Environmental Engineering Reference
In-Depth Information
to provide the technology needed to limit these emissions for the useful life of the vehicle and to
warrant the performance of these control systems.
To certify a vehicle class for exhaust emissions, the manufacturer must test a prototype vehicle
on a dynamometer following the Federal Test Procedure (FTP) (see Section 8.5.1), during which
exhaust gases are collected and later analyzed for pollutant content. Regulated pollutants include
nonmethane hydrocarbons (NMHC) or organic gases (NMOG), carbon monoxide (CO), nitrogen
oxides (NO
x
), particulate matter (PM), and formaldehyde (HCHO). The mass of each pollutant
collected from the exhaust during the test is divided by the test mileage and is reported as grams
per mile. If the prototype vehicle's exhaust emissions do not exceed the standards set for its vehicle
type, vehicles of its class and model year may then be sold by the manufacturer. The manufacturer
is further responsible for ensuring that their vehicles' control systems continue to function properly
during the life of the vehicles, currently set at 100,000 miles. Vehicles must also conform to the
exhaust emission limitations of the Supplemental Federal Test Procedure (SFTP), designed to
evaluate the effects of air conditioning load, high ambient temperature, and high vehicle speeds
(not included in the FTP) on emissions.
Evaporative emissions are tested for two conditions: one where the vehicle is at rest after
sufficient use to have brought it to operating temperature, the other for a prolonged period of
nonuse. In these tests the vehicle is enclosed in an impermeable bag of known volume, and the
organic vapor mass is subsequently determined.
In the United States, vehicle emission standards are set by the U.S. Environmental Protection
Agency in accordance with the provisions of federal air-quality legislation. The regulation is based
upon the recognition of the ubiquity and mobility of the automobile, its concentration in urban areas,
its contribution to urban and regional air-quality problems, and the ability of the manufacturer, and
not the owner, to ameliorate its emissions. In the years since the early 1970s, when regulation
was first introduced, emission standards have become more stringent as the manufacturers devised
better technologies and the difficulty of achieving desirable air quality throughout the United States
became more apparent. Given the lead time required by the manufacturers to develop new control
technologies and incorporate them in a reliable consumer product, emission standards must be set
years in advance of their attainment in new vehicles sold to the consumer.
U.S. exhaust emission standards for vehicles of model year 1996 and beyond are listed in
Table 8.6. (Emission standards for earlier model year vehicles are shown in Table 9.2.) The standards
apply for two time periods; 1996 to 2007 (Tier 1) and 2004 and beyond (Tier 2). The Tier 2 standards
are phased in over the period 2004 -2010, during which the Tier 1 standards are simultaneously
being phased out.
Tier 1 standards limit four pollutants for five vehicle classes: light-duty vehicles (LDV, which
are passenger vehicles for 12 passengers or less) and four types of light-duty trucks (LDT1, LDT2,
LDT3, and LDT4, distinguished by the gross vehicle weight rating and the loaded vehicle weight
[see Section 8.5.1]). The larger light-duty trucks are permitted greater emissions in recognition
of their greater weight-carrying capability. Like earlier standards, the Tier 1 standards apply to
several vehicle classes, but within each class each vehicle model, small or large, must meet the
same standard.
Tier 2 standards introduce a new method of limiting emissions. Like the fuel economy standard,
each manufacturer must achieve a sales-averaged NO
x
emissions for all its vehicles of 0.07 g/mile,
although individual vehicle models may emit more if they are offset by others that emit less. Each
vehicle model is certified in one of seven emission categories (denoted by Bin 1
Bin7in
Table 8.6), which ensures that the sales-averaged emission limits for pollutants other than NO
x
will
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