What are the different forms of transportation fuel?

In 2022 per the Energy Information Administration transportation consumed approximately 27% of all energy used in the United States (see Figure 1). The primary transportation energy sources are derived from fossil fuels, and the most common fuels today come from the refining of petroleum (see Figure 2). The refining process starts with crude oil and separates out the different types of hydrocarbons through the process of fractional distillation. See OWOE: How is gasoline manufactured? The three primary hydrocarbon products used for transportation are:

• Gasoline (52% of total US transportation energy sources) - Gasoline is the primary end product from petroleum and made up approximately 52% of all refined petroleum products in the US in 2022. Gasoline is a mixture of hydrocarbons which have 5-12 carbon atoms per molecule. Gasoline is primarily used as motor fuel in cars and light trucks.
• Diesel (Distillate - 22%) - Diesel fuels comprise a mixture of hydrocarbons containing 12 or more carbon atoms per molecule. Diesel is normally produced by blending two or more refinery streams such as light gas oil, heavy gas oil and kerosene. Most freight and delivery trucks, trains, buses, boats, and farm, construction, and military vehicles, and some cars and light trucks have diesel engines.
• Jet Fuel (Kerosene - 12%) - Kerosene is a mix of hydrocarbons with 10 to 18 carbon atoms per molecule. Kerosene is used as fuel for jet engines and tractors.

Less common petroleum-based transportation fuels are:

• Liquid petroleum gas (LPG) - LPG, also simply called propane or butane, is a mixture of light hydrocarbons which are gaseous at normal temperatures and pressures, and which liquefy readily at moderate pressures or reduced temperature. LPG is produced directly from natural gas as well as during the crude oil refining process. It has to be stored in a special tank under pressure. LPG burns more cleanly than gasoline and diesel, and overall greenhouse gas emissions are slightly less than gasoline.
  
  
• Compressed natural gas (CNG) - CNG is made by compressing natural gas (which is mainly composed of methane). It is stored at high pressure. CNG is mainly used for buses and commercial vehicles. CNG powered engines emit significantly fewer pollutants (carbon dioxide, unburned hydrocarbons, carbon monoxide, nitrogen oxides, sulfur oxides and particulate matter) than gasoline engines for the same distance traveled, and overall greenhouse gas emissions are on the order of 30%.
  
  
• Methanol - Methanol is a clear liquid alcohol that can be produced from natural gas, coal, and crude oil. Pure methanol can be mixed with gasoline for use in flexible-fueled vehicles (FFV). Methanol can also be produced from biomass crops such as wood and wood residues. When derived from natural gas using current technology, methanol offers at best only a small greenhouse gas emission benefit over gasoline. When derived from biomass, it can help reduce greenhouse gas emissions by displacing fossil-fuel-based fuel.

Alternative fuels which have been displacing petroleum-based sources in recent years are:

• Ethanol - Ethanol is presently the most widely used alternative fuel in the world. It is mostly produced from crops which contain sugar (e.g. sugar cane or sugar beet), or by processing starch crops (e.g. corn or wheat) or cellulose to produce sugars. The fermentation process uses yeast to convert the sugars into ethanol and CO2. Ethanol is a renewable resource, and in some cases may even be produced from biomass waste material. On an energy equivalent basis CO2 from the ethanol combustion process alone is similar to gasoline. Considering its full life-cycle emissions, taking into account the feedstock and production process, its greenhouse gas emissions can range from 30 - 180% from corn and 0 - 115% from wood as compared to the emissions from the gasoline that is being replaced.
  
  
• Hydrogen - Hydrogen is used in a fuel cell battery to generate electricity for an electric vehicle. There are two common sources for the hydrogen production: 1) hydrolysis of water using electricity, and 2) conversion of hydrocarbons through reaction with steam. For the total environmental effect of hydrogen to be positive, the electricity used for hydrolysis should be generated from renewable sources such as solar, wind or hydro-power, and the source of hydrocarbons should be from biomass. Otherwise, it is not truly a non-fossil source of fuel, and its full life-cycle greenhouse emissions can be comparable to gasoline. (See OWOE:Hydrogen.)
  
  
• Electricity - Electric vehicles in the form of pure electric cars and plug-in hybrids are beginning to make inroads into the automotive market. Depending upon electricity source, these vehicles can contribute to a reduction in greenhouse gas emissions. If they are powered by electricity from renewable sources, they are essentially zero-emission vehicles. However, if they are powered by electricity derived from the burning of coal, they actually emit more full life-cycle greenhouse gases than vehicles powered by gasoline. (See OWOE:How much more environmentally friendly is an all-electric car than a gas powered car?.)

Figure 3 (from Energy.gov) shows the breakdown of US transportation energy use by vehicle, with cars, trucks and motorcycles accounting for 53%.

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