The cetane number measures diesel fuel quality by representing the time between injection and combustion. The cetane index is a cheaper alternative, calculated using fuel density and recovery temperature. A higher rating indicates better quality and shorter lag time, while lower ratings result in increased noise and emissions. The index is less precise than the cetane number, but easier to calculate. However, it does not account for additives or impurities, so high-end engines may require cetane number testing.
The cetane number is a number used to estimate the quality and composition of diesel fuel. This number represents the time between the injection of fuel into an engine and the start of the combustion process. This time to burn figure serves as an effective means of measuring fuel quality, as better quality fuels will ignite more quickly. 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 number of a fuel serves as an alternative to the traditional cetane number used to characterize different types of diesel. To calculate the cetane number, the fuel must be burned 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 environments. To overcome the difficulties of calculating the cetane number, many organizations rely on the cetane index as a good estimate of this calculation.
Many agencies rely on standards developed by ASTM International to accurately calculate the cetane index. ASTM D4737 provides a formula based on four fuel variables. They include fuel density and recovery temperature at three cool-down intervals. While others may rely on alternative 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 rated on this scale. The higher the cetane rating, the better the fuel and the shorter the lag between injection and combustion. For lower index values, the fuel is likely to be of lower quality and will take longer to burn. Low cetane ratings are often associated with increased noise and higher emission levels than higher ratings. Standard diesel fuel used in trucks and machinery has a cetane rating of 40 to 55 on average.
One of the major advantages associated with the cetane index is its low cost and ease of calculation. While the index values are never as precise as the cetane number, they serve as a good estimate under normal conditions. One major disadvantage of index values is that they do not take into account additives or certain types of impurities in the fuel. In very precise applications, the cetane number may not be appropriate due to its imprecise nature. Those with very high-end engines may want to go ahead with cetane number testing instead of relying on an estimate.
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