Localized ignition delay prediction from temperature measurements of n-heptane in the combustion chamber of the ignition quality tester

The constant-volume combustion chamber apparatus, known as the Ignition Quality Tester (IQT (TM)), is used in some refineries to measure the ignition delay (ID), the time between fuel injection and fuel auto-ignition, of compression ignited fuels using pressure measurements, and to calculate the derived cetane number (DCN) from correlations developed with the cetane number (CN). The IQT (TM) is also used in research due to its capability to manipulate combustion controller parameters, including pressure, temperature, oxygen content and mass of fuel injection. In this study, in addition to the traditional global pressure measurements, the temperature was recorded at 46 locations inside the IQT chamber during fuel (n-heptane) injection and combustion. Two sets of experiments were conducted: n-heptane injection into nitrogen (no chemical reaction or combustion), and n-heptane injection into air (chemical reaction or combustion) and the ranges of ambient air temperatures and pressures were 530-590 degrees C and 10 - 21.4 bar, respectively. A method developed to estimate the physical and total ignition delay based on the temperature measurements is introduced in this study. The combustion initiation location was predicted from the results of the ignition delay, and it was determined that during long periods of ID, the fuel-air mixture becomes pseudo-homogeneous and the combustion initiates everywhere in the main part of the IQT chamber. At higher pressure and temperature conditions, the combustion was initiated at the end of the main part of the chamber. The validation of the ID by temperature method showed good agreement with the predicted ID from pressure measurements. The charge air temperature did not show a significant effect on the average percentage of the total physical ignition delay, while the charge pressure has a significant effect on the average percentage of the total physical ID, which increases with the charge pressure.

Automated summary

The study introduces a method to estimate ignition delay based on temperature measurements and finds that the fuel-air mixture becomes pseudo-homogeneous during long periods of ignition delay, with the charge pressure having a significant effect on the delay.