Cetane index is a number used to estimate the quality and composition of diesel fuel. This number represents the time between when fuel is injected into an engine and when the combustion process starts. This time to combustion figure serves as an effective means of measuring fuel quality, as better-quality fuels will ignite more quickly. A fast-igniting fuel generally contains fewer contaminants and impurities than one that takes longer to ignite. The cetane index gets its name from cetane, which is another name for diesel fuel.
The cetane index of a fuel serves as an alternative to the traditional cetane number that is used to characterize different types of diesel fuel. In order to calculate the cetane number, the fuel must be burnt under very precise conditions in a special cooperative fuel research engine (CFRE) unit. These units are very expensive, and can generally only be found in very specialized settings. To overcome the challenges of calculating the cetane number, many organizations rely on the cetane index as an effective estimate of this calculation.
Many agencies rely on standards developed by ASTM International to accurately calculate cetane index. ASTM D4737 provides a formula based on four fuel variables. They include the density of the fuel as well as the recovery temperature at three cooling intervals. While others may rely on alternate calculation methods, this is one of the most widely used international standards.
The purest, highest-quality cetane has an index value of 100, and all other fuels are valued against this scale. The higher the cetane index, the better the fuel, and the shorter the delay between injection and combustion. For lower index values, fuel is likely of a lower quality, and will take longer to combust. Low cetane indexes are often associated with increased noise and higher levels of emissions compared to higher index values. Standard diesel fuel used in trucks and machinery has a cetane index of 40 to 55 on average.
One of the primary advantages associated with cetane index is its low cost and easy calculation. While index values are never as precise as cetane number, they do serve as a good estimate under normal conditions. A major drawback to index values is that they don't account for additives or certain types of impurities found in fuel. In very precise applications, cetane index may not be appropriate due to its imprecise nature. Those with very high-end engines may wish to pursue cetane number testing instead of relying on an estimate.