Limitations of the use of cetane index for alternative compression ignition engine fuels

The cetane number of a fuel is an important factor in determining the quality of ignition in compression ignition (CI) engines. The significance of accurate measurement of cetane number has become even greater since the use of alternative fuels and modern CI engines. In this work, the comparison of different methods of cetane value measurement for fuels with different chemical composition such as ultra low sulfur diesel (ULSD), synthetic jet fuel (S-8) and military grade jet fuel (JP-8), trace amounts of additives and biodiesel blends under different conditions is reported. The cetane index was calculated by ASTM D4737 and ASTM D976 and the derived cetane number (DCN) was measured using an Ignition Quality Tester (IQT) as a basis of comparison with the cetane index. The best agreement among three methods was observed for ULSD, while S-8 showed the largest discrepancy. The cetane indices for S-8 were 70.2 and 67.3 calculated using D4737 and D976 respectively, while the DCN was 52.8. The addition of biodiesel to ultra low sulfur diesel (ULSD) fuel alters the chemical properties of the fuel. The derived cetane number reflected the increase in ignition quality with the addition of biodiesel while calculations for cetane index did not. The cetane indices for a commercial B20 were 45.30 and 46.70 while the DCN showed a significantly higher value of 48.50. Blending 5% oxidized biodiesel with ULSD caused an 8% increase in the derived cetane number of the blend. The cetane index of the 5% biodiesel was not significantly affected by oxidation. The effects of fuel additives on cetane measurements were reflected in the DCN measurements, but not with cetane indices.