Carbon canister

The key components in the fuel vapor control system include the fuel tank, vapor vent valves, vapor control valve, vapor tubing, the activated carbon canister, and the engine vapor management valve (VMV) [25,26], During normal vehicle operation, fuel tank vapor pressure is relieved through the use of vapor vent valves installed in the vapor dome of the fuel tank. The vent valves are designed to allow for the flow of fuel vapor from the tank, and to assure that liquid fuel does not pass through the valve. 

The vapor vent valves are connected to the tank vapor control valve, and ultimately to the carbon canister by tubing that is resistant to swelling in the presence of fuel vapors. The tubing material must also have a low HC permeation rate, so that the evaporative emissions are not increased due to release of HC molecules. The tank vapor control valve connects the carbon canister to two fuel tank vapor sources the vapor vent valve lines and a refueling vent tube. 

Fuel system components involved in the refueling process include the fuel tank, filler pipe, filler cap, vapor control valve, liquid-vapor discriminator (LVD) valve, and the carbon canister [27,28]. During vehicle refueling, which is monitored during the integrated refueling test as outlined in Fig. 1, the following operations occur in the evaporative emission control system ... 

As initially discussed in Section 3, carbon canisters are used in the automotive emission control system to temporarily store hydrocarbon vapors. The vapors are later purged into the air charge stream of the air induction system, thus regenerating the carbon canister. Carbon canister design is dependent on the characteristics of the vapors sent to the canister and the amount of purge air available. In the following section, factors that affect the performance of the evaporative emission control system will be discussed. 

The operating environmental temperature has an effect on the carbon canister performance [20]. Fig. 14 shows a 10% degradation in GWC as the environmental temperature increases from 25 to 80 °C. The hydrocarbon heel decreases by 55% during the same test. The hot environment helps to pui ge out the canister, but adsorption is reduced under the same conditions. Ideally the canister would be packaged in an area where it would not pick up heat from vehicle operation. 

Hydrocarbon vapor migration within the carbon canister is a significant factoi during the real time diurnal test procedure. The phenomenon occurs after the canister has been partially charged with fuel vapors. Initially the hydrocarbons will reside primarily in the activated carbon that is closest to the fuel vapor source. Over time, the hydrocarbons will diffuse to areas in the carbon bed with lower HC concentration. Premature break through caused by vapor migration for twc different canisters is shown in Fig. 17. The canister with the L/D ratio of 5.0 shows substantially lower bleed emissions than the canister with an L/D ratio of 3.0. 

There are many other factors that can affect the performance and on-vehicle reliability of activated carbon canisters. The following items shown in Table 6 represent some of the more important factors that must be taken into consideration when designing an evaporative emission canister. 

The use of activated carbon canisters in the control of running loss evaporative emissions will be presented through the use of an example vehicle application. In this example, the vehicle to be studied is a representative standard size sedan equipped with a 3.0 liter, V6 engine and a 72 liter (18 gallon) fuel tank. The vehicle is assumed to have an evaporative emission control system similar to the one presented in Section 3. 

The complete set of curves in Fig. 19 show that the adsorption performance of the activated carbon canister is a continuous requirement, and not an occasional need... 

The two liter carbon canister does not exhibit the HC release during the run loss portion of the test, nor does it release more than the allowable level of HC during the three day diumals. Thus, for the given vehicle configuration and the level of purge volume obtained by the vehicle, it is clear that a two liter carbon canister is required for this vehicle to pass the EPA certification requirement. This conclusion has an effect on the cost of the evaporative control system, in that the additional activated carbon volume and canister size will have an added cost, as will any additional hardware required to mount the larger canister on the vehicle. 

The comparison made in Section 6.1 demonstrates the important effect the amount of purge has on the performance of the carbon canister in terms of limiting the amoimt of HC release. This effect is also shown in the data presented in Fig. 21. In this example, the vehicle has been subjected to the same test cycle sequence as before, but in this case two different levels of purging are examined. Also, a two liter canister is used on the vehicle for the testing at both purge levels, in order to see the effect of purge level on a single canister volume. 

A key parameter in the generation of fuel vapor is the temperature level reached in the fuel tank during vehicle operation. As the temperature approaches the top of the fuel distillation curve, a sizable increase in vapor generation will occur, which severely impacts the amount of HC vapor that the carbon canister system must handle. Limiting the temperature increase in the fuel tank is an important parameter affecting the ability of the evaporative emission system to maintam allowable emission Levels. 

The example vehicle has been run through the test sequence using a two liter carbon canister and a 150 BV purge level. Fig. 22 presents the results for both a return and retum-less fuel system used in the vehicle. As shown, the fuel vapor temperature and the amount of fuel vapor generated are both lower for the retum-less system. This reduces the amount of HC adsorption required in the carbon canister, and it also reduces the amount of HC emissions in the test sequence, fhe return fuel system used with the stated purge volume and canister size emits an unacceptable level of HC during one of the diurnal sequences (2.12 grams), while the retum-less system emission values are well below the acceptable level. 

The design of activated carbon canisters for evaporative emission control is... 

The rate of vapor generation during refueling is a major parameter affecting the design of carbon canisters to meet ORVR requirements. 

The enhanced volatilization process is operated by putting contaminated soil in contact with clean air in order to transfer the contaminants from the soil into an air stream. The air stream is further treated through the use of carbon canisters, water scrubbers or afterburners to reduce air emission impacts. Four methods are available that can achieve this effect19 ... 

Another way to examine the effect of carbon particle size on kinetics is to look at the bleed emissions from a carbon canister [20,35]. Bleed emissions are those emissions that occur prior to break through. They are the result of the diffusion of gasoline vapor components that can develop during extended soak times between purge and adsorption events. 

---