5.4 Why are evaporative emissions a problem?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
5.4 Why are evaporative emissions a problem?
As tailpipe emissions are reduced due to improved exhaust emission control
systems, the hydrocarbons produced by evaporation of the gasoline during
distribution, vehicle refuelling, and from the vehicle, become more and
more significant. A recent European study found that 40% of man-made
volatile organic compounds came from vehicles [72]. Many of the problem
hydrocarbons are the aromatics and olefins that have relatively high octane
values. Any sensible strategy to reduce smog and toxic emissions will also
attack evaporative and tailpipe emissions.
The health risks to service station workers, who are continuously exposed
to refuelling emissions remain a concern [73]. Vehicles will soon be
required to trap the refuelling emissions in larger carbon canisters, as
well as the normal evaporative emissions that they already capture. This
recent decision went in favour of the oil companies, who were opposed by the
auto companies. The automobile manufacturers felt the service station
should trap the emissions. The activated carbon canisters adsorb organic
vapours, and these are subsequently desorbed from the canister and burnt in
the engine during normal operation, once certain vehicle speeds and coolant
temperatures are reached. A few activated carbons used in older vehicles
do not function efficiently with oxygenates, but carbon cannister systems
can reduce evaporative emissions by 95% from uncontrolled levels.
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