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High-Tech I/M Tests
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.
IM24O Driving Trace
6O SO 10O 12O 14O
Time (sec)
ISO ISO 2OO 22O 24O
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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 certification.
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 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. _^___
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High-Tech VM Tests
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 measurement
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
measured, 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
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ANALYZERS
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INERTIA FLYWHEiLS
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 hydrocarbons into the
atmosphere. In addition to causing HC emissions, failure of the purge system
wastes gasoline.
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High-Tech I/M Testa
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 determined 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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High-Tech VM Testa
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 rilled 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
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For Further Information
The EPA National Vehicle and Fuel Emissions Laboratory is the national center for research
and policy related to automotive pollution. Contact the Emission Planning and Strategies
Division at 2565 Plymouth Road, Ann Arbor, MI 48105; or call 313/668-4456.
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