Alternative fuels

Definition: Materials or substances that can be substituted for commonly used fossil fuels

The development of alternatives to fossil fuels (gasoline, diesel, natural gas, and coal) has been spurred by growing awareness of the environmental damage associated with the burning of fossil fuels, as well as by the knowledge that at some time in the future the earth’s supply of fossil fuels will be exhausted.

With the exception of nuclear-powered seagoing vessels, most vehicles are powered by internal combustion engines that use either gasoline or diesel fuel. Gasoline and diesel release significant amounts of greenhouse gases into the atmosphere when burned; these gases include water vapor, carbon dioxide, ozone, nitrous oxide, and methane. These gases absorb and emit radiation in the infrared range; thus they increase the earth’s temperature. In addition to the fact that the internal combustion engine burns an environment-polluting fossil fuel, it also is an inefficient method for transferring the energy stored in the fuel into propulsion. Most of the stored energy is lost in heat, which escapes through the exhaust pipe. In addition, the pistons within the engine accelerate up, stop, accelerate down, and stop with each revolution. This rapid cycle of acceleration and deceleration wastes energy. Many of the alternative fuels available are used to power internal combustion engines and thus have the same limitations as fossil fuels in this regard. Using an alternative fuel such as stored electricity does not have these limitations because it does not produce heat and it propels an electric motor, which rotates (no starting and stopping with each cycle).

Comparisons of the costs and levels of pollutant production of nonfossil fuel sources must take into account the costs associated with production of the fuels. An example is the use of corn for the production of ethanol. Raising the crop requires energy for production, such as fuel for tractors. The corn must then be fermented (yeast converts the sugar in the corn into ethanol), and the fermented product must be distilled (boiled to release the alcohol), which requires fuel to heat the still. The process increases the cost of ethanol and also produces pollution. The electricity that charges an electric vehicle may have been produced by a fossil-fuel source such as a diesel generator. Another problem with alternative fuels lies in the difficulty consumers may have in replenishing their supplies. Facilities distributing gasoline and diesel are prevalent throughout most developed nations; in contrast, sources of alternative fuels such as hydrogen and ammonia are not readily available. The ideal alternative fuel is one that is nonpolluting, cheap to produce, and easy to replenish.

Biofuels

Biofuels are derived from plant sources such as corn, sugarcane, and sugar beets; in some cases, they are blended with a fossil fuel, usually gasoline. Alcohol, methanol, butanol, biodiesel, biogas, and wood gas are all examples of biofuels.

Alcohol was initially used as a fuel in the Ford Model T automobile, which was first produced in 1908. The carburetor (a device that mixes fuel with air prior to entry into the engine) of the Model T could be adjusted to burn gasoline, ethanol, or a mixture. Many modern-day vehicles can run on a mixture of 10–15 percent ethanol and gasoline (E10, or gasohol, is 10 percent alcohol). The fuel known as E85 is a mixture of 85 percent ethanol and 15 percent gasoline; this fuel can be used only in flexible-fuel vehicles (FFVs). FFVs are designed to run on gasoline, E85, or any other gasoline-ethanol mixture. A disadvantage of ethanol is that it has approximately 34 percent less energy per volume than gasoline. Because ethanol has a high octane rating, ethanol-only engines may have relatively high compression ratios, which increases efficiency. In developed nations such as the United States, ethanol blends are available in many areas. Critics of ethanol as an alternative fuel note that it requires a large amount of agricultural land, which is diverted from producing crops used for food; also, the use of crops such as corn for ethanol production drives up food prices.

Methanol can be used as an alternative fuel, but automakers have not yet produced any vehicles that can run on it. Butanol is more similar to gasoline than ethanol and can be used in some engines designed for use with gasoline without modification.

Biodiesel can be manufactured from vegetable oils and animal fats, including recycled restaurant grease. It is slightly more expensive than diesel; however, it is a safe, biodegradable fuel that produces fewer pollutants than diesel. Diesel engines are more efficient than gasoline engines (44 percent versus 25–30 percent efficiency); thus they have better fuel economy than gasoline engines. Some diesel engines can run on 100 percent biodiesel with only minor modifications. Biodiesel can be combined with regular diesel in various concentrations (for example, B2 is 2 percent biodiesel, B5 is 5 percent biodiesel, and B20 is 20 percent biodiesel).

Biogas is produced by the biological breakdown of organic materials—for example, rotting vegetables, plant wastes, and manure produce biogas—and the energy produced varies depending on the source. Biogas can replace compressed natural gas for fueling internal combustion engines. Wood gas is another biofuel that can power an internal combustion engine. It is produced by the incomplete burning of sawdust, wood chips, coal, charcoal, or rubber. Depending on the source, the gas produced varies in energy content and contaminants. Contaminants in wood gas can foul an engine.

Electric Vehicles

At the beginning of the twentieth century, automobiles powered by steam, gasoline, and electricity were available. Electric vehicles were popular into the early 1920s, but then the automotive industry became dominated by gasoline-powered vehicles. The decline in electric-powered vehicles occurred for several reasons: road improvement allowed travel over longer distances, and the range of electric vehicles was limited; fossil fuels became cheap and plentiful; the electric starter replaced the hand crank on gasoline engines, which greatly simplified starting such engines; and mass production of automobiles by Henry Ford’s company made gasoline-powered vehicles much less expensive than electric-powered vehicles ($650 versus $1,750 average price at that time). By the end of the twentieth century, a growing emphasis on environmentally friendly energy sources encouraged the reemergence of electric vehicles and the development of hybrid vehicles powered by both gasoline (or diesel) and electricity.

A hybrid vehicle contains an electric motor that can both propel the vehicle and recharge the battery. Hybrid vehicles have achieved greater popularity than electric-only vehicles, as electric-only vehicles continue to have some of the same basic problems as earlier electric cars: limited range and higher cost than gasoline-powered or hybrid vehicles. Public recharging facilities for electric vehicles remain few and far between; furthermore, recharging takes time. The latest electric and hybrid vehicles use lithium-ion batteries rather than the lead-acid batteries used by earlier versions (and still used in gasoline and diesel vehicles). Lithium-ion batteries are much lighter than lead-acid batteries and can be molded into a variety of shapes to fit available areas. One criticism of electric vehicles, including hybrids, is that many are small and lightweight and thus less safe for passengers, in the case of collisions, than are larger gasoline-powered vehicles.

In 2021, President Joe Biden signed an executive order that directed the federal government to become a net-zero emitter of greenhouse gases by 2050. In order to reach this target, the executive order called on the federal government to fully transition to zero-emission vehicles by 2035.

Other Fuels Derived from Nonfossil Sources

Ammonia has been evaluated for use as an alternative fuel. It can run in either a spark-ignited engine (that is, a gasoline engine) or a diesel engine in which the fuel-air mixture ignites upon compression in the cylinder. Modern gasoline and diesel engines can be readily converted to run on ammonia. Although ammonia is a toxic substance, it is considered no more dangerous than gasoline or liquefied petroleum gas (LPG). Ammonia can be produced by electrical energy and has half the density of gasoline or diesel; thus it can be placed in a vehicle fuel tank in sufficient quantities to allow the vehicle to travel reasonable distances. Another advantage of ammonia is that it produces no harmful emissions; upon combustion, it produces nitrogen and water.

Compressed-air engines are piston engines that use compressed air as fuel. Air-engine-powered vehicles have been produced that have a range comparable to gasoline-powered vehicles. Compressed air is much less expensive than fossil fuels. Ambient heat (normal heat in the environment) naturally warms the cold compressed air upon the air’s release from the storage tank, increasing its efficiency. The only exhaust is cold air, which can be used to cool the interior of the vehicle.

Hydrogen vehicles can be powered by the combustion of hydrogen in the engine much as the typical gasoline engine operates. Fuel cell conversion is another method of using hydrogen; in this type of vehicle, the hydrogen is converted to electricity. The most efficient use of hydrogen to power motor vehicles involves the use of fuel cells and electric motors. Hydrogen reacts with oxygen inside the fuel cells, which produces electricity to power the motors. With either method no harmful emissions are produced, as the spent hydrogen produces only water. Hydrogen is much more expensive than fossil fuels, and it contains significantly less energy on a per-volume basis, meaning that the vehicle’s range is reduced. Experimental fuel cell vehicles have been produced, but such vehicles remain far too expensive for the average consumer.

Liquid nitrogen (LN₂) contains stored energy. Energy is used to liquefy air, then LN₂ is produced by evaporation. When LN₂ warms, nitrogen gas is produced; this gas can power a piston or turbine engine. Nitrogen-powered vehicles have been produced that have ranges comparable to gasoline-powered vehicles; these vehicles can be refueled in a matter of minutes. Nitrogen is an inert gas and makes up about 80 percent of air. It is virtually nonpolluting. Furthermore, it produces more energy than compressed air.

Oxyhydrogen is a mixture of hydrogen and oxygen gases, usually in a 2:1 ratio, the same proportion as water. Oxyhydrogen can fuel internal combustion engines, and, as in hydrogen-fueled engines, no harmful emissions are produced.

Steam was a common method of propulsion for vehicles during the early twentieth century, but, like electricity, it fell into disfavor with the advent of the electric starter, cheap gasoline, and mass production of Ford automobiles. A disadvantage of steam-powered vehicles is the time required to produce the steam. A steam engine is an external combustion engine—that is, the power is produced outside rather than inside the engine. Steam engines are less energy-efficient than gasoline engines. Fuel for steam engines can be derived from fossil fuels or from nonfossil fuel sources.

Alternative Fossil Fuels

Some fossil fuels are less polluting than gasoline or diesel, and some are in plentiful supply. Natural gas vehicles use Compressed natural gas (CNG) or, less commonly, liquefied natural gas (LNG). Internal combustion engines can be readily converted to burn natural gas. Natural gas is 60–90 percent less polluting than gasoline or diesel and produces 30–40 percent less greenhouse gases. Furthermore, it is less expensive than gasoline. Limitations of natural gas vehicles include a lack of available fueling stations and limited space for fuel, given that natural gas must be stored in cylinders, which are commonly located in the vehicle’s trunk.

Liquefied petroleum gas is suitable for fueling internal combustion engines. Like natural gas, LPG is less polluting than gasoline, with 20 percent less carbon dioxide emissions; it is also less expensive. LPG is added to a vehicle’s fuel tank through the use of a specialized filling apparatus; a limitation of LPG is the lack of fueling stations.

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