Test Methods
Cetane Number
Cetane number of diesel is determined using ASTM D 613 - Standard Test Method for Cetane Number of Diesel Fuel Oil5 which is accepted by the oil industry internationally for this purpose. This method uses a standard single cylinder, four-stroke cycle, variable compression ratio, and indirect injected diesel engine.
According to ASTM D 613, the 95% confidence limits for the difference between results obtained by two laboratories on identical test samples is 3.8 for a cetane number of 48 and 4.3 for a cetane number of 52. That is, the difference in results obtained by two laboratories for identical samples should exceed these limits in only 1 case in 20. Confidence limits for results intermediate CN between these may be obtained by linear interpolation.
Cetane Index
Two methods, ASTM D 9766 and ASTM D 4737,7 are available for determining cetane index.8 The Regulations specify only ASTM D 976. Both methods are based on same principle.
These methods are applicable for straight-run fuel, catalytically cracked stocks, and blends of the two. The correlation is best for these fuels and least satisfactory for blends containing substantial proportions of thermally cracked stocks. In addition, CI determined by these methods does not take into account the effect of using cetane improvers.
In 1982, an ASTM taskforce was set-up to study and improve on the existing method for calculating CI. This led to the development of a modified equation that was adopted as ASTM D 4737. The difference between the two methods is that whereas ASTM D 976 uses two variables, density and distillation mid-boiling point, ASTM D 4737 uses two additional variables, 10% and 90% distillation temperatures. More information about CI calculation is given in Appendix A.
Although the Regulations specify only ASTM D 976 for determining CI, it was decided to have the extra two distillation points measured in order to compare the two methods. This was done primarily to find out which method gives the better match with measured CN for New Zealand diesel. This information can then be used in deciding on he merits of ASTM D 4737 being recognised by the Regulations for compliance purposes.
The correlation of CI with CN is dependent to a large extent upon the accuracy of the measured properties. In the case of ASTM D 976 these are specific gravity and the 50% distillation point. In the case of ASTM D 4737 these are specific gravity and the 10%, 50% and 90% distillation points.
ASTM D 976 states that the expected correlation of CI with CN will be somewhat less than ±2 cetane number for 75% of the fuels evaluated. ASTM D 4737 states that it will be somewhat less than ±2 cetane number for 65% of the fuels evaluated.
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