MadSci Network: Chemistry

Re: does #2 diesel fuel have an octane rating?

Date: Tue Oct 31 10:14:20 2000
Posted By: Nik Streit, Grad student, Laboratory of Atmospheric Chemistry, Paul Scherrer Institute
Area of science: Chemistry
ID: 972494527.Ch

Diesel has an octane number of about 15-25. The reason for this seemingly terrible number compared to the ones we're used to from gasoline is the fact that diesel is combusted in a totally different type of engine. Diesel has low volatility, low knock resistance, yet high energy per volume. Because of its low knock resistance, diesel should not be used in a gasoline engine as it will destroy it very quickly and efficiently.

Generally speaking, diesel fuel ignition quality is not measured in octanes, as these are a unit for gasoline. A similar reference value, but for diesel, is the cetane rating. The higher the cetane number, the easier the fuel ignites when injected into an engine. The cetane number is determined by an engine test using two reference fuel blends of known cetane numbers. The reference fuels are prepared by blending normal cetane (n-hexadecane), having a value of 100, with heptamethyl nonane, having a value of 15. The higher the cetane rating, the higher the fuel's propensity to knock! Choosing a very high cetane number will not make the car run better, but using a fuel with too low cetane number may prevent the engine from starting or running.

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