Bioethanol is increasingly promoted as an alternative transport fuel worldwide and global production rapidly increased from 17 to 86 billion liters between 2000 and 2011 with government support such as mandates, subsidies and tax benefits...The rapid growth of ethanol use has sparked extensive research activities exploring the environmental impact of ethanol, greenhouse gas (GHG) emissions in particular, over the entire fuel life cycle, covering biomass feedstock production, ethanol conversion and the combustion of ethanol in vehicle engines.

However, most life cycle analyses (LCAs) to date have mainly focused on the production stage while assuming end-use efficiencies are the same for ethanol and gasoline despite the fact that ethanol can, in theory, affect engine efficiency and performance due to different thermo-chemical properties compared to gasoline. The resulting estimates of the net GHG impacts of ethanol could therefore be incomplete.

Several recent highly cited LCAs that have incorporated vehicle efficiency differences between gasoline and ethanol/gasoline blends could potentially be misleading. This is because they have relied on rather differing results from a limited number of sources while ignoring the vehicle-to-vehicle variations and the inherent uncertainties in vehicle efficiency measurements.

—Yan et al.

In the ES&T paper, Yan et al. set out to:

1. to examine the effect of ethanol blending on the energy efficiency of in-use vehicle models using a common metric and available empirical evidence;

2. to explore the impact of incorporating ethanol’s effect on vehicle efficiency into its LCA;

3. to determine whether potential exists for increased vehicle efficiency through ethanol use; and

4. to explore whether there is an optimal blending level for a given quantity of ethanol, for example, large quantities of low blends versus small quantities of high blends.

Effective substitution ratios for different blending levels. Credit: ACS, Yan et al. Click to enlarge.

Efficiency. To determine the vehicle efficiency effect of ethanol, they compiled paired energy efficiency data from all known studies of existing spark injection (SI)-engine vehicles operating on ethanol/gasoline blends and pure gasoline.

Most of the studies expressed vehicle efficiency in various volume-based units. Yan et al. converted all vehicle efficiency measurements into vehicle energy efficiency (VEE, km/MJ) based on fuel energy densities reported in each study.

While VEE is important, it does not directly reveal the amount of gasoline displaced by a given amount of ethanol. This is because the ethanol component may affect the conversion efficiency of the gasoline component in a blend, that is, make an engine more or less efficient in its use of the gasoline component. We therefore employ a factor called Effective Substitution Ratio (ESR) to represent the underlying substitution effects between ethanol and gasoline. For instance, the ESR is 1.5 if the use of 1 MJ ethanol leads to a savings of 1.5 MJ gasoline. The ESR is therefore a crucial scaling factor when evaluating the net effects of substituting gasoline with ethanol in terms of energy, GHG emissions and costs.

—Yan et al.

Among their general findings from the efficiency analysis were:

• VEE is on average 2.7% higher for ethanol/gasoline blends than for gasoline but varies within a wide range from 14.9% lower to 23.6% higher. While the mean VEE increases with increasing ethanol content—from 0.3% higher for E5 (5% ethanol) to 3.3% higher for E85 (85% ethanol), the ranges of VEE changes are quite large for all blends including both positive and negative changes.

• ESR can range from −1.42 to 4.36 and the overall average for all blends is 1.08. Vehicle-to-vehicle variations, differences in test conditions and the uncertainties in ESR calculations all could have contributed to this large range, the authors suggested.

  A negative ESR implies that the use of ethanol/gasoline blends considerably reduces VEE. This can be caused by poor engine tolerance of ethanol. There is a generally decreasing trend for the mean ESR with increasing ethanol content.

• Carburetor- and FI-engine vehicles on E5-E20 achieve mean ESR of 1.25 and 1.17 respectively, both with median slightly lower than the mean. The ESR spans over wide ranges for both combinations, with a 95% confidence interval (95%- CI) of 0.57:2.36 and 0.50:2.15 for carburetor- and FI-engine vehicles. respectively.

• The mean and median ESR for DI-engine vehicles on E5-E20 are 0.85 and 0.98 respectively. The mean and median ESR for DI-engine vehicles on E5-E20 would be 1.03 and 1.04 respectively if less-reliable values were excluded.

• Flex-fuel vehicles (FFVs) (including both FI and DI engines as the difference between the two was negligible) are most likely to achieve a mean ESR slightly higher than unity with a much tighter probability distribution (a 95%-CI of 0.92:1.16).

In general, carburetor-engine vehicles appear to be able to better take advantage of low blends than FI-engine vehicles while DI- engine vehicles do not seem to benefit. Nevertheless, with many fewer observations and the fact that DI engines are just beginning to enter the market, more tests are needed for DI-engine vehicles on low blends.

—Yan et al.

Lifecycle impact. The authors used Argonne’s GREET model to assess the effect of ESR on the net GHG impact of substituting conventional gasoline with corn or switchgrass ethanol, represented by the life-cycle GHG emission changes resulted from the use of 1 MJ ethanol.

They modified the model to allow incorporation of the uncertainties in ESR for low blends in regular vehicles and high blends in FFVs.

Their results indicated that the inclusion of ESR can greatly affect the net GHG impact of both corn and switchgrass ethanol, especially when ethanol is used in low blends.

The results highlighted that even though the net GHG impacts of ethanol used in low blends are likely to be modestly better than the previously estimated, they are also much more uncertain with a finite probability of being worse than previous estimates.

Given these substantial effects of ESR on the net GHG impact of ethanol observed herein, we assert that it is important and feasible to reduce the uncertainties in ESR and to take ESR into account in future stochastic LCAs to capture these potential risks that have been largely overlooked.

—Yan et al.

Probability density functions of Life-cycle GHG emission changes relative to gasoline resulted from the use of 1 MJ corn and switchgrass ethanol with parameters included for uncertainty analysis shown in brackets (default value for corn ethanol does not include LUC emissions). Credit: ACS, Yan et al. Click to enlarge.

Future potential for ethanol optimization. As part of their study, the authors touched on several promising technology pathways that might better utilize the favorable properties of ethanol. These included:

• For flex-fuel vehicles: better control of various operating parameters such as ignition and valve timing and through increased geometric CR with variable valve timing to reduce effective CR for gasoline or low blends.

• Variable geometric CR engines also have great potential for realizing the antiknock benefits of different ethanol/gasoline blends, especially for FFV.

• Dedicated ethanol (or E85) engines have the promise to improve efficiency substantially over comparable regular gasoline engines through direct-injection, increased geometric CR, variable valve timing, exhaust gas recirculation, aggressive turbocharging, and downsizing and/or downspeeding.

• Lean boosted engines with ethanol can produce efficiency higher than that offered by diesel engines.

• Increased engine efficiency at high load for ethanol/ gasoline blends (as a result of their antiknock properties) could also be explored in hybrid-electric powertrains.

• Innovative engine concepts and designs such as the Direct Injection Ethanol Boosted Gasoline Engine (DIEBGE) that improve the efficiency of gasoline use through the leverage effect of ethanol. The DIEBGE concept is to use a downsized port-injection gasoline engine with aggressive turbocharging, increased CR and ethanol as a knock suppressant to match the performance of a much larger engine, resulting in an increase in engine efficiency of 30% or more.

Policy implications. The authors noted that current biofuel policies have focused on maximizing ethanol production and consumption and reducing GHG intensity while ignoring the potential to improve vehicle efficiency through ethanol use.

In contrast, they suggested that future policies should be designed to promote appropriate engine technologies that correspond to the quantity of ethanol available in order to improve vehicle efficiency and maximize the ESR.

In general, the potential to increase ESR is much more limited for high blends than that for low blends mainly due to the diminishing leveraging effect of the ethanol component on the gasoline component. However, tailored strategies are needed for different regions depending on the current and future level of their ethanol use and changes to the vehicle fleets and fuel infrastructure may be necessary.

...Our analysis also highlights that comparing vehicle efficiency for different fuels is far from straightforward as fuel efficiency is usually reported in volumetric units such as MPG and km/l and volumetric energy content of fuels can vary considerably (and usually not measured). Recent studies have shown that even different volumetric units of fuel economy can be quite misleading and better public understanding in energy use and savings is required to realize the benefits of energy strategies. A common metric for vehicle efficiency expressed in energy and distance such as km/MJ or MJ/km could therefore be highly beneficial for both policy makers and consumers, especially with the increasingly diversifying types of fuels and vehicles.

Vehicle energy efficiency on all commercially available fuels (particularly for FFVs), when made available, could help policy makers to develop appropriate policies that optimize the fuel-energy nexus and consumers to make informed decisions. Furthermore, the actual substitution effects between different fuels and energy carriers, which have been broadly neglected, should be clearly communicated based on available evidence.

—Yan et al.

In the future, the authors noted, other alternative fuels that have different thermo-chemical properties compared with their conventional counterparts (e.g., biodiesel, methanol, FT liquids), will also need to be assessed with respect to their effects on engine efficiency and GHG impacts.

Resources

• Xiaoyu Yan, Oliver R. Inderwildi, David A. King, and Adam M. Boies (2013) Effects of Ethanol on Vehicle Energy Efficiency and Implications on Ethanol Life-Cycle Greenhouse Gas Analysis. Environmental Science & Technology doi: 10.1021/es305209a