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Liquid fuels are those combustible or energy-generating molecules that can be harnessed to create mechanical energy, usually producing kinetic energy; they also must take the shape of their container. Most liquid fuels, in widespread use, are or derived from fossil fuels; however, there are several types, such as hydrogen fuel (for automotive uses), which are also categorized as a liquid fuel. It is the fumes of Liquid fuels that are flammable instead of the fluid.

This article deals primarily with the concept of liquid fuels in relation to ground transport. However, others such as rocket fuel also play an important role in the economy.

Petroleum

Most liquid fuels used currently are produced from petroleum. The most notable of these is gasoline. Although unproven, it is generally accepted that they formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the Earth's crust.

Gasoline

Gasoline is the most widely used liquid fuel. Gasoline, as it is known in United States and Canada, or petrol in India, Britain, Australia, New Zealand, South Africa and many English-speaking countries, is made of hydrocarbon molecules forming aliphatic compounds, or chains of carbons with hydrogen atoms attached. However, many aromatic compounds (carbon chains forming rings) such as benzene are found naturally in gasoline and cause the health risks associated with prolonged exposure to the fuel.

Production of gasoline is achieved by distillation of crude oil. The desirable liquid is separated from the crude oil in refineries. Crude oil is extracted from the ground in several processes, the most commonly seen may be beam pumps. To create gasoline, petroleum must first be removed from crude oil.

Gasoline itself is actually not burned, but the fumes it creates ignite, causing the remaining liquid to evaporate. Gasoline is extremely volatile and easily combusts, making any leakage extremely dangerous. Gasoline for sale in most countries carries an octane rating. Octane is a measure of the resistance of gasoline to combusting prematurely, known as knocking. The higher the octane rating, the harder it is to burn the fuel, which allows for a higher compression ratio. Engines with a higher compression ratio produce more power (such as in race car engines). However, such engines actually require a higher octane fuel .

Diesel

Conventional diesel is similar to gasoline in that it is a mixture of aliphatic hydrocarbons extracted from petroleum. Diesel may cost more or less than gasoline, but generally costs less to produce because the extraction processes used are simpler. Many countries (particularly in Europe, as well as Canada) also have lower tax rates on diesel fuels.

After distillation, the diesel fraction is normally processed to reduce the amount of sulfur in the fuel. Sulphur causes corrosion in vehicles, acid rain and higher emissions of soot from the tail pipe (exhaust pipe). In Europe, lower sulfur levels than in the United States are legally required. However, recent US legislation will reduce the maximum sulphur content of diesel from 3,000 ppm to 500 ppm by 2007, and 15 ppm by 2010. Similar changes are also underway in Canada, Australia, New Zealand and several Asian countries.

A diesel engine is a type of internal combustion engine which ignites fuel by compressing it (which in turn raises the temperature) as opposed to using an outside source, such as a spark plug.

Kerosene

Kerosene once used in kerosene lamps as an alternative to whale oil, is today mainly used in fuel for jet engines (more technically Avtur, Jet A, Jet A-1, Jet B, JP-4, JP-5, JP-7 or JP-8). One form of the fuel known as RP-1 is burned with liquid oxygen as rocket fuel. These fuel grade kerosenes meet specifications for smoke points and freeze points.

In the mid-20th century, kerosene or "TVO" (Tractor Vaporising Oil) was used as a cheap fuel for tractors. The engine would start on gasoline, then switch over to kerosene once the engine warmed up. A "heat valve" on the manifold would route the exhaust gases around the intake pipe, heating the kerosene to the point where it can be ignited by an electrical spark.

Kerosene is sometimes used as an additive in diesel fuel to prevent gelling or waxing in cold temperatures.

Non-petroleum fossil fuels

When petroleum is not easily available, chemical processes such as the Fischer-Tropsch process can be used to produce liquid fuels from coal and/or natural gas.

Biodiesel

Biodiesel is similar to diesel, but has differences akin to those between petrol and ethanol. For instance, biodiesel has a higher cetane rating (45-60 compared to 45-50 for crude-oil-derived diesel) and it acts as a cleaning agent to get rid of dirt and deposits. It has been argued that it only becomes economically-feasible above oil prices of $80 (£40 or €60 as of late February, 2007) per barrel. This does however depend on locality, economic situation, government stance on biodiesel and a host of other factors- and it has been proven to be viable at much lower costs in some countries. Also, it gives about 10% less energy than ordinary diesel. NOTE: As with alcohols and petrol engines, taking advantage of biodiesel's high cetane rating potentially overcomes the energy deficit compared to ordinary number 2 diesel.

Alcohols

Generally, the term alcohol refers to ethanol, the first organic chemical produced by humans, but any alcohol can be burned as a fuel. Ethanol and methanol are the most common, being sufficiently inexpensive to be useful.

Methanol

Methanol is the lightest and simplest alcohol, produced from the natural gas component methane. Its application is limited due to its toxicity (similar to gasoline). Small amounts are used in some gasolines to increase the octane rating. Methanol-based fuels are used in some race cars and model airplanes.

Methanol is also called methyl alcohol or wood alcohol, the latter because it was formerly produced from the distillation of wood. It is also known by the name methyl hydrate.

Ethanol

Ethanol, also known as grain alcohol or ethyl alcohol, is most commonly used in alcoholic beverages. However, it may also be used as a fuel, most often in combination with gasoline. For the most part, it is used in a 9:1 ratio of gasoline to ethanol to reduce the negative environmental effects of gasoline.

There is increasing interest in the use of a blend of 85% fuel ethanol blended with 15% gasoline. This fuel blend called E85, has a higher fuel octane than most premium gasolines. When used in a modern Flexible fuel vehicle, it delivers more performance to the gasoline it replaces.

Ethanol for use in gasoline and industrial purposes may be called a fossil fuel because it is synthesized from the petroleum product ethylene, which is cheaper than production from fermentation of grains or sugarcane.

Butanol

Butanol is an alcohol which can be used as a fuel in most gasoline internal combustion engines without engine modification. It is typically a product of the fermentation of biomass by the bacterium Clostridium acetobutylicum (also known as the Weizmann organism). This process was first delineated by Chaim Weizmann in 1916 for the production of acetone from starch for making cordite, a smokeless gunpowder.

The advantages of butanol are its high octane rating (over 100) and high energy content, only about 10% lower than gasoline, and subsequently about 50% more energy-dense than ethanol, 100% more so than methanol. Butanol's only major disadvantages are its high flashpoint (95 °F or 35 °C), toxicity (note that toxicity levels exist but are not precisely confirmed), and the fact that the fermentation process for renewable butanol emits a foul odour. The Weizmann organism can only tolerate butanol levels up to 2% or so, compared to 14% for ethanol and yeast. Making butanol from oil produces no such odour, but the limited supply and environmental impact of oil usage defeats the purpose of alternative fuels. The cost of butanol is about $0.57-$0.58 per pound ($1250-$1320 per metric ton or $4 approx. per US gallon). Butanol is much more expensive than ethanol (approx. $1.50 per gallon) and methanol.

On June 20 2006, DuPont and BP announced that they were converting an existing ethanol plant to produce 9 million gallons of butanol per year from sugar beets. DuPont stated a goal of being competitive with oil at $30-$40 per barrel without subsidies, so the price gap with ethanol is narrowing.

Hydrogen

Liquified hydrogen is the liquid state of the element hydrogen. It is a common liquid rocket fuel for rocket applications and can be used as a fuel in an internal combustion engine or fuel cell. Various concept hydrogen vehicles have been lower volumetric energy, the hydrogen volumes needed for combustion are large. Hydrogen was liquefied for the first time by James Dewar in 1898.

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Published in July 2009.

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