottoming cycle,’ which has been  demonstrated in the Department of Energy’s SuperTruck program, and hybrids should be integral to the vocational vehicle technology package.

For instance, the paper notes, heavy-duty tractor-trailers could see a 21% to 34% improvement. Engine downsizing and reusing waste heat energy through a “bottoming cycle” will provide large fuel savings \. A bottoming cycle converts heat energy captured from the exhaust gas recirculation (EGR) loop and the exhaust stream into mechanical energy, which is then either fed back to the shaft for mechanical power or converted to electricity and used for electrical loads. Major savings are available from further improvements to aerodynamics and tires, especially on trailers and through the integration of tractor and trailer.

The ACEEE analysis, which draws from a 2010 National Academy of Sciences report and the agencies’ work on the phase 1 rule, shows that for new heavy-duty vehicles as a whole, strong Phase 2 standards could reduce fuel consumption to 26% below phase 1 levels, which results in a 37% overall reduction from pre-phase 1 levels.