U.S. ENVIRONMENTAL PROTECTION AGENCY EPA 4oo-F-93-oo5
OFFICE OF MOBILE SOURCES
High-Tech Inspection and Maintenance Tests
(Procedures and Equipment)
High- Tech Tests for High-Tech Vehicles
Modern vehicles are equipped with sophisticated emission control systems ca-
pable of minimizing pollution from exhaust and from evaporating fuel.
Inspection and Maintenance (I/M) programs have been established in many
parts of the country to ensure these systems are working properly so vehicles
retain their low pollution profiles in actual use. The 1990 Clean Air Act requires
improved I/M programs and more comprehensive testing in certain areas.
The U.S. Environmental Protection Agency (EPA) has developed three new short
tests for use in these I/M programs. The new tests are specifically designed to
measure emissions from today's high-tech vehicles.
The "high-tech" tests provide a very thorough check of vehicle emission control
systems. The sophistication of these tests enables them to determine a car's true
emissions and do a better job than current I/M tests of identifying vehicles
needing emission repair. The accuracy of the high-tech tests also ensures that
malfunctioning vehicles will be repaired to truly acceptable emission levels.
The high-tech test includes three distinct elements:
• Transient, mass emission tailpipe test ("IM240")
• Purge flow test of the evaporative canister
• Pressure test of the evaporative system
The IM240 differs from traditional I/M tests in that the emissions are measured
while the vehicle is driven on a treadmill-like device called a dynamometer.
Most I/M tests today are conducted while the vehicle is idling. A few states test
vehicles on a dynamometer, but only operate the vehicle at one speed.
In the IM240, the vehicle is operated over a driving cycle that has many different
speeds. The cycle is designed to resemble typical city driving and includes
driving modes such as acceleration and deceleration. Vehicle acceleration and
deceleration can be significant sources of emissions from malfunctioning ve-
hicles.
Hydrocarbon (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) emis-
sions are all measured during the IM240, while only HC and CO emissions are
FACT SHEET OMS-16 .
July, 1994
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High-Tech I/M Tests
measured in traditional I/M tests. HC and NOx emissions combine in the
atmosphere to form ground-level ozone, the primary component of smog.
Another important difference between the IM240 and traditional I/M tests is the
way emissions are measured. The IM240 captures the entire exhaust stream
during the test and measures the total mass of emissions from the vehicle
(grams of pollutant per mile driven). Traditional tests measure the
concentration of pollutants in exhaust (percent or ppm). Mass emissions are a
more accurate way of measuring the emission performance of large and small
engines and are more directly related to the contribution that each car makes to
air pollution. The IM240 also can measure fuel economy.
The purge and pressure tests check for proper functioning of a vehicle's
evaporative emission system. This control system prevents fuel vapors from
escaping into the atmosphere. Evaporative emissions can be a much greater
source of HC pollution than exhaust emissions, especially in hot weather when
smog levels are highest. Traditional I/M tests cannot measure evaporatative
emissions.
The evaporative emission system uses engine vacuum to draw fuel vapors in the
fuel tank and vapors temporarily stored in the evaporative canister into the
engine for combustion. The purge test determines whether this system is
functioning properly by measuring the flow of vapors into the engine during the
IM240. The pressure test checks the evaporative emission system for leaks.
IM240 Test Procedure
The IM240 begins by driving the vehicle onto the dynamometer, activating
vehicle restraints, properly placing the exhaust collection device, and positioning
the auxiliary engine cooling fan. An inspector then "drives" the vehicle according
to a prescribed cycle displayed on a video screen. The inspector follows the
driving cycle by using the accelerator pedal and the brake to speed up or slow
down just as if the vehicle were being driven on a city street. A cursor on the
video screen indicates vehicle speed. The inspector adjusts the speed to keep the
cursor on the trace. This technique is easily and quickly learned by anyone who
can drive a car.
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High-Tech I/M Tests
The length of the IM240 test varies depending on the vehicle's emissions. To
determine emission levels, second-by-second instantaneous emission
measurements are taken and integrated by a computer. Failure levels for
vehicles undergoing I/M tests are generally two to three times higher than
manufacturer certification standards for new cars. The computer continually
monitors and assesses the emission levels during each phase of the test and uses
pass/fail algorithms to identify exceptionally clean or dirty vehicles. As soon as
the emission rates indicate that a vehicle is exceptionally clean or dirty the
computer automatically notifies the inspector to stop testing. For vehicles that
are close to maximum allowable emission levels, the test may continue for a full
240 seconds. Thus, while the complete driving cycle is 240 seconds long, the
average test time will be only two to three minutes.
IM240 Test Equipment
The equipment needed to perform the IM240 is different from the equipment
used for either the idle-type I/M test or the single-speed dynamometer tests used
in some states (e.g., Arizona and Florida). These differences include
dynamometer capabilities, video driver trace monitors, special sampling systems,
and emission analyzers. In addition, the high-tech test system will use computer
controls with integrated quality assurance functions, and will be completely
automated.
The primary difference between the dynamometer used for the IM240 and those
used for single speed I/M tests is the addition of inertia flywheels. The inertia
flywheels used are based on the weight of the car being tested. They allow the
inspection test to simulate vehicle acceleration and deceleration by putting a
load on the engine. This in turn allows the measurement of emissions under
these normal driving conditions. This type of dynamometer is widely available
and is similar to the ones used by EPA and car manufacturers for new car certifi-
cation.
The selection of the inertia weight and test horsepower for an individual vehicle
will be automatically determined by computer so that the I/M inspector is only
required to drive the vehicle onto the dynamometer. Even the system used to
hold the vehicle on the dynamometer will be automatic in order to minimize test
set-up time and improve testing efficiency.
The vehicle's mass emissions are determined by collecting the entire exhaust
flow from the tailpipe with a device known as a Constant Volume Sampler
(CVS). The CVS dilutes the exhaust with fresh air and measures the flow rate of
the mixture. Mass emissions (for each second) are calculated by multiplying this
flow rate by the measured concentration of pollutants in the mixture. To arrive
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at the official test value in grams per mile, the mass emissions for each second
are added together; this sum is then divided by the distance (number of miles)
traveled over the 240-second test cycle.
The fresh air dilution is vital because it preserves the integrity of the sample
and because it protects the emission analyzers from high concentrations of water
vapor produced by the vehicle. The dilution process also allows the measure-
ment system to accommodate the differences in exhaust flow between small
engines and large engines while measuring the true amount of emissions from
each type of engine.
The dilute sample, however, lowers the concentration of pollutants to be mea-
sured, and hence requires more sensitive emission analyzers than those used by
traditional I/M programs. In addition, the method for measuring HC emission
uses a different and more accurate technique than traditional programs. HC
emissions are measured with a Flame lonization Detector (FID), while CO and
carbon dioxide emissions are measured using non-dispersive infra-red analyzers.
NOx emissions are measured with a chemiluminescense analyzer.
(COMPUTER)
V
EXHAUST
ANALYZERS
landCVsJ
INERTIA FLYWHEELS
Evaporative System Purge Test
Since 1971, fuel tanks on cars have been designed as a closed system in which
vapors that evaporate from the gasoline in the tank are not released into the
atmosphere. The system is sealed and under pressure so that excess vapors are
shunted to a container filled with charcoal known as the evaporative canister.
The evaporative system purge test is used to determine whether fuel vapor
stored in the evaporative canister and present in the fuel tank is being properly
drawn into the engine for combustion. If the purge system is not working
properly, the evaporative canister can become saturated and start to vent hydro-
carbons into the atmosphere. In addition to causing HC emissions, failure of the
purge system wastes gasoline.
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The purge test is conducted while the vehicle is undergoing the IM240 on the
dynamometer. Purge flow is measured by simply inserting a flow meter at one
end of the hose that runs between the evaporative canister and the engine.
Determination of an acceptable purge rate is based on the total volume of gas
that flows through the system during the IM240, not by instantaneous flow
rates. The vehicle must have a minimum of 1 liter of volume in order to pass.
Most cars in proper working order will accumulate as much as 25 liters during
the IM240 cycle. As soon as a vehicle exceeds 1 liter of volume, the purge test is
complete. The entire IM240 driving cycle ends as soon as final results are deter-
mined for the emission test.
The purge test requires a flow meter that can measure the total volume of flow
over the transient cycle. Additionally, hoses and universal fittings are required
to hook up the flow meter as indicated below. Finally, a computer is needed to
control the test process, collect and record the data, and determine the pass/fail
status.
Purge Test Schematic
FILLER FILLER NECK
CAP
ROLLOVER VALVE
FLOW TRANSDUCER
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Evaporative System Pressure Test
The pressure test checks the system for leaks that would allow fuel vapors to
escape into the atmosphere. A "pressure decay" method is used to monitor for
pressure losses in the system. In this method, the vapor lines to the fuel tank
and the fuel tank itself are filled with nitrogen to a pressure of 14 inches of
water (about 0.5 psi). To pressurize these components, the inspector must locate
the evaporative canister, remove the vapor line from the fuel tank, and hook up
the pressure test equipment to the vapor line. After the system is filled, the
pressure supply system is closed off and the loss in pressure is observed. If
pressure in the system remains above eight inches of water after two minutes,
the vehicle passes the test.
A source of nitrogen, a pressure gauge, a valve, and associated hoses and fittings
are needed to perform the pressure test. In addition, a computer is used to
automatically meter the nitrogen, monitor the pressure, and collect and process
the results. Algorithms will be developed to optimize the test so that a pass/fail
decision can be made in less than two minutes on most vehicles.
Pressure Test Schematic
FILLER NECK
. ROLLOVER VALVE
NITROGEN CYLINDER
For More Information:
The Office of Mobile Sources is the national center for research and policy on air
pollution from highway and off-highway motor vehicles and equipment. You
can write to us at the EPA National Vehicle and Fuel Emissions Laboratory,
2565 Plymouth Road, Ann Arbor, MI 48105. Our phone number is (313) 668-4333.
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