Liquid Fuels For Transportation
Most transportation fuels are liquids, because vehicles usually require high energy density, as occurs in liquids and solids. High power density can be provided most inexpensively by an internal combustion engine; these engines require clean-burning fuels, to keep the engine clean and minimize air pollution.
The fuels that are easiest to burn cleanly are typically liquids and gases. Thus, liquids (and gases that can be stored in liquid form) meet the requirements of being both portable and clean-burning. Also, liquids and gases can be pumped, which means handling is easily mechanized, and thus less laborious.
Read more about this topic: Biofuel
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