Is there a theoretical basis for why the cetane number of n-alkanes is higher than that of isomeric alkanes? The pattern below is merely experimentally determined; is there any reason for this?
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The cetane number of diesel fuel depends on its chemical composition. The cetane numbers of various hydrocarbons differ significantly from one another. The general rules are as follows: (1) Alkanes: Normal alkanes have the highest cetane numbers; moreover, the higher the relative molecular mass, the higher the cetane number. The cetane number of isomeric alkanes with the same carbon count is lower than that of n-alkanes. (2) Olefins: Normal olefins have a fairly high cetane number, but it is slightly lower than that of the corresponding normal alkanes. The effect of branching is similar to that of alkanes. (3) Naphthenes: The cetane number of naphthenes is lower than that of normal alkanes and normal alkenes with the same number of carbon atoms, and naphthenes with side chains have an even lower cetane number than those without side chains. (4) Aromatic hydrocarbons: Aromatic hydrocarbons with no side chains or short side chains have the lowest cetane number, and the higher the number of rings, the lower the cetane number. Aromatic hydrocarbons with longer side chains have a relatively higher cetane number, and this value increases as the length of the side chains increases. The hexadecane value of straight-chain alkylarenes with the same number of carbon atoms is higher than that of branched alkylarenes
Cetane number is an important indicator for measuring the ignition performance of diesel in compression-ignition engines. The higher the cetane number, the better the ignition performance of the diesel, the shorter the afterburning period, the more uniform the combustion, and the smoother the engine operation. A low cetane number indicates difficult combustion and ignition, a long afterburning period, and rough engine operation. However, if the cetane number is too high, a small amount of black smoke will be produced due to partial incomplete combustion. The higher the hydrogen-to-carbon ratio in the components and the greater the molecular spacing, the lower the auto-ignition temperature, the better the ignitability, and the higher the cetane number. The cetane number of diesel is related to its chemical composition and distillate composition. Among the various diesel component hydrocarbons with the same number of carbon atoms, n-alkanes have the highest cetane number, while polycyclic aromatics have the lowest cetane number; olefins and cycloalkanes fall between alkanes and aromatics in terms of cetane number. The higher the degree of isomerization of hydrocarbons and the greater the number of rings, the lower their cetane number ； For cycloalkanes and aromatic hydrocarbons, as the length of the side chain increases, the cetane number also increases; conversely, as the degree of branching of the side chain increases, the cetane number decreases. To ensure balanced operation of diesel engines, straight-run diesel with a high proportion of alkanes and a high cetane number is typically used. However, a higher cetane number is not necessarily better; a high cetane number indicates a higher content of alkanes. However, diesel with a high content of alkanes has a higher freezing point. Additionally, due to the poor thermal stability of alkanes, some of them undergo thermal decomposition in the combustion chamber during the initial stages of combustion, producing large amounts of carbon. The combustion of this carbon takes longer time, and the carbon that does not burn is released through the exhaust pipes in the form of smoke, contaminating the atmosphere and increasing fuel consumption. Therefore, the cetane number of diesel is not the higher the better; tests on engines and in actual driving conditions have shown that in order to ensure uniform combustion of diesel and prevent unnecessary increases in fuel consumption, a cetane number of 40–50 for diesel is generally sufficient to ensure the proper operation of diesel engines.