Abstract:
The effectiveness against vapor breakthrough of an adsorbent material (e.g., activated carbon granules) containing canister in an evaporative fuel emission control system is greatly increased by employing a relatively small secondary volume of adsorbent downstream of the vapor vent of the primary adsorbent volume and heating the secondary volume just prior to commencing the flow of purge air back through the two adsorbent volumes to remove adsorbed fuel and carry the purged fuel to the induction system of an associated engine. The secondary volume is heated to a temperature enabling complete purging of hydrocarbons from it and, thus, to greatly increase the capacity of that volume to prevent fuel vapor breakthrough during the subsequent engine-off fuel vapor storage cycle. The secondary volume may be contained in a common canister with the primary volume or in a secondary canister.

Description:
TECHNICAL FIELD 
     This invention pertains to evaporative fuel vapor emissions from automotive vehicles. More specifically, this invention pertains to an improved fuel vapor adsorption canister system and method of operation that reduces the breakthrough of fuel vapor to the atmosphere. 
     BACKGROUND OF THE INVENTION 
     Fuel evaporative emission control systems have been in use on automotive vehicles for over 30 years. The gasoline fuel used in many internal combustion engines is quite volatile. The fuel typically consists of a hydrocarbon mixture ranging from high volatility butane (C-4) to lower volatility C-8 to C-10 hydrocarbons. When a vehicle is parked in a warm environment during the daytime heating (i.e., diurnal heating), the temperature in the fuel tank increases. The return of hot fuel from the engine also heats the contents of the fuel tank. The vapor pressure of the heated gasoline increases and fuel vapor will flow from any opening in the fuel tank. Normally, to prevent fuel vapor loss into the atmosphere, the tank is vented through a conduit to a canister containing suitable fuel adsorbent material. High surface area activated carbon granules are widely used to temporarily adsorb the fuel vapor. 
     The fuel vapor enters the canister through a top inlet of the canister and diffuses downwardly under its own pressure and gravity into the bed of carbon granules where it is adsorbed in temporary storage. The total volume of adsorbent is specified so as to be suitable to retain a quantity of fuel vapor expected to evaporate from the fuel tank during normal or representative usage of the vehicle. 
     The canister is molded of a thermoplastic material and shaped so that ambient air can be drawn through the carbon granule bed during engine operation to purge adsorbed fuel from the surfaces of the carbon particles and carry the removed fuel vapor into the air induction system of the vehicle. Typically, a partition is formed in the canister to lengthen the flow path of vapor and air through the volume of carbon particles. Thus, the fuel vapor enters at one end of the flow path and escapes to the atmosphere at the opposite end, the vent end, if the quantity of fuel exceeds the adsorption capacity of the carbon volume. Ambient air, induced to flow through the activated carbon bed under engine intake vacuum, enters the canister at the “vent” end of the flow path. The air traverses the full length of the flow path and exits the canister with desorbed, i.e., purged fuel at the vapor inlet end of the carbon volume. Typically, neither the canister nor the purge air experience heating other than ambient heating. 
     The described emission control system obviously works in a repeating cyclical mode. When the engine is not running, fuel vapor generated by diurnal heating, or the like, flows to the canister and is adsorbed up to the capacity of the adsorbent volume. The vehicle may remain idle for several days and fuel vapor will accumulate in the canister. The initial loading will be at the inlet end of the adsorbent volume but the fuel gradually becomes distributed along the entire adsorbent bed pathway. When the vehicle engine is started and can accommodate a secondary fuel-air mixture, a purge valve is opened and purge air is drawn through the adsorbent volume. Purging can continue as long as the engine is running and the air can cause the removal of a substantial portion of the stored fuel vapor. But a portion of the adsorbed hydrocarbons remain adsorbed on the carbon. That portion is called the “heel” and it significantly limits the capacity of the carbon to adsorb additional fuel. 
     Environmental regulators are proposing lower limits on the amount of fuel vapor that can escape the evaporative emission system during a prescribed test of the system in a closed space called SHED (Sealed Housing for Evaporative Determination). For example, the California Air Resources Board (CARB) has proposed “near zero” and “zero” evaporative emission standards for automotive vehicles for year 2004. The proposed standards require near-zero fuel vapor emissions from all the sources: permeation losses through plastic fuel system parts; leaks through the fittings and joints; and canister breakthrough emissions. Reducing the emissions through the leaks involves the selection of better sealing joints and connectors or eliminating some of joints, and reducing permeation losses involves the selection of low permeability or no permeability materials, whereas reducing canister breakthrough emissions to near-zero requires new technologies in the canister design. An object of this invention is to provide a canister system, and method of operating the system, that will limit canister breakthrough emissions to less than 0.02 grams fuel loss per test. 
     SUMMARY OF THE INVENTION 
     In one aspect, this invention provides a method of increasing the adsorption capacity of an evaporative emission control system by selectively heating a small portion of the adsorption material before air purging of the canister. Granular activated carbon is a preferred adsorbent material. The adsorbent portion that is heated is located at the purge air inlet region of the canister system. Generally, only about 1% to 5%, and preferably less than 3%, by volume of the total adsorbent is heated prior to purging. But that portion is heated to a temperature at which the normal flow of ambient purge air will remove substantially all of the adsorbed fuel, including the hydrocarbon heel, during a purge cycle. When high surface area carbon granules are employed, the upstream secondary carbon volume is preferably heated to about 350° F. (177° C.). 
     A volume of carbon granules heated to 350° F. is readily stripped of gasoline vapor with a flow of ambient air of suitable duration. No hydrocarbon heel remains in the heated volume, and during the subsequent vapor loading cycle, vapor breaking through the main carbon volume is readily adsorbed in the “green,” or hydrocarbon-free, secondary volume. 
     Preferably, the relatively small secondary volume of adsorbent is thus heated prior to each purge cycle. Heating may be initiated by the engine control computer module and accomplished using vehicle battery-powered, embedded electrical heating elements, or the like. When a temperature sensor in the secondary adsorbent volume indicates that a suitable purge temperature has been reached, the engine control computer causes the heater to be shut off and the purge valve to be opened. Engine induction system vacuum induces the flow of purge air from the engine compartment, through the secondary and primary adsorbent volumes and into the operating engine. The air flow both strips the volume of hydrocarbons and cools the granules to restore their full adsorptive capacity before the next vapor load cycle. 
     The secondary volume of adsorbent may be located in the canister adjacent the air purge inlet. In this embodiment, the secondary volume is closely adjacent the main adsorbent volume and it may be preferable to provide some thermal insulation between them. In a second embodiment, the small heated volume is located in a smaller, secondary molded canister in the air inlet line with the vent valve. 
     Other objects and advantages of the invention will become more apparent from a detailed description of preferred embodiments that follows. The description will refer to the drawings that are described in the next section of this specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the evaporative fuel emissions control system of an automobile. 
     FIG. 2 is a sectional view of a carbon granule filled canister in accordance with a first embodiment of the invention. 
     FIG. 3 is a sectional view of a secondary carbon granule filled canister in accordance with a second embodiment of the invention. 
     FIG. 4 is a graph of breakthrough emissions, in grams, of butane versus butane load, in grams, for a green carbon canister and a used carbon canister. 
     FIG. 5 is a graph using the same ordinates as FIG. 4 but illustrating the effect of increasing soak periods on breakthrough emissions. 
     FIG. 6 is a graph using the same ordinates as FIG. 4 but comparing baseline canister performances with a canister utilizing the subject invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A typical evaporative fuel emissions control system  10  for an automotive vehicle is illustrated in FIG.  1 . The illustration is schematic and the components are not drawn to scale. 
     The system comprises an engine schematically indicated at block  12 . However, the engine would typically be a multi-cylinder, gasoline-powered, internal combustion engine. The operation of a modern fuel efficient, low exhaust and evaporative emissions engine is controlled using a suitable programmed digital microprocessor or computer, indicated at block  14 . The microprocessor is part of a control module that controls the operation of at least the engine and its emission controls (an engine control module, ECM) or the engine and transmission (a powertrain control module, PCM). Such control modules in various similar forms are used on millions of cars, sport utility vehicles, trucks, and the like, today. 
     When the engine is started, the control module, which is powered by the vehicle battery, not shown, starts to receive signals from many sensors on the engine, transmission and emission control devices. Line  16  from the engine  12  to control module  14  schematically depicts the flow of such signals from the various sensors on the engine. During engine operation, gasoline is delivered from a fuel tank  18  by a fuel pump (not shown, but often located in the fuel tank) through a fuel line, not shown, to a fuel rail and fuel injectors that supply fuel to each cylinder of the engine or to ports that supply groups of cylinders. The timing of the operation of the fuel injectors and the amount of fuel injected per cylinder injection event is managed by the control module  14 . The subject emission control purge system is operated in harmony with engine operation to avoid upsetting the air-to-fuel ratio in the engine. 
     Since gasoline and other fuels are quite volatile, fuel tank  18  is closed except for a vent line  20 . Tank  18  is often made of blow molded, high density polyethylene provided with a suitable interior gasoline impermeable layer(s). The tank  18  is provided with fill tube  22  with a gas cap  24  closing the gas fill end  26 . The outlet end  28  of fill tube  22  is inside tank  18  and is provided with a one-way valve  30  to prevent liquid fuel from splashing out the fill tube  22 . 
     A volume of gasoline  32  is indicated with upper surface  34 . A float-type fuel level indicator  36  provides a fuel level signal through line  38  to the control module  14 . Fuel tank pressure sensor  40  and temperature sensor  42  provide their respective data through signal transmitting lines  44  and  46 , respectively, to controller  14 . These sensors are not necessarily present in all evaporative control systems and sometimes their functions may be combined. They are sometimes used for diagnostic purposes. 
     Fuel tank  18  is provided with a vent line  20  that leads through seal  48  from the top of the tank to a fuel vapor adsorption canister  50 . Float valve  52  within the tank  18  prevents liquid gasoline from entering vapor vent line  20 . During heating of the fuel in the tank by liquid fuel returned from the hot engine (through fuel return line, not shown) or by ambient heating, hydrocarbon fuel vapor is generated from the gasoline. Vapor mixed with air flows under the vapor pressure through vent line  20  to the vapor inlet of canister  50  (see FIGS.  1  and  2 ). The vapor enters canister vapor inlet  54 , flows past granule retainer element  56  and diffuses into primary volume  57 ′ and  57 ″ of adsorptive material  58 . 
     Canister  50  is typically molded of a suitable thermoplastic polymer such as nylon. In this embodiment, canister  50  comprises four side walls, defining an internal volume of rectangular cross section (two side walls  60  shown), with an integral top  62  and a vertical internal partition  64  that extends from top  62  and the non-shown front and rear sides. Canister  50  includes a bottom closure  66  that is attached to the side walls. At the top of canister  50  is a vent opening  68  that also serves as an inlet for the flow of air during the purging of adsorbed fuel vapor from the adsorbent material  58 . Also formed in the top  62  of the canister  50  is a purge outlet  70  through which a stream of purge air and purged fuel vapor can exit the canister. 
     Connected to vent opening  68  is a vent line  72  (see FIG. 1) and solenoid actuated vent valve  74 . Vent valve  74  is normally open as shown, but upon actuation of battery powered solenoid  76 , stopper  78  is moved to cover vent opening  80 . Solenoid  76  is actuated upon command of control module  14  through signal lead  79 . The vent valve  74  is usually only closed for diagnostic purposes. 
     Purge outlet  70  is connected by purge line  82  through solenoid actuated purge valve  84  to the engine  12 . Purge valve  84  includes a battery powered solenoid  86  and stopper  88  to close purge opening  90 . Purge valve  84  is opened only by command of control module  14  through signal lead  91  when the engine  12  is running and can accommodate the fuel-laden air stream drawn through canister  50 . 
     Referring again to FIG. 2, it will be appreciated that as an airfuel mixture flows from the fuel tank through vent line  20  and through the inlet  54  into canister  50 , fuel vapor will be absorbed onto the granules of adsorbent material  58  in the canister. Gradually, the granules of activated carbon or other suitable adsorbent material will become laden with butane and heavier hydrocarbons, and the vapor will further settle into the portion of adsorbent material  58  on the left side volume  57 ′ of partition  64 . While partition  64  extends from the top  62  of the canister, it does not reach all the way to the bottom closure piece  66 . 
     Thus, there is a flow path from the granules  58  on the left side volume  57 ′ of partition  64  up into the granules on the right side volume  57 ″ of partition  64 . When vent valve  74  is open, the air-fuel mixture can ascend the adsorbent material to the right of partition  64  and gradually pass through a porous, thermal insulator separator  92  into a secondary volume  93  of adsorbent material  94 . Embedded in this secondary volume  93  of adsorbent material is an electrical heating element  96 . The secondary volume of adsorbent material  94  is held in a relatively small compartment between porous separator  92  and a granule retainer element  98 . Once both the primary volume  57 ′,  57 ″ of adsorbent material  58  and the secondary volume  93  become saturated with vapor, then vapor will accompany air exiting the canister at vent outlet  68  and pass through vent line  72  and through the open solenoid-actuated valve  74 . 
     The flow path of fuel-laden vapor entering the canister through inlet  54  is extended by partition  64  to include two vertical portions  57 ′,  57 ″ of the primary adsorbent material  58  and a smaller volume  93  of adsorbent material  94 . Although the secondary volume of adsorbent may be somewhat larger, preferably it is less than 3% of the primary absorbent volume. 
     Conversely, when the engine is operating and the control module  14  has opened purge valve  84  to permit air to be drawn past vent valve  74  through vent line  72  and into the vent/purge inlet  68 , the purge air is drawn through the extended path, i.e., through the secondary adsorbent volume  93  and the primary adsorbent volume  57 ′,  57 ″. The air stream laden with desorbed fuel vapor exits purge vent  70  and is drawn through purge line  82 , through purge valve  84 , into the induction system of the engine  12 . It is realized that the temperature of the primary adsorbent volume  58  is that of the ambient temperature of the engine compartment, also taking into consideration any heat of adsorption or desorption of the fuel vapor. However, before the purge solenoid  84  is opened, a command is issued from the control module  14  to actuate heating element  96  by battery (not shown) power until the secondary volume  93  is heated to a relatively high temperature at which substantially all fuel vapor is desorbed by the ambient air stream passing through the secondary adsorbent volume. The temperature of the secondary volume is controlled by the engine control module utilizing the temperature sensor information from temperature sensor  100 . 
     A preferred adsorbent material is activated carbon granules. As stated, whereas the carbon granules in the primary volume of the canister  58  are not especially heated, the granules in the secondary volume  93  are heated to a suitable temperature such as 350° F., at which the ambient air strips substantially all fuel vapor from the secondary volume. At this temperature the secondary volume is rapidly stripped of fuel. Further air flow cools the secondary volume for effective high capacity adsorption during a subsequent diurnal cycle. 
     A Second Embodiment 
     FIG. 3 illustrates a second embodiment of the invention in which the secondary volume  193  of adsorbent carbon granules (for example)  194  are contained within a Stage II canister  191  located in the vent line  72  between a conventional canister  150  and vent valve  74 . Canister  150  is quite similar to the canister  50  depicted in FIG. 2 (and thus the common parts are identified by the same numbers) except that the secondary volume of adsorbent is not contained within canister  150 . 
     Stage II canister  191  contains a relatively small volume of carbon granules, suitably only about two to ten percent by volume of the primary volume of granules in unheated canister  150 . Canister  191  includes the secondary volume  193  of adsorbent granules  194  and heating element  194 . Heating element  194  is tuned on by a suitable signal lead, not shown, from controller  14  (FIG. 1) prior to opening of purge vent  84  and the commencement of purge air flow. The granules are retained by porous retainers  197  and  198  that permit purge air flow through the canister  191  and vapor over from canister  150  into the secondary volume  193 . 
     Thus, air and fuel vapor overflow from canister  150  enters canister  191  and the fuel vapor is temporarily adsorbed. At a suitable time following engine startup, heater  196  is activated and the granules heated to about 350° F. using temperature sensor  200 . After the purge solenoid  84  (FIG. 1) is opened, air flows through vent valve  74  in to the hot secondary volume to fully remove all adsorbed fuel vapor. 
     Experimental 
     The subject invention is better understood following a description of the mechanism of canister breakthrough emissions. Breakthrough emissions from a green or virgin canister are very low as shown by the data in FIG.  4 . For comparison, the breakthrough emissions from a used canister are also shown in FIG.  4 . 
     Each canister contained 1850 cubic centimeters of commercial activated carbon granules specified to have an adsorption working capacity of 15 grams of butane per 100 cc granules (15BWC). The “green” canister contained unused carbon granules. The carbon granules in the “used” canister had been saturated with butane and then purged with a stream totaling ten cubic feet of ambient air. For convenience, a mixture of 40 parts by volume butane and 60 parts by volume air was used instead of gasoline vapor to load and evaluate the adsorption capacities of the canisters. Butane/air mixtures are commonly used in the canister studies, and butane loading is also used in the CARB and Federal test procedures. 
     The green canister was loaded with 40 g of butane from a 40:60 butane/air mixture to simulate the loading of a vehicle canister with gasoline vapor. The experiment was conducted in a closed container like the SHED test procedure for evaluating automotive evaporative emission systems. As the carbon was being loaded with butane from the synthetic mixture, the atmosphere around the canister was tested with a flame analyzer to detect any escape or “breakthrough” of butane. 
     FIG. 4 records the cumulative weight of breakthrough emissions of butane, in grams, versus the weight of butane being adsorbed by the respective carbon granule-filled canisters. It is seen that the green carbon canister lost only about 0.001 g of butane while being loaded with about 40 grams of butane. In fact, there was no significant breakthrough of butane from the green canister until its loading reached about 109 grams of butane. At that point in the loading process, substantially all of the butane entering the canister was escaping from its vent opening. In contrast, as FIG. 4 shows, the “used” canister permitted a significant amount of butane to break through from the beginning of the loading process. 
     The difference in performances of the green and used canisters is explained as follows. After several load/purge cycles, an adsorbent canister builds a residual hydrocarbon content, called a heel, and reaches a stable adsorption capacity which is lower than the adsorption capacity of a green canister. The residual hydrocarbon in the pores of the granules of activated carbon is difficult to remove by air purging and is, therefore, called “heel”. Breakthrough emissions from the used canister were higher than those from the green canister (FIG. 4) because of the hydrocarbon heel on the used carbon. Thus, the 40 g butane loading resulted in 0.016 g of breakthrough emissions from the used canister compared to 0.001 g from the green canister. 
     The heel is not distributed uniformly. There is less heel where the purge air enters the canister and more heel where the fuel tank vapor enters the canister. It can be observed that if a purged canister is allowed to simply stand for some time before loading (called soak or soaking), the breakthrough emissions increase further as shown by the data in FIG.  5 . During a soak period, the hydrocarbon heel redistributes more uniformly over the volume or bed of carbon. This redistribution reduces the loading capacity of the carbon even more and increases breakthrough emissions as illustrated in FIG.  5 . 
     FIG. 5 records the SHED test data of a group of substantially identical 1850 cc 15BWC canisters that had been loaded with butane from a 40:60 butane/air mixture and then purged with a total of 10 cubic feet of air flow. The first canister was then immediately tested (no soak period) during reloading with a 40:60 butane/air mixture. Subsequent canisters were tested in the same way after increasing soak periods of one, two, four and six days, respectively. It is clearly seen that the breakthrough emissions of butane increased from canisters subjected to a longer soak periods. Although each of these canister performances meet 1999 emission requirements, it is clear that they can release hydrocarbons after appreciable soak periods. 
     Similar results were obtained when the test was repeated by increasing the purge air volume to 20 cu ft. In real world and also in the CARB test, the purged canister experiences some soaks before loading; therefore, it is desirable to find ways to reduce the breakthrough emissions to near-zero from a soaked canister. By using a small secondary volume of adsorbent in series with the primary volume of fuel vapor adsorbent, the breakthrough emissions can be reduced to near-zero as demonstrated in the following experiment. 
     Heated Stage II Canister 
     Extrapolating on the very low breakthrough emissions from the green canister (FIG.  4 ), it has been found that near-zero breakthrough emissions from a used canister can be realized if there is some activated carbon without any heel at the vapor exit (e.g., volume  93  in FIG. 2 or Stage II canister  191  in FIG.  3 ). However, it is found that heel cannot be removed completely by purging with ambient air. The heel can be completely removed by heating the carbon and then purging with air. Experimental data had shown that if the carbon is heated to 350° F. and then purged with air, the heel is removed completely. Based on this observation, a heated Stage II canister was made as shown in FIG.  3 . Experiments were conducted to prove the concept by using a 200 cc Stage II canister with 200 cc of activated carbon granules identical to those used in the above-described tests. 
     The heel concentration in the Stage II canister was near-zero after every heated purge as determined by the Stage II canister weight change. Effectiveness of Stage II canister in reducing the breakthrough emissions was measured and the results are shown in FIG.  6 . For comparison, a non-heated Stage II canister system is also included in FIG.  6 . The results clearly show that the heated Stage II canister is very effective in reducing the canister breakthrough emissions to very low levels. 
     It is noted that in this early experiment the 200 cc volume of the Stage II canister was nearly ten percent of the total of the primary (1850 cc) and secondary volumes. It has been found that the volume of the Stage II canister or secondary volume can be smaller, less than 50 cc (or less than 2.5% to 3% of the total of 1900 cc), for reducing breakthrough emissions in the SHED test. This, of course, means that the energy requirement for heating the secondary volume of adsorbent can be reduced. 
     At illustrated above, the secondary volume of adsorbent may be located in the same canister as the primary volume or it may be located in a separate heated canister upstream of the primary adsorbent volume with respect to purge air flow (or downstream of the flow path during vapor storage). 
     The temperature sensors  100  (in FIG. 1) and  200  (in FIG. 2) can be eliminated by using a heater (heater  96  in FIG.  2  and heater  196  in FIG. 3) made of a self-regulating positive temperature coefficient material. 
     While the invention has been described in terms of certain preferred embodiments, it is recognized that other could readily be made by one skilled in the art. Accordingly the scope of the invention is to be considered limited by the following claims.