What is the effect of different components in gasoline and diesel on their octane and cetane numbers?

In gasoline hydrogenation, the hydrogenation and saturation of olefins reduce its octane rating； Diesel hydrogenation saturation increases its cetane number

In gasoline, saturation reactions of olefins and aromatics should be minimized to reduce losses in octane rating; octane rating is 100 for isooctane and 0 for n-heptane. Aromatics and isomerized olefins have the highest octane rating, followed by isomerized alkanes and olefins, then naphthenes, with n-alkanes having the lowest rating. The main factor affecting gasoline octane number is the olefin content, especially that of isomeric olefins. Hydrogenation saturation of diesel’s cetane number increases its cetane value; the type of catalyst used plays a key role here. Some catalysts are used for refined saturation, while others facilitate the ring-opening of polycyclic aromatics to form monocyclic structures. Alkanes have high cetane numbers – n-hexadecane has a cetane number of 100, whereas α-methylnaphthalene has a value of 0. During the reaction, olefins are hydrogenated and saturated, so few olefins remain; monocyclic aromatics, on the other hand, are relatively abundant.

Olefins in gasoline get saturated, reducing the octane number; aromatics in diesel undergo ring-opening, increasing the cetane number

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