4. Equation for Calculated Cetane Index
4.1 The Calculated Cetane Index is determined from the following equation:
where:
G = API gravity, determined by Test Method D287 or D1298,
M = mid-boiling temperature, °F, determined by Test Method D86 and corrected to standard barometric pressure,
D = density at 15°C, g/mL, determined by Test Method D1298, and
B = mid-boiling temperature, °C, determined by Test Method D86 and corrected to standard barometric pressure.
4.2 Calculated Cetane Index values for distillate fuels may be conveniently determined by means of the alignment chart in Fig.1, rather than by direct application of the equation. The method of using this chart is indicated by the illustrative example thereon.
5. Limitations of Equation
5.1 The Calculated Cetane Index equation possesses certain inherent limitations which must be recognized in its application. These are:
5.1.1 It is not applicable to fuels containing additives for raising cetane number.
5.1.2 It is not applicable to pure hydrocarbons, synthetic fuels, such as certain products derived from shale oils and tar sands, alkylates, or coal-tar products.
5.1.3 Substantial inaccuracies in correlation may occur if used for crude oils, residuals, or products having a volatility of below 500 F end point.