How Evaporative Emission Control Systems Work

By: Patrick E. George  | 

Don't let this happen to you.
Don't let this happen to you.
Thinkstock/Comstock Images/Getty Images

When we think of automotive emissions, we often think of the dirty stuff coming out of our exhaust pipes. The internal combustion process creates noxious chemicals (like CO2 and nitrogen oxides) that are released into the air as we drive. But tailpipe emissions are controlled using a variety of systems like catalytic converters and exhaust gas recirculation systems, and they're regulated by state and federal legislation.

However, did you know that there's another type of emission that comes from our cars? They're called evaporative emissions. The gasoline in your fuel tank and in you fuel lines slowly evaporates over time, releasing volatile organic compounds into the air. The Environmental Protection Agency says there are enough of these emissions to contribute to air pollution and pose a health risk to humans [source: EPA].


Because of their harmful nature, the government also regulates these evaporative emissions in new vehicles. This means that carmakers are required to install evaporative emissions control systems onto every new car and truck they build. It's nothing new, really -- these systems have been in place since the early 1970s. But as technology advances, car companies find newer and more innovative ways to mitigate pollution.

In this article, we'll tell you how evaporative emissions control systems work, and learn ways you can prevent your vehicle's fuel from evaporating under certain conditions.

Controlling Emissions

The fuel we put in our cars contains more than 150 chemicals, including benzene, toluene and sometimes even lead. These ingredients can cause dizziness, breathing problems and headaches when they're inhaled. Inhaling large amounts of gasoline fumes can even cause death. On top of all that, evaporated gasoline is one of the leading causes of smog and air pollution.

For these reasons, carmakers are required to install systems on their vehicles that help mitigate gasoline evaporations. Environmental regulation in the United States began in earnest in the early 1970s, and as a result, cars have had evaporative emission control (EVAP) systems ever since. These systems are designed to store and dispose of fuel vapors before they can escape into the atmosphere.


A typical system consists of a small canister full of charcoal, valves, hoses, vents in the fuel lines and a sealed fuel tank cap. When fuel evaporates inside the gas tank, the excess vapors are transferred to the charcoal canister. They're stored there until they can safely be transferred back to the engine to be burned with the normal air-fuel mixture.

When that's ready to happen, a valve creates a vacuum that draws the vapors into the engine. Fresh air is also drawn in through the vents and valves to mix with the vapors for better combustion. These systems can be controlled mechanically, or like on most on newer cars, through the engine's computer. The computer tells the valves when to purge the canister of vapors. This typically happens when the car is in motion, rather than at idle. It's just one example of some of the behind-the-scenes technology that you'll likely never see or feel.

As you may expect, things can go wrong with the EVAP system, too. If the canister fails to purge or does so under the wrong conditions, it can hamper the performance and emissions of your vehicle. When this happens, you may find that the entire system needs to be replaced.

Up next, we'll learn more about gasoline evaporation and discuss what you can do to reduce it in your own vehicle.

Keeping Gas Evaporations Down

There are steps you can take to prevent your gasoline from evaporating -- or at least slow the process down a little, anyway.
There are steps you can take to prevent your gasoline from evaporating -- or at least slow the process down a little, anyway.
Peter Dazeley/Photographer's Choice RF/Getty Images

It's no secret that gasoline is volatile. And it evaporates quickly, too. In fact, your car's fuel can turn from a liquid into a gas at a very fast rate -- especially when it's hot outside. This trait is bad for the environment, what with 20 percent of all hydrocarbon emissions from cars coming from fuel evaporation [source: Autoshop 101].

It's bad for your wallet, too. Unless you're an executive at an oil company, you probably aren't a fan of the way gas prices have been on a steady rise over the past few years. It's bad enough that your car burns gasoline -- why do you also have to lose more to evaporation?


The good news is that there are things you can do to prevent your gasoline from evaporating (or at least slow the process down a little). First and foremost, make sure your fuel cap is secured tightly. If you don't have a fuel cap, get one. Seriously. That old rag you stuffed in there isn't doing you any favors. Fuel can escape right out of your tank if it's not airtight.

Second, whenever possible, park in the shade during the summer months. Even though modern cars have advanced EVAP systems to prevent too much evaporation from occurring, gas does still evaporate from the tank, especially when the car is parked in the sun. This is even worse when it's extremely hot outside. Parking in the shade helps keep the entire vehicle cooler and reduces fuel evaporation.

One more thing: Buy your gas in the early morning or later at night. It's warmer in the afternoon and early evening, which means evaporation is more prevalent. That's why you're sometimes hit with that nasty gasoline smell at filling stations during the heat of the day.

Originally Published: Feb 23, 2012

Lots More Information

Related Articles

  • Autoshop 101. "Emission Subsystems." (Jan. 31, 2012)
  • U.S. Environmental Protection Agency. "Evaporative Emissions." (Jan. 31, 2012)
  • Vermont Department of Health. "Facts about Automotive Gasoline." (Jan. 31, 2012)
  • Wright, Lance. "Evaporative Control System (EVAP)." Auto Repair Help. (Jan. 31, 2012)