Isotopic Tracing of Fuel Component Carbon in the Emissions From Diesel Engines

The addition of oxygenates to diesel fuel can reduce particulate emissions, but the underlying chemical pathways for the reductions are not understood. While measurements of particulate matter (PM), unburned hydrocarbons (HC), and carbon monoxide (CO) are routine, determining the contribution of carbon atoms in the original fuel molecules to the formation of these undesired exhaust emissions has proven difficult. Using accelerator mass spectrometry (AMS) diagnostics, carbon atoms in a specific bond position in a specific fuel molecule can be labeled with carbon-14 (¹⁴C) and traced through the combustion event to determine whether they reside in PM, HC, CO, CO₂, or other emission products. This knowledge of how specific molecular structures produce certain emissions can be used to refine chemical-kinetic combustion models and to optimize fuel composition to reduce undesired emissions. Due to the high sensitivity of the technique and the lack of appreciable ¹⁴C in fossil fuels, fuels for AMS experiments can be labeled with such a small amount of ¹⁴C that they are not even considered radioactive.