The calculated carbon aromaticity index (CCAI) is an index of the ignition quality of residual fuel oil.
The running of all
internal combustion engines
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...
is dependent on the ignition quality of the fuel. For
spark-ignition engines the fuel has an
octane rating
An octane rating, or octane number, is a standard measure of a liquid fuel, fuel's ability to withstand Compression ratio, compression in an internal combustion engine without causing engine knocking. The higher the octane number, the more compres ...
. For
diesel engine
The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...
s 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
Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
and
kinematic viscosity
Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
of the fuel.
Definition
Formula for CCAI:
which is equivalent to:
Where:
D= density at 15°C (kg/m
3)
V= viscosity (cST)
t = viscosity temperature (°C)
Use
This will normally give a value somewhere between 800 and 880. The lower the value is the better the ignition quality. Fuels with a CCAI higher than 880 are often problematic or even unusable in a diesel engine. CCAI are often calculated under testing of marine fuel.
In case of high CCAI, the manufacturers recommendations and guidance limits should be consulted to ensure that the fuel falls within the permissible range for the engine type. Attention should be given to the combustion profile, peak pressures and exhaust temperatures on the Engine.
As the name suggests, CCAI is a calculation based on the density and viscosity of a given fuel. The
formula is rather complex but in general, the higher the CCAI, the poorer the ignition quality of the fuel
is considered to be. Once the CCAI goes above 860, it is an indication that some combustion problems
may occur.
Studies carried out by engine manufacturers indicate that combustion related problems caused by fuels with
high CCAI can be reduced by avoiding running the engine at part load. It is therefore suggested that
wherever possible, the engine load should be maintained above 50% and the chief engineer should listen
for indications of poor combustion (i.e. knocking). Should any such problems be noted then it is
recommended that an alternative fuel is used whilst further investigations are carried out on samples from this fuel
Combustion