How Does a Carbon Canister Work?

by John Albers

What is a Carbon Canister?

Carbon Canisters are devices found in automobiles designed to decrease the amount of air pollution the vehicle creates while at the same time increasing its fuel efficiency. Even when the vehicle's engine is turned off, hydrocarbons are produced. This occurs in the form of fuel vapor rising in the fuel tank. Carbon Canisters trap that vapor rather than allow it to escape the fuel tank, feeding it back into the engine.


Carbon Canisters are rectangular shaped boxes that sit apart from the fuel tank next to the throttle in most vehicles. There is an input port and an output port, with the two ports side by side. There are three chambers inside the canister, running in sequence from the intake to the outtake. The interior of the canister is filled with charcoal or carbon pellets. The input of the canister connects to the gas tank's vent port, while the output connects to the purge valve in the side of the vehicle's intake manifold.

How does a Carbon Canister Work?

When the vehicle is shut off, there is a pressure imbalance within the fuel tank caused by fuel being siphoned out, but no air being let back in to take up the empty space. The lower pressure in the gas tank promotes a greater rate of evaporation, letting some of the fuel become a gas. Eventually the tank's internal pressure equalizes, at which point the gas leaves the tank through the vent port and goes into the carbon canister. It's trapped there by the properties of the carbon within the canister, keeping it from escaping into the air. When the vehicle's engine starts, the sudden suction created along the intake manifold opens up the purge valve and pulls all the gaseous fuel out of the canister and burns it in the engine. The canister goes unused until the car is turned off again.

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