This paper provides an overview of the effects of blending
ethanol with gasoline for use in spark ignition engines. The
overview is written from the perspective of considering a future
ethanol-gasoline blend for use in vehicles that have been designed
to accommodate such a fuel. Therefore discussion of the effects of
ethanol-gasoline blends on older legacy vehicles is not
included.
As background, highlights of future emissions regulations are
discussed. The effects on fuel properties of blending ethanol and
gasoline are described. The substantial increase in knock
resistance and full load performance associated with the addition
of ethanol to gasoline is illustrated with example data. Aspects of
fuel efficiency enabled by increased ethanol content are reviewed,
including downsizing and downspeeding opportunities, increased
compression ratio, fundamental effects associated with ethanol
combustion, and reduced enrichment requirement at high speed/high
load conditions. The effects of ethanol content on emissions are
also reviewed, including NMOG/CO/NOX, particulate matter, toxic
compounds, and off-cycle and evaporative emissions.
Considering the engine and vehicle-related factors reviewed in
this paper, a mid-level ethanol-gasoline blend (greater than E20
and less than E40) appears to be attractive as a future fuel. To
provide high knock resistance, this fuel should be formulated using
a blendstock that retains the octane of the current blendstock used
for regular-grade E10 gasoline. Further work is needed to recommend
a specific ethanol blend level, including analysis of fuel
efficiency and CO₂ benefits for representative powertrain/vehicle
applications, and of fuel production and supply considerations.
