In Reply to: Charcoal Canister posted by Howard on August 02, 2001 at 13:00:54:
Charcoal Canister, How it Works
Do you know how the system works? What it does is take the gas vapor from when the gas heats up or is poured into the tank and then condenses it and inserts it into the intake. This makes for a wet manifold as well as non-metered air entering the intake. This system was a complete after-thought from BMW and all true Euro cars don't have it. The reason why is because the US EPA wanted to prevent the gas vapors from escaping to the atmosphere. What really they are doing is creating a situation where you are getting extra air and fuel vapor into the combustion. As you will know, gas vapor does not combust as easily as liquid, and in fact is very hard to combust, and is therefore put through the cat and out the tail pipe. This increase the emissions of the car and also robs power because of the wet manifold condition and the increase in un-metered air going into the motor.
After completely understand how this system works and what it does to the operation of the motor, you will see that it has no benifit and in fact the claim you made of having your premium evaporting should happen because of the way the system works in the European cars where the expansion tank is vented to the atomsphere. What expansion tank does in European car is that it acts as a collector for the gas vapor and then once the gas vapor converts back to liquid, it flows back into the tank. This is also shown by the placement of the expansion tank in the car. The expansion tank is placed higher than the main tank and is also far away from a hot exhaust. This allows the gas vapors to cool and therefore condense back into a liquid and flow back into the tank. Thw only reason why there is a line to the outside of the car is that in cases when you are filling up the car, you are inputing more gas than air can come out. This therefore allows for the pressure in the tank to be released and so you are able to fully fill the tank. I have had this happen on a 7-series where I was filling it up and the pump stopped, but the tank was not full. This was caused by the charcoal canister system and the fact that it doesn't allow pressure to be released from the main tank quickly in situations like filling the car. Also, the possitioning of the line port on the expansion tank that runs to the charcoal canister is also at the top of the tank. If you know about chemistry, the gas vapor will settle to the bottom of the tank because of the fact that it is heavier than the normal air. Therefore, the normal air will go out of the expansion tank, and the fuel vapor will stay in. Now you are correct in saying that when you fill the car it will smell like gas for a little bit, but you are incorrect in saying that you want positive pressure in the gas tank. While positive pressure will allow for the tranfer pump to work easier, you will not be able to fill the car fully after driving. Also the charcoal canister system does not keep positive pressure in the tank. If you have ever looked at the canister and how it works, it is open to the air on the underside. This allows for the prevention of a build-up of pressure in the tank, but it doesn't work very well.
In closing, the charcoal canister system was an after-thought by BMW to abide by the EPA laws. Personally, after reviewing how the system and the problems that it cause, such as rough idle, incorrect mixture, poor emissions and difficulty in filling, I see now reason to keep the system on the car unless it is required by the emissions laws in your state.
Written by Rob Anderson '01