Evaporative Emissions

How does it happen? Reactive hydrocarbons can escape into the air through fuel evaporation. This is a major problem and the pollution it causes can be worse than what comes out the vehicle's tailpipe.

Why? Well, we're getting pretty good at making cleaner car engines, which is good because it reduces the amount of combustion emissions our cars produce. In fact, given today's exhaust emissions controls (like catalytic converters) evaporative emissions can account for the majority of hydrocarbon emissions in many areas. This is especially true on hot days.

So when do vehicles produce evaporative emissions?

• On hot days the air temperatures heat the fuel tank and cause ventilation (release) of gas vapors;
• When you put fuel in the tank and accidentally let it run over a little, the wasted fuel evaporates into the air (this usually happens when the automatic shutoff tells you the tank is full, but for some reason you just HAVE to squeeze a little more in - not a good idea!);
• When you put fuel in the tank and the nozzle isn't all the way in, or it doesn't have a seal to keep in the vapors (called a vapor recovery nozzle), the fumes escape into the air;
• When the car is running, its hot engine exhaust system vaporizes gas; and
• This gas evaporation continues after a car is turned off, until the engine is cooled down (this is called a "hot soak").

The annoying part about evaporative emissions is that just turning your car off doesn't mean it stops polluting. If you've seen the video, in the opening scene Wanesha says "even PARKED cars evaporate hydrocarbons," - she's talking about evaporative emissions.

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