Cetane Effect on Diesel Ignition Delay Times Measured in a Constant Volume Combustion Apparatus

The key feature of diesel fuel ignition quality is ignition delay time. In the American Society for Testing and Materials standard test for cetane number measurement, (ASTM D 613) the ignition delay time is held constant while the compression ratio is varied until ignition occurs at the set time. On the other hand, commercial diesel engines have set compression ratios and therefore, the ignition delay time varies with the cetane number of the fuel. The shorter this delay time, the wider the time window over which the combustion processes are spread. This leads to a more controlled heat release rate and pressure rise, resulting in prevention of diesel knock and in lowering of emissions. High cetane fuels exhibit short ignition delay times. The Constant Volume Combustion Apparatus (CVCA) precisely measures the ignition delay time of fuels.

This study investigates the CVCA as a supplementary tool for characterization of diesel fuel ignition quality under a variety of conditions. Two different diesel fuels of similar cetane number are characterized in the CVCA. These fuels are then additized with two different cetane improvers: 2-ethylhexyl nitrate and di-tert-butyl peroxide. The respective ignition delay time measurements are made in the CVCA and partitioned into physical and chemical delay times using the pressure curve characterizing each event. Our study shows that the CVCA accurately captures the effect of cetane improver additives on the ignition delay response times for the diesel fuels tested. Partitioning of this response to the physical and chemical attributes of the cetane improver adds to the temporal resolution of where in the combustion cycle these additives impart their major effect. Ignition delay time measurements at three different temperatures enable further characterization of the fuel by Arrhenius plot analysis.

The results show that diesel fuels containing cetane improver additives can be ranked in terms of ignition quality by the CVCA ignition delay time method. CVCA also provides additional information about low and high temperature ignition behavior of different cetane fuels.