Measurement and Fractionation of Diesel Exhaust Hydrocarbons through Variable Temperature Controlled-Condensation and Flame Ionization Detection

Abstract

The industry standard for measuring hydrocarbons in engine exhaust is the flame ionization detector (FID). FID can measure total hydrocarbon concentrations but is limited in that it is not equally sensitive to all hydrocarbons present in exhaust. Instruments capable of measuring individual gas species such as gas chromatography with mass spectrometry (GC/MS) are expensive and sensitive to sample conditioning. A need exists for a simple and relatively inexpensive method to estimate the distribution of lube oil, partially burned, and unburned hydrocarbons present in the exhaust of internal combustion engines. In this work, a custom-designed variable temperature sample conditioner was developed for use with two fast-response FIDs to determine the condensable fraction of hydrocarbons present in engine exhaust over a range of temperatures. A correction model was developed to compensate for water condensing from the exhaust, nitrogen and carbon dioxide dissolving in the condensing water, and the reduced response factor of a FID in the presence of water. Sample conditioner data processed with the correction model were used to analyze low temperature combustion (LTC) and conventional engine operating modes for diesel fuel and various biodiesel blends. A theoretical model predicting the condensation of unburned biodiesel as a function of temperature was created using Antoine's equation and was used to validate the hypothesis that the middle weight condensing hydrocarbons were largely unburned fuel. The model was also used to estimate a relative response factor of 0.65 for the unburned biodiesel, resulting in a response factor of 0.77 for the total HC in the exhaust using the FID tested.