This paper presents engine dynamometer testing and modeling
analysis of ethanol compared to gasoline at part load conditions
where the engine was not knock-limited with either fuel. The
purpose of this work was to confirm the efficiency improvement for
ethanol reported in published papers, and to quantify the
components of the improvement.
Testing comparing E85 to E0 gasoline was conducted in an
alternating back-to-back manner with multiple data points for each
fuel to establish high confidence in the measured results.
Approximately 4% relative improvement in brake thermal efficiency
(BTE) was measured at three speed-load points.
Effects on BTE due to pumping work and emissions were quantified
based on the measured engine data, and accounted for only a small
portion of the difference. Approximately half of the improvement
was accounted for by the fact that the heat of vaporization (HoV)
of the fuel detracts from the heat release measured in the
combustion bomb used in the determination of heating value, but
does not detract from the heat released during combustion in the
engine. Engine modeling indicated that the remaining difference in
BTE is due to lower burned gas temperatures and consequently lower
heat transfer losses. The lower temperatures are due to greater
charge cooling and to lower adiabatic flame temperature.
CO₂ emissions at part load are reduced about 7% for ethanol
compared to gasoline. Approximately 4% CO₂ benefit is due to
improved thermal efficiency, and about 3% is due to the increased
hydrogen-to-carbon ratio (H/C) of ethanol.
