IGNITION QUALITY OF FUELS

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The ignition quality of a fuel dictates the time delay that occurs between the start of fuel injection, and the commencement of ignition in a cylinder. Thus a fuel with a high ignition quality will have a shorter delay period, and visa versa for the low ignition quality fuel.

The ignition of the fuel is dependant on the ease of which the hydrocarbon molecule can be broken away or 'attacked'. The long thin chains of the paraffin are more easily broken down than the compact ring structures of the aromatics (benzene ring).

Thus the amount of aromatics within the fuel structure has a direct relationship on the ignition performance. However measuring fuel aromaticity is difficult, so the concept of CCAI (Calculated Carbon Aromaticity Index) has been used for indicating ignition quality.

CCAI is a ranking of ignition qualities of different types of residual fuels on the basis of known specification properties of density and viscosity. The rating of a high CCAI value 870-950 indicates a low ignition quality fuel, which will give long ignition delays.

Different delays in the ignition of the fuel will affect the rate of pressure rise during the initial period of combustion. Low CCAI rating - This will have a shorter delay between injection and ignition. The rate of pressure rise within the cylinder will be lower than normal producing a lower peak pressure. High CCAI rating- This will have a greater delay between injection and ignition. The rate of pressure rise once ignition does occur will be quicker and greater due to the larger amount of vapour in the cylinder. Peak pressure will be greater, although there is little change on the power developed or fuel consumption, if the fuel injection point remains the same.

The most noticeable effect of a long delay period is increased engine noise, diesel 'knock', and 'rougher' running particularly at the lighter loads.

To reduce the ignition delay the following changes can be made:

1.         Increasing the compression ratio

2.         Increasing the load on the engine

3.         Retard the fuel injection point

4.         Increasing the charge air temperature

5.         Reduce the amount of fuel injected during the ignition delay-cam profile

The ignition delay period is short compared to the injection period for low speed engines, meaning that only little fuel is injected before ignition. Faster running engines may inject all the fuel before ignition causing detonation. Both NSD and MAN B&W have stated that their slow speed engines are not affected by the effects of poor ignition quality fuels, and all the published data relating to damage to engines comes from the medium speed engine manufacturers.