propulsion power, either together or independently." General Motors has avoided the use of the term "hybrid" when describing its Voltec designs, even after the carmaker revealed that in some cases the combustion engine provided some assist at very high speeds. This contribution from the gas engine is only at high speeds. Normal driving there is no contribution and the car runs all electric. Instead General Motors describes the Volt as an electric vehicle equipped with a "range extending" gasoline-powered internal combustion engine (ICE) as a genset and therefore dubbed the Volt an "Extended Range Electric Vehicle" or E-REV. In a January 2011 interview, the Chevy Volt's Global Chief Engineer, Pamela Fletcher, referred to the Volt as "an electric car with extended range." According to the Society of Automotive Engineers (SAE) definitions, the Volt is a plug-in hybrid vehicle due to the combination of an internal combustion engine and two electric motors, along with a battery that can accept off-board energy. As a plug-in hybrid, the Volt can operate as a purely electric vehicle for the first in charge-depleting mode. When the battery capacity drops below a pre-established threshold from full charge the vehicle enters charge-sustaining mode, and the Volt's control system selects the most efficient combination of two electric motors, one powered by the propulsion system battery pack reserve and one by the combustion generator, to improve performance and boost high-speed efficiency. History Concept vehicle The Chevrolet Volt concept car debuted at the January 2007 North American International Auto Show, becoming the first-ever series plug-in hybrid concept car shown by a major car manufacturer. The Volt concept vehicle had four doors with a rear liftgate and seating for four passengers. This was a significant change in design when compared to the General Motors EV1 of the 1990s, which only seated two to reduce weight and to make the necessary room for the lead-acid battery pack. The top speed was also increased on the Volt, from the electronically limited to . The battery pack size was reduced, from about in volume in the EV1, to in the Volt. General Motors' then-Vice-Chairman for global product development Robert Lutz was looking for a "Prius fighter", a leapfrog product like Apple's iPod. The two-seater sports car being developed by Tesla, the Tesla Roadster (2008), and the rapid advancement of lithium-ion battery technology inspired him to push the carmaker to develop the Volt after the