Future blending options for ethanol in gasoline include continuation of low-level blends (E0-E15), greater use of E85 in flexible fuel vehicles (FFVs), or new use of midlevel blends (E20-E40) in FFVs or in new vehicles designed with midlevel blend-capability. Clearly there are many considerations for determining the best future fuel strategy, including fuel properties, refining sector implications, vehicle compatibility, refueling infrastructure compatibility, and transition timing. One consideration is the impact of ethanol content on tailpipe emissions of NO_x, volatile organic compounds (VOC), CO, and particulate matter (PM) and hence on urban air quality.

… Higher ethanol content in gasoline affects several funda- mental fuel properties that can impact emissions, including increased oxygen content, decreased volumetric energy content typically measured as net heating value (NHV), increased heat of vaporization (HoV), and other volatility changes. These changes can have positive or negative effects that can depend on engine design, hardware, and control strategy. In addition to direct emissions impacts, higher ethanol content fuel can also provide more efficient combustion and overall engine operation under part-load conditions10 and under knock-limited higher-load conditions. Designing and calibrating for optimal performance on midlevel ethanol blends represents a new consideration for FFV design. FFVs have historically been designed for operation on E0/E10 and E85 and are certified for emissions compliance by testing with E0 and E85.

… To improve the understanding of the effects of fuel ethanol content on vehicle emissions a comprehensive study was conducted using a 2006 model year FFV on a chassis dynamometer equipped for measurement of both engine-out and tailpipe emissions.… We report the effect of fuel ethanol content on engine-out and tailpipe emissions and develop a novel conceptual framework for explaining the emission trends in terms of fuel chemistry and vehicle calibration effects.

—Hubbard

Broadly, with increasing ethanol content in the fuel, they found that the tailpipe emissions of ethanol, acetaldehyde, formaldehyde, methane, and ammonia increased; NO_x and NMHC decreased; while CO, ethene, and N₂O emissions were not discernibly affected.

NMOG and THC emissions displayed a pronounced minimum with midlevel (E20–E40) ethanol blends; 25–35% lower than for E0 or E80. Emissions of NO_x decreased by approximately 50% as the ethanol content increased from E0 to E30–E40, with no further decrease seen with E55 or E80.

The chemistry of a fuel impacts the emissions from an engine, with the most obvious example being unburned or partially burned fuel which is a major component of engine exhaust. Engine-out exhaust contains typically 1−3% unburned or partially burned organic fuel components. The emissions control system then removes 95−99% of these organic compounds.23

The general trend of increased ethanol and acetaldehyde emissions, and decreased NMHC emissions, with increased ethanol blend level is expected based on the dominant role of fuel chemistry in determining the tailpipe emissions of these species—e.g., via the unburned or partially burned fuel which is a major component of engine exhaust.

Comparison of the NO_x data shows the dominant role of engine calibration on the tailpipe emissions of NO_x. Upon sensing greater ethanol content in the fuel, the calibration prescribed different engine operating conditions that yielded higher engine-out and catalyst temperatures during the critical cold-start period, which improved the NO_x conversion efficiency of the exhaust after-treatment system.

There is a pronounced minimum of THC and NMOG emissions for midlevel blends reflecting the combined impact of fuel chemistry and engine calibration effects.

It is important to recognize the importance of both fuel chemistry and engine calibration in determining tailpipe emissions and to consider the fuel and vehicle together as a system. There are important differences in the nature of the fuel chemistry and engine calibration effects. The fuel chemistry effect is fundamental and the same qualitative emissions trend of increased ethanol, acetaldehyde, and CH₄ emissions and decreased NMHC emissions will be seen for all FFVs from all manufacturers. In contrast, the impacts of engine calibration on emissions depend on the calibration strategy and will be specific for specific vehicle models. Different manufacturers adopt different strategies and even the same manufacturer can adopt different strategies for different models.

Hence, in contrast to the situation for NMHC, ethanol, acetaldehyde, and CH₄, it is not possible to make general statements regarding the effect of fuel ethanol content on tailpipe emissions of NO_x, THC, and NMOG for the current or future FFV fleet based on testing of a small number of existing vehicle models. The effect of fuel ethanol content on NO_x, THC, and NMOG emissions for different vehicle models is not predictable without detailed information on engine calibrations of each model.

—Hubbard et al.

Ozone reduction in highly polluted urban areas generally requires reduction of the emissions of both NO_x and organic compounds, the Ford team noted. Irrespective of ethanol blend level, the effect of increasingly stringent emission standards and vehicle fleet turnover is that emissions of NO_x and NMOG from the on-road FFV fleet will continue to decrease substantially in the future.

A secondary question is whether this decrease would be slightly enhanced, or slightly degraded, as a result of the use of fuel with increased ethanol content in the on-road FFV fleet. It is not possible to provide a definitive answer to this question, but two points are worthy of note. First, any such effect will be small compared to the larger trend driven by emission regulations and fleet turn over noted above. Second, the minimum in NO_x and NMOG emissions for midlevel ethanol blends for the FFV tested in the present work points to future opportunities for emission reductions from FFVs.

—Hubbard et al.

Resources

• Carolyn P. Hubbard, James E. Anderson, and Timothy J. Wallington (2013) “Ethanol and Air Quality: Influence of Fuel Ethanol Content on Emissions and Fuel Economy of Flexible Fuel Vehicles.” Environmental Science & Technology doi: 10.1021/es404041v