Reducing Sample Loss in Measurement of Heat of Vaporization of Ethanol/Gasoline Blends by Differential Scanning Calorimetry/Thermogravimetric Analysis

Higher gasoline heat of vaporization (HOV) can enable higher compression-ratio, direct-injection, spark-ignition engines by providing evaporative cooling that effectively increases fuel knock resistance. Methods to directly measure this fuel property in complex gasoline samples are not well developed. This study aimed to further improve a differential scanning calorimetry/thermogravimetric analysis (DSC/TGA) method to measure the total and partial HOV of gasoline. Ten market gasoline samples were chosen to have a wide range of properties to assess the method’s capability across the entire volatility range, with an emphasis on understanding how well the method captures the initial 10% of sample evaporation and how much sample is left unevaporated at the end of the experiment. Modifications to both the sample preparation/introduction method and the instrument itself were made to reduce initial sample losses, which included HOV measurements at 10°C and 5°C (in a cold chamber) and under (uncontrolled) ambient conditions. Experimental results from the DSC/TGA were compared to calculate total HOV results based on detailed hydrocarbon analysis (DHA). Results from the two methods agreed very well, with a ≤5% difference in many cases. The repeatability of the method was also investigated by analyzing samples in triplicate at the three temperatures investigated. One valuable conclusion was that the lower temperatures of 10°C and 5°C enabled more reproducible measurements for both total and partial HOV. This improved precision may result from more reliable temperature control in the cold chamber versus ambient laboratory temperature control. Additionally, cold chamber experiments, due to ergonomic limitations, did not allow for the use of a lid on the sample pan. The reproducibility of the evaporation rate was highly dependent on the pan/lid fit, which can vary significantly, such that elimination of the lid improved measurement precision, while operation at sub-ambient temperature slowed the evaporation rate. Results detailing sample preparation, instrument modifications, and a detailed comparison of total and partial HOV are presented.