Fuels Technical Information

The carbon residue of a fuel is the tendency to form carbon deposits under high temperature conditions in an inert atmosphere, and may be expressed commonly as Micro Carbon Residue (MCR) or alternatively Conradson Carbon Residue (CCR).

It should be noted that numerically MCR is effectively the same as CCR. The overall relationship between actual diesel engine performance and carbon residue is poor, however, the carbon residue value is considered by some to give an indication of the combustibility and carbonaceous deposit forming tendencies of a fuel.

From ISO 8217 it will be seen that some residual grades have the same visocity parameter but different limits for various other parameters. With respect to carbon residue this applies to RM 10, RM 25 and RM 35 and a tighter limit exists for RMA 10 and RMB 10, RME 25 and RMG 35. However, it must be appreciated that there is only a limited availability of such fuels and in some geographical locations the availability is zero.

Fuels with a high carbon residue value may cause problems in older engines when they are operating under part load conditions. Whilst one of the light intermediate fuels referred to above may have an acceptable carbon residue value, the actual fuel delivered may have other characteristics which could have an adverse effect on the engine, specifically with reference to ignition quality.

The carbon residue value of a fuel depends on the refinery processes employed in its manufacture. For straight run residual fuels the value is typically 10-12 % m/m, while for fuels derived from secondary conversion processing, the value depends on the severity of the processes applied.