Vehicle Fuel Vapor Systems

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. 

A vehicle fuel vapor control system must be designed to meet both driving and refueling emission level requirements. Due to the nature of hydrocarbon adsorption, this emission control is a continuous operation. 

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. 

The key components in the fuel vapor control system include the fuel tank, vapor vent valves, vapor control valve, vapor tubing, the activated carbon canister, and the engine vapor management valve (VMV) [25,26], During normal vehicle operation, fuel tank vapor pressure is relieved through the use of vapor vent valves installed in the vapor dome of the fuel tank. The vent valves are designed to allow for the flow of fuel vapor from the tank, and to assure that liquid fuel does not pass through the valve. 

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]. 

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 example vehicle has been run through the test sequence using a two liter carbon canister and a 150 BV purge level. Fig. 22 presents the results for both a return and retum-less fuel system used in the vehicle. As shown, the fuel vapor temperature and the amount of fuel vapor generated are both lower for the retum-less system. This reduces the amount of HC adsorption required in the carbon canister, and it also reduces the amount of HC emissions in the test sequence, fhe return fuel system used with the stated purge volume and canister size emits an unacceptable level of HC during one of the diurnal sequences (2.12 grams), while the retum-less system emission values are well below the acceptable level. 

The generation of arr pollutants, including VOC s, from automotive vehicles was identified to come from two principal sources vehicle exhaust emissions, and fuel system evaporative emissions [4], Evaporative emissions are defined as the automotive fuel vapors generated and released from the vehicle s fuel system due to the interactions of the specific 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 vast majority of propane fuel systems used on light-duty vehicles to date have been of the mechanical-control type that meter propane in proportion to the amount of air used by the engine (air-valve and venturi-type mixers ). While these systems work well, their capabilities have been overshadowed by gasoline fuel injection systems and often lag behind gasoline systems in terms of acceleration, driveability, and cold-start performance. Chrysler Canada and one European equipment manufacturer offer liquid propane injection systems that are direct analogs to gasoline port fuel injection systems. These systems should have inherent performance advantages compared to the vaporized propane fuel systems. 

CNG fuel systems can thus be subjected to compressor oil and condensed water vapor that would not occur otherwise if the gas had not been compressed. The compressor oil is probably more of an operating problem than a materials compatibility problem, though new CNG vehicle fuel systems that use multi-point fuel injectors may encounter some problems. 

Most CNG refueling systems also should include a dryer that removes water vapor, foreign matter, and hydrogen sulfide from the natural gas before it is compressed. The water vapor can condense in the vehicle fuel system, causing corrosion, especially if hydrogen sulfide is present. In some parts of the country where the natural gas is known to be consistently very clean, a dryer may not be necessary, but in most cases it is prudent to include one. 

NFPA 88B—Standard for Repair Garages requires that areas below grade used for repair vehicles have a forced ventilation system capable of continuously removing at least 0.75 cubic feet of air per minute few each square foot (cfm/sq.ft.) of floor space [5.8]. This ventilation requirement helps prevent accumulation of heavier-than-air fuel vapors which could accumulate in below-grade areas. 

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