Heptamethylnonane, cetane number

The quality of diesel fuel is measured using the cetane number, a measure of the tendency of a diesel fuel to knock in a diesel engine, and the scale, from which the cetane number is derived, is based on the ignition characteristics of two hydrocarbons (1) n-hexadecane (cetane) and (2) 2,3,4,5,6,7,8-heptamethylnonane. 

Either primary or secondary reference fuels can be utilized in the cetane engine when determining the cetane number of distillate fuel. Primary reference fuels are n-cetane and heptamethylnonane. Secondary reference fuels are identified as T Fuel and U Fuel. The characteristics of each of these fuels are outlined in TABLE 4-7. 

This number is determined from the percentage of n-cetane which must be mixed with heptamethylnonane to give the same ignition performance as the fuel being tested. The cetane number can be obtained by using the following calculation ... 

Cetane Number—The ignition quality of a diesel fuel measured using an engine test specified in ASTM Method D 613. Cetane number is determined using two pure hydrocarbon reference fuels cetane, which has a cetane rating of 100 and heptamethylnonane (also called isocetane), which has a cetane rating of IS [2.2]. 

The cetane number of a diesel fuel is defined as the whole number nearest to the value determined by calculation from the percentage by volume of normal cetane (cetane No. 100) in a blend with heptamethylnonane (cetane No. = 15) which matches the ignition quality of the test fuel when compared by this method. The matching blend percentages to the first decimal are inserted in the following equation to obtain the cetane number ... 

The capacity of diesel fuels for autoignition is characterized by the cetane number. Commercial diesel fuels have cetane numbers of between 48 and 55. This number is measured with the aid of a CFR diesel engine by comparison with mixtures of primary reference fuels cetane or n-hexadecane has a cetane number of 100 (by definition), and heptamethylnonane has a cetane number of 15. A binary mixture of these two species containing X % by volume of cetane has therefore a cetane number equal to X + 0.15 (100-X). 

The reference fuels for ASTM D613, which describes the test method for cetane number, are n-cetane, a-methylnaphthalene, and heptamethylnonane, for which cetane numbers are defined to be 100, 0, and 15, respectively. Table 31 shows cetane numbers for selected pure compounds. 

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