Cetane Number of biodiesel

The ignition quality of a fuel can be deduced through its cetane number. A fuel with good ignition quality has a high cetane number, where the ignition delay period between the start of fuel injection and the onset of auto ignition is short. Cetane number of biodiesel varies with the feed stock used, but it is generally in the higher end of the typical diesel fuel range.

The value of cetane number is found to generally increase with increasing carbon chain length. Vegetable oils have a low cetane number. This is due to the presence of bulkier molecules in the triglycerides which have a high viscosity.

Methyl esters (biodiesel) are observed to possess a higher CN when compared to their corresponding oils. Methyl esters have a lower viscosity than their corresponding vegetable oils. Biodiesel records high CN values. These values are higher than that of diesel fuel with CN of 52. The high CN of biodiesel may be influenced by their characteristics of the feed stock.

Factors that affect the CN in the biodiesel are, e.g. the number of carbon atoms of the original fatty acids, the number of double bonds and the ester yield. Non edible oils blends have CN values close to those of the diesel fuel samples. This is due to the presence of diesel in blends. It is clear that, the higher the density the lower the Cetane number.

There exists a relation between cetane number and properties of biodiesel so a correlation was developed which is given as the following equation:

CN = K5 + K4 ν + K3 HV + K2 FP + K1 ρ

where:

K1, K2, K3, K4, K5 are constants,  ν is kinematic viscosity (mm2 /sec), HV is heating value (MJ/kg), FP is flash point (°C), ρ is density (kg/l).

The above equation shows the relationship between the Cetane number and the thermal properties. 

The average measured values of CN obtained compare well with the predicated values using the equation.  The equation is used to predict the CN values of different biodiesels consisting of different thermal properties. The coefficient of determination is 0.9, which indicates that the CN can be predicted with 90% accuracy based on thermal properties of biofuel.