Efficiency technology potential for heavy-duty diesel vehicles in the United States through 2035

Heavy-duty vehicles in the United States are subject to increasingly stringent fuel efficiency and greenhouse gas standards. Set by the Environmental Protection Agency (EPA) and the National Highway Traffic Safety Administration, the current Phase 2 standards extend through model year 2027. Most heavy-duty vehicles on the road today are powered by diesel-fueled internal combustion engines and no existing national policy requires a change in this status quo.

This analysis considers the potential for technologies to improve the efficiency of internal combustion engine vehicles beyond the Phase 2 standards and out to model year 2035. We focus on two key vehicle segments: Class 8 high-roof sleeper cab tractor trucks and Class 6–7 multipurpose vocational vehicles. Drawing on previous ICCT and EPA analysis, recent literature, and the latest SuperTruck achievements, we simulate efficiency technologies using the EPA’s Greenhous Gas Emissions Model (GEM). We estimate technology costs relative to low-cost compliance with the Phase 2 standard.

For both vehicle segments, engine efficiency improvements deliver the largest efficiency benefits. Low rolling resistance tires, idle reduction technologies, and tractor-trailer aerodynamic improvements are also significant but we assume some of these advances are already deployed to achieve low-cost compliance in model year 2027. For the Class 8 tractor-trailer, we identify cost-effective potential for a 29% increase in fuel economy and a 24% decrease in per ton-mile CO₂ emissions compared to Phase 2 standards, at a marginal cost of $5,200. Adding mild hybridization and transmission-enabled strategies like engine downspeeding and downsizing, which require more than two years to recoup upfront costs, increases per ton-mile efficiency gains by 10 percentage points and brings the marginal cost up to $29,300. However, the efficiency benefits of these more expensive technologies are largely erased if trailer technologies are removed, which lowers fuel economy from 13.8 to 12.1 mpg. For the Class 6–7 vocational vehicle, we identify potential for a 44% increase in fuel economy and a 34% decrease in per ton-mile CO₂ emissions from Phase 2. This technology package has a marginal cost of $15,400 relative to the standard and a cumulative payback period of less than two years.

With the entrance of zero-emission vehicles into the heavy-duty market, future standards may consider the technology potential for both conventional and zero- emission powertrains to reduce greenhouse gas emissions. Given the long lifetime of heavy-duty vehicles, fuel efficiency standards for new diesel vehicles cast a long shadow. Achieving ambitious climate targets requires that the last generations of internal combustion engine vehicles be as fuel-efficient and low-emitting as possible.

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