Impact and Control of Canister Bleed Emissions
`
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`SAE Homa > Papers > By Evant > SAE 2001 World Congress
`Impact and Control of Canister Bleed Emissions
`
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`Paper Number; 2001-01-0733
`DOI: 10.427112001-01-0733
`
`Citation:
`
`'Mlliama, R. and Clontz, C., "Impact and Control of Canister Bleed Emissions," SAE Technical
`Poper 2001-01-0733, 2001, doi:10.4271/2001-01-0733.
`
`Author(s):
`
`Ro~,or S. \Mlllams • Westvaco Corp.
`C. Roid Clontz - Westvaco Corp.
`
`Abstract:
`
`Vi8wAn
`
`Current EPA and CARB regulaUona allow a maximum of 2.0 lf(e1t for Hot Soak + Diurnal
`evaporative emissions. The State of California haa adopted LEV II regulation• that will
`decrease the evaporative emi11ion1 standard to 0.5 g,ie,t 1tartlng In the 2004 model year.
`These regulation• also indude a Zero Emi11ion Vehicle or ZEV program. The ZEV program
`allows car manufacturera to substitute vehicle& that meet the SULEV tail pipe emi11ion
`standards and have zero fuel evaporative emi11iona for electric vehidea. The Increased
`stringency of th010 regulation, ha. necessitated significant decree1eo in hydrocarton
`emi11ion1 from eveporetive emission canisters. For exomple, cani1ter vont emiosions may be
`a! levels of 100-300 mg,iest for a vehicle that meets !he rumint regulations. However, canister
`emission terget11hould be 50 mg/teat and less for LEV II and 10 mg,1e1t and less for zero
`evaporalive emi .. ion vehicles. Emiaaiona at this level are not due to • lack of adsorptive
`capacity in the ceni1tor, but rather are due to diffusion of hydrocarbon species. These
`emissions are often referred to as bleed emissions,
`
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`• SAE 2001 Wo~d Congress, 5 maart 2001, Detroit, Michigan, United Stetes
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`BASF-1012
`U.S. Patent No. RE38,844
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`

`

`Impact and Control of Canister Bleed Emissions
`
`2001-01-0733
`
`Roger S. Willlams and C. Reid Clontz
`Westvaco Corporation, Covington, VA
`
`Copyright O 2001 Society of Au!omo!ive Engineers, Inc.
`
`ABSTRACT
`
`Current EPA and CAAB regulations allow a maximum of
`2.0 gltest for Hot Soak + Diurnal evaporative emissions.
`The State of California has adopted LEV II regulations
`that will decrease the evaporative emissions standard to
`0.5 g/test starting in the 2004 model year. These
`regulations also include a Zero Emission Vehicle or ZEV
`program. The ZEV program allows car manufacturers to
`substitute vehicles that meet
`the SULEV tail pipe
`emission standards and have zero fuel evaporative
`emissions for electric vehicles. The increased stringency
`regulations has necessitated significant
`of
`these
`decreases in hydrocarbon emissions from evaporative
`For example, canister vent
`emission canisters.
`emissions may be at levels of 100-300 mg/test for a
`vehicle that meets the current regulations. However,
`canister emission targets should be 50 mg/test and less
`for LEV II and 1 O mg/test and less for zero evaporative
`emission vehicles. Emissions at this level are not due to
`a !ack of adsorptive capacity in the canister, but rather
`are due to diffusion of hydrocarbon species. These
`emissions are often referred to as bleed emissions.
`
`A technique was developed to study the level of bleed
`emissions specifically from canister vent ports. Canister
`design and purge volume were shown to have a
`significant impact on bleed emissions. Further, the
`incorporation of a small auxiliary chamber in series with
`the primary canister was shown to decrease bleed
`emissions significantly.
`
`INTRODUCTION
`
`Activated carbon has been used since the eaiiy 1970's to
`capture gasoline vapors from vehicle fuel vapor systems.
`The activated carbon is housed in canisters and is
`required to adsorb gasoline vapors at high efficiency and
`release them during the purge regeneration cycle. The
`canisters are used as part of a system to control
`evaporative emissions.
`
`This paper is concerned with hydrocarbon em1ss1ons
`from evaporative emission canister atmosphere ports
`specifically during diurnal testing. Diurnal emissions
`occur while a vehicle is parked and the fuel tank is
`
`

`

`heated due to daily temperature changes. Both the
`(GARB) and
`California Air Resources Board
`the
`Protection Agency
`(EPA)
`Environmental
`have
`established test standards that measure and control hot
`soak and diurnal emissions during real-time, two- or
`three-day procedures. These procedures, adopted for
`1995 (GARB) and 1996 (EPA) model year vehicles, have
`identical standards of 2.0 g/test maximum emissions for
`the two and three-day tests in addition to the SHED
`measured emissions from a 1 hour Hot Soak test. The
`procedures differ somewhat in their temperature profile
`and test fuel volatility.
`
`Recently, CARB adopted LEV II regulations that will
`decrease the evaporative emissions standard for all cars
`and light trucks to as low as 0.5 g/test starting with the
`2004 model year. Additionally, the LEV II program calls
`for 1 0% of the 2003 model year fleet to be Zero
`Emissions Vehicles (ZEV or battery electric vehicles).
`The
`law allows
`the car manufacturer to substitute
`vehicles with very low tailpipe emissions (SULEV) and
`zero fuel evaporative emissions to satisfy some of the
`ZEV requirement. The increased stringency of these
`regulations has necessitated significant decreases in
`hydrocarbon emissions
`from evaporative emissions
`canisters. For example, under the current standard,
`emissions from carbon canisters could be as high as 0.1
`- 0.3 g/test for an in-compliance vehicle. However,
`canister emissions targets for "near-zero" evaporative
`emissions vehicles (0.5 g/test standard) should be 50
`mg/test and less. Target canister emissions levels for
`"zero-evap" vehicles (0.0 g/test) are in the 3-10 mg/test
`range. Emissions at this level are not due to a lack of
`adsorptive capacity in the canister, but are due to
`diffusion. These emissions are often referred to as bleed
`emissions.
`
`Bleed emissions are those emissions from the canister
`that occur prior to breakthrough. Breakthrough is defined
`by ASTM D 2652 as "the first appearance in the effluent
`of an adsorbate of interest under specified conditions."
`In the automotive carbon application, breakthrough is
`usually considered to be that point when 2.0 g of
`hydrocarbon have been emitted from the canister or
`when the total hydrocarbon concentration in the effluent
`reaches 5000 ppm.
`
`Canister bleed emissions during diurnal testing are rarely
`due to carbon bed inefficiencies but are rather due to
`diffusion. That is, the bleed emissions most likely consist
`of hydrocarbons that were already adsorbed in the
`canister prior to the start of the diurnal. Repeated
`loading/purging
`canisters produce
`of
`carbon
`a
`hydrocarbon concentration gradient within the carbon
`bed (1,2). The adsorbed hydrocarbon concentration will
`tend to be higher near the fuel tank port of the canister
`(vapor inlet) and gradually decreases as the vapor flow
`path approaches and exits the canister atmosphere port
`(purge inlet). There will exist some concentration of
`hydrocarbon at the atmosphere port, even after extended
`
`purging. The amount of hydrocarbon remammg after
`purge is referred to as hydrocarbon heel.
`
`During extended soak periods, the hydrocarbon vapors
`will tend to diffuse from areas of higher concentration to
`areas of lower concentration. Thus, the concentration of
`hydrocarbons in the section of the carbon bed near the
`atmosphere port will increase with time. During diurnal
`loading, a mixture of air and gasoline vapor enters the
`canister through
`the tank port. Virtually all of the
`gasoline vapor is adsorbed by the carbon within a
`specific region of the bed known as the mass transfer or
`adsorption zone. The adsorption zone will move in the
`direction of vapor flow with time. On the downstream
`side of the adsorption zone, a small volume of clean air
`will move through the remainder of the bed, potentially
`causing bleed emissions.
`
`Diffusion can be described by Fick's Law, one form of
`which is:
`
`Amount of Diffusion == dn/dt = - A * D * dC/dx ,
`
`Where, n = moles of a species
`t = time
`A = cross sectional area
`D = diffusion coefficient of a species
`(temperature dependent)
`C = concentration
`x ::::: diffusion path length
`
`Although an exhaustive examination of this law is not
`within the scope of this paper, inspection of it with
`respect to evaporative emissions canisters yields several
`characteristics that affect bleed emissions.
`
`The first characteristic that affects bleed em1ss1ons is
`time. Bleed emissions are important in evaporative
`emissions testing because of the potential for long soak
`times prior to and during the real-time diurnal. Although
`important, most of the time elements in the procedure
`are fixed and cannot be changed.
`
`Another characteristic that affects bleed emissions is the
`concentration gradient. This can be viewed in two ways.
`First, the concentration gradient from one portion of the
`canister to another affects the diffusion rate. This is
`controlled primarily by the amount of purge used.
`Second,
`the hydrocarbon
`concentration gradient
`between the hydrocarbons adsorbed in the carbon pores
`and the hydrocarbons in the gas phase affects the
`diffusion rate between the phases. Although this is also
`controlled by the amount of purge used, the pore size
`distribution of
`the carbon used will
`influence
`the
`equilibrium. Thus, a well-purged canister filled with a
`high-capacity, low-heel carbon is desired to reduce bleed
`emissions.
`
`Two characteristics that can be considered together are
`the diffusion path length and the cross-sectional area.
`
`

`

`Both are elements of canister design. Decreasing the
`cross-sectional area can be accomplished by simply
`narrowing
`the canister bed profile or effectively by
`placing a plate with restricted open area across the
`carbon bed. Both of these options will increase the
`pressure drop, a critical canister parameter particularly
`important for Onboard Refueling Vapor Recovery and
`should be considered carefully. Therefore, a canister
`with an optimized length-to-diameter ratio is desired to
`reduce bleed emissions.
`
`Two final characteristics that affect bleed emissions are
`temperature and hydrocarbon species. The diffusion
`coefficient is temperature-dependent and is unique for a
`given species.
`Since the temperatures of the test
`conditions are fixed, there is little reason to consider their
`effect However, the effect of the hydrocarbon species is
`important consideration.
`like species,
`an
`For
`the
`increasing
`diffusion coefficient will decrease with
`molecular weight. For example, n-butane will have a
`higher diffusion coefficient than will n-pentane. Prior to
`the diurnal test, the canister is preconditioned by loading
`to either breakthrough or 1.5 times breakthrough with
`50% n-butane. Butane is the lightest major component
`of gasoline vapor (3) and has a diffusion coefficient
`greater than that of the other major components of
`gasoline vapor. That is, butane will have a higher
`diffusion rate compared to "gasoline vapor." Thus, it is
`quite important to purge as much of the butane loaded
`during preconditioning as possible.
`
`In this paper, we will examine the effects of soak time,
`purge volume, canister geometry, and carbon type on
`bleed emissions.
`
`EXPERIMENTAL METHODS
`
`Bleed emissions were evaluated using three different test
`procedures. Each test featured the following common
`elements: an initial canister pre-conditioning, vapor load,
`purge followed by a time-controlled soak, and emissions
`measurement during a
`final canister vapor
`load
`simulating a diurnal loading event
`
`GASOLINE VAPOR AND BUTANE CYCLING - Gasoline
`vapor cycling was performed using automated cycle test
`equipment that precisely controlled and monitored all
`testing conditions. Gasoline vapors were generated by
`bubbling air at a rate of 200 ml/min through 2 liters of 9.0
`RVP-certified test gasoline heated to 36°C. Under these
`conditions, the vapor generation rate was approximately
`40 g/hr and
`the hydrocarbon concentration was
`approximately 50% by volume. The vapors were sent to
`the canister until a breakthrough concentration of 5000
`ppm was detected using a flame ionization detector
`(FID).
`If breakthrough was not detected after 100
`minutes of vapor
`loading,
`liquid gasoline was
`the
`replaced with a fresh 2-liter aliquot. After breakthrough
`was detected, the canister was purged for a specified
`
`volume, typically 300 bed volumes, using dry air at a rate
`of 15 liters per minute. The soak time between load and
`purge events was no more than 5 minutes. The ambient
`temperature of the equipment,
`including the canister
`storage compartment, was maintained at 25°C during all
`stages of the testing.
`
`Butane vapor cycling was also performed using
`automated cycle test equipment. Butane, diluted with
`laboratory air to a concentration of 50% by volume, was
`delivered to the canister at a rate of 40 g/hr. The
`breakthrough concentration used was 5000 ppm. After
`breakthrough was detected, the canister was purged with
`dry air, typically for 300 bed volumes, at a rate of 15
`I/min. The soak time between load and purge events was
`no more than 5 minutes. The ambient temperature of the
`equipment,
`including
`the butane gas cylinder, was
`maintained at 25"C during all stages of the testing.
`
`BWC BLEED TEST - The BWC Bleed Test involved
`measuring emissions from the atmosphere port of the
`canister using a flame ionization detector while loading
`the canister with butane. The goal in performing the
`BWC bleed
`test was
`to evaluate carbon bleed
`performance under controlled conditions.
`
`A 1.0-liter test canister was used for all BWC bleed
`testing. The properties of the test canister are described
`the experimental methods.
`elsewhere
`in
`The
`temperature of the entire system including canister and
`butane fuel source was controlled at 25°C. Canister pre(cid:173)
`conditioning included three butane adsorb/purge cycles
`with loading to 5000-ppm breakthrough and 300 bed
`volumes of purge according to the procedures described
`above. Following the three conditioning cycles, the
`canister was soaked for a specified length of time. After
`the soak, the canister was again loaded with 50% butane
`vapors to breakthrough and the canister emissions were
`measured and recorded during the course of the loading
`using a flame ionization detector.
`
`DIURNAL TESTS - Two separate test procedures were
`developed for measuring bleed emissions under diurnal
`temperature conditions. Both of these tests involved
`measuring emissions directly from the canister vent
`rather than from the entire vehicle as in a SHED test, or
`from an entire fuel system, as with a mini-SHED.
`
`The first test, referred to as the compressed diurnal test,
`was developed for measuring canister bleed emissions
`during a one-day diurnal using the 11-hour temperature
`ramp segment of the CARB temperature profile. A 1.0-
`liter test canister was used in combination with a 60-liter
`fuel tank and 9.0 R.V.P. fuel in order to load the carbon
`bed to a near-breakthrough condition within an 11-hour
`test period. The compressed test introduced the CARB
`temperature profile as a variable without the time
`requirements of the two- and three-day diurnal test.
`However, the results are useful only on a relative basis,
`
`

`

`temperature ramp from 18.3° C to 40.6° C, the canister
`weight and emissions were measured several times.
`During the portion of the diurnal when the temperature
`decreased, the Tedlar bag was removed in order to allow
`the system to back purge.
`
`The removed Tedlar bag was filled to a known volume
`with nitrogen.
`The hydrocarbon concentration was
`determined by evacuation of the bag contents into an
`FID. Once
`the concentration and volume were
`determined, the mass of hydrocarbon was calculated and
`recorded.
`
`0
`
`24
`llme, hours
`
`48
`
`72
`
`6
`
`Tedllr Bag
`
`Canlntr
`
`liQ..Li!er
`Fu•ITank
`
`Figure 1. CARB temperature profile and simulated
`diurnal test setup.
`
`•
`IN-USE CANISTERS
`in-use
`In some cases,
`evaporative emission canisters were used to measure
`Automotive Testing
`diurnal
`bleed
`em1ss1ons.
`Laboratories Inc. was contracted for procurement of the
`canisters. The canisters were obtained from in-use
`vehicles, with odometer readings ranging from 16,000 km
`to 53,000 km. The nominal size of the canisters tested
`was about 2.0 liters. The canisters were filled with wood(cid:173)
`based carbon from Westvaco:-"1ne· carbon ~had- a
`nomlnai spe~ltieclnew-AST~fiiutan~ -work111·· ,_ca "acf ..
`(BVVC) of 15 gf~L ·~ ·~·.,~- .... --~·c , .... w."'-··--·-·-··-JL .• ...E ...... !L
`
`because the canister and tank size combinations were
`not realistic and the test time was shortened. Also, the
`vapor generation rates were much higher using 9.0 RVP
`fuel as compared to using 7.0 RVP fuel.
`
`the compressed diurnal
`Pre-conditioning
`test
`for
`consisted of six gasoline vapor load / purge cycles with
`300 bed volumes of purge and a loading rate of 40 g/hr
`as described previously. Within 2 hours of the final
`gasoline vapor purge, the canister was loaded with 50%
`butane vapors in air at a rate of 40 g/hr until a
`breakthrough of 5000 ppm was measured by a flame
`ionization detector. The canister was soaked for 60
`minutes before the final purge. The final purge was
`performed at a rate of 15 liters per minute with dry air for
`300 bed volumes. Following the purge, the canister was
`soaked at 20° C for 24 hours before starting the diurnal
`test.
`
`A second diurnal test procedure, referred to as the
`simulated real-time diurnal test, subjected a fuel tank and
`canister to the entire two-day or three-day CARB diumal
`temperature profile. A 60-liter fuel tank was used in
`combination with commercial automotive canisters,
`typically with a nominal size of about 2 liters. The fuel
`used for the simulated real-time diurnal test was a 7.0
`fuel as specified
`RVP Phase
`II
`in
`the CARB test
`procedures. This test was designed to simulate the
`diurnal portion of the CARB vehicle emissions test
`procedure
`in order to generate quantitative canister
`emissions data.
`
`Pre-conditioning for the simulated real-time diurnal test
`included multiple gasoline vapor load / purge cycles as
`described above. The loading rate for the gasoline
`cycles was 40 g/hr, and the most typical purge was 400
`bed volumes. Following the gasoline cycles, the canister
`loaded with 50% butane vapor to 5000 ppm
`was
`breakthrough in preparation for a two-day diurnal test
`and was loaded to 1.5 times the nominal butane working
`capacity in preparation for a three day diurnal test After
`the butane load, the canister was allowed to soak for 60
`minutes before purging for a specified volume. Typical
`purge volumes were 400 bed volumes in preparation for
`the three-day diurnal test and 150 bed volumes for the
`two-day diurnal test. Following the purge, the canister
`was soaked at 20 °C for 24 hours before starting the
`diurnal test.
`
`After the 24-hour soak, for both the compressed and real
`time diurnal tests, the canister was attached to a
`commercial 60-liter, steel fuel tank containing 24 liters of
`certified test fuel, previously equilibrated to 18.3 °C
`overnight. A 9.0 RVP fuel was used for the compressed
`diurnal test and a 7.0 RVP fuel was used for the real-time
`diurnal test.
`A Tedlar bag was attached
`to the
`atmosphere port of the canister as shown in Figure 1 to
`collect the hydrocarbon emissions. During the 11-hour
`
`

`

`TEST CANISTERS
`
`In some cases, test canisters were filled with virgin
`carbon samples and evaluated for bleed emissions
`performance. The test canisters were 1.0 liter in size.
`Each of these canisters contained a partition separating
`the carbon into two beds. The length-to-diameter (UD)
`ratio for the test canisters was 3.5 unless otherwise
`stated. The partition was located either in the middle of
`the canister creating equal vapor and atmosphere
`chambers or was located in the canister so that the ratio
`between vapor and atmosphere chambers was 2:1.
`Placement of the partition in the canister is referenced
`along with data obtained using the test canisters.
`
`RESULTS AND DISCUSSION
`
`EFFECT OF SOAK TIME
`
`The BWC Bleed Test was used to examine the effect of
`soak time. The results from the test are shown in Figure
`2. A 1.0-liter test canister was exposed to repeated
`butane load/purge cycles under the conditions described
`above. At the end of a purge cycle, the canister was
`allowed to soak for 1 hour prior to the next load step.
`The plot shows the emissions as a function of canister
`weight gain. The data for the 1-hour soak is a typical
`breakthrough curve for a carbon canister. That is, the
`emissions from the canister vent are essentially non(cid:173)
`detectable during loading. This is consistently true in the
`absence of long soak times after purge and is evidence
`that bleed emissions are not the result of canister
`inefficiencies.
`
`I:
`C.
`Q.
`,ii
`i::
`Cl ·;
`.!!!
`E
`w
`1:::1
`~
`.!
`m
`
`6000
`
`45110
`
`3000
`
`1500
`
`0
`
`0
`
`24-Hmlr )
`
`Saal!
`
`1-Hoor
`$ml!
`
`20
`
`40
`Weight Gain, g/L
`
`60
`
`80
`
`Figure 2. BWC bleed test effect of soak time.
`
`The second set of data in Figure 2 show the emissions
`for the same canister when the soak time after purge
`was lengthened to 24 hours.
`In this case, the emissions
`from the canister during loading were immediate and
`somewhat consistent until breakthrough. Due to the long
`soak time, significant diffusion occurred within the carbon
`
`bed leading to the emission of butane that was adsorbed
`on the carbon during previous loading.
`
`EFFECT OF PURGE
`
`The simulated real-time diurnal test was used to study
`the effect of purge volume on bleed emissions. A two(cid:173)
`liter, in-use canister was preconditioned as described
`above for the 2-day simulated diurnal. The final purge
`volume after the butane load to breakthrough was varied
`from 200 to 1200 bed volumes using a constant rate of
`15 I/min. The test results are shown in Figure 3.
`
`300
`
`250 ·
`
`0
`E 200
`,,,;
`C
`_g
`
`150
`
`tQ • ·e
`w
`l
`iii
`
`[l0ay 1
`111Day2
`
`-
`
`100
`
`50
`
`0
`
`200
`
`.,~-,--L--
`600
`BOO
`400
`Purge, Bed Volumes
`
`1200
`
`Figure 3. Simulated real-time diurnal; effect of purge
`volume.
`
`The graph shows the bleed emissions as a function of
`purge volume after 24 hours of soak time for a 2-day
`simulated diurnal test The data show the strong
`influence of purge on bleed emissions. At 200 bed
`volumes purge, the bleed emissions for the second day
`were 270 mg. The emissions decreased to 31.6 mg at
`400 bed volumes purge. Further decreases to levels of
`12.9, 6.9, and 2.1 mg were found for 600, 800, and 1200
`bed volumes purge respectively.
`
`The effect of purge volume on bleed emissions is greater
`than
`its effect on working capacity.
`For example,
`increasing the purge volume from 200 to 400 bed
`volumes will increase the working capacity of the canister
`by about 20% (4). However, this same increase in purge
`volume will decrease bleed emissions by nearly 90%.
`Further,
`it appears that meeting canister emissions
`targets for LEV II can be achieved by increasing purge
`volume alone. Though it should be noted that increased
`purge volume could
`lead to drivability and tailpipe
`emissions concerns.
`
`Increased purge volume requires increased purge time
`and/or increased purge rate. On the vehicle, purge
`occurs during the drive cycle portions of the evaporative
`
`

`

`emissions test These drive cycles are fixed, making
`increased purge time difficult Considering this, the effect
`of purge rate on bleed emissions was examined using a
`simulated 3-day diurnal test.
`
`Figure 4 gives the bleed emissions after 24 hours of soak
`time for an in-use canister purged for 400 and 600 bed
`volumes at different rates.
`The purge rates were
`adjusted so that the different final purge volumes were
`achieved in the same time. A rate of 15 I/min was used
`for 400 bed volumes and a rate of 22.5 I/min was used
`for 600 bed volumes. The bleed emissions after 400 bed
`third day as
`volumes purge was 60.3 mg on
`the
`compared to 15.7 mg for 600 bed volumes purge. The
`data indicate that increased purge volumes achieved
`through increased purge rates reduce bleed emissions
`significantly.
`
`80 ~ - - - - - - - - - - - - - - - -
`
`!:ll
`E 60 +--··(cid:173)
`.,,·
`
`C
`0
`'iii
`.! 40 +--(cid:173)
`E w
`i
`.!! 20 m
`
`111D11y1
`I! Day2
`1111 Day 3 ,__ ___ _,
`
`0
`
`400 Bed Voluffll!S
`15 l.itef!I per Minute
`
`600 Bl!!d Volumes
`22.5 Liters per Minll'l:ll>
`
`Figure 4. Simulated real-time diurnal test; effect of purge
`rate and volume.
`
`EFFECT OF CANISTER GEOMETRY
`
`The compressed diurnal test was used to study the effect
`of canister geometry on bleed emissions. Three one-liter
`canisters with different geometries were used (Figure 5).
`
`m
`
`1.0 Liter
`UD"' 3.5
`Equal Partition
`
`UI Liter
`LJD:4.5
`Equal Partition
`
`rn
`
`1.0 Liter
`213: 1/3
`Unequal Partition
`
`Figure 5. 1.0-liter test canisters.
`
`first canister was rectangular with a partition
`The
`completely separating two carbon beds of equal volume.
`The length-to-diameter (UD) ratio of the canister was
`3.5. The second canister also had a partition completely
`and equally separating the two carbon beds. The UD of
`this canister was increased to 4.5. The third canister had
`the same dimensions as the first one, except that the
`partition was moved such that the vapor inlet carbon
`section had twice the volume as that of the purge inlet
`section. The same 15 g/dl BWC wood-based carbon
`was used in all of the canisters.
`
`The results of the diurnal test are shown in Figure 6. The
`equally-partitioned canister with an UD of 3.5 had bleed
`emissions of 290 mg under
`the conditions used.
`Increasing the UD to 4.5 was found to decrease the
`bleed emissions to 154 mg. Moving the partition in the
`UD=3.5 canister from 50:50 to 2/3:1/3 further decreased
`the bleed emissions to 121 mg. Please note that bleed
`emissions results obtained under the compressed diurnal
`test conditions are somewhat higher than those found
`using the simulated diurnal test. Nonetheless, the data
`reflect the significant impact that canister geometry has
`on bleed emissions.
`
`Equal
`Partition
`I.JD= 3.5
`
`Equal
`Partition
`I.JD"' 4.5
`
`213 : 113
`Partition
`I.JD=3.5
`
`Figure 6. Compressed diurnal test effect of canister
`geometry.
`
`It is interesting to note that the effect of canister UD is
`it is for working
`different for bleed emissions than
`capacity. Previous test results showed, that as the
`canister UD is increased from 1.0 to 3.5, the gasoline
`working capacity (40 g/hr loading rate) is increased by
`nearly 40% for 2 mm carbon pellets (5). However,
`increased canister UD's above 3.5 had much less of an
`impact.
`loading
`The effect appears
`to be
`rate
`dependent. Under ORVR conditions (50 g/min loading
`rate), an increase in UD from 1.0 to 3.5 resulted in only a
`1 O"/r, gain in working capacity (6). Based on these data,
`it was assumed that an increase in canister UD from 1.0
`to 3.5 would impact bleed emissions as it did working
`capacity. Therefore, an investigation of the effect of
`geometry for UD's greater than 3.5 was performed.
`
`

`

`Additional optimization of canister geometry may yield
`further reductions in bleed emissions.
`
`EFFECT OF CARBON TYPE
`
`The compressed diurnal test was used to study the effect
`of carbon type on bleed emissions. Three different
`carbons, all with ASTM Butane Working Capacities iii "the .
`11.0 - 12.0 g/dl range, were used. One liter of each
`carbon type was subjected to 6 adsorb/purge cycles
`using gasoline vapors, followed by preconditioning with
`butane as previously described.
`The heel was
`determined after the butane preconditioning. The results
`are shown in Figure 7.
`
`The three carbon types differed in the raw material used
`and in the activation conditions. This resulted in different
`It has been shown that carbon
`pore size distributions.
`pore size distribution directly affects both working
`capacity and hydrocarbon heel development (1,4) ..
`The wood-based carbon had a hydrocarbon heel of 58
`g/1 ;fter-cycfin-g, s1gmf1cantly lower tfian 1:fielieels for" the
`coal-based and olive-based carbons. The wood-based
`carbon also had the lowest measured bleeaemissfcins,
`at 121 mg· as coriii;iare-a-fu779 and 299 mg for the coal(cid:173)
`based and olive-based carbons respectively. The data
`suggests
`is a relationship between
`that
`there
`the
`hydrocarbon heel and bleed emissions.
`
`Also under consideration was the influence of aging and
`heel composition on bleed emissions. A previous study
`showed that continued laboratory load/purge cycles with
`gasoline vapors alter the composition of the heel (2).
`Specifically, it was shown that the average molecular
`weight of the hydrocarbon heel increased with increased
`aging. As discussed earlier, higher molecular weight
`hydrocarbons tend to have lower diffusion coefficients as
`compared
`lower molecular weight hydrocarbons.
`to
`Thus, carbon canisters exposed to significant gasoline
`vapor aging should have higher hydrocarbon heels and
`lower bleed emissions than canisters exposed to less
`aging.
`
`and hydrocarbon
`Bleed
`heels were
`emissions
`determined for samples of wood-based carbons with
`BWC's of 11 and 15 g/dl using the compressed diurnal
`test. The results were compared after 6 and 200
`gasoline vapor aging cycles and are displayed in Figure
`8. After 6 cycles, the heel and bleed emissions for the
`15-g/dl BWC carbon were 85.1 g/I and 154 mg. These
`values are higher than the 48-g/l heel and 60.8 mg of
`bleed emissions found for the 11-g/dl BWC carbon.
`
`However, after 200 aging cycles, the heel and bleed
`emissions found for the 15 g/dl BWC carbon were 121 g/1
`and 34 mg. The heel and bleed emissions for the 11-g/dl
`BWC carbon were 60 g/1 and 41 mg after aging. As
`expected, the heel levels for both carbons increased
`while the bleed emissions decreased with aging. Also,
`the bleed emissions after aging were similar for both
`carbons.
`
`Wood Base
`Heel =58 g/l
`
`Coal Base
`Heel =84 g/1
`
`Olive Base
`Heel =80 g/!
`
`Figure 7. Compressed diurnal test effect of carbon type.
`
`It is often necessary to utiliie an ac:tiya.t_ed carbon with
`very high. working capacity. Currently,
`the higtiest
`working capacity carbon ·available commercially is a 15
`g/dl BWC wood-based carbon. High working capacity is
`accomplished by increasing the pore volume in the
`critical pore size range (1 ). However, this can also result
`in an increase in pore volume of pore sizes associated
`with heel. For this reason, the effect of increased
`working capacity and heel on bleed emissions was
`studied.
`
`Jr
`
`rli 120 I "' 80
`w I 40
`
`0
`
`NewCllrbon
`15BWC
`H111111