A review study commissioned by the Urban Air Initiative—a non-profit organization focused on increasing the use of ethanol in the gasoline supply to replace aromatic compounds in gasoline—has found that measured and modeled effects of ethanol blending on gaseous and particulate emissions have varied widely between studies, to the point that it is difficult to reach any summary conclusions on ethanol’s emissions effects.
The study, conducted by Future Fuel Strategies, reviewed, assessed and compared nearly 100 different peer-reviewed vehicle emission studies. Many of the studies were organized by the Coordinated Research Council (CRC), which has the support of the petroleum industry and is relied upon by the EPA for fuels and emissions data. In addition to the Urban Air Initiative-sponsored study, the consultants have written two different technical papers, one of which is already in peer review. The results of the analysis will also be presented at the CRC Real World Emission Workshop in March in Long Beach, California.
Among the findings of the study:
Major studies, particularly those carried out by EPA and the CRC have highlighted ethanol’s potential to raise PM and either raise or maintain NOx. However, the first major study of new gasoline direct injection (GDI) engines did not find that NOx had a statistical increase.
One reason for inconsistent results is that automotive technology is rapidly evolving, especially with the advent of the GDI engine. Vehicles may interact with specific fuels and test cycles in different ways.
The blending of fuels used in the studies reviewed represents a major cause of differences in conclusions and draws into question applicability to real-world predictions. This is a major study finding here, and that is something that needs to be highlighted to the affected industries (including the ethanol industry) and policymakers alike.
Many of the blends used in emissions studies do not reflect typical makeup of in- use fuels. They are not reflective of real-world fuel blending that happens at the refinery or terminal. This is certainly the case in studies where "match blended" fuels were used to test a few select parameters, such as ethanol.
In match blending, it is just not possible to add ethanol to a blendstock for oxygenate blending (BOB) and hold all other properties constant. Matching select properties such as aromatics or T50 requires addition of higher emission aromatic streams which significantly contribute to emissions. The match blending essentially alters the emission characteristics (higher emissions) of the E0 fuel, increasing the emissions of the match blended ethanol blend and misrepresenting study conclusions on the impact of ethanol. Further, the blending of different hydrocarbon groups and the blending of ethanol produces nonlinear property responses, causing added uncertainty.
Splash blended fuels provide a study fuel closer than match blending to real world fuel blending but ignores any AKI effects on emissions and fails to account for the reduction in fuel aromatics results in real world blending, and its impact on emissions.
Variability between studies in itself suggests that many studies should not be used to predict real-world emissions effects, and the causes of the variability between studies are also likely to cause differences in emissions effects between most study conclusions and the real-world application.
Study conclusions are suited to real-world predictions only if the study addresses the fuels of interest holistically. This means all appropriate parameters in a multivariate model derived from the study must be employed. For example, real-world ethanol blends typically have reduced T50 and lower aromatics than an E0, and all three property changes must be considered in using a model to predict PM or NOx.
Studies adopt different parameters for planning, blending and modeling, stressing a lack of agreement on root cause of emissions. Variation in emissions is assigned to parameters, but those assignments vary between studies. Reselection of one parameter may change the attribution of emissions to another parameter, such as ethanol level. This is important, yet not accounted for, explained or acknowledged in many studies reviewed.
Future studies seeking to clarify real world ethanol level effects on emissions, whether multivariate analyses or direct comparisons, should seek to use fuels at different blend levels that are derived from real refinery streams or thoroughly represent real world fuels, without imposing limiting parameters in the blending. They should also consider refinery economics and not blend just high-value streams. Suggestions for potential study designs are incorporated in the report.
Care must be taken in addressing emissions from mid blends, insofar as the step in properties from E0 to E10 is very different than the step from E10 to E15 or to E20. Linear extrapolation is not possible, though some studies have not defined their model limits.