Diesel compression-ignition engines. Diesel CI engines offer about a 20–25 percent fuel consumption benefit over gasoline SI engines (when adjusted for the energy density of diesel fuel). There are opportunities for further efficiency improvements that could reduce the fuel consumption of new diesel-engine vehicles relative to current diesel vehicles by about 10 percent by 2020 and an additional 10–15 percent by 2030. New technologies are emerging for after-treatment that reduce emissions of particulate matter and nitrogen oxides to levels comparable to those of SI engines. The primary challenges for diesel engines in the United States are the added costs and fuel penalties (of about 3–6 percent) associated with those after-treatment systems (Bandivadekar et al., 2008; Johnson, 2008, 2009; Ricardo, Inc., 2008).
Gasoline hybrid-electric vehicles. HEVs combine an internal-combustion engine (ICE) with a battery-electric motor/generator system. Their primary efficiency benefits derive from smaller engines, regenerative braking, elimination of idling, and optimization of engine operating conditions. Hybrid vehicles span a range of technologies and fuel-economy levels. Diesel HEVs are also under development.
Plug-in hybrid-electric vehicles. PHEVs have larger batteries than regular hybrids do, and they can be recharged from an external source of electricity. They also require a larger electric motor and higher-capacity power electronics. Hybrid vehicles, including PHEVs, are designed to allow all-electric operation powered by the battery. The driving range with all-electric power depends on factors such as the size of the battery, the weight of the vehicle, and the driving cycle. Unlike a hybrid-electric vehicle, a PHEV’s external power connection can recharge the battery when the vehicle is at rest and plugged in; the internal combustion engine can also recharge the battery, provide power to the wheels, or both, extending vehicle range. The capacity of the battery and the distance of the trip(s) determine gasoline savings; current hybrids in commercial production have a range of less than 10 miles on all-electric power. The vast majority of U.S. vehicles are driven less than 40–60 miles per day in normal operation. Thus, a battery that can power the vehicle for 40 to 60 miles could substantially reduce petroleum consumption for this duty cycle. Commercial PHEVs with a variety of ranges on all-electric power will likely be introduced to the U.S. market over the next 5 years. However, success