Evaporative emissions

These are carbon monoxide, CO, unburned hydrocarbons (HC), and the nitrogen oxides, NO. In the U.S.A., a program called Auto/Oil (Burns et al., 1992), conducted by automotive manufacturers and petroleum companies, examined the effect of overall parameters of fuel composition on evaporative emissions and in the exhaust gases. The variables examined were the aromatics content between 20 and 45%, the olefins content between 5 and 20%, the MTBE content between 0 and 15% and finally the distillation end point between 138 and 182°C (more exactly, the 95% distilled point). 

Me Arragher, J.S. and al. (1990), The effects of temperature and fuel volatility on evaporative emissions from european cars . /. Mech. Eng., No. 394/028, London. 

On-Board Diagnostics. State of California regulations require that vehicle engines and exhaust emission control systems be monitored by an on-board system to assure continued functional performance. The program is called OBD-II, and requires that engine misfire, the catalytic converter, and the evaporative emission control system be monitored (101). The U.S. EPA is expected to adopt a similar regulation. 

Evaporative Emission. Fumes emitted from stored fuel or fuel left in the fuel dehvery system are also regulated by U.S. EPA standards. Gasoline consists of a variety of hydrocarbons ranging from high volatility butane (C-4) to lower volatility C-8 to C-10 hydrocarbons. The high volatility HCs are necessary for cold start, and are especially necessary for temperatures below which choking is needed to start the engine. Stored fuel and fuel left in the fuel system evaporates into the atmosphere. 

Emission control for natural gas fueled engines consists of either the same principal components as used for gasoline or those designed for lean combustion. No evaporative emission control is required. 

The emission control system for LPG is the same as is used for gasoline fueled engines with the exception of the fuel metering system. No evaporative emission system is required. Both Pt—Rh and Pd—Rh catalysts are good for emission control of LPG fuel exhaust. Pt provides the lowest light off temperature for C Hg. The sulfur content of LPG is also very low so that Pd catalysts perform very well. 

H. M. Haskey, W. R. Cadman, and T. E. Liberty, The Development of a Real-Time Evaporative Emission Test, SAE 901110, Society of Automotive Engineers, Warrendale, Pa., 1990. 

The Effiects of Temperature andFuel Volatility on Vehicle Evaporative Emissions, Report No 90/1, Concawe, Bmssels, Belgium, 1990. 

Crankcase emissions no crankcase emissions shall be discharged into the ambient atmosphere. Fuel evaporative emissions 2 gm per test procedure for vehicles beginning with 1972. 

Evaporative emissions from the fuel tank and carburetor have been controlled on all 1971 and later model automobiles sold in the United States. This has been accomplished by either a vapor recovery system which uses the crankcase of the engine for the storage of the hydrocarbon vapors or an adsorption and regeneration system using a canister of activated carbon to trap the vapors and hold them until such time as a fresh air purge through the canister carries the vapors to the induction system for burning in the combustion chamber. 

The generation of au pollutants, ineluding VOC s, from automotive vehieles was identified to eome from two prineipal sourees vehiele exhaust emissions, and fuel system evaporative emissions [4], Evaporative emissions are defmed as the automotive fuel vapors generated and released from the vehiele s fuel system due to the interactions of the speeific fuel in use, the fuel system characteristics, and environmental factors. The sources of the evaporative emissions are discussed below and, as presented m the remainder of this chapter, control of these evaporative emissions are the focus of the application of activated carbon technology in automotive systems. 

The early carbon trap and SHED methods measured two components of evaporative emissions. Hot soak emissions were measured for a one hour period immediately after a vehicle had been driven on a prescribed cycle and the engine turned off. Diurnal emissions were also measured during a one hour event where the fuel tank was artificially heated. The one hour fuel temperature heat build was an accelerated test that was developed to represent a full day temperature heat build. 

The latest CARB/EPA procedures require diurnal emissions to be measured during a real time, three day test that exposes the complete vehicle to daily temperature fluctuations. This test method has been employed to more accurately reflect the real world diurnal emissions that occur. Rutming loss emission measurements were also initiated in the latest test procedures. Evaporative emissions are measured... 

The following countries also have evaporative emission regulations Canada, European Economic Cormnunity (EEC), Japan, Brazil, Mexico, Australia, South Korea. Regulations in these countries have requirements that are typically less stnngent than the U.S. imperatives. Table 1 depicts the chronology of evaporative emission regulation developments in the United States. 

A current vehicle fuel system designed for evaporative emission control should address enhanced SHED, running loss, and ORVR emission level requirements (see Table 1). A typical vehicle fuel system is shown in Fig. 4. The primary functions of the system are to store the liquid and vapor phases of the fuel with acceptable loss levels, and to pump liquid fuel to the engine for vehicle operation. The operation of the various components in the fuel system, and how they work to minimize evaporative losses during both driving and refueling events, is described below. 

Fig. 4. Representative vehicle fuel system with evaporative emission control...

The vapor vent valves are connected to the tank vapor control valve, and ultimately to the carbon canister by tubing that is resistant to swelling in the presence of fuel vapors. The tubing material must also have a low HC permeation rate, so that the evaporative emissions are not increased due to release of HC molecules. The tank vapor control valve connects the carbon canister to two fuel tank vapor sources the vapor vent valve lines and a refueling vent tube. 

During vehicle operations similar to those experienced during the three day diurnal evaporative test outlined in Fig. 1, the following operations occur in the evaporative emission control system ... 

Fuel system components involved in the refueling process include the fuel tank, filler pipe, filler cap, vapor control valve, liquid-vapor discriminator (LVD) valve, and the carbon canister [27,28]. During vehicle refueling, which is monitored during the integrated refueling test as outlined in Fig. 1, the following operations occur in the evaporative emission control system ... 

Evaporative emissions from vehicle fuel systems have been found to be a complex mixture of aliphatic, olefinic, and aromatic hydrocarbons [20,24,33]. However, the fuel vapor has been shown to consist primarily of five light paraffins with normal boiling points below 50 °C propane, isobutane, n-butane, isopentane, and n-pentane [33]. These five hydrocarbons represent the more volatile components of gasoline, and they constitute from 70 to 80 per cent mass of the total fuel vapor [24,33]. 

Once the heel has been established in the carbon bed, the adsorption of the fuel vapor is characterized by the adsorption of the dominant light hydrocarbons composing the majority of the hydrocarbon stream. Thus it is common in the study of evaporative emission adsorption to assume that the fuel vapor behaves as if it were a single light aliphatic hydrocarbon component. The predominant light hydrocarbon found in evaporative emission streams is n-butane [20,33]. Representative isotherms for the adsorption of n-butane on activated carbon pellets, at two different temperatures, are shown in Fig. 8. The pressure range covered in the Fig. 8, zero to 101 kPa, is representative of the partial pressures encountered in vehicle fuel vapor systems, which operate in the ambient pressure range. 

As initially discussed in Section 3, carbon canisters are used in the automotive emission control system to temporarily store hydrocarbon vapors. The vapors are later purged into the air charge stream of the air induction system, thus regenerating the carbon canister. Carbon canister design is dependent on the characteristics of the vapors sent to the canister and the amount of purge air available. In the following section, factors that affect the performance of the evaporative emission control system will be discussed. 

There are many other factors that can affect the performance and on-vehicle reliability of activated carbon canisters. The following items shown in Table 6 represent some of the more important factors that must be taken into consideration when designing an evaporative emission canister. 

The use of activated carbon canisters in the control of running loss evaporative emissions will be presented through the use of an example vehicle application. In this example, the vehicle to be studied is a representative standard size sedan equipped with a 3.0 liter, V6 engine and a 72 liter (18 gallon) fuel tank. The vehicle is assumed to have an evaporative emission control system similar to the one presented in Section 3. 

A key parameter in the generation of fuel vapor is the temperature level reached in the fuel tank during vehicle operation. As the temperature approaches the top of the fuel distillation curve, a sizable increase in vapor generation will occur, which severely impacts the amount of HC vapor that the carbon canister system must handle. Limiting the temperature increase in the fuel tank is an important parameter affecting the ability of the evaporative emission system to maintam allowable emission Levels. 

The role of activated earbon in the eontrol of automotive evaporative emissions is... 

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