Evaporative Emission Control Technologies

for Gasoline Powered Vehicles

December 2010

Manufacturers of Emission Controls Association

2020 N. 14th St. * Suite 220 * Arlington, VA 22201

www.meca.org

www.dieselretrofit.org
TABLE OF CONTENTS

Executive Summary........................................................................................................................ 1

1.0 Introduction............................................................................................................................... 3

2.0 Current Evaporative Emissions Regulations in the U.S. .......................................................... 5

2.1 ARB’s Proposed LEV III Evaporative Emissions Requirements................................. 7

2.2 Test Methods for Measuring Evaporative Emissions................................................... 9

 

3.0 Technologies to Control Evaporative Emissions.................................................................... 10 

 3.1 Permeation Controls.................................................................................................... 11

 3.2 Carbon Canisters......................................................................................................... 12

 3.3 Activated Carbon ........................................................................................................ 14

 3.4 Air Intake Systems (AIS)............................................................................................ 16

 3.5 On-Board Refueling Vapor Recovery ........................................................................ 17

 

4.0 On-Board Diagnostic Requirements....................................................................................... 18

5.0 Conclusions............................................................................................................................. 18

6.0 References............................................................................................................................... 18
LIST OF FIGURES

Figure 1: Emissions Contributions for PZEV and LEV II Vehicles............................................... 5

Figure 2. CARB LEV II and PZEV Evaporative Requirements ................................................... 7

Figure 3. Test set-up for a bleed emissions or mini-rig test.......................................................... 10

Figure 4. Fuel system with emission control technology ............................................................. 11

Figure 5. Examples of evaporative carbon canisters .................................................................... 12

Figure 6. Inside of EVAP canister................................................................................................ 13

Figure 7. Canister bleed emissions with and without a scrubber.................................................. 14

Figure 8. Pore size depiction in activated carbon particle ........................................................... 15

Figure 9. Examples of extruded carbon honeycombs .................................................................. 16

Figure 10. Examples of carbon AIS systems ............................................................................... 16

Figure 11. Examples of carbon AIS systems ............................................................................... 17

Figure 12. Air Induction System performance on 4 and 8 cyl. engine ........................................ 17

LIST OF TABLES

Table I: CARB LEV II Evaporative Emission Standards .......................................................... 6

Table II: U.S. EPA Tier II Evaporative Emission Standards....................................................... 6

Table III: Option 1 of ARB’s Proposed LEV III Evaporative Standards..................................... 8

Table IV: Option 2 of ARB’s Proposed LEV III Evaporative Standards...................................... 8
1

Executive Summary

Evaporative emission control technology was the first to be used on passenger vehicles as 

a way to control smog forming hydrocarbons in the early 1960’s. Evaporative emissions from 

motor vehicles constitute about half of the reactive organic gas (ROG) inventory in California 1

and nearly 40% in the Northeastern United States. Ozone is considered to be a respiratory 

irritant harmful to humans and plants and is regulated by the U.S. EPA which sets a National 

Ambient Air Quality Standard (NAAQS) for ozone concentration. Areas that are out of 

attainment or experience concentrations above this ozone concentration, like California, regulate 

evaporative emissions to reduce ground-level ozone. In addition to the negative health effects of 

ground level ozone, it is also a greenhouse gas. 

Evaporative emissions are broken down into five primary sources; diurnal, running loss, 

hot soak, permeation and refueling. The magnitude of the relative components depends greatly 

on the engine design, fuel delivery and application. A brief description of the major types of 

evaporative emissions is given below.

Diurnal emissions result from the evaporation of gasoline due to temperature fluctuation 

during the day and night. 

Running loss emissions represent gasoline that is vaporized from the engine and fuel 

system while in operation.

Hot Soak emissions occur during the first hour that the vehicle is parked after normal 

operation.

Permeation occurs continuously once the polymer components of the fuel system become

saturated with fuel. 

Refueling emissions occur as gasoline is pumped into the tank displacing the gasoline 

rich vapor.

The function of the automobile evaporative emission control system is to block or 

capture the above sources of vaporized hydrocarbons and prevent their release into the 

atmosphere. There are varying levels of complexity and efficacy of these controls with the most 

advanced systems equipped on partial zero emission vehicles being certified to California’s 

PZEV standard under the state’s LEV II emission standards.

Companies that manufacture evaporative emission controls have responded to the 

challenge of reducing VOC emissions from gasoline powered vehicles. Through their efforts, a 

wide range of cost-effective technologies have been developed to block HC emissions via the 

above mechanisms. Manufacturers of Emission Controls Association (MECA) member 

companies, together with engine manufacturers, have worked together to meet California’s 

PZEV requirements on over 50 light-duty vehicles and employed evaporative canisters on 

motorcycles and marine engines. 
2

Interest in evaporative emissions control has grown considerably in recent years around 

the world. MECA is engaged with California on their LEV III regulation that proposes to extend 

the most advanced evaporative controls across the entire on-road, light and medium-duty vehicle 

fleet. This document has been prepared to supplement information already made available by 

MECA on emission control technologies and provides an overview of the types of technologies 

being developed for new gasoline fueled cars and trucks.

Today’s cleanest gasoline vehicles, certified to California’s PZEV emission limits require 

near zero evaporative emissions and include additional technologies such as canister scrubbers to 

virtually eliminate bleed emissions from the carbon canisters during periods of low purge. Some 

vehicles also incorporate carbon based air-intake HC traps to prevent engine breathing losses 

from escaping through the intake manifold and air induction system (AIS) after the engine is shut 

off.

Today, viable emission control technologies exist to reduce fuel system based HC 

evaporative emissions from all types of spark-ignited engines including small handheld 

equipment up to large spark-ignited (LSI) vehicles. Applications include marine and recreational 

off-road vehicles. The major technologies that control permeation emissions include: 

• Fuel tanks made of low permeation polymers

• Multilayer co-extruded hoses 

• Low permeation seals and gaskets

Technologies designed to control diurnal, hot soak and refueling HC emissions include: 

• Advanced carbon canisters

• High working capacity activated carbon

• Honeycomb carbons scrubbers

• Air induction system (AIS) HC traps

The most stringent evaporative emission control regulations are enforced in the United 

States. Vehicles certified to California’s PZEV low emission vehicle standards must 

demonstrate near zero evaporative emissions from the fuel system at 0.054 g/test using a rig test 

of a vehicle’s fuel system. The California Air Resources Board (CARB) is proposing to extend 

these requirements across the entire light-duty and medium-duty passenger vehicle fleet 

(<14,000 lbs GVWR) by 2022 as part of their LEV III regulations.

Demands on vehicle manufacturers to achieve higher fuel efficiency through the use of 

smaller displacement, boosted engines and hybrid electric powertrains will create challenging 

operating conditions for evaporative emission control technologies. The lower purge volumes 

that result from smaller displacement engines or hybrid systems under partial or full electric 

drive will require the development of specialty carbon adsorbents and advanced canister designs 

to achieve the lowest evaporative emissions demanded by future regulations. Gasoline vehicles 

in other parts of the world and SI off-road equipment everywhere can benefit from much of the 

same technologies applied to passenger vehicles in the U.S. This paper will describe the types of 

technologies that are being used to meet the current and future evaporative emission regulations.
3

1.0 Introduction

Evaporative emissions from gasoline powered vehicles consist of volatile organic 

compounds (VOCs), or hydrocarbons, released from the fuel system, rubber and other plastic 

components of the vehicle, such as tires, plastic interior and exterior trim, carpeting etc. This 

paper will focus on the hydrocarbon vapors emitted from the fuel system as these represent the 

major sources of evaporative emissions from mobile vehicles and equipment. This paper will 

further focus on fuel-related evaporative emissions from light-duty vehicles. Some of the same 

types of technologies that are discussed here for passenger cars have been successfully applied to 

other spark-ignited off-road vehicles and equipment and will not be covered here. A more 

detailed discussion of regulations and evaporative control technologies in these applications can 

be found in MECA’s white papers specific to those applications available at www.meca.org. 

Evaporative emissions from motor vehicles constitute about half of the reactive organic 

gas (ROG) inventory in California 1 and nearly 40% in the Northeastern United States. 

Evaporative emissions depend on ambient temperatures and the vapor pressure of the fuel 

supply. VOCs and other hydrocarbons (HC), such as those emitted from the tailpipe of motor 

vehicles, react with NOx in the presence of sunlight to form photochemical smog which is the 

primary component of ground level ozone. Ozone is considered to be a respiratory irritant 

harmful to humans and plants and is regulated by the U.S. EPA which sets a National Ambient 

Air Quality Standard (NAAQS) for ozone concentration. Areas that are out of attainment or 

experience concentrations above this ozone concentration, like California, regulate evaporative 

emissions to reduce ground-level ozone. In addition to the negative health effects of ground 

level ozone, it is also a greenhouse gas. 

Evaporative emissions are broken down into five primary sources; diurnal, running loss, 

hot soak, permeation and refueling. The magnitude of the relative components depends greatly 

on the engine design, fuel delivery and application. 

Diurnal emissions result from evaporation of gasoline due to temperature fluctuation 

during the day and night. As the fuel tank warms during the day or while parked in the sun, 

expansion of the gasoline vapor above the liquid is vented from the tank. During the night, as 

the tank cools, fresh air is pulled into the tank to mix with the gasoline vapors to start the process 

again the next day.

Gasoline vapors coming from the engine and fuel system while in operation are known as 

running losses. The primary source of running loss emissions in earlier passenger cars was from 

the carburetor. Modern vehicles may emit small amounts of emissions from the fuel cap and 

vapor canister. Most of these HCs end up being captured by the evaporative emission control 

system and routed back through the intake of the engine to be consumed during combustion.

Emissions attributed to the hot soak occur during the first hour that the vehicle is parked 

after normal operation. During this time, the engine remains hot and, without convective 

cooling, continues to heat the fuel system. This source is primarily attributed to carbureted 

vehicles and equipment. In modern vehicles equipped with fuel injectors and carbon canisters, 
4

the hot soak vapors would be captured and stored until the next time the engine is running. At 

this time they would be purged into the intake air of the engine and combusted.

Permeation of gasoline molecules can occur during normal operation or extended periods 

of parking a vehicle. This is caused by the saturation of the plastic and rubber components of the 

fuel system such as plastic fuel tanks and rubber hoses and results in relatively constant 

permeation through these components. Significant advances in polymer chemistry and laminated 

forming techniques have significantly reduced permeation as a source of emissions in modern 

vehicles. 

Refueling emissions can occur as gasoline is pumped into the tank displacing the gasoline 

rich vapor causing it to be emitted into the atmosphere. In modern passenger vehicles, these 

vapors are stored in the EVAP canister and purged into the intake air of the engine to be 

combusted. 

The function of the automobile evaporative emission control system is to block or capture 

the above sources of vaporized hydrocarbons and prevent their release into the atmosphere. 

There are varying levels of complexity and efficacy of these controls with the most advanced 

systems equipped on partial zero emission vehicles (PZEV) being certified to California’s 

tightest evaporative standard under the state’s LEV II and future LEV III emission standards. 

Evaporative emission controls were the first type of emission control system on 

automobiles and is often the least expensive approach to controlling VOC and HC emissions 

from gasoline powered engines. In the 1960s positive crankcase ventilation systems were 

introduced to capture crankcase vapors and prevent them from being vented to the atmosphere. 

The technology has advanced to include carbon canisters, low permeation hoses, active purge 

systems and on-board vehicle refueling (ORVR) controls. Eventually on-board diagnostic 

sensors were integrated to insure proper functioning of the entire fuel evaporative control system. 

Hot soak emissions were a problem in carbureted engines releasing gasoline vapors from the fuel 

bowl as the temperature in the engine compartment increased after shut down. Initially air filter 

assemblies were used to capture these emissions and later carbon canisters were connected with a 

hose to the fuel bowl. The introduction of fuel injection greatly reduced evaporative emissions 

due to hot soak and running loss as they isolated the fuel from the atmosphere at all time except 

for refueling. On-board refueling vapor recovery (ORVR) systems have been required on all 

passenger vehicles in the U.S. since 2000 and capture vapors emitted from the tank during 

refueling in the carbon canister and later emit them into the combustion chamber during canister 

purging. 

Today’s cleanest gasoline vehicles, certified to California’s PZEV emission limits require 

near zero evaporative emissions and include additional technologies such as canister scrubbers to 

virtually eliminate bleed emissions from the carbon canisters during periods of low purge. Some 

vehicles also incorporate carbon based air-intake HC traps to prevent engine breathing losses 

from escaping through the intake manifold and air induction system (AIS) after the engine is shut 

off. Figure 1 shows the benefit that advanced evaporative controls can have on the relative 

amounts of evaporative emissions contributed from different parts of a vehicle. The bar chart 

quantifies the mass of evaporative emissions that contribute to the SHED test measurement for a 
5

typical LEV II and PZEV certified vehicle. The background emissions represent the non-fuel 

related evaporative emissions from carpets, rubber and plastic parts. The emissions that are 

readily impacted by advanced evaporative control technology are those emitted by the air 

induction system (AIS) and fuel system. Carbon AIS traps can reduce emissions by 0.10 to 0.15 

g/test and incorporating canisters with auxiliary scrubbers onto a LEV II vehicle reduces tank 

venting emissions by as much as 0.1 g/test. The advanced emission control technologies used on 

PZEV vehicles are very effective and leave background emissions as the major contributor 

(65%) to the total evaporative emissions.

Figure 1: Relative contribution to full vehicle emissions from different sources for PZEV 

and LEVII certified vehicles.

After a brief review of the evaporative emission regulations in the United States, this 

paper will describe the specific evaporative control system technologies in greater detail. 

2.0 Current Evaporative Emission Regulations in the U.S.

California has enforced evaporative emissions on light-duty vehicles since 1970. The 

current requirements under CARB’s low emission vehicle program were fully implemented in 

2006 for the entire fleet. The evaporative emission standards are shown below for the three-day 

diurnal-plus-hot soak test and the two-day diurnal-plus-hot-soak test. The standards are 

expressed in total vehicle HC evaporative emissions and include both fuel and non-fuel vehicle 

emissions. The three-day diurnal test insures that running loss emissions, high temperature hot 

soak and diurnal emissions are controlled whereas the two-day diurnal test verifies that the 

canister is well purged during vehicle operation. For 2001 and newer vehicles, additional testing 

of the ORVR system is required to insure that refueling emissions do not exceed 0.2 g/gal. of 

fuel dispensed. The standards apply to gasoline-fueled, liquefied-petroleum-gas-fueled, and 

alcohol-fueled passenger cars, light-duty trucks, medium-duty vehicles, and heavy-duty vehicles. 
6

Included in these requirements are flexible-fuel vehicles, dual-fuel vehicles, hybrid-electric 

vehicles, and zero-emission vehicles with fuel fired heaters. Table I shows the evaporative 

emission limits for the current California LEV II standards.

Table I: CARB LEV II Evaporative Emission Standards

The standards were also adopted by U.S. EPA as part of their Tier II light-duty vehicle 

standards with a different phase-in period. Both agencies introduced the standards in 2004 with 

requirements for 40% of the fleet in California and 25% federally. California completed the 

phase in by 2006 with full harmonization by both agencies in 2009 for evaporative emissions. 

The fleet average evaporative limits for EPA’s Tier II standards are shown in Table II. The 

evaporative emission control system standards include a useful-life requirement of 15 years or 

150,000 miles, whichever comes first.

Table II: Evaporative Emission Limits under EPA’s Tier II Standards

Vehicle Type Model Year 

3 day Diurnal + Hot 

Soak 

(g/test) 

Supplemental 

2 day Diurnal + Hot 

Soak 

(g/test) 

Running 

Loss 

(g/mi) 

Federal 

LDV/LLDTs 2004 0.95 1.20 0.05 

HLDTs 2004 1.20 1.50 0.05 

MDPVs 2004 1.40 1.75 0.05 

LDV 2009 0.50 0.65 0.05 

LLDT 2009 0.65 0.85 0.05 

HLDT 2010 0.90 1.15 0.05 

MDPV 2010 1.00 1.25 0.05 

California allows manufacturers to certify to their zero-fuel evaporative standards as a 

way to generate credits toward meeting the states zero-emission vehicle (ZEV) requirements. 

The PZEV requirements are compared to the LEV II limits for passenger cars and light-duty 
7

trucks in Figure 2. In addition to tighter limits for the diurnal tests, manufacturers must also 

meet a near zero (0.054 g/test) limit for the fuel system using a rig test. The EVAP test 

procedures will be described in greater detail in Section 2.1.

Figure 2: CARB LEVII and PZEV Evaporative Requirements

Meeting the PZEV limits requires the use of advanced fuel and evaporative control 

systems that are capable of essentially eliminating fuel-related evaporative emissions. These 

advanced technologies include a pressurized and sealed fuel system such as an insulated, bladder 

fuel tank, larger chambered carbon canister equipped with a bleed emissions scrubber and an 

AIS carbon filter. A more detailed description of LEVII and PZEV technologies will be 

provided in Section 3.0.

2.1 ARB’s Proposed Evaporative Emission Requirements for LEV III

ARB is in the final stages of finalizing their evaporative emission requirements as part of 

their LEV III proposal that will be presented to the Board in April of 2011. ARB is proposing to 

extend “zero” evaporative emission technology to all light-duty and medium-duty vehicles. 

Phase-in of these proposed vehicle evaporative emission requirements would begin

with model year 2018 and be complete by model year 2022. The phase-in would require that 

60% of the fleet is in compliance in the 2018-19 time frame ramping up to 80% compliance by 

2020 to 2021 and full coverage of the fleet by 2022. To certify to the new standards ARB is also 

proposing to require the use of an E10 certification fuel starting with the 2014 model

year for new vehicles. Because all gasoline fuel in California contains 10% ethanol, this will 

bring certification data more in-line with real world emissions. All certifications would need to 

use the E10 fuel starting in model year 2018.
8

Vehicle manufacturers would have two options for evaporative emissions compliance. 

The first option is to comply with the current “zero” evaporative, whole vehicle standard that 

varies with vehicle weight and use the current complete fuel system rig test to confirm that the 

fuel system has no more than 0.054 g/test of evaporative emissions. 

Table III: Option 1 of ARB’s Proposed LEV III Evaporative Standards 

The second option allows manufacturers to comply with a lower whole vehicle emissions 

standard and use a new bleed emissions test protocol to confirm that the carbon canister bleed 

emissions are less than or equal to 0.02 g/test for all light-duty vehicle classes and less than or 

equal to 0.03 g/test for medium-duty and heavy-duty vehicle classes (Table 4). Within this 

second option, manufacturers would be allowed to comply with a fleet average whole vehicle 

standard within each weight class. Manufacturers will be required to report the highest value of 

the 3-Day Diurnal and 2-Day Diurnal plus hot soak for a given certification test vehicle. The 

test procedures for the fuel system rig test and bleed emissions test will be described in Section 

2.2. Evaporative emission system certification for hybrid electric vehicles will have specific test 

procedures as these vehicles can have low canister purge rates relative to non-hybrid vehicles.

Table IV: Option 2 of ARB’s Proposed LEV III Evaporative Standards 

Europe and other countries around the world have implemented evaporative emission 

standards on automobiles and other gasoline powered vehicles since 1983. Most of these have 

followed the European standards for fuels and emission limits and will not be discussed here. 
9

These EURO based regulations for evaporative emissions are generally less stringent than U.S. 

or California regulations. Evaporative limits have also been established for non-automotive 

applications including motorcycles, small and large off-road spark-ignited engines as well as 

marine engines. Although these will not be covered here, some information on evaporative 

emission standards and technologies for non-automotive applications can be found in MECA’s 

small engine2 and motorcycle3 white papers.

2.2 Test Methods for Measuring Evaporative Emissions

The apparatus used to evaluate the diurnal and hot soak emissions from automobiles is 

known as a SHED or Sealed Housing for Evaporative Determination. To conduct the 

measurements, the vehicle is driven into the chamber and the doors are sealed. The SHED is 

equipped with analyzers to measure HC emissions inside the chamber at various intervals. A 

flame ionization detector (FID) measures the total hydrocarbons at the beginning and end of each 

test to determine emissions from the hot soak and each diurnal day. An infrared detector is used 

to distinguish between fuel based HC’s and non-fuel based HCs such as those from polymers, 

rubber and plastics. The two-day plus hot soak test consists of running the vehicle on a Federal 

Test Procedure (FTP) drive cycle with a loaded carbon canister followed by a one-hour hot soak 

at elevated temperature and monitoring over two-diurnal days. The result that is reported is the 

total of the hot soak and the highest value from one of the diurnal days. The canister is preloaded with a mixture of butane and nitrogen to simulate the state of the canister if the vehicle is 

not driven for several days or recently refueled. The hot soak simulates parking the vehicle at 

72 ο

F after driving. The temperature range for the diurnal day to represent a California ambient 

environment consists of a soak at 65 οF followed by a 12 hour ramp up to 105 οF and a 

subsequent 12 hour cool down to 65 ο

F. The U.S. EPA diurnal cycle consists of a temperature 

cycle from 72 οF to 95 οF. 

The three-day diurnal test for CARB includes an additional running loss test which 

involves an FTP drive cycle performed at an ambient temperature of 105 ο

F. Additional 

emissions measurements are taken at the filler cap and canister to make sure that the purge 

system is functioning properly and emissions are contained during operation. Following the 

running loss test, the vehicle emissions are measured after a one-hour hot soak at 105 ο

F 

followed by three days of diurnal cycles as in the two-day test. The results that are reported for 

the three-day diurnal test combine the emissions measured during the hot soak and the highest of 

the three diurnal days. 

In order to meet the PZEV emission requirements, manufacturers must demonstrate 

elimination of fuel system related evaporative emissions. The zero-evaporative standard has 

two components that must be tested separately. First, the whole vehicle must be tested in a 

SHED for the two and three-day diurnal to show that emissions from all HC sources do not 

exceed 0.35 g/test for passenger cars under LEV II and Tier 2. Larger vehicles are allowed 

higher whole vehicle limits as shown in Figure 1. Second, to test only the emissions from the 

fuel system components, a rig test is used. For this test, the fuel system of the vehicle is 

reassembled inside the SHED and subjected to diurnal testing. The emissions from this test must 

not exceed 0.054 g/test.
10

The rig test requires that a support frame and the entire fuel system be assembled inside a 

SHED apparatus. The assembly includes the fuel tank, fuel metering system, including injectors 

and fuel vapor control system such as the canister. The testing takes over 160 days to complete 

including time to age the components and a 140 day stabilization period. The actual SHED 

diurnal testing takes about 1-2 weeks. The bleed emissions test or mini-rig test that ARB has 

included under compliance option 2 of the LEV III proposal was suggested by MECA as a way 

to insure against backsliding by de-contenting advanced canister technologies from PZEV 

vehicles that might occur if only a whole vehicle test was required. Because a whole vehicle test 

combines evaporative emissions from the fuel system with non-fuel related VOCs, without a 

separate fuel system test, manufacturers could trade-off higher fuel related emissions for lower 

non-fuel emissions to pass a higher vehicle limit. This may result in the removal of the best 

available canister technologies and lead to higher overall evaporative emissions during real 

world operation.

The bleed emissions test is much simpler because it only tests the critical fuel vapor 

control components in a SHED apparatus.5

 The components involved in the test include the fuel 

tank, tank vent lines and carbon canister (Figure 3). Under this test, the canister can be aged and 

prepared in under 2 weeks resulting in a total test time of only two and a half weeks. Only the 

canister venting emissions are measured and included to meet the 0.02 g/test limit as proposed 

for LEV III. 

Figure 3: Test set-up for a bleed emissions or mini-rig test

3.0 Technologies to Control Evaporative Emissions

Evaporative emissions from vehicles can be significantly reduced by addressing the 

primary sources of HC leakage from the fuel system such as permeation, diurnal, and refueling. 

The technologies employed in modern fuel systems to address these mechanisms will be the 

topic of this section. The sources of these emissions are better illustrated in Figure 4 which 

depicts a typical fuel system.
11

Figure 4: Fuel system with emission control technology. Circles identify common joints 

and connectors as sources of liquid and vapor leakage.

3.1 Permeation Controls

Permeation emissions can be addressed by reducing the permeability of plastics and 

polymers to gasoline in either the liquid or vapor phase. This can be accomplished through both 

design and selection of materials. Reducing the number of joints and connectors in a fuel system 

and the design of the fuel tank are some of the ways to reduce fuel leakage and permeation. Fuel 

system design is not within the scope of this paper but we will touch briefly on materials of 

construction as a way to control permeation. This is important in tanks, hoses, seals and 

connectors. Although metal tanks offer the highest barrier to permeation, they add weight and 

limit the shape necessary to meet stringent packaging requirements. Advanced tanks consist of 

coextruded, multilayer construction with a barrier layer of ethylene vinyl alcohol and 

fluoropolymers to reduce permeation. Furthermore, polymers can be treated via sulfonation or 

fluorination to further reduce permeability. 

Similar approaches of material selection can be applied to fuel hoses, seals, fuel caps and 

gaskets used within the fuel system. The use of coextruded, low permeation polymers such as 

nylon, fluoropolymers, and fluoroelastomers can be employed in fuel lines to significantly 

reduce permeation emissions. Special challenges in permeation emissions and materials 

compatibility have resulted since the introduction of ethanol blends in gasoline. The newest 

vehicles and, in particular, Flexible Fuel Vehicles (FFV) are equipped with the lowest 

permeation materials in the fuel tanks, hoses, seals and gaskets. Older vehicles and, in particular, 

spark-ignited off road engines like those used in lawn equipment, marine engines, recreational 

motorcycles and ATVs still use conventional fuel system materials which are not compatible 

with ethanol levels above 10%. There is a concern that if this equipment and vehicles are fueled 

with ethanol-gasoline blends greater than E10 they will result in significant emissions of 

hydrocarbons into the atmosphere contributing to ozone formation. Furthermore these engines 
12

are not calibrated to operate on higher ethanol blends. The low permeation technology discussed 

here is equally effective in reducing evaporative emissions from all types of gasoline powered 

vehicles and equipment but require technology forcing regulations for implementation.

3.2 Carbon Canisters

One of the essential components of the evaporative emission control system is the carbon 

canister. The canisters employed on automobiles, and other gasoline powered vehicles and 

equipment, are similar and consist of a plastic housing containing high surface area carbon 

adsorbent material. Hydrocarbon molecules are attracted to the non-polar surface of the 

activated carbon and stored within the pores by physical adsorption or physisorption. Canister 

filling occurs during diurnal events and refueling. Canisters come in many shapes and sizes and 

are proportional to the volume of vapor generated in the fuel tank (Figure 5). 

Figure 5: Examples of the shapes and sizes of EVAP canisters for on-road and off-road 

applications.

Advanced canisters employ multiple chambers and specially designed carbon adsorbents to 

achieve very low or zero evaporative emissions depending on the level of evaporative emission 

that must be achieved. The carbon technology will be discussed in Section 3.2.1. As HC vapors 

are forced out of the tank during heating or refueling, they enter the first chamber of the canister 

and pass through to the second chamber (see Figure 6). 
13

Figure 6: Inside of EVAP canister without carbon adsorbent to show chambers and 

adsorption/desorption process.

Canister purging occurs during engine operation. In early automotive canister designs 

and more recent off-road applications such as spark-ignited marine engines which employ carbon 

canisters, the purging occurs passively by the vacuum created during cooling of the tank and 

fuel. The tank vacuum will pull clean ambient air through the canister causing desorption of the 

hydrocarbon molecules from the carbon surfaces. The vacuum created during engine operation 

pulls air through the carbon bed resulting in desorption of HCs and drawing them into the intake 

of the engine. A sudden surge in HCs into the combustion chamber can result in excessive 

hydrocarbons in the tailpipe. Control of the vapor concentration of the purge air stream by the 

engine control system is necessary to meet both exhaust and evaporative emission standards. 

Proper metering of purge flow using the purge valve and restricting vapor flow from the fuel 

tank with a flow management orifice are methods that are used to prevent large HC vapor 

transients from entering the intake of the engine but yet allow high gas flow during refueling 

events. 

Important elements of canister design used to control bleed emissions include crosssectional area and pressure drop. The length-to-diameter (L/D) ratio of the canister can be 

optimized to increase diffusion path length while minimizing pressure drop.5 The working 

capacity is another important design parameter for carbon canisters that is also affected by the 

L/D ratio. Working capacity represents the amount of hydrocarbons by weight that can be 

maintained within the activated carbon charge of the canister under a well defined test procedure. 

In general the L/D ratio is optimized for working capacity without compromising backpressure 

within the constraints of packaging. Increasing the L/D ratio of the canister increases the 

Atmosphere

Adsorption

Desorption
14

localized airflow over the carbon bed during purging resulting in more effective removal of the 

adsorbed HCs.5

 Carbon canisters are very effective and extremely durable control technologies 

with little or no deterioration of performance over the full useful life of the vehicle. 

The zero evaporative emissions required of PZEV vehicles has put engineering focus on 

the small levels of bleed emissions from the canister which occur during diurnal loading. Bleed 

emissions are those that that pass through the canister prior to breakthrough. Breakthrough 

represents the first appearance of HCs in the air from a canister. In automotive applications, this 

is the point when the total concentration of HC emitted from the canister reaches 5000 ppm. 

Bleed emissions represent adsorbed hydrocarbons that desorb and diffuse from the canister. 

Bleed emissions are a function of soak time, concentration gradient, temperature and 

hydrocarbon species.

Additionally, smaller displacement engines, or partial electric operation like that of 

gasoline-electric hybrids will result in less available purge air to clean the carbon canister. The 

canister designs for these applications use a third, smaller chamber filled with specially designed, 

easy to purge carbons, known as a scrubber, to effectively capture these canister bleed emissions. 

This chamber is referred to as the auxiliary chamber and is filled with a high capacity carbon that 

is often extruded into a honeycomb shape to maximize geometric surface area while minimizing 

flow restriction. The emission reduction benefits of incorporating a scrubber or auxiliary

chamber on a carbon canister are shown in Figure 7. Bleed emissions are reduced by 95 to 295 

mg/test depending on the level of purge. A low purge design would represent a hybrid vehicle 

whereas a normal purge would be a conventional PZEV design. The most advanced canisters 

being developed for the lowest purge vehicles such as plug-in hybrids may incorporate a heat 

transfer medium to warm the carbon and improve the purge efficiency.

Figure 7: Canister bleed emissions with and without a scrubber

3.3 Activated Carbon

At the core of the canister function is the activated carbon that is charged inside the 

chambers. Carbon is available in different particle sizes and working capacities. The particle 

size or granule size controls the back pressure whereas the working capacity is a function of 

surface area and porosity. The working capacity describes the activation level of the carbon and 
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is expressed as Butane Working Capacity in units of grams of butane adsorbed per 100 cc of 

carbon. Marine applications require a special water proof pelletized carbon. The porosity of 

carbon is described as a function of pore sizes including micropores (< 20 angstroms), 

mesopores (20-50 angstroms) and macropores (>500 angstroms). Vapor migration into the 

carbon particle occurs via gas phase and surface diffusion of the hydrocarbon molecules 

(Figure 8). Hydrocarbon molecules are driven to migrate and redistribute within the pores of the 

carbon by a combination of concentration gradient and surface energy. 

Another important property of the activated carbon is the heel or the residual 

hydrocarbons remaining on the carbon after purging. The pore size distribution of the carbon 

directly affects both the working capacity and heel of the carbon. High working capacity is 

achieved by increasing the pore volume within a critical size range depending on the size of the 

hydrocarbon molecules being adsorbed. A smaller pore size range is associated with the heel as 

pores of this size range trap the hydrocarbons and prevent them from being purged. High 

activity carbons that have a high working capacity also have a tendency for stronger adsorption 

or heel during purging. This can lead to higher diurnal emissions. Advanced carbon designs that 

release HCs easily with a small volume of purge air are best suited for smaller engines and 

hybrid powertrains. These carbons tend to have a lower working capacity and are not ideal for 

use in the entire canister. Bleed emissions are best reduced by using an activated carbon where 

the working capacity and low heel are optimized. The shape must be controlled to minimize 

back pressure or flow restriction. The carbon used in the auxiliary chamber of PZEV canisters is 

typically extruded into small honeycomb monoliths as shown in Figure 9.

Figure 8: Pore types and diffusion mechanisms within an activated carbon particle.

Macropores

Micropores

Mesopores
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Figure 9: Examples of extruded activated carbon honeycombs like those used in auxiliary 

chambers of PZEV canisters.

3.4 Air Intake Systems (AIS)

When the engine is shut off, the concentration of hydrocarbons in the cylinders and 

intake manifold is higher than the concentration upstream of the throttle body and air intake. In 

the absence of intake airflow, fuel vapors will migrate past the air induction system and into the 

atmosphere. These emissions are on the order of 0.1 g per diurnal with some additional losses 

during hot-soak cycles. Because of the zero evaporative requirement on PZEV vehicles, these 

emissions will be detected in the SHED test and must be controlled. These slow moving bleed 

emissions can be captured by incorporating a small hydrocarbon trap into the air induction 

system of the vehicle. The earliest designs utilized activated carbon within the air cleaner 

element. The size and shape of engine air induction traps can vary from honey combs to carbon 

coated paper or thin panels of activated carbon (Figure 10). In some applications, zeolite based 

coatings have been applied to metal honeycomb substrates to control air intake evaporative 

emissions. This design offers low pressure drop, high HC capture efficiency and clean 

desorption. An example of this technology is shown in Figure 11.

Figure 10: Example of carbon shapes used in vehicle air intake systems
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Figure 11: Zeolite based AIS hydrocarbon trap

Because these traps are not intended to capture a large amount of hydrocarbons, their 

working capacity is extremely low and they have a very low pressure drop. They are extremely 

effective in reducing the HC emissions by 100-200 mg/test which is significant when trying to 

meet near zero PZEV limits. Figure 12 compares the AIS emissions from a small four cylinder 

as well as a large eight cylinder vehicle both with and without air induction traps. This test 

isolated the air induction emissions from the other types of emissions in a SHED test to show 

that these traps can be up to 90% efficient in capturing emissions from the air intake system.

Figure 12: Air induction system emissions with and without an AIS carbon trap

3.5 On-Board Refueling Vapor Recovery

On-board refueling vapor recovery (ORVR) systems are designed to capture 

hydrocarbons dispersed in the vapor of the fuel tank that are displaced during refueling. 

Although the heart of the system is the carbon canister, there are a number of other valves and 

seals to prevent escape of vapor through the fuel filler pipe and preventing liquid gasoline from 

exiting the fuel tank when tipped beyond horizontal. The displaced vapor is directed into the 

carbon EVAP canister and trapped. During engine operation, fresh air is purged through the 

canister to regenerate the carbon so that it is ready for subsequent fueling or diurnal events. The 

purged vapors are consumed in the combustion process. Today, all new passenger vehicles 

manufactured in North America are equipped with ORVR systems.
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4.0 On-Board Diagnostic Requirements

Modern EVAP control systems incorporate on-board diagnostic controls that test the fuel 

system integrity and insure that they are functioning properly. Fuel tank pressure sensors are 

being used to alert the driver when a fuel cap is missing or not sealed properly. This functions 

by applying a slight vacuum to the fuel tank and monitors if it is maintained. On OBDII 

equipped vehicles, the system will perform an integrity check during normal operation. If the 

leak rate exceeds a limit value, the Malfunction Indicator Lamp (MIL) will be illuminated. 

During canister purge, the Powertrain Control Module (PCM) monitors the HC air 

mixture going into the combustion chamber so that the overall air/fuel ratio does not impact the 

vehicle tailpipe emissions. The PCM must control the fuel delivery to each individual cylinder. 

The exact control of purge emissions in Flex Fuel Vehicles (FFV) poses challenges as they are 

able to use multiple fuels with various vapor pressures.

5.0 Conclusion

• Evaporative emissions from mobile sources have raised health and environmental 

concerns, but a number of technologies exist that can greatly reduce VOC emissions 

from gasoline-powered vehicles and equipment.

• Regulations are necessary to prevent back-sliding and decontenting of technology 

from vehicles and insure that the best available evaporative controls continue to be 

installed on vehicles. 

• Low permeability rubber and polymers are being deployed in fuel tanks, hoses, seals 

and gaskets of modern fuel systems to minimize permeation losses.

• Carbon canisters remain at the heart of vehicle EVAP controls to capture fueling, 

diurnal and hot soak emissions of hydrocarbons.

• EVAP controls offer an extremely effective and durable means to control ozone 

forming HC emissions over the life of the vehicle.

• PZEV and hybrid vehicles use the most advanced carbon technology as part of the 

auxiliary chamber in PZEV canisters and air induction systems to achieve near-zero 

evaporative emissions despite low purge volumes.

• The types of technologies deployed on PZEV vehicles can be implemented within the 

entire gasoline vehicle fleet and beyond to off-road spark-ignited vehicles and 

engines that have yet to benefit from EVAP emission controls.

6.0 References

1) ARB LEV II Staff report: http://www.arb.ca.gov/regact/LEVii/isor.pdf

2) MECA Publication: Emission Control of Small Spark-ignited Off-Road Engines and 

Equipment, http://www.meca.org/galleries/defaultfile/SORE%20white%20paper%200109%20FINAL.pdf
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3) MECA Publication: Emission Control of Two and Three Wheeled Vehicles, 

http://www.meca.org/galleries/defaultfile/Motorcycle%20whitepaper%20final%20081908.pdf

4) Technologies for Near-Zero-Emission Gasoline-Powered Vehicles, F. Zhao, SAE 

International, 2007

5) Impact and control of Canister Bleed Emissions, SAE paper 2001-01-0733