International Experience and Best Practices - Clean Air Initiative

a diesel I/M program at all. 126 In contrast, the EU requires I/M testing using opacity
measures for all diesel-powered cars and trucks.
As noted earlier, a test procedure for PM suitable for an I/M program remains elusive,
yet PM is widely acknowledged as posing the largest health risk of any vehicle pollutant.
Smoke opacity meters are commonly used and accurately measure something that the
public cares about: visible diesel smoke. Most experts agree that there is no correlation
between a vehicle’s emissions of PM and its smoke opacity. However, a contrary view
holds that there is some correlation in developing countries due to lower penetration of
advanced diesel technologies. 127 Regardless of this debate, experts generally agree that
policymakers should use opacity testing, arguing that it can serve as a useful screen
for major engine malfunctioning or tampering (which is likely but not certain to result
in increased PM emissions) and that reduction of visible smoke still serves a public
purpose.
Several choices for opacity test procedures are available. 128 Testing an idling diesel
engine is not recommended because smoke levels at idling speed are nearly always low
regardless of the condition of the vehicle. A common no-load test is “snap acceleration”
(or “free acceleration”): in which the engine is revved up once to high speed quickly (with
vehicle stationary) and smoke is measured. A more complex test is the “snap-idle” test
where the engine is revved several times and an average smoke value is derived.
Aiming for an accurate and replicable snap-idle test, a committee organized by the
Society of Automotive Engineers (SAE) developed a standardized procedure for this test
known as “J1667.” 129 Nonetheless, accuracy and replicability remain a problem with both
the snap acceleration and snap-idle tests. The acceleration(s) are performed manually
and should be guided by the vehicle’s tachometer measuring engine rpm. If they are not
done properly, the emission measurements will vary substantially. Accidental dilution of
the exhaust stream will result in low readings. These same factors leave the snap and
snap-idle test open to fraud. If the engine is revved to less than full speed (a “lazy” snap),
lower emissions will result. 130 Temporary adjustments to the fuel injection pump can also
lower emissions. 131 There are also concerns that the snap-idle test poses some risk of
damage to older diesel engines. This increases the likelihood that drivers and testers will
not conduct the test according to its design specifications.
A variety of loaded tests are available for diesel trucks. In the “road load” test, the
dynamometer simulates steady driving at moderate speed. The “lug down” test loads the
126
Many states have a less formal program that tries to catch gross diesel emitters at truck stops and
weigh stations (see Chapter 4). U.S. attention to diesel may increase with tighter PM standards coming
into effect. Joe Pedelty and Dan Meszler, personal communications.
127
See Gilliam, op. cit., pp. 171-173, and Erlandsson and Walsh, 2003, op. cit., p. 27 for the conventional
view. In contrast, Weaver argues that the lack of correlation is partly a function of the emissions effects
of turbochargers and puff limiters which have a sizable presence in industrialized countries but relatively
low market penetration in developing countries (personal communication).
128
For a detailed and concise summary of diesel test options, see Weaver and Chan, op. cit., 2003, pp.
32-40.
129
Society of Automotive Engineers, SAE Standard J1667, February 1996.
130
Weaver, C. and L. Chan, op. cit., p. 38.
131
Gwilliam, K. et al, op. cit., pp. 170-171.
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