ASTM D4737 Test Method for Calculated Cetane Index by Four Variable Equation
5. Procedure
5.1 Determine the density of the fuel at 15°C to the nearest 0.0001 g/mL, as described in Test Method D1298 or Test Method D4052.
5.2 Determine the 10 %, 50 %, and 90 % recovery temperatures of the fuel, as described in Test Method D86.
5.3 Test Method D2887 maybe used as an alternative to Test Method D86 to determine the 10%, 50%, 90% recovery temperatures of the fuel.
5.3.1 If Test Method D2887 is used, convert the Test Method D2887 data to estimated Test Method D86 data following Appendix X5, Correlation of Jet and Diesel Fuel, of Test Method D2887 and use the estimated Test Method D86 data in place of actual Test Method D86 data in the calculations.
5.3.2 Provision for use of Test Method D2887 data in this test method is intended to facilitate its use in determining compliance with Specification D975 requirements. If this test method is used for purposes other than Specification D975 compliance, the use of estimated Test Method D86 data should be reviewed to ensure it is acceptable.
6. Calculation or Interpretation of Results
6.1 Compute the Calculated Cetane Index by Four Variable Equation using the equation given in 3.1 (Procedure A) for Grades 1-D S15, 1-D S500, 1-D S5000, 2-D S5000, and 4-D. The calculation of Procedure A is more easily performed using a computer or programmable hand calculator. Round the value obtained to the nearest one-tenth. Compute the Calculated Cetane Index by Four Variable Equation using the equation given in 3.3. (Procedure B) for Grade 2-D S15 and 2-D S500.
6.1.1 Calculated Cetane Index by Four Variable Equation (Procedure A) can also be easily determined by means of the nomographs (applicable to Procedure A only) appearing in Figs. 1-3. Fig. 1 is used to estimate the cetane number of a fuel based on its density at 15°C and its 50 % recovery temperature. Fig. 2 is used to determine a correction for the estimate from Fig. 1 to account for deviations in the density and the 90 % recovery temperature of the fuel from average values. Fig. 3 is used to determine a second correction for the estimate from Fig. 1 to account for deviations in the 10 % and the 90 % recovery temperatures of the fuel from average values. The corrections determined from Fig. 2 and Fig. 3 are summed algebraically with the cetane number estimate from Fig. 1 to find the Calculated Cetane Index by Four Variable Equation (Procedure A). The method of using these nomographs is indicated by the illustrative example shown below and on Figs. 1-3.
6.2 The Calculated Cetane Index by Four Variable Equation possesses certain inherent limitations which must be recognized in its application. These are as follows:
6.2.1 It is not applicable to fuels containing additives for raising the cetane number.
6.2.2 It is not applicable to pure hydrocarbons, nor to non-petroleum fuels derived from coal.
6.2.3 It is not applicable to fuels containing biodiesel as defined by Specification D6751.
6.2.4 Substantial inaccuracies in correlation may occur if the equation is applied to residual fuels or crude oils.
