Reference Fuel

a liquid hydrocarbon or blend of liquid hydrocarbons used to evaluate the performance characteristics of fuels. Reference fuels make it possible to determine the antiknock rating (octane number and gasoline performance rating) and the spontaneous combustion rating (cetane number) of fuels for piston engines as well as the flame intensity rating (luminometer number) for gas turbine fuels. They may be classified as primary and secondary types. The former are blends of isooctane (octane number = 100) and n-heptane (octane number = 0), of cetane (cetane number = 100) and α-methylnaphthalene (cetane number = 0), and of isooctane (luminometer number = 100) and tetrahydronaphthalene (luminometer number = 0).

Because of the high cost of primary reference fuels, cheaper secondary reference fuels are often used instead and calibrated against the primary fuels. A secondary reference fuel used to determine octane numbers may be a blend of technical-grade isooctane (octane number = 98–99) and straight-run gasoline (octane number not less than 68) or white spirits (octane number = 17–27). The secondary reference fuel used to determine cetane numbers is a blend of straight-run diesel fuel from a paraffinbase petroleum (cetane number not less than 55) and a highly aromatic diesel fraction of pyrolysis products (cetane number not more than 20). Gasoline performance ratings are determined from comparison with a reference fuel consisting of either pure technical-grade isooctane or a blend of same with tetraethyl lead (0.3–0.8 g per kg of isooctane) or with n-heptane (5–10 percent by volume).

In some cases evaluations are made of such fuel operating characteristics as antiwear properties and the tendency to form carbon deposits. For such purposes, the reference fuel used (in addition to those already described) may be n-pentadecane or a blend of 85 percent (by weight) cetane and 15 percent α-methylnaphthalene.

Reference fuels are also used in comparative bench tests of engines. The petroleum fractions used for such purposes conform to special standards that define density range, viscosity, boiling point, vapor pressure, calorific value, and sulfur content.