1. Field of the Invention
The present invention relates to a fuel evaporative emission treatment system for preventing evaporative fuel from dissipating into the atmosphere from an engine fuel system.
2. Description of the Related Art
Engines installed in motor vehicles and the like emit harmful substances such as carbon monoxide, nitrogen oxides and hydrocarbons. For example, unburned hydrocarbon (HC) gas contained in blowby gas or in exhaust gas is emitted to the atmosphere, and also evaporative fuel gas produced in the fuel tank and containing HC as its main component is dissipated into the atmosphere. Motor vehicles are, therefore, equipped with systems for suppressing the emission of harmful substances, such as a blowby gas recirculation system, an exhaust gas purification system, a fuel evaporative emission treatment system, and an ORVR (Onboard Refueling Vapor Recovery) system. Further, in some gas stations, a special refueling system capable of sucking in evaporative fuel is installed in order to suppress the emission of harmful substances.
The fuel evaporative emission treatment system serves to prevent evaporative fuel from dissipating into the atmosphere from the fuel tank, and typically comprises a canister 6 having an adsorbent such as activated charcoal 8 filled in a container 7 thereof for adsorbing HC components in the evaporative fuel, as shown in FIG. 1. The container 7 has an inlet port 7a communicating with a fuel tank 1, an outlet port 7b communicating with the suction pipe of an engine, and a vent port 7c opening to the atmosphere. In this canister storage-type fuel evaporative emission treatment system, when the engine is at rest, evaporative fuel gas is introduced into the canister 6 through a vent hose 5 and HC components in the gas are adsorbed by the activated charcoal 8. When the engine is operated thereafter, the negative pressure of suction air produced in the suction pipe acts upon the outlet port 7b, permitting purge air to be introduced through the vent port 7c. Consequently, the HC components of evaporative fuel gas adsorbed by the activated charcoal 8 are separated therefrom by the purge air, and the separated HC components are discharged into the suction pipe together with the purge air. The HC components thus discharged into the suction pipe burn together with air-fuel mixture in engine cylinders, thus preventing the emission of evaporative fuel into the atmosphere.
In FIG. 1, reference numeral 3 denotes a fuel-cut valve for preventing the oversupply of fuel to the fuel tank 1, 4 denotes a rollover valve for preventing the leak of fuel from the fuel tank 1 in case of rollover of the motor vehicle, and 9 denotes a two-way valve arranged in a vent hose 5' extending between the fuel tank 1 and an intermediate portion of the vent hose 5. If evaporation of fuel takes place in the fuel tank 1 while the fuel-cut valve 3 is closed, the internal pressure of the fuel tank rises. In this case, the two-way valve 9 opens and permits the evaporative fuel to be discharged to the canister 6 through the vent hoses 5' and 5.
When fuel is supplied to the fuel tank 1 from a feed gun 10 inserted into a filler neck 2, negative pressure is produced inside the filler neck 2 due to aspiration (suction) induced by the discharge of fuel from the feed gun 10. Consequently, the outside air is drawn into the filler neck 2 through the gap between the filler neck 2 and the feed gun 10. The outside air then flows into the fuel tank 1 together with fuel discharged from the feed gun 10. The air thus introduced accelerates the evaporation of fuel in the fuel tank 1 and increases the quantity of evaporative fuel gas in the tank 1.
If a quantity of evaporative fuel gas exceeding the HC adsorption capacity of the canister 6 is introduced into the canister 6, HC components in the evaporative fuel gas that are not adsorbed by the canister 6 are dissipated into the atmosphere through the vent port 7c.
In order to prevent the dissipation of increased evaporative fuel, the canister capacity may be increased so that it can cope with an increase in the quantity of evaporative fuel caused by the inflow of air. However, a large-capacity canister takes up a large installation space and is heavy in weight, which is not preferable from the viewpoint of downsizing and weight reduction of motor vehicles. Also, large-sized canisters may not conform to certain regulations regarding the prevention of dissipation of evaporative fuel. Further, if a large-sized canister is used, an increased quantity of HC components in evaporative fuel is introduced into the engine from the canister. This makes the air-fuel ratio control on the engine side difficult and also imposes a severer burden on the exhaust gas purification measures.
To eliminate the drawbacks, the gap between the filler neck and the feed gun may be closed by a seal section arranged at the filler neck, to thereby prevent or reduce the flow of air into the fuel tank during refueling, as proposed in Unexamined Japanese Patent Publication No. 2-102822. However, the seal used in this type of system must be made of an elastic member etc. having fuel resistance because it is brought into close contact with the outer surface of the feed nozzle, increasing the cost. Further, since the feed nozzle is inserted into the seal section each time fuel is supplied, the sealing performance lowers after long use.
The ORVR system serves to prevent the dissipation of evaporative fuel gas from the fuel tank, and has a fuel trap 2c in the filler neck 2, as shown in FIG. 2. The fuel trap 2c is formed by concavely bending an end portion of the filler neck 2 located inside the fuel tank 1. The fuel trap 2c is located at a lower level than a fuel outlet opening 2b of the filler neck 2; therefore, fuel remains in the trap 2c even when the level of the fuel in the fuel tank 1 becomes lower than the fuel outlet opening 2b. Namely, fuel always exists in the fuel trap 2c, and the trapped fuel serves to block the communication between the fuel tank 1 and the outside of the same via the filler neck 2 at all times. Accordingly, even when a fuel inlet opening 2a of the filler neck 2 is opened at the time of re-fueling, the evaporative fuel gas in the upper space of the fuel tank 1 above the fuel level is never dissipated into the atmosphere through the filler neck 2.
However, also in the fuel tank equipped with the ORVR system, if the outside air is drawn through the filler neck 2 into the fuel tank 1 during refueling, the quantity of evaporative fuel gas in the fuel tank 1 increases due to the inflow of air. Accordingly, in the event a quantity of evaporative fuel gas exceeding the adsorption capacity of the canister 6 is introduced into the canister 6, the evaporative fuel gas is dissipated into the atmosphere through the canister 6.
The refueling system having the function of sucking in evaporative fuel is intended to prevent evaporative fuel gas from being discharged into the atmosphere from the fuel tank during refueling. As shown in FIG. 3, this refueling system includes a feed gun 10, a double pipe-type fuel feed hose 111, a fuel storage tank 112, and a bellows-type suction duct 113. The suction duct 113 is attached to the feed gun 10 so as to surround a feed nozzle 10a of the gun 10. When the feed nozzle 10a is inserted into the filler neck 2, the fuel inlet opening 2a of the filler neck 2 is hermetically closed by the suction duct 113. While in this state, the refueling system is operated, whereupon fuel in the fuel storage tank 112 is introduced to the feed nozzle 10 through the inner tube of the fuel feed hose 111, and then is supplied from the nozzle 10a into the fuel tank 1 through the filler neck 2. On the other hand, the evaporative fuel gas in the fuel tank 1 is drawn by suction into the fuel storage tank 112 through the filler neck 2, the gap between the feed nozzle 10a and the suction duct 113, and the outer tube of the fuel feed hose 111. Consequently, the dissipation of evaporative fuel gas from the fuel tank 1 into the atmosphere is prevented.
Some refueling systems having the above evaporative fuel suction function are associated with an autostop mechanism for preventing the oversupply of fuel. The autostop mechanism is designed to automatically stop the supply of fuel when a detection hole 10b formed in the feed nozzle 10a is submerged in fuel.
The refueling system having both the evaporative fuel suction function and the automatic fuel supply stopping function can sometimes automatically stop the fuel supply while fuel is supplied to the fuel tank, though the stop of fuel supply is actually not required. Namely, in this refueling system, an excessive negative pressure can be produced within the filler neck 2 in the vicinity of the feed nozzle 10a. In such cases, fuel flows back toward the feed nozzle 10a and enters the detection hole 10b, actuating the autostop mechanism.
To eliminate the disadvantage, a fuel tank apparatus is proposed in Unexamined Japanese Patent Publication No. 2-102822 wherein a branch vent tube diverging from a vent tube (corresponding to the element 5 in FIGS. 1 and 2) is connected to the fuel inlet side of the filler neck, and a solenoid valve, which opens during refueling, is arranged in the branch vent tube. During refueling, the evaporative fuel gas in the fuel tank is introduced into the filler neck through the branch vent tube so that the internal pressure of the filler neck in the vicinity of the feed nozzle may be increased to thereby prevent undesired actuation of the autostop mechanism. In this proposed apparatus, however, the internal pressure of the filler neck occasionally rises to a level considerably higher than the atmospheric pressure. If the pressure within the filler neck increases to such high level while fuel is supplied from a normal refueling system having no evaporative fuel suction function, a significant quantity of the evaporative fuel gas introduced into the filler neck via the branch vent tube is dissipated into the atmosphere through the gap between the filler neck and the feed gun.
Therefore, in the proposed apparatus, a seal section for preventing the dissipation of evaporative fuel gas is arranged in the filler neck at a location upstream of the position where the branch vent tube opens into the filler neck. However, seals excellent in both fuel resistance and airtightness are expensive, and the sealing performance lowers after long use, causing leak of the evaporative fuel.