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United States Department of Agriculture

Agricultural Research Service

Title: Cetane Numbers of Fatty Compounds: Influence of Compound Structure and of Various Potential Cetane Improvers

Authors
item Knothe, Gerhard
item Bagby, Marvin - RETIRED, USDA/ARS
item Ryan, Thomas - SWRI, SAN ANTONIO, TX

Submitted to: Society of Automotive Engineers Transaction
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 19, 1997
Publication Date: N/A

Interpretive Summary: The cetane number (CN) is a prime indicator of the quality of a diesel fuel. The CN is related to the speed with which ignition of a fuel in a diesel engine begins. The higher the CN, the faster ignition will begin and vice versa. Fuel additives called cetane improvers which increase CN are known to reduce certain emissions, for example nitrous oxides (Nox). With biodiesel (methyl ester derivatives of vegetable oils) continuously gaining momentum as an alternative diesel fuel, it is important to study the cetane numbers of the fatty compounds that make up biodiesel. Biodiesel also produces lower exhaust emissions than conventional diesel fuel with the exception of Nox. The influence of the structure of the compounds comprising biodiesel was studied. Furthermore, new cetane-improving materials were identified. Some of them have previously unknown effects on the cetane numbers of fatty acids and esters. For example, in one case the CN of the fatty acid is higher than that of the corresponding esters. These results offer the possibility of tailoring the cetane improver to the predominant fatty compound in a biodiesel mixture. The cetane improvers identified here are a possible route to reducing Nox emissions of biodiesel.

Technical Abstract: Biodiesel is a mixture of esters (usually methyl esters) of fatty acids found in the triglycerides of vegetable oils. The different fatty compounds comprising biodiesel possess different ignition properties. To investigate and potentially improve these properties, the cetane numbers of various fatty acids and esters were determined in a Constant Volume Combustion Apparatus. The cetane numbers range from 20.4 for linolenic acid to 80.1 for butyl stearate. The cetane numbers depend on the number of CH2 groups as well as the number of double bonds and other factors. Various oxygenated compounds were studied for their potential of improving the cetane numbers of fatty compounds. Several potential cetane improvers with ignition delay properties giving calculated cetane numbers over 100 were identified. The effect of these cetane improvers depended on their concentration and also on the fatty material investigated. In one case, the cetane number of the acid was increased more than that of the corresponding esters. The effect also depends on the corresponding esters. The effect also depends on the nature of the ester. These results offer the possibility of tailoring cetane improvers to the nature of component fatty compounds in biodiesel. The cetane improving additives are a potential route for reducing the exhaust emissions of biodiesel, for example Nox.

Last Modified: 9/21/2014
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