In the technology analysis for the 2025 corporate average fuel economy (CAFE) and greenhouse gas standards, the U.S. Environmental Protection Agency and the National Highway Transportation and Safety Administration projected that naturally aspirated engines would have little or no place in the 2025 vehicle fleet.
But since their analyses were completed in 2012, naturally aspirated engines have improved in terms of fuel efficiency more, and more rapidly, than the agencies projected, thanks to innovative technologies such as high-compression engines with cooled exhaust gas recirculation, improved stop-start systems, and dynamic cylinder deactivation. And the cost of those technologies is tending to fall below the agencies' projections.
This is a significant trend, because while naturally aspirated engines cannot fully match the efficiency gains of downsized turbocharged engines or hybrids and electric vehicles, the recent innovations in more conventional engine designs do provide considerable efficiency benefits that had not been fully factored into the 2017–2025 CAFE standards, and at much lower costs than more advanced technologies.
Manufacturers are using these technologies in varied combinations, with each other and with many other innovations in vehicle design. That heterogeneity makes it impossible to pin a precise number on the contribution naturally aspirated engines are likely to make to overall gains in vehicle efficiency in the next decade. But it is certainly reasonable to expect that future naturally aspirated engines should be able to reduce passenger vehicle fuel consumption by close to 20%.
It seems likely that naturally aspirated engines will offer lower cost technology alternatives for up to a quarter of the U.S. vehicle market in 2025. The effect will be to multiply the practical technology choices available to manufacturers for raising average fuel economy, and bring down the costs of meeting the 2025 CAFE standard.