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Bunker Fuel Combustion

Combustion of a residual fuel is a multi-stage process of which one part is the ignition quality of the fuel. The most common method of assessing this aspect is by an empirical equation involving density and viscosity, known as the Calculated Carbon Aromaticity Index (CCAI). Of the two parameters, density has the major effect. The incidence of fuels with a CCAI exceeding 870 (the maximum for RMG and RMK residual fuels under ISO8217: 2010) is very low.

Combustion slice

Fuel takes a finite time from the start of the injection to the start of combustion. During this period, fuel is intimately mixed with the hot compressed air in the cylinder where it begins to vaporise. After a short delay known as the ignition delay, the heat of compression causes spontaneous ignition to occur.
Combustion graphRapid uncontrolled bunker fuel combustion follows as the accumulated vapour formed during the initial injection phase is vigorously burned. The longer the ignition delay, the more fuel will have been injected and vaporised during this “pre-mixed” phase and the more explosive will be the initial combustion. The second phase or “diffusion burning” phase of bunker fuel combustion is controlled by how rapidly the oxygen and remaining vaporised fuel can be mixed as the initial supply of oxygen near the fuel droplets has been used during the pre-mixed combustion. Rapid pre-mixed combustion causes very rapid rates of pressure rise in the cylinder resulting in shock waves, which can cause broken piston rings and overheating of metal surfaces. Large diesel engines are designed to withstand a certain rate of pressure rise within the cylinder although the figure will vary between different designs.

Pressure rise implications

Ignition performance requirements of residual fuels in large diesel engines are primarily determined by engine type and, more significantly, engine operating conditions. Fuel factors influence ignition characteristics to a much lesser extent.  A CCAI value that may be problematical for an engine under some conditions may perform quite satisfactorily under others. Needless to say CCAI has now been included in ISO 8217:2010 in order to avoid residual fuel oils with uncharacteristic density/viscosity relationships, which can lead to an extended ignition delay. In particular engine operation under part load conditions using high CCAI fuel should be avoided.

CCAI and CII are empirical attempts to estimate how long the fuel will take from injection to ignition and by implication the likelihood of engine damage. After calculating the CCAI or CII of a fuel, the operator must then judge the acceptability of that fuel for effective operation in the engine. Variations of engine load, rated speed and design affect the likelihood of poor combustion, hence it is impossible to give precise figures that apply to all engines. If required, further guidance regarding acceptable ignition quality values should be obtained from the engine manufacturer.

CCAI can be automatically calculated in both the Viscometer and Density Meter

Off-line Bunker Fuel

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