Have
you noticed that gas stations no longer have the strong smell of
gasoline as in years past? This
is a direct effect of the technology and regulations that led to signs
at the pumps that say “Do Not Top Off Tank.”
Most
of us who were driving (and buying gas) before 1995 would continue
pumping after the automatic shut-off kicked in because we wanted to
get in every last possible drop or to round off the price. If you’ve not yet kicked
that habit, please do so immediately!
At today’s gas prices you are stuffing perhaps an extra 0.28
gallons into the tank at the double expense of:
- increasing the air pollution from your car
- possibly causing damage to your vehicle’s emission
control system
Every gas tank
contains both liquid gasoline and gasoline vapors. As the tank heats up during the day, the pressure
in it increases. This
rising tank pressure forces vapor out of the tank—slightly more than
2% of all hydrocarbon (HC) emissions from an automobile are from the
gas tank. In cars built after 1970 these vapors (that used to
escape into the surrounding air) are collected in an evaporator
canister containing activated carbon.
Activated carbon is capable of holding up to 35% of its own
weight in fuel vapor by holding it on the surface of the charcoal
granules like a magnet.
Because this closed system collects vapors forced out of
the gas tank when the tank is filled and the use of vapor-recovery
nozzle design has increased, gas stations no longer have the
distinctive smell of gasoline.
These vapors are stored in the vapor canister until the
engine is started. As
soon as the engine is run above idle speed the engine vacuum purges
the canister by drawing the vapors into the engine where they’re
burned as part of the air/fuel mixture.
Now that you
know all this, you will better appreciate the important part… when you top off your gas
tank, you are actually overfilling it.
Overfilling the tank can also result in fuel spillage or spit
back. Spit back can
result in gasoline splashing on you, and both spillage and spit back
cause wasted gasoline that evaporates, adding to total HC emissions in
the atmosphere and contributing to smog formation.
Topping off and overfilling the tank can prevent the
evaporative emission control system from working properly by forcing
liquid fuel into the evaporative vapor lines and into the carbon
canister. Any amount of
liquid fuel will contaminate the activated charcoal and cause a loss
in its ability to hold onto the vapor.
Once the charcoal is contaminated from too much gasoline, the
system is effectively ruined. Unless
the canister is replaced, the vapor emissions into the atmosphere are
greatly increased for the rest of the life of the car.
