`

`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
`
`2
`
`

`

`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.
`
`3
`
`

`

`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,
`
`4
`
`

`

`

`

`

`

`

`

`

`

`

`

`Figure 12. Simulated real-time diurnal test; effect of
`auxiliary chamber geometry.
`
`indicate that the addition of an auxiliary
`The data
`the vent side of the primary canister
`to
`chamber
`substantially reduces bleed emissions.
`The simple
`addition of an auxiliary chamber may help achieve
`canister emissions targets for LEV II applications without
`a significant increase in purge. The data also suggest
`that the design of the auxiliary chamber will need to be
`optimized for best performance. One aspect of auxiliary
`canister design is the form of acfivatei:1-cafuo-ffusea.
`Camon- pellets, sucn·· as··~ those ···used in. automotive
`canisters, can be used
`to
`lower bleed emissions.
`However,
`their use in auxiliary chambers may also
`introduce a significant amount of flow restriction, which
`may be unsuitable for ORVR applications. As an
`alternative,
`high
`carbon-content monoliths,
`or
`honeycombs, have been developed by Westvaco for use
`in auxiliary chambers. The honeycombs have very low
`restriction and have been demonstrated
`flow
`to
`significantly reduce bleed emissions.
`In fact, the data
`presented in Figures 11 and 12 were developed using
`carbon honeycombs.
`
`CONCLUSION
`
`LEV II regulations necessitate significant decreases in
`hydrocarbon emissions
`from evaporative emission
`canisters. The emissions from canisters are not due to a
`lack of adsorptive capacity, but are due to bleed
`emissions. Bleed emissions can be reduced by
`
`•
`
`Increasing the purge volume
`
`• Optimizing the canister geometry
`
`• Using high-capacity/low-heel wood-based
`carbons
`
`• Utilizing an auxiliary chamber
`
`• Optimizing the auxiliary chamber
`
`ACKNOWLEDGMENT
`
`The authors wish to express their sincere appreciation to
`Joe W. Snead, who aided in the design, operation and
`maintenance of the equipment used in the study. Jerry
`Rucker provided assistance performing laboratory testing
`when needed. Dr. H. Ray Johnson provided support and
`encouragement. Catherine K. Morgan provided great
`help in the coordination of the manuscript with SAE. Erin
`Higinbotham of Automotive Testing Laboratories was
`very helpful in providing in-use canisters.
`
`REFERENCES
`
`1. H. R. Johnson and R. S. Williams, "Performance of
`in Evaporative Loss Control
`Activated Carbon
`Systems," SAE Technical Paper 902119, October
`1990.
`2. H. ltakura, N. Kato, T. Kohama, Y. Hyoudou, and T.
`Murai, "Studies on Carbon Canisters to Satisfy LEV II
`EVAP Regulations," SAE Technical Paper 2000-01-
`0895.
`3. R. L. Furey and K. L. Perry, "Composition and
`Reactivity of Fuel Vapor Emissions from Gasoline(cid:173)
`Oxygenate Blends," SAE Technical Paper 912429,
`October 1991.
`4. P. J. Johnson, D. J. Setsuda, and R. S. Williams,
`"Activated Carbons
`for Automotive Applications,"
`from Carbon Materials for Advanced Technologies,
`Chap. XII, Ed. by T. D. Burchell, Pergamon Press,
`Oxford U.K., 1999.
`5. A. S. Williams, Westvaco Evaporative Emissions
`Seminar, 1991, copies of data available upon
`request.
`6. R. S. Williams and C. Reid Clontz, Westvaco
`Evaporative Emissions Seminar, 1998, copies of
`data available upon request.
`
`10
`
`