The use of fossil fuels to power engines has been common practice for decades. As different modes of transportation were designed, different fuels were developed to fulfill the requirements of the many types of engines, both on the ground and in the air.
In general, when you consider types of fuel, there is a trade-off between the energy density (BTU value) and the flashpoint. The primary fuel types used in ground transportation are diesel and gasoline. On average, a gallon of diesel produces 130,000 BTU, while a gallon of gasoline produces about 112,000 BTU. A visual comparison shows diesel to be heavier and more oily. BTU values vary based on external temperature.
Historically, diesel produced higher emissions including sulfur, but that is changing due to increased regulation and improvements in the refining process. As of December 1, 2010, all highway fuel sellers in the U.S. are required to sell ultra low sulfur diesel (ULSD). Another distinction in the U.S., diesel is sold dyed and un-dyed. Dyed diesel has a red additive and is intended to identify non-highway fuels used in farming and other off-road activities. Un-dyed highway diesel has much stricter quality standards for contaminants and emissions and is subject to highway taxes.
The flashpoint of a fuel is the point at which it gives off vapor that can be ignited by an open flame. Transportation regulations distinguish fuels as flammable or combustible. Flammable fuels have a flashpoint below 100 degrees F. and combustible fuels have a flashpoint above 100 degrees F. Diesel flashpoints range from about 125 degrees F. to 210 degrees F. The greater stability of diesel makes it safer to transport and handle. Gasoline, by comparison, has a flashpoint of minus 45 degrees F. This is why gas stations all have warning signs about smoking or open flames near gasoline. Sparks and static electricity can easily ignite gasoline fumes.
Aviation fuels fall into two categories, aviation gasoline, commonly shortened as “avgas,” and jet fuel.
Avgas is quite similar to automotive gasoline, except for its octane rating. Avgas must have a lower vapor pressure than automotive gasoline, because it must stay in a liquid state, even at high altitudes. The octane of avgas is supplemented with tetraethyl lead (TEL). Octane is the measure of the fuel’s resistance to pre-ignition, meaning the chance of combustion occurring before the spark. The most common form of avgas has an octane rating of 100 and is dyed blue for easy recognition. Its flashpoint is the same as automotive gasoline, thus there is significant danger of combustion if it is not handled carefully. Avgas produces about 112,000 BTU per gallon.
The most common jet fuels in use are named Jet A (U.S.) and Jet A-1 (international), and they are kerosene grade fuels with a flashpoint of 100 degrees F. Jet fuel produces about 125,000 BTU per gallon. The difference in engine structure of a jet turbine requires higher power, especially at take-off, and the large fuel tanks of modern jetliners could present an elevated fire risk if they stored low flashpoint fuels, thus the choice of a higher energy density and higher flashpoint fuel. Jet fuels are subject to rigorous testing for impurities, and fuels that fail testing are resold for ground applications.
Avgas is sold for many more individual small airplanes, but jet fuel is sold in much larger volumes because the jumbo jets use such large quantities for each flight. All aviation fuels are supplemented with small quantities of additives such as corrosion inhibitors, static dissipaters, anti-freeze, and others.
There are as many different types of fuel as there are engines that use the fuel. Dozens of grades of gasoline, diesel, avgas, and jet fuel are on the market for specific applications. Up until recently, they all shared the characteristic of being petroleum-based, but even this is changing with the introduction of bio-fuels and other alternative fuels. In the future, the specifications of the fuels used on the ground and in the air may be radically different than what is common today.