Working Paper

Diesel Engines

The technology assessments conducted by the Environmental Protection Agency and the National Highway Traffic Safety Administration to inform the 2017–2025 passenger vehicle fuel economy and greenhouse gas emissions regulations were conducted five years ago. Since then, innovations in vehicle technologies have come rapidly, enabled by computer-aided design tools and electronic engine controls.

Diesel vehicles remain a tiny fraction of the overall U.S. fleet. Despite their small share, they have several features which make them attractive to both automakers and consumers. Several engine and emission control system improvements are leading to lower cost and higher benefit dieselization options. These developments include better turbochargers, engine downsizing and downspeeding, higher pressure fuel lines and more capable injectors, a suite of thermal management and friction-reducing technologies, and reduced-cost exhaust aftertreatment devices. As with gasoline engines, diesel engines may soon benefit from 48V electrical systems and electric boosting (with superchargers or turbochargers).

The combination of improved engines and less expensive aftertreatment will likely lead to advanced diesels costing around $300 less than the rulemaking anticipated in 2025, while providing the same, or greater, reductions in fuel consumption. Adding mild hybridization and electric boosting increases both costs and benefits, at a minor increase in cost per percent fuel consumption reduction.