In spite of the significant progress that battery technology has experienced in the last 20 years, the battery is still the most challenging technology in the design of hybrid vehicles.

All production hybrid vehicles used batteries employing nickel-metal-hydride (NiMH) chemistry. It is anticipated that the NiMH battery will be replaced by Li-ion batteries in the near future. The acceptability of today’s hybrid vehicles has been shown to be strongly dependent on the price of gasoline, as evidenced by the rapid growth of hybrid sales in 2008, when gasoline prices were high, and the fact that hybrid sales dropped dramatically in early 2009 when prices returned to lower values.
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Light-duty compression-ignition (CI) engines operating on diesel fuels have the highest thermodynamic cycle efficiency of all light-duty engine types. The CI diesel thermodynamic cycle efficiency advantage over the more common SI gasoline engine stems from three major factors: the CI’s use of lean mixtures, its lack of throttling of the intake charge, and its higher compression ratios. In a CI diesel engine-equipped vehicle, there is an additional benefit of reduced volumetric fuel consumption (e.g., gal/100 miles) because diesel fuel provides more energy per gallon than gasoline.

Lean mixtures, whose expansions are thermodynamically more efficient because of their higher ratio of specific heats, are enabled by the CI diesel combustion process. In this process, diesel fuel, which has chemical and physical properties such that it self-ignites readily, is injected into the cylinder late in the compression stroke. Ignition occurs following atomization of the fuel jet into small droplets that vaporize and mix, creating pockets of heterogeneous combustible mixtures.
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