Basics first, because "vapor pressure" seems to evade understanding often. Everyone has opened a gas can to the experience of a "chuff" of pressure being released. That was gasoline vapor. Things which evaporate easily, like gasoline, evaporate inside closed containers, like gas tanks, to the degree that their vapor pressure level is reached, then they just sit there, the pressure remaining. Open the tank, poof, close it, new pressure builds up again. Each time, a bit of liquid gasoline is lost as vapor.
How high is the vapor pressure? Depends only on temperature of the fuel and it's enclosure. Back as a teenager, learning how to break things, gas tank caps just had a tiny hole in them, they were "vented". So, fuel evaporated slowly from within the tank, all the time. Along the way, perhaps mid to late '50s, the mfrs. began placing a check valve in the caps, crude at first, often failing, though no one knew it. The check valve prevented vapor from escaping the tank, but opened as fuel was withdrawn for the engine, to allow for replacement of the fuel withdrawn with fresh air from outside the tank. Good system. Only time some vapor escaped was when the cap was removed to refill the tank.
Then, came the charcoal canister, connected to the tank by a tube, the pressure forcing it's way into the activated charcoal, which absorbed it. I know nothing more about that process, but originally, the canister was not connected to the engine in any way, that I know of. Sometime between carburetors and computers, the canisters were hooked up to dump the their fuel vapor occasionally into the engine, where it was burned. Today, a tank pressure sensor "tells" the computer when vapor pressure reaches a certain level, and computer then opens a solenoid valve allowing "purging" the canister into the engine. I think.
What are we gaining by doing this, environmentally, compared to the old, closed-tank system allowing air to replace the fuel being used? imp
How high is the vapor pressure? Depends only on temperature of the fuel and it's enclosure. Back as a teenager, learning how to break things, gas tank caps just had a tiny hole in them, they were "vented". So, fuel evaporated slowly from within the tank, all the time. Along the way, perhaps mid to late '50s, the mfrs. began placing a check valve in the caps, crude at first, often failing, though no one knew it. The check valve prevented vapor from escaping the tank, but opened as fuel was withdrawn for the engine, to allow for replacement of the fuel withdrawn with fresh air from outside the tank. Good system. Only time some vapor escaped was when the cap was removed to refill the tank.
Then, came the charcoal canister, connected to the tank by a tube, the pressure forcing it's way into the activated charcoal, which absorbed it. I know nothing more about that process, but originally, the canister was not connected to the engine in any way, that I know of. Sometime between carburetors and computers, the canisters were hooked up to dump the their fuel vapor occasionally into the engine, where it was burned. Today, a tank pressure sensor "tells" the computer when vapor pressure reaches a certain level, and computer then opens a solenoid valve allowing "purging" the canister into the engine. I think.
What are we gaining by doing this, environmentally, compared to the old, closed-tank system allowing air to replace the fuel being used? imp