UC Riverside team characterizes impact on PM of fuels with varying aromatics and octane rating; benefit of increased ethanol fraction
Researchers at the University of California-Riverside have characterized the effect of decreased aromatic content fuels combusted in advanced vehicle technologies on emissions of particulate matter (PM). In a paper in the ACS journal Environmental Science & Technology, they present the changes in PM emissions for different fuels, engine technologies, and operating conditions. Among their findings is that an increased ethanol fraction in gasoline could help reduce PM mass and black carbon (BC) from gasoline direct injection engines (GDI).
Typical commercial gasoline comprises varying concentrations of aromatic hydrocarbons and octane ratings; the impacts on PM such as black carbon (BC) and water-soluble and insoluble particle compositions of these differences are not well-defined. The UC Riverside study tested seven 2012 model year vehicles, including one port fuel injection (PFI) configured hybrid vehicle; one PFI vehicle; and six GDI vehicles.
Broadly, PM mass and particle number (PN) emissions are higher for the GDI vehicles compared to their PFI counterparts. This finding is described in detail in a paper published earlier in Environmental Science & Technology. (Karavalakis et al.) That paper also detailed statistically significant increases in carbon monoxide, non-methane hydrocarbon, particulate matter (PM) mass, particle number, and black carbon emissions with increasing aromatics content.)
To characterize the impact on PM, the researchers drove each vehicle on the Unified transient testing cycle (UC) using four different fuels. Three fuels had a constant octane rating of 87 with varied aromatic concentrations at 15%, 25%, and 35%. A fourth fuel with a 91 octane rating contained 35% aromatics. They measured BC, PM mass, surface tension, and water-soluble organic mass (WSOM) fractions; the water-insoluble mass (WIM) fraction of the vehicle emissions was estimated.
The found that increasing fuel aromatic content increased BC emission factors (EFs) of transient cycles; however higher octane ratings in fuels reduce BC emissions. BC concentrations were 5-6 times higher for the GDI vehicles than the PFI and hybrid vehicles, suggesting a potential climate impact for increased GDI vehicle production.
Vehicle steady-state testing showed that the hygroscopicity of PM emissions at high speeds (70 mph; κ > 1) are much larger than emissions at low speeds (30 mph; κ < 0.1). Iso-paraffin content in the fuels was correlated to a decrease in WSOM emissions. Both aromatic content and vehicle speed increase the amount of hygroscopic material found in particle emissions.
In the next decade, higher ethanol and lower aromatic content in gasoline may become more common due to the renewable fuels standard (RFS). Our results show that reduced aromatic concentrations are associated with reduced PM mass and (more importantly) reduced BC from GDI vehicles. Thus, increasing the ethanol fraction in gasoline could help to reduce climate and human health impacts attributed to particle emissions from GDI vehicles. Future work should explore the effects of specific aromatic compounds that affect particle emissions whereas this study only correlated results with the total aromatic concentration. As ethanol concentrations are increased in the US, the higher octane fuel could effectively decrease BC emissions from the high PM emitting GDI vehicles thus helping to minimize BC.
High vehicular speeds impact the particle hygroscopicity parameter κ and the fraction of water-insoluble particles. This trend was shown during the UC and vehicular steady-state speeds. Water-soluble particles have been shown to have negative effects on human health. This is a concern for communities near major freeways where high speeds are typical. In addition, higher vehicle speeds and the increased water-soluble PM emissions found in this study might also have implications on CCN activity. Future studies are needed to elucidate the mechanism connecting vehicles speed/load and the amount of water-insoluble particle emissions. Future work should investigate the fuel and lubricating oil contribution to the particle emissions as the lubricating oil may have a more significant contribution to the particle emissions at higher speeds.
Daniel Z. Short, Diep Vu, Thomas D. Durbin, Georgios Karavalakis, and Akua Asa-Awuku (2015) “Components of Particle Emissions from Light-Duty Spark-Ignition Vehicles with Varying Aromatic Content and Octane Rating in Gasoline” Environmental Science & Technology doi: 10.1021/acs.est.5b03138
Georgios Karavalakis, Daniel Short, Diep Vu, Robert Russell, Maryam Hajbabaei, Akua Asa-Awuku, and Thomas D. Durbin (2015) “Evaluating the Effects of Aromatics Content in Gasoline on Gaseous and Particulate Matter Emissions from SI-PFI and SIDI Vehicles” Environmental Science & Technology 49 (11), 7021-7031 doi: 10.1021/es5061726