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Federal Register / Vol. 86, No. 122 / Tuesday, June 29, 2021 / Proposed Rules This list is not intended to be exhaustive, but rather provides a guide for readers regarding entities likely to be regulated by this action. If you have questions regarding the applicability of this action to a particular entity, consult the person listed in the FOR FURTHER
INFORMATION CONTACT section.
What action is the Agency taking?
This action proposes to amend the regulations that implement our air pollutant emission standards for heavyduty engines and vehicles. The proposed amendments in this notice of proposed rulemaking include corrections, clarifications, additional flexibilities, and adjustment factors to the Greenhouse gas Emissions Model GEM compliance tool for heavy-duty vehicles. These amendments would modify the existing test procedure for heavy-duty highway engines and vehicles and apply to the measurement of CO2 emissions. EPA published a proposed rule on May 12, 2020 85 FR
28153 Technical Amendments proposed rule. EPA is issuing a final rulemaking Technical Amendments final rule relating to most revisions proposed in the Technical Amendments proposed rule, published in the Final Rules section of this issue of the Federal Register, titled Improvements for Heavy-Duty Engine and Vehicle Test Procedures, and other technical amendments, docket number EPA
HQOAR20190307; FRL1001852
OAR.
This action supplements the Technical Amendments proposed rule for only certain specific aspects of revising GEM by proposing several amendments to the model, after consideration of comments solicited and received on the Technical Amendments proposed rule, including a proposed revision to address concerns raised regarding potential stringency impacts that may result from changes to GEM.

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What are the incremental costs and benefits of this action?
This action is limited in scope and does not have significant economic or environmental impacts. EPA has therefore not estimated the potential costs or benefits of this notice of proposed rulemaking.
II. Greenhouse Gas Emissions Model GEM Background GEM is a computer application that estimates the greenhouse gas GHG
emissions and fuel efficiency performance of specific aspects of heavy-duty vehicles. GEM uses several vehicle-specific inputs, such as engine fuel maps, aerodynamic drag
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coefficients, and vehicle weight ratings, to simulate vehicle and engine operation and model the amount of CO2
emitted over multiple duty cycles for tractors and vocational vehicles. The resulting CO2 values over these cycles are weighted by GEM to provide a composite GEM score. GEM version 3.0
was used to set standards in the Greenhouse Gas Emissions and Fuel Efficiency Standards for Mediumand Heavy-Duty Engines and Vehicles-Phase 2 Phase 2 rulemaking 81 FR 73478.
For purposes of determining compliance, composite GEM scores are compared to the applicable Phase 2
vehicle standard.
In the Technical Amendments proposed rule, we proposed several amendments to GEM 3.0, including corrections, clarifications, and additional flexibilities in a revised version of the model, GEM 3.5 85 FR
28145. EPA also requested comment on whether any differences in GEM output values resulting from changes to the model would impact the effective stringency of the program and, if so, whether EPA should revise the GEM
model itself or address such impacts via regulations see 85 FR 28145.
Comments received in response to the NRPM supported most of the proposed updates to GEM and requested additional revisions to further improve the model.2 The California Air Resources Board CARB provided comment on this topic, stating the importance of GEM results being consistent with the current program standards to avoid affecting program stringency. CARB recommended that EPA revise GEM in order to ensure stringency is maintained.3
After considering the comments received, EPA applied further potential changes to GEM 3.5 and released a new development version of GEM, GEM 3.7, to the public for download and review by stakeholders to evaluate and assess the performance of this revised model.
GEM 3.7 incorporated some corrections and improvements relative to the proposed version GEM 3.5, as noted in the corresponding memorandum in the rulemaking docket.4
2 See Improvements for Heavy-Duty Engine and Vehicle Test Procedures, and other Technical Amendments Response to Comments, Publication Number: EPA420R20026, December 2020.
Chapter 2 of the Response to Comments provides additional details on the amendments, clarifications requested by commenters, and our responses to most of the comments to the NPRM.
3 California Air Resources Board, Docket number EPAHQOAR201902070030.
4 Nelson, Brian. Memorandum to Docket EPA
HQOAR20190307. Development version of GEM and adjustment factors. October 23, 2020.
Docket number EPAHQOAR201903070083.

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While evaluating GEM 3.7, we found differences in the output values for some tractor and vocational vehicles compared to the output values from GEM 3.0 used to set the Phase 2 CO2
standards. To understand the differences between GEM 3.0 and GEM
3.7, we recreated the process used in 2016 to determine the numerical level of the Phase 2 standards. Without an adjustment to the resulting GEM output value, these differences in GEM output values when compared to the Phase 2
final rule could be considered an effective change in stringency. In light of GEM 3.7 output differences and considering CARBs comment, we identified adjustment factors in that same docketed memo that could be applied to the unrounded GEM 3.7
output to better ensure effective stringency of the standards is maintained.5
The Truck and Engine Manufacturers Association EMA requested additional time for its members to review the potential updates to the model in GEM
3.7 and evaluate the impact of the adjustment factors made available.6
As described in the Technical Amendments final rule, EPA released a revised version of GEM i.e., GEM 3.5.1
that corrected three errors in GEM 3.5
and finalized provisions to specify GEM
3.5.1 without adjustment factors as the compliance tool for meeting Phase 2
standards.7 GEM 3.5.1 includes the following updates to GEM 3.5:
Corrected duty cycle weighting factors for vocational vehicles in the Heavy Heavy-Duty Multipurpose subcategory.
Corrected an idle map error when the cycle average engine fuel mapping procedure is used for all three drive cycles.
Corrected a functional error that unnecessarily required manufacturers to include transmission power loss data when using the option to enter a unique instead of default k-factor for the torque converter.
Also available online: https www3.epa.gov/otaq/
gem-p2v3.7-release-memo-2020-10-23.pdf.
5 Id.
6 Charmley, Bill. Memorandum to Docket EPA
HQOAR20190307. EPA discussions with the Truck and Engine Manufacturers Association, and with the California Air Resources Board, regarding Highway Heavy-Duty Technical Amendments.
December 14, 2020. Docket Number EPAHQ
OAR201903070092.
7 See the notice of final rulemaking for Improvements for Heavy-Duty Engine and Vehicle Test Procedures, and other Technical Amendments published in the Final Rules section of todays Federal Register, titled Improvements for Heavy-Duty Engine and Vehicle Test Procedures, docket number EPAHQOAR2019
0307; FRL1001852OAR.

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