The Calculated Ignition Index (CII) is an index of the ignition quality of residual fuel oil. It is used to determine the suitability of heavy fuel oil for (marine) engines.
Background
The effective and efficient running of
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. Incorrect or out of specification fuel can cause problems or severe damage to the engines and associated equipment.
The effects may include: corrosion, abrasive wear, clogged cylinder valves, fuel equipment damage, fuel pump failure, premature ignition, ignition failure, explosion, or engine failure.
Several indices are used to characterise fuels. 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 engines
The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the die ...
it depends on the type of fuel, for distillate fuels the
cetane numbers are used. Cetane numbers are tested using a special test engine, however, the existing test engine was not intended for residual fuels. For residual fuel oil two auxiliary indexes have been developed: the Calculated Ignition Index (CII) and
Calculated Carbon Aromaticity Index (CCAI).
CII index
The calculated ignition index (CII), together with the calculated carbon aromaticity index (CCAI), are empirical indicators which describe the characteristics or properties of a fuel.
Both CII and CCAI 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. CII was developed by
BP to calculate the autoignition capacity of
heavy fuel oils (HFO). It is calculated using the measured kinematic viscosity ''V'' (cSt or mm
2/s) of a given fuel determined at temperature ''t'' (°C) and the density ρ
15 at 15°C (kg/m
3).
Definition
Formula for CII:
Where:
D = density at 15°C (kg/m
3)
V = kinematic viscosity (cSt)
T =kinematic viscosity temperature (°C)
A CCAI and CII calculator is available on several websites.
Use
CII was designed to produce numbers of the same order as the cetane index number for distillate fuels. The relationship between fuel properties and CII number is shown in the table.
CII gives an indication of the quality of heavy fuel oil. Certain practical steps can be taken to ensure the continuing quality. This includes close supervision of fuel, including tests during bunkering using the viscosity data to determine the CII and CCAI. Avoid mixing fuels; checking water content as the water settles out; monitoring the condition and performance of engines.
References
{{reflist
Combustion