This test method is intended for use as a guide in cases where an experimental determination of heat of combustion is not available and cannot be made conveniently, and where an estimate is considered satisfactory. It is not intended as a substitute for experimental measurements of heat of combustion (Note 2).

Note 2—The procedures for the experimental determination of the gross and net heats of combustion are described in Test Methods D240 and D4809.

The net heat of combustion is a factor in the performance of all aviation fuels. Because the exhaust of aircraft engines contains uncondensed water vapors, the energy released by fuel in vaporizing water cannot be recovered and must be subtracted from gross heat of combustion determinations to calculate net heat of combustion. For high performance weight-limited aircraft, the net heat of combustion per unit mass and the mass of fuel loaded determine the total safe range. The proper operation of the aircraft engine also requires a certain minimum net energy of combustion per unit volume of fuel delivered.

Because the heat of combustion of hydrocarbon fuel-mixtures are slowly varying functions of the physical properties of the mixtures, the heat of combustion of the mixtures can often be estimated with adequate accuracy from simple field tests of density and aniline point temperature, without the elaborate apparatus needed for calorimetry.

The empirical quadratic equation for the net heat of combustion of a sulfur-free fuel was derived by the method of least squares from accurate measurements on fuels, most of which conformed to specifications for fuels found in Note 1 and were chosen to cover a range of values of properties. Those fuels not meeting specifications were chosen to extend the range of densities and aniline-point temperatures above and below the specification limits to avoid end effects. The sulfur correction was found by a simultaneous least-squares regression analysis of sulfur-containing fuels among those tested.

1.1 This test method covers the estimation of the net heat of combustion at constant pressure in metric (SI) units, megajoules per kilogram.

1.2 This test method is purely empirical, and it is applicable only to liquid hydrocarbon fuels derived by normal refining processes from conventional crude oil which conform to the requirements of specifications for aviation gasolines or aircraft turbine and jet engine fuels of limited boiling ranges and compositions as described in Note 1.

Note 1—The estimation of the net heat of combustion of a hydrocarbon fuel from its aniline point temperature and density is justifiable only when the fuel belongs to a well-defined class for which a relationship between these quantities has been derived from accurate experimental measurements on representative samples of that class. Even in this class, the possibility that the estimates can be in error by large amounts for individual fuels should be recognized. The JP-8 fuel, although not experimentally tested, has properties similar to JP-5 and Jet A fuels and can be considered in the same class. The classes of fuels used to establish the correlation presented in this test method are represented by the following applications:

1.3 The net heat of combustion can also be estimated by Test Method D1405. Test Method D1405 requires calculation of one of four equations dependent on the fuel type with the precision equivalent to that of this test method.

1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.