poolboy said:
But what mechanism contains the vapors in the cannister while all this expansion is going on?
The process is called
adsorption. Basically it's a "partial pressure" kind of thing; the concentration of hydrocarbons stuck to the surface of the carbon adjusts to match the concentration in the air passing through. Thus when more or less pure vapor is going into the canister, it gets adsorbed onto the surface of the charcoal. (The surface is much more than what you see; the "activation" process involves creating millions of tiny fissures in the carbon chunks so there is a great deal of surface involved.) But later when the engine is running, it sucks fresh air through the canister, and the adsorbed hydrocarbons go back into the air and get sucked into the engine to be burned.<div class="ubbcode-block"><div class="ubbcode-header">Quote:] I'm not sure of this[/QUOTE] Well, I'm sure my Dad could give you a much more convincing lecture with examples (he's a retired chemist) but the basic process is used for a lot of things, so I believe it works. It's not perfect, but even if it's only trapping 90% of the vapors (and I believe the percentage is much higher), it still seems worthwhile to me.<div class="ubbcode-block"><div class="ubbcode-header">Quote:] Despite the purge it might get from the activation of the anti run-on valve[/QUOTE]Just for clarity, the ARS is not part of the purge process; and in fact most cars (including my Stags) don't have one. The purge happens continously all the time the engine is running; while the ARS is activated only when the key is turned off and the engine is still turning.
What the ARS does is apply vacuum to the float bowls, to starve the engine of fuel so it cannot run-on with the key off. The carbon canister just happens to be a convenient place to do this, since it already has vacuum and the link to the float bowls.<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I wonder how long it takes for the granules to become saturated to the point of being ineffective.[/QUOTE]Since they are cleaned every time the engine is run, in theory they never become saturated and ineffective. However, eventually they do become fouled and clogged with dirt and so on carried in during the purge process (which is why most canisters have a filter in the bottom of them that should be changed occasionally). That's also why Triumph recommended the canister be changed every 48,000 miles.
I've heard from several people that have solved "fuel smell" problems in their garages by replacing 30 year old carbon canisters. Must be doing something for them
If you've followed my posts, you'll know that I'm dubious about most of the emission controls from the 70s. They reduce power and increase fuel consumption (and CO2 emissions) with dubious benefit. But carbon canisters are an exception, IMO. They really do help without hurting (except of course for all the hoses & added complexity involved).