In these days of burning residual fuels in our ships, various types of fuel related problems occur. These can, while being evident, be a considerable 'pain in the neck' for the engine crew and for the operator of the vessel. Hence, the measures to be taken from the owner's side to
avoid these hick-ups are to specify as appropriate as possible the grade of fuel required for 'his' engine. And in case of a dispute, to be a subscriber to a
recognized fuel analyzing scheme. Sampling procedures for receiving fuel should
be accepted by all involved parties.
Let us dwell for a moment on the issue of ordering bunkers and the specification that normally is submitted to
ensure the correct grade is received on board. A number of parameters are normally mentioned, such as; density max, viscosity max, sulphur max, poue point etc. There is however a parameter rarely being mentioned in these specifications and that is the CCAI, Calculated Carbon Aromaticity Index, which gives a value on the Ignition Quality for residual fuels, since these grades cannot be verified by methods used for distillates, i.e. Diesel Index, Cetane Index and Cetane Number.
Accepted method for determination of the ignition quality of residual fuels is currently not available. It has, however, been empirically established that there is a relationship between the density, the viscosity and the ignition performance and the Shell-developed CCAI is the one presently most accepted for indicating ignition delay, although there is also a BP- developed Calculated Ignition Index (CII). CCAI gives an idea of how much the ignition is delayed, the higher the index, the longer the delay. The CCAI can be determined, with limited accuracy, by the enclosed nomogram AAAA
The combustion starts with a short delay already when a small amount of the fuel has been injected and therefore the remaining quantity injected burns in a controlled manner. If,however, the delay is long, a
large amount is injected before the combustion starts,producing a quick and violent raise of pressure. This produces the characteristic"diesel knock". The problem is generally related to medium speed diesel engines when burning blended fuels less than 220 cSt. and problems seems to appear in the CCAI-span 850-890. See enclosed diagram BBBB If
it is required (necessary) to
operate the engine within this span the stresses on the engine components might increase considerably and special attention should be paid to:
0 Connecting rod big-end and bearing shells.
0 Main bearing shells
0 Pistons(particularly
composite pistons)
0 Piston rings and liners
0 Cylinder head with studs and gaskets
0 Tie bolts
0 Intake and exhaust valves
To alleviate the effect of the ignition delay, the ambition should be to keep the engine load within 50 - 85 %
and to
maintain the inlet air temperature as high as practically possible and through pre-heating prior start-up(the CCAI problems are accentuated on a cooler engine, hence a known scenario is the vessel makes it to port but the engine can not be restarted upon departure due to fouled/clogged piston rings, poppet valves and turbocharger).With the violent increase of combustion pressure, when operating on fuels delaying the ignition, the rate of
blow-by will increase and it goes without saying that the lub.oil quality must be optimal to cope with the additional load imposed on the bearings.
So, by
way of
conclusion, if the shipowner is operating engines which are sensitive to ignition
quality he would be wise to order fuels with a CCAI limit or to set density and viscosity limits which will control the CCAI.
Special care to CCAI is needed when a ship is forced to use low viscosity fuels (below
180 cSt), due to heating limitations. If the density of these fuels is high the CCAI will be too high and ignition problems may be encountered.
