Carbon canister for use in evaporative emission control system of internal combustion engine

First and second chambers are coaxially arranged and have substantially the same cross sectional area. First and second activated charcoal masses are respectively received in the first and second chambers. A labyrinth structure is arranged between respective first ends of the first and second chambers. An atmospheric air inlet port is provided by a second end of the second chamber. A third chamber is arranged beside the coaxially arranged first and second chambers. The third chamber has a first end positioned near a second end of the first chamber and a second end positioned near the second end of the second chamber. A third activated charcoal mass is received in the third chamber. A connector passage extends between the second end of the first chamber and the first end of the third chamber to provide a fluid connection between the first and third chambers. A fuel vapor inlet port is provided by the second end of the third chamber, and a fuel vapor outlet port is also provided by the second end of the third chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to an evaporative emission control system of an internal combustion engine, and more particularly to a carbon canister which is practically employed in the evaporative emission control system.

2. Description of the Related Art

Hitherto, for suppressing atmospheric pollution from motor vehicles powered by internal combustion engines, various evaporative emission control systems have been proposed and put into practical use. Some of them are of a type which employs a carbon canister to capture any fuel vapors (viz., HC) coming from the fuel tank. That is, the carbon canister prevents the vapors from escaping into the atmosphere. The carbon canister generally comprises a canister case which is filled with activated charcoal mass which adsorbs the fuel vapors. The canister case is formed at one end with an atmospheric air inlet port and at the other with both a fuel vapor inlet port and a fuel vapor outlet port. These three ports are communicated through flow passages defined in the activated charcoal mass.

Upon stopping of the engine, fuel vapors from the fuel tank are led into the canister through the fuel vapor inlet port and adsorbed (or trapped) by the activated charcoal mass. Only air that has left the fuel vapors therefrom is discharged to the atmosphere through the atmospheric air inlet port.

While, under operation of the engine with a canister purging mode, a certain negative pressure is applied to the interior of the canister from an intake system of the engine through the fuel vapor outlet port. With this, atmospheric air is led into the canister through the atmospheric air inlet port to pick up the trapped fuel vapors and carry the same to an intake manifold of the intake system of the engine through the fuel vapor outlet port. The fuel vapors thus led to the intake manifold become part of the air/fuel mixture entering the engine cylinders to burn. The action of clearing the trapped fuel vapors from the canister is called “purging”. The air used for purging the canister (more specifically, the activated charcoal mass received therein) is called “purging air”.

Due to inherent construction of the carbon canister, the trapped fuel vapors therein have such a concentration distribution characteristic that the fuel vapor concentration lowers as approaching the atmospheric air inlet port. However, because of the shape of the canister wherein the activated carbon is packed in a continuous space in the canister case, a so-called vapor migration phenomenon takes place wherein due to adsorption equilibrium, the trapped fuel vapors diffuse and move toward a lower concentration zone, that is, toward the atmospheric air inlet port. Thus, undesired leakage of the fuel vapors into the atmosphere increases with passing of time.

SUMMARY OF THE INVENTION

For solving the above-mentioned undesired leakage of the fuel vapors, an improved carbon canister is proposed by Japanese Laid-open Patent Application (Tokkai) 2003-003914. The carbon canister of this publication has first and second vapor trapping chambers arranged in a vapor flow passage which leads to an atmospheric air inlet port. However, even this improved carbon canister fails to provide the evaporative emission control system with a satisfied performance. Actually, the carbon canister shows a considerable pressure loss between the first and second vapor trapping chambers because a cross sectional area of the second vapor trapping is considerably small as compared with that of the first vapor tramping chamber.

It is therefore an object of the present invention to provide a carbon canister for use in an evaporative emission control system of an automotive internal combustion engine, which is free of the above-mentioned shortcoming.

According to the present invention, there is provided a carbon canister for use in an evaporative emission control system of an automotive internal combustion engine, in which undesired vapor migration phenomenon is minimized and undesired pressure drop between two vapor trapping chambers is minimized.

In accordance with a first aspect of the present invention, there is provided a carbon canister which comprises first and second chambers which are coaxially arranged and have substantially the same cross sectional area; first and second activated charcoal masses respectively received in the first and second chambers; a labyrinth structure arranged between respective first ends of the first and second chambers so that the first and second chambers are connected through a limited fluid communication; an atmospheric air inlet port provided by a second end of the second chamber; a third chamber arranged beside the coaxially arranged first and second chambers, the third chamber having a first end positioned near a second end of the first chamber and a second end positioned near the second end of the second chamber; a third activated charcoal mass received in the third chamber; a connector passage extending between the second end of the first chamber and the first end of the third chamber to provide a fluid connection between the first and third chambers; a fuel vapor inlet port provided by the second end of the third chamber; and a fuel vapor outlet port provided by the second end of the third chamber.

In accordance with a second aspect of the present invention, there is provided an evaporative emission control system of a motor vehicle powered by an internal combustion engine, which comprises a carbon canister including first and second chambers which are coaxially arranged and have substantially the same cross sectional area; first and second activated charcoal masses respectively received in the first and second chambers; a labyrinth structure arranged between respective first ends of the first and second chambers so that the first and second chambers are connected through a limited fluid communication; an atmospheric air inlet port provided by a second end of the second chamber; a third chamber arranged beside the coaxially arranged first and second chambers, the third chamber having a first end positioned near a second end of the first chamber and a second end positioned near the second end of the second chamber; a third activated charcoal mass received in the third chamber; a connector passage extending between the second end of the first chamber and the first end of the third chamber to provide a fluid connection between the first and third chambers; a fuel vapor inlet port provided by the second end of the third chamber; and a fuel vapor outlet port provided by the second end of the third chamber; a charging pipe extending from a fuel tank of the vehicle to the fuel vapor inlet port of the third chamber; and a purge pipe extending from a negative pressure producing area of an intake pipe of the engine to the fuel vapor outlet port of the third chamber.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, three embodiments100,200and300of the present invention will be described in detail with reference to the accompanying drawings.

For ease of understanding, various directional terms, such as, right, left, upper, lower, rightward and the like are used in the following description. However, such terms are to be understood with respect to only a drawing or drawings on which a corresponding part or portion is shown.

Referring toFIGS. 1to7, particularlyFIGS. 1 and 2, there is shown a carbon canister100which is a first embodiment of the present invention.

As is best shown inFIG. 2, carbon canister100comprises a generally cylindrical case12of a molded plastic, which includes a first hollow portion13and a second hollow portion14which are disposed on each other and extend in parallel with each other.

These two hollow portions13and14have respective left open ends which are integrally connected to spaced portions of a connector passage portion15. Thus, a generally U-shaped passage17is defined in and by the plastic case12, which comprises an interior of first hollow portion13, that of connector passage portion15and that of second hollow portion14.

As shown, first and second hollow portions13and14have a reinforcing rib16integrally interposed therebetween.

As shown inFIG. 1, first hollow portion13is formed at a right end thereof with an atmospheric air inlet port18.

Within first hollow portion13, there are packed a first activated charcoal mass21and a second activated charcoal mass23which are arranged in series in such a manner that the second activated charcoal mass23is positioned between first activated charcoal mass23and atmospheric air inlet port18. Preferably, the vapor adsorbing/releasing ability (or working capacity) of the second activated charcoal mass23is higher than that of the first activated charcoal mass21.

Within second hollow portion14, there is packed a third activated charcoal mass31which functions to selectively adsorb and release fuel vapors, as will be described in detail hereinafter.

Second hollow portion14is formed at a right end thereof with both a fuel vapor inlet port19and a fuel vapor outlet port20.

As will be understood fromFIG. 1, upon stop of an associated internal combustion engine “ENG”, fuel vapors in a fuel tank1is led into second hollow portion14through a charging pipe2and fuel vapor inlet port19and trapped by activated charcoal mass31packed therein. Any fuel vapors which have slipped through activated charcoal mass31are led to first hollow portion13and trapped by first activated charcoal mass21and second activated charcoal mass23. Air in first hollow portion13, which has the fuel vapors sufficiently released therefrom, is gently discharged to the atmosphere through atmospheric air inlet port18and an air inlet pipe3.

Under operation of engine “ENG” with a canister purging mode, a negative pressure produced in an intake pipe4downstream of a throttle valve4ais applied to the interior of carbon canister100through a purge pipe5and fuel vapor outlet port20. With this application of negative pressure to the carbon canister100, atmospheric air is led into the interior of carbon canister100through air inlet pipe3and air inlet port18. Due to this air introduction into carbon canister100, the fuel vapors are released from activated charcoal masses21,23and31and led into intake pipe4together with the atmospheric air through purge pipe5and finally burnt in each combustion chamber6of engine “ENG”.

Installed in purge pipe5is an electromagnetic valve7by which an amount of the fuel vapors directed toward intake pipe4and a timing of feeding the fuel vapors to intake pipe4are electronically controlled or adjusted. As shown, the valve7is controlled by an engine control unit8which has a microcomputer installed therein. That is, the amount of fuel vapors directed toward intake pipe4and the fuel vapor feeding timing are controlled in accordance with an operation condition of the engine “ENG”. If desired, the valve7may be of a mechanical type which enforcedly opens/closes purge pipe5in accordance with a magnitude of the negative pressure in intake pipe4.

If desired, charging pipe2may be provided with a negative pressure cut valve (viz., check valve), which shuts charging pipe2when the interior of carbon canister100shows a negative pressure higher than a predetermined degree.

By processing an information signal from an all range type exhaust air/fuel ratio sensor9installed in an exhaust system, engine control unit8controls, in a feedback manner, an air/fuel ratio of air/fuel mixture fed to combustion chambers6. More specifically, engine control unit8controls an operation of fuel injectors10through which a fuel is injected for cylinders of the engine “ENG”. It is to be noted that the all range type exhaust air/fuel ratio sensor9can issue a continuous output in accordance with the exhaust air/fuel ratio in the exhaust gas.

As is seen from the drawing, atmospheric air inlet port18, fuel vapor inlet port19and fuel vapor outlet port20are all arranged at the right end, that is, at the same end of the canister100. That is, these three ports18,19and20are placed at the same side, which facilitates the work for piping these ports18,29and20to associated parts without need of a larger space.

As is best seen fromFIG. 2, first hollow portion13of the case12comprises a first cylindrical chamber22in which the first activated charcoal mass21is packed, a second cylindrical chamber24in which the second activated charcoal mass23is packed and a cylindrical labyrinth structure25which is arranged between first and second cylindrical chambers22and24.

It is to be noted that first and second cylindrical chambers22and24have a substantially same cross sectional area.

As is described hereinabove, the vapor adsorbing/releasing ability (or working capacity) of the second activated charcoal mass23is higher than that of the first activated charcoal mass21. Generally, the vapor adsorbing/releasing ability of activated charcoal mass increases as the specific heat of the same increases.

As shown inFIG. 2, first cylindrical chamber22is equipped at left and right ends thereof with first and second filter members26and27respectively.

Like the above, second cylindrical chamber24is equipped at left and right ends thereof with third and fourth filters28and29respectively.

Cylindrical labyrinth structure25is arranged between second and third filters27and28, which connects first and second cylindrical chambers22and24with a limited fluid communication.

As is best seen fromFIG. 3, for the limited fluid communication between first and second cylindrical chambers22and24, cylindrical labyrinth structure25has thin and zig-zag passages defined therein.

Referring back toFIG. 2, a first coil spring30is arranged at a left end of first hollow portion13, by which a unit including first filter member26, first activated charcoal mass21, second filter member27, cylindrical labyrinth structure25, third filter member28, second activated charcoal mass23and fourth filter member29is constantly pressed rightward against a shoulder portion (no numeral) provided behind atmospheric air inlet port18. With this, the unit is steadily held in first hollow portion13.

Activated charcoal mass21in first cylindrical chamber22is of a crushed granulated type, and activated charcoal mass23in second cylindrical chamber24is of a briquett type.

As is seen from the graph ofFIG. 4, the vapor adsorbing/releasing ability (or working capacity) of activated charcoal mass23is higher than that of activated charcoal mass21.

Referring back toFIG. 2, second hollow portion14has a third cylindrical chamber32in which the third activated charcoal mass31is packed. As is seen from the drawing, third cylindrical chamber31is larger in size than the above-mentioned first and second cylindrical chambers22and24. Activated charcoal mass31in third cylindrical chamber31is the crushed granulated type and thus somewhat poorer in vapor adsorbing/releasing ability than the activated charcoal mass23in second cylindrical chamber24.

As shown in the drawing, third cylindrical chamber32is equipped at a left end thereof with a fifth filter member33, and at a right end thereof with sixth and seventh filter members34and35. Sixth filter member34is put in a base part of fuel vapor inlet port19and seventh filter member35is put in a base part of fuel vapor outlet port20, as shown.

A second coil spring36is arranged at a left end of third cylindrical chamber32, by which a unit including fifth filter member33, the third activated charcoal mass31, sixth filter member34and seventh filter member35is constantly pressed rightward against a partition wall37provided between and behind fuel vapor inlet port19and fuel vapor outlet port20, as shown. With this, the unit is steadily held in third cylindrical chamber32of second hollow portion14.

Partition wall37is integral with second hollow portion14and comprises a first seat portion38by which sixth filter member34is held and a second seat portion39by which seventh filter member35is held.

As is seen fromFIG. 2, first and second seat portions38and39are arranged at different positions with respect to an axial direction of second hollow portion14. In the illustrated embodiment, second seat portion39is positioned away from connector passage portion15as compared with first seat portion38.

As is seen from this drawing, fuel vapor inlet port19and fuel vapor outlet port20are communicated through the third activated charcoal mass31and sixth and seventh filter members34and35.

The above-mentioned first, second, third, fourth, fifth, sixth and seventh filter members26,27,28,29,33,34and35are of a permeable layered type made of polyurethane foam, non-woven fabric or the like.

As has been described hereinafore, in the case12, there is defined a generally U-shaped passage17in and along which the three activated charcoal masses23,21and31are arranged in series in the above-mentioned manner. Accordingly, a compact size of the case12and a sufficient length of passage17are both achieved at the same time in the carbon canister100of the present invention.

As has been mentioned hereinabove, first and second cylindrical chambers22and24of first hollow portion13have a substantially same cross sectional area.

It is now to be noted that the rate (viz., L/D) of the axial length (L) of first cylindrical chamber22to the diameter (D) of the same is substantially the same as that of third cylindrical chamber32. As has been mentioned hereinabove, in these first and third cylindrical chambers22and32, there are disposed the same kind of activated charcoal masses21and31.

It is also to be noted that the L/D rate of second cylindrical chamber24is smaller than that of first cylindrical chamber22(or third cylindrical chamber32). As has been mentioned hereinabove, in the second cylindrical chamber24, there is packed the activated charcoal mass23that is superior to the activated charcoal mass21or31in the vapor adsorbing/releasing ability.

In first and third cylindrical chambers22and32, the L/D rate is from about 2 to about 5. While, in second cylindrical chamber24, the L/D rate is smaller than 1.

That is, in the first embodiment100, the following inequalities are satisfied by the first, second and third cylindrical chambers22,24and32:
2≦L1/D1≦5  (1)
L2/D2<1  (2)
2≦L3/D3≦5  (3)
wherein:L1: axial length of first cylindrical chamber22D1: diameter of first cylindrical chamber22L2: axial length of second cylindrical chamber24D2: diameter of second cylindrical chamber24L3: axial length of third cylindrical chamber32D3: diameter of third cylindrical chamber32

FIG. 5is a graph depicting vapor adsorbing/releasing ability and pressure drop of a test sample of cylindrical carbon canister with respect to the L/D rate.

As is understood from this graph, the vapor adsorbing/releasing ability increases with increase of the L/D rate. However, with increase of the L/D rate, the pressure drop also increases. That is, with decrease of the L/D rate, the pressure drop decreases and the vapor adsorbing/releasing ability decreases.

In view of the characteristics of the tested cylindrical carbon canister depicted by the graph ofFIG. 5, the following fact has been revealed.

That is, in order to effectively suppress leakage of fuel vapors from atmospheric air inlet port18while suppressing increase of the pressure drop, it is preferable that the L/D rate of second cylindrical chamber24is set lower than that of first cylindrical chamber22. Furthermore, it is preferable that even when a certain amount of dust is deposited in each of cylindrical chambers22and24, the interior of first hollow portion13is prevented from showing an excessive pressure drop.

Considering these preferable matters, the above-mentioned L/D rate setting for first, second and third cylindrical chambers22,24and32have been determined by the inventor. If the chambers22,24and32have each a cross sectional shape other than the circle, the diameter of a circle that has the same area as the cross sectional shape should be used for “D” of the L/D rate.

Furthermore, preferably, the amount of second activated charcoal mass23is set smaller than 2% to 20% of that of the first activated charcoal mass21or that of the third activated charcoal mass31.

In the following, operation of carbon canister100of the first embodiment will be described with reference to FIG.1.

For ease of explanation on the operation, the following description will be commenced with respect to a condition wherein engine “ENG” has just stopped.

Upon stop of the engine “ENG”, fuel vapors in fuel tank1flows into second hollow portion14of canister100through charging pipe2and fuel vapor inlet port19and is directed toward atmospheric air inlet port18through the U-shaped passage17. This flow of the fuel vapors toward the air inlet port18is enhanced particularly when the internal temperature of fuel tank1is high. During the flow in U-shaped passage17, the fuel vapors are adsorbed by the third activated charcoal mass31in third cylindrical chamber32. Any fuel vapors which have slipped through the activated charcoal mass31of third cylindrical chamber32are led through connector passage portion15into first cylindrical chamber22where the fuel vapors are adsorbed by the first activated charcoal mass21. Almost all of the fuel vapors from third cylindrical chamber32are trapped by this first activated charcoal mass21of first cylindrical chamber22. However, if any fuel vapors which have slipped through the activated charcoal mass21are present, they are directed toward the second activated charcoal mass23of second cylindrical chamber24through cylindrical labyrinth structure25.

However, due to provision of labyrinth structure25, the flow speed of the fuel vapors toward the second activated charcoal mass23of second cylindrical chamber24is reduced. This enhances the fuel vapor adsorption by first activated charcoal mass21in first cylindrical chamber22. In second cylindrical chamber24, the remaining fuel vapors are adsorbed by the second activated charcoal mass23while leaving air that is directed toward the atmosphere through atmospheric air inlet port18and air inlet pipe3.

As is mentioned hereinabove, the fuel vapors from fuel tank1are forced to flow through the third activated charcoal mass31, the first activated charcoal mass21and the second activated charcoal mass23. Thus, almost all of the fuel vapors are adsorbed by carbon canister100, and thus, leakage of the fuel vapors into the atmosphere is suppressed or at least minimized. Furthermore, since activated charcoal mass23in second cylindrical chamber24has a higher vapor adsorbing/releasing ability, the undesired leakage of the fuel vapors is much assuredly suppressed.

While, under operation of the engine “ENG” with a canister purging mode, purging is carried out in carbon canister100. That is, under such operation of the engine “ENG”, atmospheric air is introduced into carbon canister100through atmospheric air inlet port18because of the power of the negative pressure applied to the interior of the carbon canister100from intake pipe4of the engine “ENG”. During flow in and along the U-shaped passage17toward fuel vapor outlet port20, the atmospheric air picks up the trapped fuel vapors from all of the second activated charcoal mass23, first activated charcoal mass21and third activated charcoal mass31and carries the same to intake pipe4for burning the same in the engine cylinders.

In the following, various advantageous features provided by carbon canister100of the first embodiment will be described.

Since labyrinth structure25is provided between first and second activated charcoal masses21and23, the undesired fuel vapor migration from first cylindrical chamber22to second cylindrical chamber24is greatly obstructed or at least minimized under stop of the engine “ENG”, and thus, the leakage of the fuel vapors into the atmosphere is greatly lowered.

Since first and second cylindrical chambers22and24have substantially the same cross sectional area, undesired pressure drop between these two chambers22and24is minimized.

Since second activated charcoal mass23that has a higher vapor absorbing/releasing ability is positioned just behind atmospheric air inlet port18, purging of the second activated charcoal mass23is quickly carried out. Thus, at early stage of the purging mode, the second activated charcoal mass23can exhibit a full-release of fuel vapors therefrom. This is quite advantageous for obstructing the vapor leakage into the atmosphere that would take place upon stop of the engine “ENG”.

FIG. 6is a graph depicting the results of an evaporation test (or vapor leakage test). In the test, three types of carbon canisters “a1”, “a2” and “a3” were examined in which the amount of leaked fuel vapors was measured in each canister “a1”, “a2” or “a3”. The tested carbon canisters were a first canister “a1” that contained only a normal activated charcoal mass, a second canister “a2” that contained a high specific heat activated charcoal mass and the normal activated charcoal mass and a third canister “a3” that contained a high effective activated charcoal mass and the normal activated charcoal mass. As is seen from this graph, second and third canisters “a2” and “a3” showed a higher emission suppression performance than first canister “a1”. This proves that the combination of first and second activated charcoal masses21and23which are different in vapor absorbing/releasing ability can exhibit a high emission suppression performance.

FIG. 7is a graph depicting a relationship between an amount of purging air (viz., atmospheric air led into an activated charcoal mass) and the vapor adsorbing/releasing ability of the activated charcoal mass. As is understood from this graph, with increase of the purging air, the vapor adsorbing/releasing ability of the activated charcoal mass increases. Thus, when carbon canister100is fed with a larger amount of atmospheric air under the canister purging mode, second, first and third activated charcoal masses23,21and31can effectively release the trapped fuel vapors therefrom.

The amount of purging air can be increased by expanding the engine operation range for the canister purging mode.

In the illustrated feedback type engine control system (seeFIG. 1) using the all range type exhaust air/fuel ratio sensor9that detects the exhaust air/fuel ratio in a linear manner, a larger amount of atmospheric air can be fed to carbon canister100as compared with another feedback type engine control system that uses an oxygen sensor that detects the oxygen concentration in the exhaust gas.

As is seen fromFIG. 1, between fuel vapor inlet port19and fuel vapor outlet port20, there is placed the third activated charcoal mass31. Accordingly, when, with the fuel vapors kept flowing from fuel tank1toward carbon canister100after stop of the engine “ENG”, the engine “ENG” starts again, the fuel vapors from fuel tank1are prevented from being directly led to intake pipe4. That is, upon starting of the engine “ENG”, the fuel vapors are inevitably treated by the third activated charcoal mass31before being transferred to intake pipe4, and thus, undesired exhaust emission impact, which induces an abnormally richer condition of air/fuel mixture, is suppressed.

If desired, one of first and second cylindrical chambers22and24may have another labyrinth structure installed therein. In this case, the vapor migration phenomenon is much assuredly suppressed.

Referring toFIG. 8, there is shown a carbon canister200which is a second embodiment of the present invention.

Since the second embodiment200is similar in construction to the above-mentioned first embodiment100, only portions that are different from those of the first embodiment100will be described in detail in the following.

As is understood from the drawing, in second cylindrical chamber24at a position close to atmospheric air inlet port18, there is disposed a fourth activated charcoal mass52. More specifically, the fourth activated charcoal mass52is formed into a honeycomb structure and an eighth filter member51is put between the fourth activated charcoal mass52and second activated charcoal mass23. That is, due to provision of eighth filter member51in second cylindrical chamber24, a fourth cylindrical chamber53is defined in which the honeycomb type activated charcoal mass52is disposed.

In this second embodiment200, the L/D rate of first cylindrical chamber22and that of second cylindrical chamber24are both from about 2 to about 4. In third cylindrical chamber32, the L/D rate is from about 2 to about 5.

That is, the following inequalities are satisfied by the second embodiment200:
2≦L1/D1≦4  (4)
2≦L2/D2<4  (5)
2≦L3/D3≦5  (6)
wherein:L1: axial length of first cylindrical chamber22D1: diameter of first cylindrical chamber22L2: axial length of second cylindrical chamber24D2: diameter of second cylindrical chamber24L3: axial length of third cylindrical chamber32D3: diameter of third cylindrical chamber32

Like the other filter members26,27,28,29,33,34and35, eighth filter member51is of a permeable layered type made of polyurethane foam, non-woven fabric or the like.

Due to addition of fourth activated charcoal mass52, the undesired leakage of the fuel vapors into the atmosphere is much assuredly suppressed. This carbon canister200is suitable for an evaporative emission control system incorporated with a hybrid type motor vehicle because the internal combustion engine of such vehicle has a less time for carrying out the purging mode for a carbon canister.

For examining the performance of carbon canister200of the second embodiment, a comparison test was carried out between the carbon canister200and a known carbon canister200X as shown in FIG.9. The known carbon canister200X comprises generally two parallel cylindrical chambers22and32which are connected through a connector passage portion15, each chamber22or32being filled with the activated charcoal mass21or31of crushed granulated type. For the comparison, the two carbon canisters200and200X were subjected to an evaporation test (or vapor leakage test) on a test bench, wherein for each carbon canister200or200X, the amount of leaked fuel vapors was measured on a first day when the canister200or200X was substantially new and on a second day when 24 hours had passed from the first day.

The results of the comparison test is shown by the graph of FIG.10. As shown, carbon canister200of the second embodiment showed an excellent emission reduction performance as compared with the related canister200X.

Referring toFIG. 11, there is shown a carbon canister300which is a third embodiment of the present invention.

Since the third embodiment300is similar in construction to the above-mentioned first embodiment100, only portions that are different from those of the first embodiment100will be described in detail in the following.

As is understood from the drawing, from atmospheric air inlet port18, there extends a pipe63in which a fourth activated charcoal mass52is disposed. More specifically, the fourth activated charcoal mass52is formed into a honeycomb structure and sandwiched between ninth and tenth filter members64and65. That is, in the pipe63, there is defined a fourth cylindrical chamber53in which the honeycomb type activated charcoal mass52is disposed.

In this third embodiment300, the L/D rate of first cylindrical chamber22and that of second cylindrical chamber24are both from about 2 to about 4. In third cylindrical chamber32, the L/D rate is from about 2 to about 5.

That is, the following inequalities are satisfied by the third embodiment300:
2≦L1/D1≦4  (7)
2≦L2/D2<4  (8)
2≦L3/D3≦5  (9)
wherein:L1: axial length of first cylindrical chamber22D1: diameter of first cylindrical chamber22L2: axial length of second cylindrical chamber24D2: diameter of second cylindrical chamber24L3: axial length of third cylindrical chamber32D3: diameter of third cylindrical chamber32

Like the other filter members26,27,28,29,33,34and35, ninth and tenth filter members64and65are of a permeable layered type made of polyurethane foam, non-woven fabric or the like.

Due to addition of fourth activated charcoal mass52, the undesired leakage of the fuel vapors into the atmosphere is much assuredly prevented. For the above-mentioned same reason, the carbon canister300is suitable for an evaporative emission control system incorporated with a hybrid type motor vehicle.

The entire contents of Japanese Patent Application 2003-178910 filed Jun. 24, 2003 are incorporated herein by reference.

Although the invention has been described above with reference to the embodiments of the invention, the invention is not limited to such embodiments as described above. Various modifications and variations of such embodiments may be carried out by those skilled in the art, in light of the above description.