Cetane value

An important characteristic of diesel fuels is their ability to autoignite, quantified by the cetane value. Generally the cetane value is higher for biodiesel, resulting in smoother running of the engine with less noise. 

LCO quality was determined by two different methods. Before 2-D chromatography was available, liquid product aromatics were measured for all samples using a variation of the ASTMD 5186-96 method. A selected number of samples were cut between 221°C and 343°C using a TBP column to produce an LCO fraction for direct ASTMD 3747 cetane index determination. The cetane values of the LCO cuts were then correlated to the total liquid product aromatics (Figure 2.1) and the correlation was used to estimate the LCO quality of the other samples. 

Cetane Number—The cetane number (C.N.) of a fuel is the percentage by volume of normal cetane in a mixture of cetane and alpha-methylnaphthalene which matches the unknown fuel in ignition quality when compared with a standard diesel engine under specified conditions. The C.N. scale ranges from 0 to 100 C.N. for fuels equivalent in ignition quality to alpha-methylnaphthalene and cetane, respectively. For routine-testing, secondary reference fuels having cetane values of about 25 and 74 are blended in any desired proportion. 

The methyl esters are usually straight chain hydrocarbons and have relatively high cetane values shown in Table 4.3. The ethyl esters are between 2 and 5 units higher than the corresponding methyl esters. Such diesel fuel, though effective, is still too expensive and not competitive with petroleum-based fuels. 

Ethanol and methanol have cetane values from 0 to 5 making them poor compression ignition fuels for a diesel engine. However, alcohol-diesel fuel blends have been used successfully, and emulsifiers have been added to help blend the two components which have limited miscibility. 

The differences between the measured cetane numbers and the cetane indices calculated by the above formula are relatively small for cetane indices between 40 and 55. Nevertheless, for diesel fuels containing a pro-cetane additive, the CCl will be much less than the actual value (refer to paragraph d). 

Some empirical formulas have been devised to link the cetane index to the Diesel Index (D.l.) or even directly to the aniline point. We will cite two such formulas here in order to illustrate their comparative values C ... 

Cetane number is measured in a single-cylinder laboratory engine, but cetane index (Cl) is more commonly used. Cetane index is a calculated value and correlates adequately with the cetane number. Most refiners use the ASTM equation (Method D-976-80) to calculate the cetane index. The equation uses the 50% boiling point and API gravity (Example 6-1). 

Cetane Number is a numerical indication of a fuel s (kerosene, dic.sel, heating oil) ignition quality. Cetane number is measured in a single-cylinder engine, whereas cetane index is a calculated value. 

As shown above, CN and ON are important properties of diesel and gasoline fuels, respectively. Although a set of large databases are available, cetane and octane values of many individual compounds are still missing. Therefore, the prediction tools we have developed are very useful in correlating the molecular structures with properties of fuels. In this section, the predicted CN and ON values are implemented to construct the most effective catalytic strategies to optimize CN for diesel and ON for gasoline. 

Vegetable all methyl esters (biodiesel) Kinematic viscosity Cetane no. Lender heeling value (MJJkg) Cloud point l°C Pour point <"C) Flesh point fC) Denelly (kgfl)... 

Conversion of heterocyclic aromatic compounds or other aromatic fuel components to paraffins by hydroprocessing can help increase the cetane number of diesel fuel. Fuel paraffins have significantly higher cetane number values than fuel aromatics. For this reason, the cetane number of ultra-low sulfur diesel fuel will typically be one to five numbers greater than higher-sulfur grade fuels. 

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