Quick answer
Cetane index is a calculated estimate based on fuel properties like density and distillation range. Cetane number is measured via an engine test. Both gauge diesel ignition quality, but the index is an indirect method.
Detailed answer
The cetane index and the cetane number both measure diesel’s ignition readiness, but they’re derived differently. The cetane number uses an actual engine test—often a CFR (Cooperative Fuel Research) engine—to compare the test diesel against reference fuels. This direct approach is highly accurate but expensive. The cetane index, on the other hand, is a formula that uses properties like density and distillation data to estimate ignition quality. It’s quicker and cheaper but might not account for additives or unique blends. So, a fuel’s measured cetane number can be higher than its calculated index if ignition-improving additives are present. Both metrics help ensure diesel meets performance and emission standards; they’re just different roads to the same goal: a fuel that ignites reliably under compression.
