

U.S. DEPARTMENT OF TRANSPORTATION

NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION

1200 NEW JERSEY AVENUE SE.

WASHINGTON DC, 20590	UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

OFFICE OF AIR AND RADIATION

NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY

2000 TRAVERWOOD DRIVE

ANN ARBOR, MI  48105-2498



July 18, 2016

MEMORANDUM

SUBJECT:	Impact of Additional Weight Due to Trailer Aerodynamic Devices

FROM:	Jessica Brakora, Engineer, Assessment and Standards Division

Office of Transportation and Air Quality

TO:	Greenhouse Gas Emissions Standards and Fuel Efficiency Standards for
Medium- and Heavy-Duty Engines and Vehicles - Phase 2 - Dockets
NHTSA-2014-0132 and EPA-HQ-OAR-2014-0827



The agencies evaluated the impact of additional vehicle weight due to
the use of aerodynamic devices as calculated by EPA’s Greenhouse gas
Emissions Model (GEM).  This memo describes the analysis.

In GEM, users simulate aerodynamic improvements by supplying a delta CdA
value from aerodynamic testing.  GEM does not account for any additional
weight of aerodynamic devices when calculating CO2 emissions for
trailers.  While we based our performance standards on a set of
aerodynamic devices, in practice, manufacturers may choose from a
variety of devices or trailer design changes.  It would be in inaccurate
to specify a single weight for each aerodynamic bin and would be an
additional burden to require manufacturers to obtain and verify the
weight change for all aerodynamic improvements applied.  Additionally,
as shown in the analysis that follows, the impact of additional weight,
in terms of CO2 emissions of our simulated vehicle, is relatively small.
  The impact of potentially increased vehicle miles traveled (VMT) due
to trailers that weigh-out is not addressed in this memo, but can be
found in Section 5.1 of our Response to Comments document.

In this analysis, we modeled two vehicles: a Class 8, 6x4 high roof
sleeper cab pulling a 53-foot dry van (long dry van) and a Class 7, 4x2
high roof day cab pulling a 28-foot dry van (short dry van).  In each
case, we kept the trailer tire rolling resistance set to 5.1 kg/ton,
with zero weight reduction applied and no tire pressure system
installed. 

In the first set of runs, we modeled the impact of aerodynamic
improvements.  These values are reflective of the results expected from
the version of GEM finalized under this rulemaking and used to generate
the GEM-based compliance equation for the trailer program.  The CO2
results in   REF _Ref457304144 \h  Figure 1  are shown for the bins we
expect most manufacturers will use to meet the standards for long (  REF
_Ref457304144 \h  Figure 1  (a)) and short (  REF _Ref457304144 \h 
Figure 1  (b)) box vans.  The delta CdA value for Bin II is 0.1, Bin III
is 0.4, and Bin VI is 1.4 for the trailer program (see 40 CFR 1037.515).
 It can be seen that each bin of increased aerodynamic performance
results in a lower CO2 value, as expected. 

 (a)						(b)

Figure   SEQ Figure \* ARABIC  1 :  Impact of Aerodynamic Improvements
for Common Bins Used in the Trailer Program for Long Box Vans (a) and
Short Box Vans (b)

Separately, we modeled the impact of additional weight, with no
aerodynamic improvements and similar weights as those of the aerodynamic
devices evaluated in our trailer test program.  To adjust this weight,
we changed the default values used within the GEM code, and therefore
could not use the GEM-based equation that most trailer manufacturers
would use for compliance.  Note that the weight of the 28-foot van skirt
is half the weight of the 53-foot van skirt.  

Table   SEQ Table \* ARABIC  1 :  Additional Weight Applied to Simulated
Vehicles Based on Trailer Length

Trailer Length	Aerodynamic Bin	Example Technology	Assumed Weight (lb)

28-foot	Bin I / Baseline	None	0

	Bin II	Skirts	100 

	Bin III	Skirts + Gap	400

53-foot	Bin I / Baseline	None	0

	Bin III	Skirts	200

	Bin VI	Skirts + Tail	500



The two plots in   REF _Ref457296798 \h  \* MERGEFORMAT  Figure 1  show
the impact of these additional weights on the CO2 results from each
simulated vehicle.  The additional weight does produce more CO2. 
However, a doubling of the expected Bin VI weight to 1000 lb for long
dry vans would only increase CO2 emissions by 1.1% (  REF _Ref457296798
\h  Figure 1 (a)).  A similar doubling of the Bin III weight for short
dry vans would increase CO2 emissions by 1.4%.  

 

(a)						(b)

Figure   SEQ Figure \* ARABIC  2 :  Impact of Additional Vehicle Weight
for Simulated Long Dry Vans (a) and Short Dry Vans (b)

The analysis was repeated to include both the aerodynamic improvements
and the associated additional weight specified in   REF _Ref457304780 \h
 Table 1 .    REF _Ref457306273 \h  Figure 3  and   REF _Ref457306760 \h
 Figure 4  compare the results that consider aerodynamic improvements
only to those considering both aerodynamic improvements and additional
weight.  In   REF _Ref457306273 \h  Figure 3 , the additional weight
creates the largest difference for long dry vans with Bin VI aerodynamic
performance, but the 0.5 kg/ton-mi increase in CO2 is less than 1%
different than the value without considering weight.  Similarly,   REF
_Ref457306760 \h  Figure 4  shows that the 0.9 kg/ton-mi increased CO2
for Bin III short box van aerodynamic improvements is also less than 1%
compared to the aerodynamics-only result.  We note that the addition of
aerodynamic technologies to both van lengths results in a net
improvement in CO2 emissions, even if we consider the added weight of
the technologies.

 

Figure   SEQ Figure \* ARABIC  3 :  Impact of Aerodynamic Improvements
and Additional Weight for Long Dry Vans

 

Figure   SEQ Figure \* ARABIC  4 :  Impact of Aerodynamic Improvements
and Additional Weight for Short Dry Vans

The agencies are not requiring the use of specific technologies on
trailers covered by our Phase 2 design standards.  As a result, many
aerodynamic technology options exist for manufacturers to apply to meet
the standards.  The agencies cannot predict all of the possible
technologies that will meet a given performance level and it would be
inaccurate to choose a single value to represent each aerodynamic bin. 
In addition, the results presented above show a relatively small impact
of including the effect of additional weight on CO2 emissions.  As a
result, the agencies cannot justify including the additional weight of
potential aerodynamic improvements in GEM’s CO2 emissions calculations
for the Phase 2 trailer program.

Attachment:  MemoAttachment_Trailer_AeroWeightEffect.xlsx

Impact of Additional Weight Due to Trailer Aerodynamics, Page   PAGE  3 
of   NUMPAGES   4 

