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Carbon Residue in Fuel Oils

The carbon residue of a fuel is the tendency of carbon deposits to form under high temperature conditions in an inert atmosphere. It is well known that the correlation between carbon residue and diesel engine performance is poor. However, in the absence of any other parameter, this property is included in fuel specifications and is generally considered to give an approximate indication of the carbonaceous deposit-forming tendencies of the fuel. Numerous factors can affect the combustion process in diesel engines, including engine loading, engine tuning and the ignition qualities of the fuel. All these factors have an effect on the deposit tendencies of a particular fuel.

The carbon residue value of a fuel depends on the refinery processes employed in its manufacture. For straight run fuels, the value is typically 10 - 12% m/m, while for fuels from secondary refining processing, the value depends on the severity of the processes applied. On a global basis, this value is typically 15 - 16%, however in some areas it can be as high as 20% m/m.

Modern engines tolerant to a wide range of MCR valves. However, some older engines, typically of the 1970s, are such that difficulties may be experienced burning fuels with an MCR greater than 12% m/m, especially at low loads. Above this level, there is likely to be increased carbonaceous deposit which will affect the performance of the engine. Operational experience has shown that the present generation of large, medium and slow speed engines designed for residual fuel can tolerate a wide range of MCR values without any adverse effect.

> 20 %	High and may be problematic and cause increased fouling
10 - 12 %	Straight run fuels
15 - 16 %	Average and acceptable in modern engines