Effects of Ethanol on Vehicle Energy Efficiency and Implications on Ethanol Life-Cycle Greenhouse Gas Analysis

Abstract

Bioethanol is the world's largest-produced alternative to petroleum-derived transportation fuels due to its compatibility within existing spark-ignition engines and its relatively mature production technology. Despite its success, questions remain over the greenhouse gas (GHG) implications of fuel ethanol use with many studies showing significant impacts of differences in land use, feedstock, and refinery operation. While most efforts to quantify life-cycle GHG impacts have focused on the production stage, a few recent studies have acknowledged the effect of ethanol on engine performance and incorporated these effects into the fuel life cycle. These studies have broadly asserted that vehicle efficiency increases with ethanol use to justify reducing the GHG impact of ethanol. These results seem to conflict with the general notion that ethanol decreases the fuel efficiency (or increases the fuel consumption) of vehicles due to the lower volumetric energy content of ethanol when compared to gasoline. Here we argue that due to the increased emphasis on alternative fuels with drastically differing energy densities, vehicle efficiency should be evaluated based on energy rather than volume. When done so, we show that efficiency of existing vehicles can be affected by ethanol content, but these impacts can serve to have both positive and negative effects and are highly uncertain (ranging from -15% to +24%). As a result, uncertainties in the net GHG effect of ethanol, particularly when used in a low-level blend with gasoline, are considerably larger than previously estimated (standard deviations increase by >10% and >200% when used in high and low blends, respectively). Technical options exist to improve vehicle efficiency through smarter use of ethanol though changes to the vehicle fleets and fuel infrastructure would be required. Future biofuel policies should promote synergies between the vehicle and fuel industries in order to maximize the society-wise benefits or minimize the risks of adverse impacts of ethanol.

Performances and emissions of a 4-stroke motorcycle fuelled with ethanol/gasoline blends

“…Another important aspect is the reduction of the greenhouse gases (GHG) emissions involved in the use of bioethanol fuel. The lower carbon content per unit of volume and the type of source used during ethanol production causes, as a net effect, result in a lower emission of carbon dioxide, which is the major GHG from transport [4]. The main disadvantage is related to the lower vapour pressure of bioethanol, which makes a cold start of the engine difficult.…”

The impact of ethanol and iso-butanol blends on gaseous and particulate emissions from two passenger cars equipped with spray-guided and wall-guided direct injection SI (spark ignition) engines

“…However, there are several drawbacks with the use of ethanol as gasoline extender. These include ethanol's lower energy content (26.8 MJ/kg) compared to gasoline (42.7 MJ/kg), the increase in RVP (Reid vapor pressure), and the inability to transport it through pipelines due to risk of water-induced phase separation [12,13].…”

Oil-saving pathways until 2030 for road freight transportation in China based on a cost-optimization model

“…Compared with internal combustion truck using oil-based fuels, the fuel consumption rate of alternative fuels and technology was evaluated based on energy rather than on volume because of the significantly different energy densities and vehicle efficiency [35]. The energy unit of this study is oil equivalent.…”