CCAI & CII

The Calculated Carbon Aromaticity Index (CCAI) is an index of the ignition quality of residual fuel oil. The running of all internal combustion engines is dependent on the ignition quality of the fuel.

For spark-ignition engines the fuel has an octane rating.

For diesel engines it depends on the type of fuel, for distillate fuels the cetane numbers are used. Cetane numbers are tested using a special test engine and the existing engine was not made for residual fuels.

For residual fuel oil two other empirical indexes are used: CCAI and Calculated Ignition Index (CII). Both CCAI and CII are calculated from the density and kinematic viscosity of the fuel.

CCAI= D - 81 -141x Log [Log [kV + 0.85]] - 483 x Log [(t+273) / 323]

Where: D= density at 15°C (kg/m3); V= viscosity (cSt); t = viscosity temperature (°C).

Calculated Carbon Aromaticity index will normally give a value between 800 and 880. The lower the value is the better the ignition quality.

Fuels with a CCAI more than 880 are often problematic or even unusable in a diesel engine. CCAI are often calculated under testing of marine fuel.

The Calculated Ignition Index (CII) is an index of the ignition quality of residual fuel oil.

CII = (270.795 + 0.1038T) – 0.254565 D + 23.708 log log(V+0.7)

Where:D = density at 15°C (kg/m³); V = viscosity (cSt); T = viscosity temperature (°C).
CII was designed to give out numbers in the same order as the cetane index for distillate fuels.