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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. The figure gives guidance in relation to CCAI for a number of engine types. This data is derived from the results of engine simulations and published performance criteria.

Comment

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

Info Centre >> Fuel and Lube Oil Technical Manual >> 4. Fuel Oil - Test Results >> 4.16 Combustion

4.16 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 is in the order of 0.2% , whilst those in the range 870-860 are less than 3%.

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. Rapid uncontrolled 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 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, 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.

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. It is for this reason that no general limits for ignition quality can be applied, since a value which may be problematical to one engine under adverse conditions may perform quite satisfactorily in many other circumstances. Engine operation under part load conditions using high CCAI fuel should be avoided.

On a world-wide basis, there are some residual fuels with unusual burning characteristics which impose an additional load on the ring/liner interface. If this is not controlled, there is a danger that excessive maintenance will be incurred.