FUEL MATTERS BoatU.S. SPECIAL REPORT
BY BOB ADRIANCE
THREE ETHANOL MYTHS CLARIFIED It’s time we get to the bottom of how E10 is affecting our engines
hough it’s just been a few years since ethanol began
to be widely used in the United States, a lot has been
written about its properties, the problems it’s created,
and how to best cope with its possible effects. Some of
the advice has been based on science, some on hearsay.
While E10 is not an ideal fuel — and E15 could cause serious problems for marine engines — at least a few myths about ethanol have
arisen with the potential to do more harm than good:
MYTH #1: ETHANOL-ENHANCED GASOLINE (E10) LOSES
OCTANE MUCH FASTER THAN REGULAR GASOLINE.
Many mechanics believe that octane loss during winter storage could be great enough
to damage an engine when it’s run in the spring. These same mechanics will often
recommend leaving the tank almost empty so that fresh gasoline can be added in the
spring to raise depleted octane levels. While all gasoline loses octane as it ages, ethanol-enhanced gasoline loses octane at about the same rate as regular gasoline, according
to Jim Simnick, a technical advisor at BP Global Fuels Technology, and Lew Gibbs, a
senior engineering consultant and Chevron Fellow. The two men have over 75 years
of combined experience working with gasoline and both agree that the loss of octane
over the winter would not be sufficient to damage an engine. Note, however, to keep
any gasoline, including E10, as fresh as possible, they said it’s good practice to always
add fuel stabilizer — an antioxidant — before the boat will be idle for long periods.
The recommendation to leave a tank mostly empty is bad advice; it could sig-
nificantly increase the amount of water that gets into the tank. When enough water
enters through the vent, the ethanol will separate (“phase separate”) from the gasoline.
Leaving a tank mostly empty does three things to increase the chances of phase sepa-
It increases the volume of open space in the tank (its “lung capacity”) so it can
“breathe in” damaging moist air. An almost-empty tank leaves more space on tank walls
for condensation to form. Leaving less gasoline in the tank means there will be less
ethanol to absorb the condensation.