Liquid fuels are those combustible or energy-generating molecules which can be harnessed to create mechanical energy, which in turn usually produces kinetic energy, and which also must take the shape of their container. Most liquid fuels in widespread use also fall under the category of fossil fuels; however, there are several types, including next-generation hydrogen-fuel test cars that utilize liquid fuels which do not fall under this category, as well as ethanol.
This article deals primarily with the concept of liquid fuels in relation to ground transport. However, the usefulness of liquid fuels in the development of rocketry cannot be overlooked.
Main Article: Fossil Fuel
Fossil fuels which are also liquid fuels come from dead animals and plants which died many millions of years ago. The most notable of these is gasoline.
Main Article: Petrol
Gasoline is the most widely used liquid fuel. Gasoline (known as petrol in Britain, Canada, Australia, New Zealand, 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. Oil is extremely volatile and easily combusts, making any leakage extremely dangerous. Oil 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 octance fuel.
Main Article: Diesel
Diesel is similar to gasoline in that it is a hydrocarbon aliphatic compound. Diesel is also extracted from petroleum. Diesel may cost more or less than gasoline, but generally is cheaper because the extraction process is simpler and requires less time and money in refineries. Many countries (particularly in Europe) also have lower tax rates on diesel fuels. Diesel fuel must, however, be purified by reducing the amount of sulfur which causes such phenomena as acid rain and higher emissions of soot from the tail pipe (exhaust pipe). Diesel fuel in Europe, due to legal conditions, is generally much lower in sulfur than diesel produced in the United States. However, on May 11, President George Bush signed into law a bill that forces diesel, which now contains a maximum of 3,000 ppm sulfur to reduce to 500 ppm by 2007, and 15 ppm by 2010. Similar changes are also underway in Australia, New Zealand and several Asian countries.
Diesel fuel, unlike gasoline, is only for use in diesel engines. 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.
Main Article: Ethanol, alcohol as a fuel
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.
Ethanol for use in gasoline and industrial purposes may be called a fossil fuel because it is separated from the petroleum product ethylene, which is cheaper than production from fermentation of grains or sugarcane.
Main Article: Methanol
Methanol is produced from the natural gas byproduct methane. Its main application is limited due to the weighty environmental impacts that come with it. Small amounts are used in some gasolines to make them burn less readily (the same idea behind octane levels). Methanol is also called wood alcohol, because it is like ethanol and alcohol and may be created through wood (though this is not a general practice for commercial production). Methanol fuels are used mainly in some race cars and model airplanes.
Main Article: Hydrogen car
Hydrogen as a fuel is a feasible option for future use as a fuel. Liquid hydrogen is an important consideration because it has a higher density than its gaseous counterpart. Liquid hydrogen would be stored in cryogenic tanks. Its application would be most useful in fuel cells where hydrogen would react with oxygen (obviously this is readily available in the air) to create electricity which would power the vehicle.
Unfortunately, widespread use of liquid hydrogen is several decades away. Their application is plagued with several serious problems including production, which may still involve fossil fuels, durability of the fuel cells to common roadway conditions such as bumps, and existing cars cannot easily be converted to run on hydrogen fuel. For a more detailed explanation see the Hydrogen car article.