The On-Road Heavy-Duty Emissions Measurement System (OHMS), previously known as
the Streamlined Heavy-Duty Emissions Determination (SHED) system, is an innovative
technology that can be used for the characterization of emissions of heavy-duty trucks
while they are in use. The technology comprises exhaust collection systems, vehicle
monitoring equipment, and gas and particle analyzers housed in a large tent-like or
shed structure and a trailer or room for the analyzers. Emissions are measured by
sampling exhaust generated as the truck passes through the test setup.
In 2012, the Texas A&M Transportation Institute (TTI) and the University of Denver (DU)
conducted a pilot study assessing the applicability of the SHED/OHMS to a potential
heavy-duty diesel inspection and maintenance (I/M) program in the Dallas-Fort Worth
region. This study established the OHMS as a viable option to characterize emissions of
a relatively large number of trucks, with results that were comparable to those obtained
using portable emissions measurement systems (PEMS).
The OHMS design, with a gabled roof and central sampling tube, was
deployed at a field study location in New Waverly, Texas. This was the same location as
the 2012 pilot study, and data were collected from 935 trucks in just over a two-week
period. The emissions of oxides of nitrogen (NOx) and particulate matter (PM) were
analyzed for these test trucks, and compared against vehicle Model Year (MY) categories
where information was available. The findings were consistent with the results of the
initial pilot testing, and indicated that vehicles classified as high emitters when
compared to their MY category were responsible for approximately 21 percent of NOx
emissions and 38 percent of PM emissions of the entire fleet.
OHMS Installed at Field Location in New Waverly Tx.
The findings from the testing at the test site and in the field provided several insights
into the successful installation and operation of the OHMS. From a technical
perspective, further research is needed in areas such as the measurement of emissions
from vehicles with low exhaust stack configurations, or for the application of OHMS to
light-duty vehicles.
From a programmatic perspective, the study findings reinforced the conclusion that the
OHMS could serve as a viable platform for an I/M or vehicle screening program. Several
aspects of the implementation of such a program, such as the target fleet, deployment
location(s), definition of a high-emitting vehicle, funding, and enforcement mechanisms,
require further consideration by local agencies and stakeholders.
