CCAI – Calculated Carbon Aromaticity Index – This is one important parameter which every shipboard engineer and the Technical Superintendents should watch critically in every fuel quality test certificate.
A vessel experienced severe knocking sound from the main engine and later experienced numerous piston seizures. Temporary repairs were executed, but the vessel’s C/E did not realise that the problems experienced might have something to do with the fuel oil properties and opted to continue running the engine at reduced RPM with the same fuel oil until the vessel reached a port of refuge. When the engine was opened up severe damages were discovered to all cylinder units. Main bearings had to be renewed and the crankshaft’s main bearing journals had to be polished in addition to numerous of different parts inspected/overhauled. The damage repairs amounted to USD 2 million and involved 36 days off-hire. During repairs it was felt that the damage seen had similarities to that which could have been caused by fuel oil with poor ignition and combustion properties and a decision was made to perform an ignition quality test.
The diagram below illustrates the results from the fuel oil ignition tests performed with the FIA 100/3, compared with a reference curve illustrating test results for ’normal‘ fuel oil.
The CCAI is a marine fuel parameter which is an indicator of ignition quality. The CCAI gives an idea of how much the ignition is delayed. The higher the CCAI, the longer the ignition delay.
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 rise of pressure. This produces the characteristic “diesel knock”.
The problem is generally prevalent in medium/high speed diesel engines when burning blended fuels and the problems seems to appear in the CCAI range 850 to 890. Ignition delay is indicated by CCAI greater than 840 for medium-speed engines and greater than 870 for low-speed engines.
Fuel oil ignition and combustion quality is not yet part of the ISO 8217:2005 and yet most vessels test fuel basis ISO 8217:2005 specification. When it comes to ISO 8217:2010 – we have a CCAI limit of 870 for RMG 380 grade. CCAI is a marine fuel parameter in the ISO 8217:2010/2012 specification and recognized as an indicator of ignition quality.
The Calculated Carbon Aromatic Index (CCAI) has historically been the default method of estimating heavy fuel oil ignition quality. The fuel oil density and viscosity are the key parameters needed for calculating the CCAI and the number 860 has for years been considered the limit for an acceptable ignition quality for a trunk piston engine.
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:
- Connecting rod big-end and bearing shells,
- Main bearing shells,
- Pistons (especially composite pistons),
- Piston rings and liners,
- Cylinder head with studs and gaskets,
- Tie Bolts,
- 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. With violent increase of combustion pressure, when operating on fuels delaying the ignition, the rate of blow-by will increase.
With fuels having high CCAI, it is to be noted that Medium and high speed diesel engines and in general engines of older design may not operate satisfactorily on such fuels, in particular at low and part load. Please contact engine maker for engine specific operational guidance. Every engine manual will clearly specify the operational limits with regards to CCAI.
- The engine builder should be contacted for detailed guidance as to the degree of sensitivity of specific engines.
- Observe closely for any adverse impact on engine operation, e.g. abnormal engine knocking, starting difficulties, erratic running behaviour, sticking exhaust valves and/or increased black smoke. If any of these are observed switch to another fuel or apply one or more of the following remedial actions which may ease or eliminate the problems:
- Increase the load wherever practicable and avoid continuous low load operation. If low load operation is unavoidable, it is highly recommended to maintain high charge air temperatures – but within the limits permitted by the engine maker.
- Use of Additives which can improve combustion quality
- Blending with other fuel distillates (Emergency operation only)
Blending with a distillate fuel oil in the range of 5 -10% may improve the ignition quality of the fuel. Blending of a residual fuel oil with a paraffinic distillate fuel may result in the precipitation of asphaltenes due to incompatibility of the blend components. A ‘Spot test’ should, at least, be undertaken prior to blending to check that the fuels are compatible. Additionally, producing a homogenous blend of a residual fuel oil and a distillate fuel without a blending unit may be difficult to achieve. A spare tank from where the mixed fuels may be re-circulated through a transfer pump or similar is recommended.
Viswa Lab recommends testing the suspect fuel for FIA 100/FCA test to determine the ignition and combustion properties and characteristics. When the above generic recommendations are employed, it is essential to let Viswa Lab know when every step is executed.
Please note: a careful study of specific shipboard conditions are essential and the above recommendations are generic.
Typical engine problems experienced when using a fuel oil with poor ignition properties are: –
- Difficulties or complete failure in starting the engine
- Undesirable peak pressures which can lead to blow by and collapse of piston rings
- Unstable operation and loss of power – Varying revolutions, which are highly undesirable for the operation of auxiliary engines
- Increased deposits in the combustion area and in the exhaust gas system, including turbocharger and boiler
- Increased emissions of NOx.
- In a worst case scenario poor fuel oil ignition and combustion properties can render the engine inoperative and compromise the safe operation of the ship.
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Source: Viswa Lab Fuel Problem Support Team & Gard