Since the primary function of aviation turbine fuel (jet fuel) is to power an aircraft, energy content and combustion quality are key fuel performance properties. Other significant performance properties are stability, lubricity, fluidity, volatility, non-corrosivity, and cleanliness. Besides providing a source of energy, fuel is also used as a hydraulic fluid in engine control systems and as a coolant for certain fuel system components.
An aircraft turbine engine generates power by converting chemical energy stored in the fuel into a combination of mechanical energy and heat. Since space is at a premium in most aircraft, the amount of energy contained in a given quantity of fuel is important. The energy content of jet fuel can be measured: It is the heat released (also called the heat of combustion) when a known quantity of fuel is burned under specific conditions. The amount of heat released depends on whether the water formed during combustion remains in the vapor phase or is condensed to a liquid. If the water is condensed to the liquid phase, giving up its heat of vaporization in the process, the energy released is called the gross energy content. The net energy content is lower because the water remains in the gaseous phase (water vapor). Since engines exhaust water as vapor, net energy content is the appropriate value for comparing fuels.
Energy content can be expressed either gravimetrically (energy per unit mass of fuel) or volumetrically (energy per unit volume of fuel). The International Metric (SI) units are megajoules per kilogram (MJ/kg) and megajoules per liter (MJ/L). In the United States, the gravimetric unit is British thermal units per pound (Btu/lb), and the volumetric unit is British thermal units per gallon (Btu/gal). Because the energy contents of individual hydrocarbons can differ, jet fuel composition has some effect on energy content (see page 25). The effect is usually predicted by fuel density, which is also a function of composition. Generally, less dense jet fuels have a higher gravimetric energy content, and more dense jet fuels have a higher volumetric energy content. This effect is more pronounced when different types of fuel are compared.-
Fuels differ in density, and therefore, in energy content per unit weight or unit volume. Less dense fuels, such as avgas, have a higher energy content per unit weight and a lower energy content per unit volume. The relationships are reversed for more dense fuels.
Which is preferred for aviation fuel, a higher density fuel with a higher volumetric energy content? Or a lower density fuel with a higher gravimetric energy content? The answer is obvious for aircraft that take off with their fuel tanks full, as most military aircraft do. A fuel with a high volumetric energy content maximizes the energy that can be stored in a fixed volume and thus provides the longest flight range. The answer is not so obvious for commercial airliners, most of which don’t fill their fuel tanks before each flight. Instead, they take on enough fuel to reach their intended destination, plus an adequate safety margin. In this situation, is it more advantageous to use a less dense fuel with a high gravimetric energy content to minimize fuel weight? Or does the increased range provided by the same volume of a more dense fuel with a high volumetric energy content offset the added weight? The relationship among these variables is complex, and beyond the scope of this Review. But, in most cases, it
turns out that the answer is the same as for the “full tank” case: A more dense fuel with a high volumetric energy content is preferred. Jet fuel is a commodity product that is typically bought and sold by volume, with no price adjustment for density or energy content.