ABSTRACT

With increasingly stringent Federal Emissions Standards, automobile manufacturers have been required to decrease both tail pipe emissions and the evaporative emissions of volatile organic compounds (VOC).  Some of these VOCs are hydrocarbons (gasoline and oil), which vaporize and escape from an automobile’s engine and fuel system. Automobile manufacturers have addressed this issue with the implementation of Evaporative (EVAP) Emissions Systems.  This article will describe the major system components and explain how the EVAP systems operate.

INTRODUCTION

The purpose of the automobile evaporative emission systems is to reduce or eliminate the release of vaporized hydrocarbons (HC) into the atmosphere.  The major source of these HC vapor emissions is the automobile fuel system.  Though not the only source of evaporative emissions (rubber parts and tires contribute), fuel system emissions are the major producers of VOCs and are, therefore, the topic of this paper.

EVAP emission control systems began to appear on automobiles when it was discovered that HC vapors in the atmosphere contributed to the formation of photochemical smog.  Exposure to these vapors was found to lead to a variety of respiratory ailments under certain conditions of prolonged exposure.  As the magnitude of the problem became greater, state governments (most notably California) and the federal government began to legislate increasingly stringent standards for control of these automotive emissions.

EARLY EVAP SYSTEMS

The introduction of positive crankcase ventilation (PCV) in the 1960’s marked the beginning of evaporative emission control systems in the automobile.  The initial purpose of the PCV system was to capture crankcase vapors and prevent them from being vented into the atmosphere through the road draft tubes in use at the time.  These escaping vapors were often visible and thought by many to be the primary cause of smog.

Combustion chamber gasses escape into the crankcase through a process known as “blow-by”.  Blow-by occurs when the compressed fuel/air mixture is allowed to pass the seal created between the piston ring and the cylinder wall.  In doing so these gasses become trapped in the crankcase where hydrocarbons are then allowed to vent into the atmosphere. 

Another concern of combustion gasses in the crankcase is that they increase Ph in the oil, making it acidic.  An engine containing oil with high Ph will cause accelerated wear on vital components such as bearings and seals.  The addition of the PCV valve was the point at which the government first began to address evaporative emissions.

The PCV valve meters the return of the crankcase vapors to the engine’s intake manifold.  The vapors then mix with the engine’s intake air and/or fuel/air mixture and re-enter the combustion chamber to be burned.  This ensures that all of the HC vapors are exposed to the combustion process, thus eliminating HC emissions from the crankcase to the atmosphere. 

The purpose of all Evaporative (EVAP) Emissions Systems is to prevent the release of VOC.  The main concern is Hydrocarbons (HC’s) or unburned gasoline vapors.  Hydrocarbons are released from gasoline in the form of a vapor, and if the fuel system is not air tight, these vapors can then escape into the atmosphere.  The problem with unburned hydrocarbons is that they aid in the production of photochemical smog.  

There are four major ways in which these gasoline vapors are produced and allowed to escape.  The first way is through diurnal evaporation.  This occurs during the daylight hours when the fuel is heated by an increase in ambient temperature.  The rise in temperature increases vaporization.  The second way that HC’s escape is through running losses.  This is the result of heat in the engine compartment from the exhaust system and the operation of the engine, both of which cause fuel vaporization. The third concern is from “hot soak”.   After an engine is turned off, the radiant heat will cause gasoline vaporization for an extended period (as long as one hour).  The fourth source of HC production occurs during the time of refueling.  Fuel vapors are always present in the fuel tank.  When liquid fuel is added to the tank, it displaces the vapors by venting them into the atmosphere.

Early evaporative emissions systems contained many of the same components as today’s systems.  The primary function of the evaporative emissions system is to prevent hydrocarbons from being released into the atmosphere and store them until they can be reintroduced into the intake air stream at a later time.  The vapors are stored in a charcoal canister.  The charcoal in the canister provides a surface area onto which the fuel vapors can be adsorbed and stored.  These stored vapors can then be released back into the incoming air charge when certain criteria are met during vehicle operation.  The fact that charcoal is a carbon makes it ideal for use in the fuel system.  Carbons are attracted to other carbons.  Because of this fact, hydrocarbons will form loose chemical bonds to the charcoal in the carbon canister.

The release of fuel vapors is accomplished by a canister purge solenoid, which is placed in series between the canister and the engine’s intake manifold.  This solenoid controls the quantity and rate of vapors being released by cycling on and off in what is called a duty cycle.  A duty cycle is the solenoid’s “on time”(open) compared to its “off time” (closed) as a percent of the cycle.  The vehicle’s Powertrain Control Module (PCM) determines the duty cycle of the purge solenoid.  

The PCM receives feedback from the exhaust oxygen sensor (O2S) located in the exhaust stream.  The O2S is usually a voltage-generating device.  Most commonly, when an O2S senses a rich air/fuel ratio (one below 14.7-1 such as 13.0-1) it will produce a high voltage between .6 and 1 volt.  A lean air/fuel ratio (one above 14.7-1 such as 15.0-1) will produce a low voltage between .1 and .4 volts.  An average voltage of .45 to .5 would be representative of a nearly stoichiometric air-fuel ratio (approximately 14.7 lbs. of air to 1 lb. of fuel). 

With O2S feedback the canister purge solenoid can be activated when the vehicle is capable of running lean, most commonly at warm engine cruise.  As the canister purge solenoid opens, it permits the vacuum in the engine’s intake manifold to draw in fuel vapors. These vapors will enrich the lean air/fuel mixture and the purge will be shut off once the O2S senses this change.

In early systems, “hot soak” of the engine was a major producer of HC emissions. Hot soak is the period after engine shutdown when the engine temperature will actually increase due to the lack of coolant flow through the engine.  During this time, vehicles equipped with carburetors experience fuel evaporation from the fuel bowl(s).  To address this problem, engineers looked initially to the vehicle’s air filter assembly to capture these stray vapors.  Later models connected the fuel bowl to the carbon canister with a hose, and controlled its venting with a valve.  With the implementation of fuel injection, this problem has been dramatically reduced because no fuel is directly exposed to the atmosphere at any time other than during refueling.  

One major source of HC emissions in both systems is fuel tank venting.  During the course of a day, the tank heats and cools, causing pressure variations.  As fuel expands and evaporates in the heat of the day, it builds a positive pressure.  The fuel system must be vented to prevent components in the fuel system from leaking as a result of system pressures becoming too high.  And as the fuel is consumed, air must be allowed to enter the tank to prevent the collapse of the tank resulting from the lowered internal pressure.   

One component that has been added to the fuel system is a vented fuel cap.  These caps are designed to allow air to enter the tank during a period of low pressure, or vacuum, while resisting the release of air containing HC during times of high pressure.  Pressurized air/HC in the system still needed somewhere to go.  The charcoal canister was the answer.  The reason charcoal was chosen as the storage medium was for its vast surface area and its ability to adsorb the excess vapors in the fuel system. 

MODERN ADDITIONS TO THE EVAP SYSTEM

The basic components of the modern EVAP system are essentially much the same as the older systems.  A major modification/addition to older systems is that of the computer controls.  A function of the new EVAP system is testing fuel system integrity.  In older systems, if the gas cap were left off the vehicle could be driven around all day before the driver realized it, allowing hydrocarbons to escape the system through the uncapped filler neck.  

To address this problem, manufacturers are now beginning to install fuel tank pressure sensors.  These sensors will allow the PCM to apply a vacuum to the fuel tank to see if it is capable of maintaining a system vacuum.  If the system is unable to maintain a vacuum, it has the potential to emit HC into the atmosphere, and it will illuminate an indicator lamp in the driver’s information center.  This light is most commonly marked as a gas cap light or gas door light. 

The pressure sensors are commonly supplied with a 5-volt reference voltage and will usually read between 1.3-1.7 volts at atmospheric pressure.  In the newest Onboard Diagnostic System Generation 2 (OBD II) systems, the PCM will perform a system integrity test during the normal operation of the vehicle.  If the system pressure leak rate exceeds a given value, the system will illuminate the Malfunction Indicator Lamp (MIL) on the driver’s information center.  Once the problem has been addressed and corrected, the MIL will be turned off by the PCM.  In most cases a failed EVAP system test will illuminate the MIL as well as trigger a Diagnostic Trouble Code (DTC).

During the time of canister purge, the system draws a vacuum through the carbon canister and allows fresh air to enter it.  The purpose of allowing fresh air into the carbon canister is to oxygenate the stored hydrocarbons before they enter the engine.   The mixture of HC and air being purged from the carbon canister is monitored closely by the PCM to assure that the lowest possible effect on vehicle tailpipe emissions is maintained. 

With the introduction of OBD II, the exact control of the emissions system is more crucial than ever.  A federal mandate requires that a vehicle’s PCM be able to control the fuel delivery to each individual cylinder in the engine.  If the PCM detects a misfire it must be able to determine which cylinder is misfiring and shut down the fuel delivery to that particular cylinder.  The same sort of attention has been paid to the evaporative emissions system.  The PCM needs to be able to determine system pressure and be able to perform a system integrity test.  These tests are simply to ensure that there are no unnecessary releases of hydrocarbons into the atmosphere.  

A new problem that automobile manufacturers are encountering is that of making an evaporative emissions system for the new Flex Fuel Vehicles (FFV).  These vehicles pose potential problems because they are able to use multiple fuels each producing vapor at different temperatures.  A vehicle’s PCM needs to be able to recognize the type of fuel being used and be able to compensate and adapt the operating strategies to make the systems work effectively. 

The PCM is able to make these calculations through feedback from the fuel system sensors.  On FFV the vehicle is equipped with a flex fuel sensor (FFS), which is able to determine the type of fuel being used by determining the dielectric constant, temperature and conductivity of the fuel.  This sensor is supplied with a five-volt reference signal.  A change in the fuel’s dielectric constant or conductivity will produce a variance in the frequency output.  For example, in Ford Motor Companyâ vehicles, if the FFS were to produce a 50Hz signal, it would mean that the fuel mixture contains 0% methanol.  A 150Hz signal means that the fuel is 100% methanol.  This information is important because of the different boiling points as well as the different Reid Vapor Pressures or (RVP).  The PCM also needs to know what type of fuel is being used so that it may adapt the air/fuel ratio to the specific fuel.  All of the above will affect when and how much the PCM will cycle the purge solenoid. 

One major addition to the modern EVAP systems is the tank mounted vapor management valve.  The purpose of this valve is to vent HC’s from the fuel tank directly into the storage canister in order to prevent the pressurized build up of hydrocarbons in the fuel tank.  The addition of this valve reduces the amount of hydrocarbons that are allowed to escape into the atmosphere as the fuel cap is removed at the time of refueling.  The integrity of the sensors is monitored by the PCM, which evaluates the status of the duty cycled output drivers.  The Vapor Management Valve (VMV) is one of the closely monitored sensors.   If the electronics in this system fails to meet the system criteria, the PCM will set a DTC and illuminate the MIL.  The criteria that need to be met are preprogrammed into the PCM.  

CONCLUSION

As today’s vehicles become more complex, the ill effect each has on the environment will continue to decrease.  As automotive manufacturers begin to experiment with electric and gasoline/electric hybrids, the need for EVAP systems will begin to decline.  With more and more automotive manufacturers looking to the future when all automotive manufacturers may be required to produce zero emissions vehicles, it is likely that sooner than we think, the gasoline-powered car will be a thing of the past.  Until that time comes, the government is making sure that the environment is damaged as little as possible. 

Since the introduction of evaporative emissions systems, the tailpipe emissions from vehicles have been dramatically reduced.  With continuing developments in technology, these numbers will continue to decline.  The introduction of PCV in the 1960’s marked the coming of the end for the large displacement V8 muscle cars, and the beginning of an era of aerodynamic economy cars.  Today’s emissions technology is allowing designers to put some of the muscle car back into the economy sector of the market. 

As emissions standards become even more stringent, automobile manufacturers will be required to find new ways to make their vehicles cleaner running.  Eventually some limit will be found and engineers will be forced to make alternative fuels into standard fuels.  Until that time comes, however, they will continue to make the best of  current automotive technology.  As technology inproves, engineers will develop more powerful, more environmentally friendly and more efficient vehicles. One of the fuels that is today used as a flex fuel may soon become a standard fuel. Replacement fuels will most likely be that of a renewable source which produce little or no harmful tailpipe emissions.

BIBLIOGRAPHY

Barney, Joseph C.  Electronic Fuel Injection Operation and Diagnosis.

Training and publications dept. Ford Parts and Service Division, 1987

Brady, Robert N.   Automotive Electronics and Computer Systems.

Prentice Hall, 2001

Fuel Systems and Emissions Controls Fourth Edition.  Dupuy, Richard K. “ed.”

Chek-Chart Publications, 2000

King, Dick H.   Computerized Engine Controls Fifth Edition.

Delmar/Thomson Learning, 1999

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