International Experience and Best Practices - Clean Air Initiative

The measurement of emissions from a vehicle is far more complicated than simply
hooking up a gas analyzer to the tailpipe. Emissions vary greatly among similar vehicles
and even an individual vehicle’s emissions can vary tremendously under different
operating conditions. As noted earlier, any attempt to quantify a vehicle’s emissions
necessarily involves assumptions, modeling, and sampling.
The U.S., European Union (EU), and Japan have each developed elaborate test
procedures for carefully measuring the mass of emissions from vehicles during simulated
driving conditions. Regulators conduct these tests in laboratories on vehicles fresh from
the assembly line to verify that they meet increasingly rigorous emission standards for
new vehicles. Engineers place a vehicle on a dynamometer and put it through a driving
cycle of different speeds and with different weights or “loads,” while a gas analyzer
measures the physical mass of emissions (e.g., in grams per kilometer traveled). The
driving cycle is designed to approximate real world driving by typical vehicles. The
resulting emission estimates are only as good as the assumptions underlying the driving
cycle, and researchers periodically modify these assumptions to reflect actual drivers’
behavior as accurately as possible. These drive cycles can last 20 to 40 minutes or
more.
These lengthy test procedures have become the benchmark against which less elaborate
and inherently less accurate I/M test procedures are compared. I/M programs need tests
that are less expensive and last no more than 10 minutes (probably the upper limit of
what drivers will accept in waiting time). Engineers have developed a range of options for
I/M tests and policymakers face trade-offs in choosing among them. 95
At one end of the range are “no-load tests” in which the engine merely idles and the test
duration can be as short as thirty seconds. This type of test can include idling at both
a low and high speed or revving the engine several times. These simple, low-cost test
procedures, when properly done, can serve as rough screening devices for high-emitting
vehicles. However, the results are sometimes inconsistent and prone to manipulation and
suffer from high error rates yielding false failures.
At the other end of the range are “loaded tests” that use sophisticated, higher-cost
equipment and test cycles that take up to 10 minutes, operating the vehicle on a
dynamometer with simulated engine loads. “Steady state loaded tests” run the vehicle
on a dynamometer at a steady speed and load and are relatively easier to administer.
“Transient loaded tests” run the vehicle through a simulated driving cycle of varying
speeds and loads and are more technically demanding and take about twice as long.
Loaded tests are more accurate than no-load tests, and more difficult to manipulate,
but the total time a driver needs to spend at the I/M facility can be 20 minutes or longer
(and he must drive to and from the facility). On-board diagnostic (OBD) equipment
offers a potential alternative to direct emission measurement by verifying the operation
of emission controls on a vehicle (see Section 3.2.2 below). OBD can also cut the time
needed to conduct the inspection.
95 For a detailed but concise summary of the range of options, see Wash, Michael, op. cit., 2003,
44
Appendix C.