Exhaust Emission Impacts of Replacing Heavy Aromatic Hydrocarbons in Gasoline with Alternate Octane Sources
High-boiling aromatics have been shown to be the primary contributor to particulate matter (PM) emissions from gasoline engines. EPA, in collaboration with the Environment and Climate Change Canada and seven automakers, conducted a gasoline vehicle emissions test program to quantify the emissions impact of replacing a portion of high-boiling aromatics with lower-boiling aromatics, ethanol, and high-octane aliphatic blending components.
The test program included ten high-sales U.S & Canadian light-duty spark-ignited (SI) test vehicles, tested at nine emission labs, using standardized vehicle emissions tests (FTP and US06), over a set of five specialty-blended test gasolines.
The results indicate the potential for significant tailpipe PM reductions from light duty gasoline vehicles when high-boiling aromatics are replaced with other high-octane blending components. Switching to fuels in which high-boiling aromatics were reduced from 7.4% volume to 4.2-4.5% volume yielded an average PM emission reduction percentage of 35-45% on the FTP Composite cycle and 20-25% on the US06 cycle in this study.
The program also measured regulated gaseous emissions, which are of interest when considering broader air quality impacts of fuel formulation changes. No increase in emissions of nitrogen oxides (NOx), non-methane organic gases (NMOG), nor carbon dioxide (CO2) was observed for the test fleet when replacing a portion of heavy aromatics with alternate octane sources.
In addition to the emission test program, a parallel effort explored the potential utility of the ASTM D7096 SimDis test method in quantifying the heavy hydrocarbon content of gasoline blends. This work assessed sources of variability in SimDis results and developed procedural recommendations to improve repeatability and reproducibility focusing on the high-boiling (T90+) tail of gasoline. Validation studies done at EPA and General Motors laboratories showed that reproducibility values well below those published by ASTM can be achieved.
This report also presents SimDis and PM Index results for 80 US market gasoline samples and discusses the application of SimDis endpoint temperatures as way of limiting PM Index of market fuel. The report linked below includes a detailed description of the optimized D7096 test method in Appendix A.
- Exhaust Emission Impacts of Replacing Heavy Aromatic Hydrocarbons in Gasoline with Alternate Octane Sources (pdf) (3.2 MB, April 2023, EPA-420-R-23-008)
- Supplemental emissions and fuel data (xlsx)
- Assessment and Optimization of ASTM D7096 Simulated Distillation for Quantifying Heavy Hydrocarbons in Gasoline (pdf) (1.1 MB, April 2023, EPA-420-R-23-009)