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Track Testing for SmartWay Verification
What is track testing?
Track testing is a method for measuring fuel economy performance with full-scale vehicles and technologies
in controlled conditions. For the EPA SmartWay Program, a modified version of the Society of Automotive
Engineers (SAE) Jl 321 Track Test procedure is used to assess the fuel savings of certain aerodynamic trailer
technologies. These protocols use closed tracks that can accommodate tractor-trailers at highway speeds.
Example of a track used for testing large trucks, courtesy of Pecos Research
and Testing Center (PRTC)
During SmartWay track testing,
two or more comparable
class 8 tractor-trailers follow
the same driving profile at
constant highway speed (65
mph) cruise conditions. Trucks
must be appropriately spaced
to avoid impacting each other's
aerodynamic performance. To
begin testing, all trucks perform
a "baseline" run without any
aerodynamic technologies.
Then, they conduct "test"
runs where one truck remains
unmodified as a "control" and
one or more of the remaining
"test" trucks are equipped with
aerodynamic technologies.
Each truck is equipped with a
portable fuel tank so fuel consumption can be directly measured by weighing the tanks after each run. Fuel
savings is quantified by the relative change in fuel used by the "test" truck(s) (with aerodynamic devices)
compared to the unmodified "control" truck (no devices). By comparing fuel saved by the "test" truck(s) to
any fuel use changes in the "control" truck (operating at the same time on the same track under similar
conditions), this method reduces certain weather and track-related variables.
What kind of measurements can I expect
from track testing?
This test method directly measures fuel consumption
instead of assessing an aerodynamic drag coefficient.
The difference in fuel use between "baseline" and "test"
segments for the control and test truck(s) determines the
percent fuel saved by the device. Additionally, the statistical
uncertainty is computed to indicate whether projected
fuel savings can be attributed to the aerodynamic
technologies or the degree of variability in the
measurement.
Continued
U.S. EPA SmartWay | EPA-420-F-15-OTI | www.epa.gov/smartway
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SmartWay8
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Track Testing for SmartWay Verification
Continued
How is the 2O14 updated SmartWay track testing procedures different
from what was done prior to 2O14?
Similar to industry truck test procedure updates, the 2014 SmartWay track test includes a number of updates.
Prior to 2014, SmartWay verification track testing was based on industry methods commonly referred to as
Society of Automotive Engineers (SAE) J1321 procedures adopted in 1986. For the new 2014 SmartWay
track test, EPA updated the protocol to incorporate the more recent (2012) version of the SAE Jl 321
procedure, overlaid with new SmartWay testing enhancements, such as tractor-trailer set-up specifications.
These new enhancements will improve truck comparisons by limiting some sources of variability. One main
addition is to require more than one complete track test in order to achieve SmartWay verification. This is an
industry recommended practice and will give fleets additional confidence that the aerodynamic technology(s)
verified with this procedure will perform comparably in similar conditions on the road.
How does the track testing results compare to that of other tests?
Each test method for SmartWay verification has advantages and disadvantages relative to the others. Below
are some considerations:
Track tests use real tractor-trailers so the complexities of an operating truck are built-in. This is also the
only SmartWay test that directly measures fuel savings without using a factor to convert aerodynamic drag
improvement to a projected fuel savings. However, operating full-sized trucks in an outdoor environment
adds variability. This makes it important to limit factors that could increase variability (e.g., weather, engine
response) by adhering rigorously to the test method and closely monitoring and documenting testing
conditions.
Coasfdown tests use real tractor-trailers and limit engine and drivetrain influences by disengaging them -
to more directly isolate and measure air resistance. To minimize the variability resulting from external factors
(e.g., changes in road grade, weather, vehicle set-up), it's important to follow the protocol conditions closely
and to thoroughly monitor and document testing.
Wind tunnel tests provide excellent repeatability due to the greater control of the environmental factors.
Test facilities can also apply simulated side winds to evaluate how an aerodynamic device may perform in
on-road conditions. Because changes can be made to the trailer (or tractor) body relatively quickly, scale
wind tunnels are often used for design work. However, it is critical to represent the tractor-trailer and device
with as much realism and fidelity to detail as possible, in order to produce results that represent real-world
performance. For some aerodynamic devices, it may not be possible to scale the representation down
enough to fit onto a small truck model.
Computational fluid dynamics (CFD) tools provide excellent repeatability by simulating on-road wind
conditions (like the wind tunnel) and generate animated visual simulations of airflow and wind interactions
around a truck. These animated visualizations can illustrate how a device works to reduce air resistance,
instead of simply calculating a projected fuel savings. CFD tools can be limited in the amount of tractor-
trailer and environmental complexity that can be represented in the computer model,
so it is important to understand the limitations of what is and is not included in a CFD
computer simulation.
In general, all these methods produce results that simulate highway cruise
conditions and must be appropriately interpreted when applying to a fleet's
unique driving profile. The testing organization and/or SmartWay can help
fleets understand how their on-road fuel savings may differ.
For more information: www.epa.gov/smartway/forpartners/technology.htm
or Tech_Center@epa.gov.
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