CCAI: calculated carbon aromaticity index

The Calculated Carbon Aromaticity Index is a number derived from a residual fuel’s density and viscosity, used as a specification limit in ISO 8217:2024. It is a maximum in all three residual tables: 860 on the 20 mm2/s grades and 870 on every other residual grade.

The formula sits at Clause 6.2 of the standard and takes three inputs: density at 15 degrees C in kg/m3, kinematic viscosity in mm2/s, and the temperature in degrees C at which that viscosity was measured. The familiar shipboard short form, density minus 141 times the log log of viscosity plus 0.85, minus 81, is that formula with the temperature term collapsed, and it is valid only where the viscosity was measured at 50 degrees C. The standard attributes the index to work presented at the 22nd CIMAC Congress in 1998.

What CCAI is for is narrower than its reputation. Annex C.2 states it is included in the tables to avoid residual fuels with uncharacteristic density-viscosity relationships, and CIMAC records that it was incorporated as a means of identifying anomalous fuels rather than as an indicator of ignition performance, and was never intended as a go or no-go characteristic. Fuels of similar CCAI can behave very differently in the cylinder, which is why the standard points at IP 541 for actual ignition assessment. The full article will cover the formula, its reproducibility, the limits by grade, and the note that its applicability to bio-residual fuels has not been established.