The highly preferred SAE Recommended Practice for measuring evaporative emissions from fuel systems of passenger cars and light trucks is the enclosure technique detailed in SAE J171. The sensitivity and accuracy of the enclosure technique is superior to that of the trap method. This recommended practice is retained for historical reference and for use with older vehicles imported into the United States of America. In addition, this trap method is referenced in SAE J171a for making running loss measurements which cannot practically be made in an enclosure. Emissions are measured during a sequence of laboratory tests that simulate typical vehicle usage in a metropolitan area during summer months:
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(1)
A 1 h soak representing one diurnal cycle in which temperature of fuel in the vehicle’s tank is raised from 60 to 84°F (15.6 to 28.9 °C).
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(2)
A 7.5 mile (12.1 km) run on a chassis dynamometer.
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(3)
A 1 h hot soak immediately following the 7.5 mile (12.1 km) run.
The method for measuring weight of fuel vapors emitted during the test employs activated carbon traps connected to the fuel system at locations where vapors are expected to escape. Vapors from these openings are adsorbed by the traps, and the gain in weight of the traps represents the fuel evaporative emissions.
The test sequence and method for measuring emissions are applicable to vehicles either with or without systems or devices to control fuel evaporative emissions. Although they have been used successfully with a wide range of vehicles equipped with a variety of control devices, they should not be applied indiscriminately to new or unique vehicles or fuel systems. For example, based on experience that temperature excursions of the fuel tank in parked vehicles follow those of ambient air, the test sequence prescribes heating of the fuel tank to simulate a diurnal soak. Any control system designed to alter the relation between fuel and ambient temperatures will not be properly evaluated in the test sequences prescribed.
The recommended practice includes the following sections:
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1. Definitions
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2. Test Fuel
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3. Test Facilities and Equipment
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4. Measurement Method
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5. Preparation of Test Vehicle and Fuel System
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6. Test Sequence
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7. Information and Data to be Recorded
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8. Presentation of Data