Cetane index and ignition quality

Cetane index is a calculated estimate of a distillate fuel ignition quality, derived from density and distillation data, and cetane number is the measured equivalent determined on a test engine. Both describe ignition delay, the interval between injection and the start of combustion, and a low value produces rough running, hard knock, elevated cylinder pressure rise rates and difficulty starting a cold engine.

The full article will set out what ISO 8217:2024 requires and where the requirement changed. The petroleum distillate grades carry a minimum cetane index by ISO 4264: 45.0 for DMX, 40.0 for DMA and DMZ, and 35.0 for DMB. The bio-distillate grades DFA, DFZ and DFB instead carry a minimum cetane number, 40.0 for the first two and 35.0 for DFB, which the 2024 edition added because the index correlation was developed on petroleum distillate and does not hold for a fatty acid methyl ester blend.

The article will explain why the ASTM calculated cetane index methods that the industry quotes are not the methods ISO 8217 specifies, cover the derived and indicated cetane methods used where an engine test is impractical, and state plainly why neither cetane measure applies to residual fuel, which is characterised instead by the calculated carbon aromaticity index. It links to marine gas oil .