From the advent of the modern automobile until the beginning of the 20th century, almost every such contraption ran on the same basic fuel. From the most no-frills compact car to the most imposing "18-wheeler" or "semi" tractor-trailer on the U.S. Interstate highway system, motor vehicles have been overwhelmingly powered by fossil fuels – mostly gasoline and diesel fuel, which are both types of petroleum.

This has mainly been an issue of economics; alternatives to traditional fuels to power the world's transportation economy have been around for longer than cars and most other familiar machines and pieces of equipment, but gas, despite constant media chatter about its price, has traditionally been very cheap compared to other options.

In the first decades of the 2000s, an earnest quest for renewable energy sources was prompted by increasing evidence that the effects of anthropogenic (i.e., human-caused) climate change are expected to be more severe, and strike some areas sooner, than originally anticipated. As a result, ethanol has emerged as one of the most popular kinds of biofuels.

Biofuels are fuels derived from living things. Fossil fuels are ultimately derived from things that were living in prehistoric times, but biofuels are made from things that are alive right now. When living things die, their physical remains fall into the realm of so-called "biomatter" or "biomass." Because this mass comes from living things, it is rich in carbon, like fossil fuels. But because of how biofuels are utilized, their impact on the environment is minimal.

Biofuels can come from both plant and animal sources, with most in the U.S. derived from crops used for other purposes (e.g., corn and sugar cane). In general, biofuels make use of chemical process (e.g., fermentation ) as well as physical processes (e.g., heat) to break down the starches, sugars and other molecules in plants. The resulting products are then refined to produce a fuel that cars or other vehicles can use.

In addition to ethanol, which complements regular gasoline, biodiesel is available as an alternative to regular diesel fuel. Biodiesel is made from both animal sources, such as cooking grease, and plant sources, such as vegetable oil.

Ethanol, also called ethyl alcohol, has the chemical formula C₂H₅O, often written CH₃CH₂OH to offer more information about its physical structure. It is the simple, symmetrical hydrocarbon ethane (C₂H₆, or CH₃CH₃) with a hydroxyl (–OH) group at one end in place of one of the three hydrogen atoms (–H).

The advantages of using ethanol as a fuel include reducing foreign dependence on fuel by cutting back on the total amount of petroleum used via substitution; emissions from combustion that are less hazardous to the environment; job creation in rural areas rich in farmland; and the absence of a need for any special kind of fueling equipment.

Among the disadvantages of using ethanol as a source of fuel are its low fuel economy (i.e., you get fewer miles to the gallon). This is presently the chief limitation on its use. Also, many fuel stations in the U.S. are not set up for regular ethanol users (just as electric car charging stations remained comparatively, if not prohibitively, rare as of 2019).

• Existing disadvantages of ethanol and biodiesel are expected to be ironed out as more companies invest in the expansion of renewables.

The two primary types of ethanol sold in the U.S. as of 2019 were almost mirror images of other in terms of their content. One is E10, which is 10 percent ethanol and 90 percent ordinary gasoline, while the other, E85, flips the ratio in favor of a heavy preponderance of ethanol. Only certain kinds of cars have engines that are capable of running on a fuel that contains so little traditional gasoline, so this type of ethanol is often more clearly marked.

Cellulosic ethanol is made from parts of the plant that are ordinarily discarded. Cellulose is a kind of starch that humans cannot digest and offers solidity to a variety of plants, but it is no longer being ignored as a part of plants humans can't benefit from using.