International Experience and Best Practices - Clean Air Initiative

BOX 3-2. MARYLAND’S DIVERSITY IN I/M TEST PROCEDURES
• On-Board Diagnostics (OBD) Test: 1996 or newer model year vehicles weighing
less than 8,501 pounds and equipped with a computer controlled engine and fuel
system.
• Loaded Test (IM240): 1984 to 1995 model year vehicles weighing less than
10,000 pounds and 1996 or newer model year vehicles weighing between 8,501
and 9,999 pounds.
• No-load (Idle) Test: 1977 to 1983 model year vehicles; vehicles with a carbureted
fuel system; and trucks weighing 10,000 pounds or more regardless of model year.
What is the bottom line? Experts consider best practice in auto testing to be no-load
testing for cars that are not equipped with catalytic converters, and loaded testing for
cars with this emission control technology. 122 Using dynamometers, test facility staff
with low technical skill are capable of conducting short steady-state/single-load tests,
achieving acceptable accuracy in measurement and holding costs down. Transient
loaded tests, those that run the vehicle through simulated driving cycles and loads, are
longer, costlier, and require relatively skilled staff and might be deferred in developing
countries until conditions warrant. 123 Applying these best practices to a diverse auto fleet
results in a complicated I/M system, again lending itself to centralized testing.
3.2.3. Diesel Trucks
Billowing black smoke from aging diesel trucks and buses is a common sight in the
developing world. 124 Although their numbers may be relatively small, their high emission
rates and high mileage combine to produce a disproportionate share of total emissions.
The principal emissions from large diesel vehicles are PM, HC, NOx, and SO2. In
addition, most citizens consider the visible smoke a nuisance. NOx emissions vary little
with maintenance, being dependent primarily on engine design, and SO2 emissions vary
with sulfur content of the fuel. However, I/M programs can reduce smoke (or PM) and HC
emissions. 125
In the U.S., I/M testing for diesel trucks has received far less attention than in the rest of
the world. This reflects the fact that the U.S. diesel truck fleet is relatively new and well
maintained, hence the impact of an I/M program would be relatively small. The small
number of diesel automobiles in the U.S. also results in little attention to diesel I/M.
Reinforcing this is the fact that reduction in the pollutants that cause health-threatening
ozone and smog–NOx and HC, not PM—has been the main driver of I/M programs
in the U.S.. For instance, USEPA models for Clean Air Act compliance do not contain
elements that quantify the effect of diesel I/M programs. Only a handful of states have
122 Asian Development Bank, op. cit., p. 18.
123 Rogers, John, 2001, op. cit., p. 6.
124 This report does not explicitly address commercial trucks that are gasoline-powered, however, test
procedures for these vehicles should be similar to those for automobiles.
125 Weaver and Chan, 2003, op. cit., p. 32.
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