Liquid transportation fuels generally fall into four broad categories:
Gasoline, Diesel, Jet Fuel, Marine Fuel
Each fuel must meet a certain set of specifications to be used. Specifications may be set by industry consortia, independent organizations (such as ASTM or API in the U.S.), or government regulation. These are generally set at a national or regional level, although there is broad consistency globally. The specifications are largely determined by the performance requirements of the engine where they are to be used. Some key differences are:
Gasoline engines are driven by spark ignition (the spark plug). The most important fuel property is the octane number – an empirically determined comparison of the combustion properties of a fuel compared to iso-octane. This indicates the ability of the fuel not to ignite prematurely before the spark timing, as this results in knocking and loss of engine performance. The octane number is highly sensitive to the detailed structure of the molecule. Ethanol, the leading biofuel, has excellent octane. It works as a blend up to 10 percent with gasoline in current vehicles, but is too corrosive to ship via pipeline and can damage engine parts at higher levels.
Diesel engines do not have a spark; rather the fuel-air mixture is ignited by the temperatures and pressures generated by the compression stroke of the engine. Diesel fuel performance is determined by the cetane number. This number tells you how quickly the mixture will combust once ignition occurs; a higher number indicates faster combustion and better engine performance. Just as with octane number, the cetane number depends very strongly on the detailed structure of the molecules. Existing biodiesel works, but because it is produced from vegetable or animal raw material supplies are limited. At large scale it would compete with food crops for agricultural land. Also significant chemical processing to meet diesel specs is necessary and that adds to costs and greenhouse gas emissions.
Jet engines have a constant flame, with fresh fuel injected continuously into the flame zone. Thus, ignition is not a key feature because the flame is continuous. For aviation the most important properties are energy density of the fuel per unit volume and weight, and the ability of the fuel to remain free flowing at the very low temperatures found at high altitudes. Energy density is a key because the airplane can only carry a fixed volume of fuel, and can also only carry so much weight. Most easily produced biofuels are problematic for aviation because they either contain oxygen, which adds weight and reduces energy content compared to a hydrocarbon, or their freezing point is too high. Because of its unique requirements, there is no practical alternative to liquid hydrocarbon fuels.
Marine fuel comes from the heaviest part of the oil barrel, and users are generally driven by low cost. The most common marine oils are only somewhat lighter than bitumen (used to make asphalt), and have to be heated in order to flow. Currently no technology developers are targeting the civilian marine fuel market. The U.S. Navy is exploring the potential.