Guidance on Quantifying NOx Benefits
for Cetane Improvement Programs for Use
in SIPs and Transportation Conformity
SEPA
United States
Environmental Protection
Agency
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Guidance on Quantifying NOx Benefits
for Cetane Improvement Programs for Use
in SIPs and Transportation Conformity
Transportation and Climate Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
United States
Environmental Protection
^1 Agency
EPA-420-B-23-006
February 2023
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Table of Contents
SECTION 1. INTRODUCTION 4
1.1 What is the purpose of this document? 4
1.2 What is a "cetane improvement program"? 5
1.3 How WOULD "federal preemption" apply to state adoption of cetane improvement programs?
6
1.4 Who can I contact for more information? 7
1.5 Does this guidance create new requirements? 8
SECTION 2. FEDERAL CRITERIA FOR SIPS AND TRANSPORTATION CONFORMITY 9
2.1 What are the basic criteria for using emission reductions in SIPs? 9
2.2 What should be considered when estimating emission reductions for SIP purposes? ... 10
2.3 What should a State submit to EPA to meet the criteria for incorporating a cetane
IMPROVEMENT PROGRAM IN A SIP? 10
2.4 What are the basic criteria for using emission reductions in transportation conformity
DETERMINATIONS? 11
2.5 How SHOULD STATES ENFORCE A CETANE IMPROVEMENT PROGRAM? 12
2.6 What types of penalties can be assessed for not complying with CAA requirements? 13
SECTION 3. PER-VEHICLE OR PER-ENGINE NOX BENEFITS OF CETANE IMPROVEMENT
ADDITIVES 14
3.1 Overview 14
3.2 How IS THE CONSTANT 'K1 CALCULATED FOR HIGHWAY VEHICLES? 15
3.3 How IS THE CONSTANT'K'CALCULATED FOR NONROAD DIESEL ENGINES? 15
3.4 HOW IS THE REFERENCE CETANE (RC) CALCULATED? 16
3.5 HOW IS ADDITIZED CETANE (AC) CALCULATED? 17
SECTION 4. CALCULATING IN-USE FLEET-WIDE NOX BENEFITS 20
4.1 Overview 20
4.2 How IS PROGRAM FACTOR F1 CALCULATED? 20
4.3 How IS PROGRAM FACTOR F2 CALCULATED? 21
4.4 How IS PROGRAM FACTOR F3 CALCULATED? 22
4.5 How IS PROGRAM FACTOR F4 CALCULATED? 23
SECTION 5. CALCULATING TONS OF NOX REDUCED 26
APPENDIX 1. NATIONAL AVERAGE DISTRIBUTION OF NONROAD DIESEL ENGINES BY
CERTIFICATION TIER 28
APPENDIX 2. NATIONAL AVERAGE DISTRIBUTION OF NONROAD DIESEL ENGINES BY
CERTIFICATION TIER AND EQUIPMENT SECTOR 29
APPENDIX 3. CETANE RESPONSE FUNCTION 36
APPENDIX 4. BASE CETANE VALUE 37
APPENDIX 5. EXAMPLE EMISSION REDUCTION CALCULATION 38
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Section 1. Introduction
Note: As used in this document, the terms "you" and "your" refer to a state or states and
the terms "we," "us" and "our" refer to the Environmental Protection Agency (EPA).
1.1 What is the purpose of this document?
The purpose of this document is to provide an update to EPA's guidance on the
emissions reductions attributable to programs that control the cetane level in diesel fuel.
In June 2004, EPA issued the original guidance on (the "2004 Guidance").1 This update
maintains much of the 2004 Guidance, such as program design considerations, use of
emission reductions for state implementation plans (SIPs) and transportation conformity,
and how to calculate NOx emissions reductions. However, certain aspects of the 2004
Guidance need to be updated to reflect changes in fleet composition and control
technology that have occurred since it was released, resulting in a significant decline in
the emission reductions from cetane programs. For example, the 2004 Guidance contains
information on calculating emission reductions from highway diesel engines for years
through 2020 and indicates that increased cetane levels may reduce emissions from all
nonroad engines in use in 2004.
Specifically, this update of the 2004 Guidance provides information on how to
calculate emissions reductions in 2021 and beyond from both highway engines and
nonroad engines. As discussed in the 2004 Guidance, cetane improvement programs only
provide emission reductions from pre-2003 model year heavy-duty diesel highway
vehicles and nonroad diesel engines that are sensitive to cetane enhancement, which are
generally pre-Tier 3 nonroad diesel engines. This update reflects the significant decrease
in the number of these highway and nonroad diesel engines that will benefit from a cetane
improvement program as pre-2003 highway diesel engines and nonroad diesel engines
that may benefit from a cetane improvement program (i.e., pre-Tier 3 engines) make up
much smaller portions of the in-use population than they did 15 to 20 years ago.
Therefore, we are providing this update to the 2004 Guidance as the number and use of
such highway and nonroad diesel engines and the benefits of cetane improvement
programs has substantially decreased and will continue to decrease in the future.
Appendix 5 contains a sample calculation of the emission reductions from a cetane
improvement program for a hypothetical Severe ozone nonattainment area for the 2008
ozone national ambient air quality standard (NAAQS). The example shows that for the
hypothetical area with 30 tons/day of nitrogen oxide (NOx) emissions from all highway
diesel engines in regulatory classes 41-49 in 2026, a cetane improvement program may
reduce NOx emissions by 0.07 tons/day.
This guidance should help state and local air quality agencies that have either
existing cetane improvement programs approved in their state implementation plans
(SIPs) or may be considering adopting similar programs. Metropolitan planning
1 Guidance for Quantifying NOx Benefits for Cetane Improvement Programs for Use in SIPs and Transportation
Conformity, June 2004, EPA420-B-04-005.
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organizations (MPOs) for nonattainment and maintenance areas where such programs
have been included in the SIP should also use this guidance for calculating emissions
reductions in 2021 and beyond for transportation conformity analyses. At this time,
cetane improvement programs are approved in SIPs in two states, California and Texas.
States considering adopting new cetane improvement programs should be aware of the
significant decrease in potential reductions as well as the limitations of adopting such fuel
programs due to changes in CAA preemption requirements since 2004, as discussed
further in Section 1.3. This guidance updates and supersedes the 2004 Guidance.
1.2 What is a "cetane improvement program"?
A cetane improvement program calls for the use of cetane additives in diesel fuel to
increase the cetane number. Generally, increases in the cetane number result in reduced
emissions of NOx from certain heavy-duty highway diesel engines and certain diesel powered
nonroad engines. For heavy-duty highway diesel engines, the emission reductions are
attributable to pre-2003 model year engines. For nonroad diesel engines, the emission reductions
are attributable to engines that are certified to pre-Tier 3 emissions standards. Cetane
improvement additives include 2-ethylhexylnitrate and di-tertiary butyl peroxide used at diesel
fuel concentrations of generally less than 1 volume percent (or 1 vol%). The numerical standard
of such a program can take one of three different forms:
Type 1: Total cetane number standard:
This type of standard sets a per-gallon minimum value for the sum of natural (base)
cetane number and the increase in cetane number due to additives. An example might be
50 or 55. For areas where the natural cetane has historically been low (e.g., 42), this type
of standard could require significantly more additives and thus produce significantly
more benefits than for areas where the natural cetane has historically been high (e.g., 47).
As a result, the NOx benefits of this type of program could vary substantially from one
area to another depending on the historical natural cetane.
Type 2: Cetane number increase standard.
This type of standard sets a per-gallon minimum value for the increase in cetane number
due to the use of additives. An example might be 5 or 10. Although the NOx benefits of
this type of program are also dependent on the natural cetane which varies by area, this
type of program generally produces similar levels of benefits from one area to another as
the relative increase in total cetane would be the same.
Type 3: Cetane additive concentration standard.
This type of standard sets a per-gallon minimum value for the concentration of a
particular type of cetane improver additive. An example might be 0.15 volume percent of
2-ethylhexylnitrate, or 0.20 volume percent of di-tertiary butyl peroxide. This type of
program uses a "proxy property" to represent the true cetane number increase, and the
increased uncertainty associated with proxy properties may reduce the NOx benefits that
can be claimed from this type of program. See Section 4 below for more details.
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The standard set under a cetane improvement program should generally be on a
per-gallon basis. Standards that apply on an average basis may be acceptable if
appropriate compliance and enforcement mechanisms are established.
1.3 How would "federal preemption" apply to state adoption of cetane improvement
programs?
In general, the Clean Air Act (CAA) provides that states are preempted from adopting
their own fuel control requirements with respect to a fuel characteristic or component that EPA
has regulated unless it is identical to the federal requirements.2 However, EPA may waive
preemption under certain circumstances, as discussed below.
State adoption of motor vehicle fuel requirements is controlled by CAA section
211(c)(4). Section 211(c)(4)(A) prohibits States from prescribing or attempting to enforce any
"control or prohibition respecting" a "characteristic or component of a fuel or fuel additive" if
EPA has promulgated a control or prohibition applicable to such characteristic or component
under section 211(c)(1).
In 1989, EPA promulgated regulations requiring diesel fuel to meet a maximum
aromatics level of 35 percent or in the alternative a minimum cetane index specification of 40.3
This requirement provides the context for evaluating whether state cetane improvement
programs are preempted. Determining whether a state cetane improvement program is
preempted will depend in large part on the specific details of the state program. For instance, a
state cetane improvement program could be structured around increases in the cetane number
rather than changes to the cetane index or aromatics content. While the cetane number of
unadditized diesel fuel has an impact on the cetane index of that fuel, increasing the cetane
number using additives would not necessarily affect either the cetane index or aromatics level.
Thus, a state program structured to increase the cetane number without affecting cetane index or
aromatic levels is less likely to be considered preempted, while a state program that does affect
cetane index or aromatic levels is more likely to be preempted.
Section 211(c)(4)(C) also provides a mechanism for obtaining a waiver from this
prohibition for a nonidentical state standard contained in a SIP where the standard is "necessary
to achieve" the primary or secondary NAAQS that the SIP implements. Specifically, EPA can
approve such a SIP provision as necessary if the Administrator finds that "no other measures that
would bring about timely attainment exist," or that "other measures exist and are technically
possible to implement, but are unreasonable or impracticable."
However, states considering adopting a new cetane improvement program should be
aware that Congress amended the fuel preemption requirements in CAA section 211(c)(4)(C) in
2005. This CAA amendment required EPA to determine the number of fuels approved into SIPs
2 Pursuant to CAA section 211(c)(4)(B) California "may at any time prescribe and enforce, for the purpose of motor
vehicle emission control, a control or prohibition respecting any fuel or fuel additive."
3 Fuel Quality Regulations for Highway Diesel Fuel Sold in 1993 and Later Calendar Years, Final Rule, 54 FR
35276 (August 24, 1989). See also, 40 CFR 1090.305.
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as of September 1, 2004, and to publish a list of such fuels in the Federal Register. The
published list included the state in which the fuel has been approved into the SIP and the
Petroleum Administration Defense District (PADD) in which the fuel is used. The CAA
amendment prohibits EPA from increasing the number of SIP-approved fuel programs above the
number that existed on September 1, 2004. EPA published the required list of SIP-approved fuel
programs on December 28, 2006 (78 FR 78192).4 In that notice, EPA stated that:
We (EPA) cannot approve a state fuel into a SIP unless the fuel is already in an existing
SIP within that PADD, with the exception of a 7.0 psi RVP fuel.5 (71 FR 78193)
EPA also stated in that notice that:
if there is 'room on the list,' we (EPA) could approve for states within PADD 5 a fuel
program that is in California's SIP, without violating the PADD restriction. CARB fuels
are approved into California's SIP. (71 FR 78196)
Two states — California, which is in PADD 5, and Texas, which is in PADD 3 — have
SIP-approved cetane improvement programs.6 The CAA's PADD restriction (as noted above)
means that states in PADD 3 could adopt a new cetane improvement program, identical to the
SIP-approved Texas low emission diesel program, and states in PADD 5 could adopt a cetane
program identical to California's SIP-approved cetane improvement regulations provided that
states in these PADDs meet all other requirements for SIP-approved fuels in CAA sections
21 l(c)(4)(C)(i) and 21 l(c)(4)(C)(v). EPA could not waive preemption under CAA section
21 l(c)(4)(C)(i) and approve a cetane improvement program into a state's SIP if the state is in
PADDs 1, 2 or 4.
1.4 Who can I contact for more information?
A state or local agency with specific questions about an approved cetane improvement
program or about incorporating such a program into its SIP should contact its Regional Office.
You can find a list of EPA Regional Office contacts at https://www.epa.gov/transportation-air-
pollution-and-climate-change/office-transportation-and-air-qualitv-contacts in Section 16.2 at the
end of the document.
A state or local agency with specific questions about including emission reductions from
a cetane improvement program in a transportation conformity determination should contact its
Regional Office. You can find a list of EPA Regional Office transportation conformity contacts
at: https://www.epa.gov/state-and-local-transportation/epa-regional-contacts-regarding-state-
and-1 ocal -transportati on.
4 On December 4, 2020, EPA published in the Federal Register an updated listing of the fuels approved in SIPs.
(See 85 FR 78412.)
5 See CAA section 211(c)(4)(C)(v)(V).
6 The states in PADD 3 are: Alabama, Arkansas, Louisiana, Mississippi, New Mexico, and Texas. The states in
PADD 5 are: Arizona, California, Nevada, Oregon, and Washington.
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For general questions about this guidance, please see the Office of Transportation and Air
Quality Contacts by Topic document available at https://www.epa.gov/transportation-air-
pollution-and-climate-change/office-transportation-and-air-qualitv-contacts. A contact person is
listed under "Cetane Improvement Programs".
Additional information regarding state and local transportation air quality planning
resources can be found on EPA's website at: https://www.epa.gov/state-and-local-transportation.
1.5 Does this guidance create new requirements?
This guidance does not create any new requirements. The CAA and the regulations
described in this document contain legally binding requirements. This guidance is not a
substitute for those provisions or regulations, nor is it a regulation in itself. Thus, it does not
impose legally binding requirements on EPA, states, or the regulated community, and may not
apply to a particular situation based upon the circumstances. EPA retains the discretion to adopt
approaches on a case-by-case basis that may differ from this guidance but still comply with the
statute and applicable regulations. This guidance may be revised periodically without public
notice.
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Section 2. Federal Criteria for SIPs and Transportation Conformity
2.1 What are the basic criteria for using emission reductions in SIPs?
In order to be approved as a control measure which provides additional emission
reductions in a SIP, a cetane improvement program would need to be consistent with SIP
reasonable further progress (RFP), attainment, or maintenance requirements and other CAA
requirements, as appropriate. The program must provide emission reductions that meet the basic
SIP requirements described below. SIP requirements are mandatory requirements under CAA
section 110.
Quantifiable -
SIP Requirement: The emission reductions are quantifiable if they are measured in a
reliable manner and can be replicated (e.g., the assumptions, methods, and results used to
quantify emission reductions can be understood). Emission reductions must be calculated for the
time period during which the reductions will occur and will be used for SIP purposes.
When quantifying the emission reductions from a cetane improvement program, you will
need to document the emission reductions and provide all relevant data to EPA for review.
Sections 3 through 5 of this document provide a recommended method for quantifying emission
reductions.
Surplus -
SIP Requirement: Emission reductions are considered "surplus" if they are not otherwise
relied on to meet other applicable RFP, attainment, or maintenance requirements for that
particular NAAQS pollutant (i.e., there can be no double-counting of emission reductions). In
the event that the cetane improvement program is used to meet such air quality related program
requirements, they are no longer surplus and may not be used as additional emission reductions.
Emissions from the vehicles, engines, or equipment subject to the cetane improvement program
must be in the applicable mobile source emissions inventory before the emission reductions from
such a program can be used for RFP, attainment or maintenance in a SIP.
Federally Enforceable -
SIP Requirement: A SIP project must be enforceable. Depending on how the emission
reductions are to be used, control measures must be enforceable through a SIP. Where the
emission reductions are part of a rule or regulation for SIP purposes, they are considered
federally enforceable only if they meet all of the following criteria:
• Emission reductions are independently verifiable.
• Violations are defined, as appropriate, e.g., an example of a violation is failing to
implement as required by the regulation.
• The state and EPA have the ability to enforce the measure if violations occur.
• Those liable for violations can be identified.
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• Citizens have access to all the emissions-related information obtained from the responsible
party.
• Citizens can file lawsuits against the responsible party for violations.
• Violations are practicably enforceable in accordance with EPA guidance on practicable
enforceability.
• A complete schedule to implement and enforce the measure has been adopted by the
implementing agency or agencies.
Permanent -
SIP Requirement: The emission reductions from the cetane improvement program must
be permanent throughout the time period that the reductions are used in the applicable SIP.
Adequately Supported -
SIP Requirement: The state must demonstrate that it has adequate funding, personnel,
implementation authority, and other resources to implement the project on schedule.
2.2 What should be considered when estimating emission reductions for SIP purposes?
For SIP RFP, attainment or maintenance strategies, the emission reductions which are
produced from the cetane improvement program can be estimated by applying the following
criteria:
• Where necessary, emission reductions need to account for seasonality. For example, if a
control measure is only applied during the summer ozone season, then only reductions
which take place during that season may be credited in a SIP.
• Emission reductions would be commensurate with the number of and activity from
onroad and nonroad diesel engines using fuel from a cetane improvement program for a
given analysis year. As noted earlier, the number of such engines from older fleets has
significantly declined since the release of the previous 2004 Guidance.
As required by Clean Air Act section 172(c)(3) and EPA's regulation at 40 CFR 51.112(a),
States must use the latest planning assumptions available at the time that the SIP is developed
to estimate emission reductions attributable to the cetane improvement program. State and
local agencies should contact their EPA Regional Office for any specific questions regarding
an area's cetane improvement program. Contact information may be found in Section 1.4 of
this guidance.
2.3 What should a State submit to EPA to meet the criteria for incorporating a cetane
improvement program in a SIP?
The state should submit to EPA a written document which:
• Identifies and describes the cetane-related control measure and its implementation
schedule to reduce emissions within a specific time period;
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• Contains estimates of emission reductions attributable to the measure, including all
relevant technical support documentation for the estimates. The state must rely on the
most recent information available at the time the SIP is developed pursuant to CAA
section 172(c)(3) and 40 CFR 51.112(a);
• Contains federally enforceable procedures to implement, track, and monitor the measure
as applicable;
• Enforceably commits to monitor, evaluate, and report the resulting emission reductions of
the measure as applicable;
• Meets all other requirements for SIP revisions including under CAA sections 110, 172
and 175 A, as applicable; and
• In the case of a cetane improvement program which is federally preempted, meets the
requirements of CAA section 211(c)(4)(C).
2.4 What are the basic criteria for using emission reductions in transportation
conformity determinations?
The transportation conformity rule describes the specific requirements for including
emission reductions from onroad mobile control measures in a transportation conformity
determination. Transportation conformity is required under CAA section 176(c) (42 U.S.C.
7506(c)) to ensure that federally supported highway and transit project activities are consistent
with ("conform to") the purpose of the SIP. EPA's transportation conformity rule (40 CFR Parts
51.390 and Part 93, Subpart A) establishes the criteria and procedures for determining whether
transportation plans, transportation improvement programs (TIPs) or projects conform to the SIP.
If the emission reductions from a cetane improvement program have been accounted for
in the SIP's motor vehicle emissions budget ("budget"), the MPO would also include the
emission reductions from the program to the extent it is being implemented, when estimating
regional emissions for a transportation conformity determination.7
To include the emission reductions from a cetane improvement program in a conformity
analysis, the appropriate jurisdictions must have made the appropriate level of commitment to
the measure, as described in 40 CFR 93.122(a). The appropriate level of commitment varies
according to the requirements outlined in 40 CFR 93.122(a), and under those provisions, for a
cetane improvement program that requires a regulatory action to be implemented, it can be
included in a conformity determination if one of the following has occurred:
• The regulatory action for the program is already adopted by the enforcing jurisdiction
(e.g., a state has adopted a rule to require the cetane improvement program);
7 The terms motor vehicle emissions budget and metropolitan planning organization (MPO) are defined the
transportation conformity regulations. (See 40 CFR 93.101.)
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• The program has been included in the approved SIP; or
• There is a written commitment to implement the program in a submitted SIP with a motor
vehicle emissions budget that EPA has found adequate.
Note that 40 CFR 93.118 describes the process and criteria that EPA considers when determining
whether submitted SIP budgets may be found adequate and used for transportation conformity
purposes prior to EPA's final action on the submitted SIP.
Whatever the case, any emission reductions can only be applied in a transportation
conformity determination for the time period or years in which the cetane improvement program
will be implemented. Written commitments must come from the agency with the authority to
implement the cetane improvement program as required by 40 CFR 93.122(a)(3)(iii). The latest
emissions model and planning assumptions must also be used when calculating emission
reductions, according to 40 CFR 93.110 and 93.111.
The interagency consultation process must be utilized (as required by 40 CFR 93.105) to
discuss the methods and assumptions used to quantify the reductions from the cetane
improvement program. Sections 3 through 5 of this document describe how to quantify emission
reductions.
2.5 How should states enforce a cetane improvement program?
The state should design a compliance and enforcement program to ensure that fuel with
cetane improvement additives is being provided to and sold within the designated geographic
boundaries of the mandated program area. The assurance of NOx benefits being generated
within the program area depends on the rigor of this compliance and enforcement program. This
guidance does not specify all elements of such a program that might be necessary, but instead
lists several areas that should be considered.
The compliance and enforcement program associated with a cetane improvement
program should be designed generally to provide a high degree of confidence that the diesel fuel
with a specified amount of improvement in cetane number due to the use of additives is actually
sold to end users within a pre-specified geographic area. To accomplish this, mechanisms may
need to be instituted to track where the cetane improver is being added to the fuel and
subsequently what avenues that fuel takes (pipelines and delivery tanker trucks, for example) in
order to be delivered to final dispensing stations within the mandated area. Mechanisms that
might be necessary to achieve confidence that this is occurring could include:
• Batch-by-batch tracking of volumes
• Segregation of additized fuel from nonadditized fuel
• Detailed recordkeeping requirements, including Product Transfer Documents
• Periodic reporting requirements
• Requirements for sampling and testing of fuel before and after cetane improver is added
• Surveys of fuel quality within the mandated area
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A compliance and enforcement program may also include requirements that particular approved
sampling methods be used and may also specify the liability provisions applicable to all parties
in the fuel distribution system and the penalties associated with noncompliance with the
established cetane standard.
Finally, a cetane improvement program is most easily enforceable if the standard it
establishes can be checked against any given batch of fuel. Generally, this means that the
standard should be set on a per-gallon basis, not an averaging basis. If a state wishes to set an
average standard, it should design a compliance and enforcement program that adequately deals
with the inherent and additional uncertainty associated with average standards.
2.6 What types of penalties can be assessed for not complying with SIP requirements?
Use of this guidance does not relieve the obligation to comply with all otherwise
applicable CAA requirements, including those pertaining to the crediting of emission reductions
for SIPs, including attainment or maintenance strategies. Violations of SIP requirements are
enforceable by the State. Additionally, violations of SIP requirements are subject to federal
administrative, civil, and/or criminal enforcement under CAA section 113, as well as to citizen
suits under CAA section 304. The full range of penalty and injunctive relief options remain
available to the federal or State government (or citizens) bringing the enforcement action.
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Section 3. Per-vehicle or Per-engine NOx Benefits of Cetane Improvement
Additives
3.1 Overview
In order to estimate the NOx benefits of cetane improver additives for an in-use fleet, you
should first have an estimate of the NOx benefits for a single highway vehicle or nonroad engine
using cetane-enhanced diesel fuel. The fleet-wide NOx benefits may differ from the per-vehicle
or nonroad engine benefits due to such issues as migration of vehicles into and out of the cetane
program area, the use of proxy fuel properties, etc. These issues are addressed separately in
Section 4 below.
The per-vehicle NOx benefits of cetane improver additives are generally represented as a
percent reduction in NOx emissions for a given increase in cetane number. There are several
potential sources for these benefit estimates, including testing completed under Environmental
Technology Verification (ETV) protocols, independent data sets (as reviewed and approved by
EPA), and EPA technical reports.8 This report provides estimates of per-vehicle NOx benefits
using the following equation, which we will refer to as (EQ 1):
(%NOx)pv = k x 100% x {1 - exp[ - 0.015151 x AC (EQ 1)
+ 0.000169 x AC2
+ 0.000223 x AC x RC]}
Where:
(%NOx)pv = Per-vehicle percent reduction in NOx emissions9
k = Constant representing fraction of NOx inventory associated with
cetane-sensitive diesel trucks or nonroad engines, as described later in
this section
AC = Additized cetane; the increase in cetane number due to the use of
additives
RC = Reference cetane; the natural (unadditized) cetane number of the fuel
prior to implementation of the cetane program
The EPA Technical Report from which equation (EQ 1) is taken contains a detailed description
of the data on which the analysis was based and the associated methodology. The Technical
Report also contains a discussion of the explanatory power of equation (EQ 1) through
comparisons to alternative models, correlating predicted and observed values, and estimating
model uncertainty.
8 For example, The Effect of Cetane Number Increase Due to Additives on NOx Emissions from Heavy-Duty
Highway Engines - Final Technical Report (EPA-420-R-03-002. February 20031.
9 Although equation (EQ 1) is written for highway vehicles, this same equation can be used for calculating per-
engine NOx benefits of cetane improver additives for nonroad diesel engines.
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The per-vehicle NOx benefits of cetane improver additives for diesel highway vehicles
(or per-engine NOx benefits for nonroad engines) can be estimated from equation (EQ 1) if you
have values for k, additized cetane, and reference cetane. The specific values will depend on the
cetane additive program being implemented, the fleet mix in the program area, and the quality of
diesel fuel prior to program implementation. Guidelines for determining values for k, additized
cetane, and reference cetane follow. Note that states should perform separate calculations for
highway and nonroad NOx emissions reductions.
3.2 How is the constant 'k' calculated for highway vehicles?
States, including states that currently have cetane improvement programs approved in
their SIPs, should calculate area-specific "k" values for highway vehicles for each ozone
nonattainment area where a cetane improvement program is or may be implemented and for each
relevant year. For example, if a state has a SIP-approved cetane improvement program that is
implemented in an area that is classified as Severe for the 2008 ozone NAAQS and the state
plans to include emissions reductions from the cetane improvement program in the RFP plans
and the attainment plan for that area, the state should calculate an area-specific "k" value for the
base year, if the cetane improvement program was in effect in the base year, the RFP and the
2026 attainment year.
The state should calculate area specific "k" values by determining the total amount of
vehicle miles traveled (VMT) in the area in the relevant analysis year (e.g., 2026 for a Severe
nonattainment area for the 2008 ozone NAAQS) attributable to heavy-duty diesel highway
engines of all ages in regulatory classes 41-49 and the total amount of VMT in the area in the
same years attributable to heavy-duty diesel highway vehicles that are model year 2002 and older
in regulatory classes 41-49. The constant "k" for the area for that year for highway vehicles
would then equal:
konroad area VMThD Diesel MY2002 and older/VMTHD Diesel All MY
Where:
konroad area = the constant "k" for the specific area and year.
VMThd Diesel MY2002 and older = the VMT attributable to heavy-duty diesel highway
vehicles (regulatory classes 41-49) in the area and in the
analysis year that are MY 2002 or older.
VMThd Diesel ah my — the VMT attributable to all heavy-duty diesel highway
vehicles (regulatory classes 41-49) in the area and in the
analysis year.
This calculation will result in values of "k" that decrease over time due to fleet turnover,
resulting in a corresponding decrease in NOx emission reductions over time.
3.3 How is the constant 'k' calculated for nonroad diesel engines?
States, including states that currently have cetane improvement programs approved in
their SIPs, should calculate area specific "k" values for nonroad diesel engines for each ozone
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nonattainment area where a cetane improvement program is or may be implemented and for each
relevant analysis year. For example, if a state has a SIP-approved cetane improvement program
that is implemented in an area that is classified as Severe for the 2008 ozone NAAQS and the
state plans to include emissions reductions from the cetane improvement program in the RFP
plans and the attainment plan for that area, the state should calculate an area-specific "k" value
for nonroad diesel engines for the base year, if the cetane improvement program was in effect in
the base year, the RFP years, and the 2026 attainment year.
NOx emissions controls such as exhaust gas recirculation and selective catalytic
reduction, which are insensitive to increases in cetane levels, became prevalent with the
introduction of engines certified to Tier 3 and Tier 4 emissions standards. One way to calculate
an area specific "k" for nonroad diesel engines is to calculate the fraction of pre-1988 through
Tier 2 engines remaining in the fleet in the subject area in the given analysis year. For example:
knonroad area (fDiesei pre-1988 + fDiesel Tier 0 + fDiesei Tier 1 + f Diesel Tier 2)/fDiesel Total
Where:
knonroad area = the constant "k" for nonroad diesel engines for the specific area and
year.
fDiesei Tierx = the number of nonroad diesel engines certified to pre-1988, Tier 0, Tier
1 and Tier 2 emissions standards in the area in the analysis year.
fDiesei Total = the total number of nonroad diesel engines in the area and analysis year
This calculation will result in values of "k" that decrease over time resulting in a corresponding
decrease in NOx emission reductions over time. To assist states, EPA has provided information
on the national fractions of nonroad diesel engines certified to each Tier of emissions standards
(see Appendix 1) and similar information broken down by sector (e.g., construction and lawn
and garden) (see Appendix 2).10 The information is available for 2025, 2026 and 2030. Based
on this national information, EPA estimates that in 2026, the national average "k" for nonroad
diesel engines certified to pre-Tier 3 emission standards equals 0.14. States should consider
local information on the number pieces of diesel powered nonroad equipment certified to each
Tier of emissions standards in the area because these fractions will vary with location. States
should also consider whether to estimate emission reductions from all nonroad diesel engines or
from individual sectors of diesel engines that utilize the cetane improvement program. Finally,
the value of "k" will continue to decrease over time as diesel engines certified to pre-Tier 3
emission standards are retired.
3.4 How is the reference cetane (RC) calculated?
This is the average natural (unadditized) cetane number of diesel fuel prior to
implementation of the cetane improvement program for highway vehicles or nonroad engines.
RC does not represent the cetane number of the unadditized base fuel after the program has been
10 Information on the fraction of nonroad diesel engines certified to various Tiers is from a national run of the
MOVES3 nonroad module. Additional information on the fleet turnover algorithm in the nonroad module is
available at: https://nepis.epa.gov/Exe/ZvPDF.cgi?Dockev=P10081RV.pdf.
16
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implemented (referred to here as BC: Base cetane), because refiners may change the quality of
the unadditized fuel if they are required to use cetane improver additives, or the natural cetane
may change incidentally as a result of compliance with ultra-low sulfur standards. The value of
RC for highway diesel fuel should be determined and specified separately from that for nonroad
diesel fuel. There are three options for specifying a value for RC for use in the equation above:
1) In areas for which pre-existing survey data is available, you may use an average from the
most recent year prior to program implementation as the default value for RC.
2) You may conduct a survey of diesel fuel parameters in the program area prior to the
implementation of the cetane improvement program.
3) You may use a default RC value of 47 to represent highway diesel fuel. For nonroad
diesel fuel, you may use a default RC value of 45 through calendar year 2007, and a
default RC value of 47 thereafter.
Survey data may provide values for total cetane number that are composed of
contributions from both natural (unadditized) cetane and a pre-existing cetane improver additive.
Since RC is intended to only represent the unadditized portion of total cetane number, any
contributions to the total cetane number from cetane improver additives should be separated out
and accounted for separately when using equation (EQ 1). See further discussion below. If the
default value for RC is used or if the available survey data does not permit quantification of the
amount of cetane improver additives already in the fuel, 1 cetane number could be assumed to
have resulted from the presence of pre-existing cetane improver additives.
3.5 How is additized cetane (AC) calculated?
This is the increase in cetane number that results from the addition of cetane improvers to
an unadditized base fuel for highway vehicles or nonroad engines. However, in order for the
NOx emissions effect equation (EQ 1) above to represent the cetane program correctly, the value
for AC should be corrected for any changes in the natural cetane prior to and after program
implementation. Additized cetane should thus be calculated from the following equation (EQ 2)
once the program has been implemented:
AC = ACm + BC - RC
(EQ 2)
Where:
ACm
AC
RC
= Value of additized cetane used to calculate (%NOx)pv via equation (EQ
1)
= Value of additized cetane actually measured after program
implementation; generally total cetane of additized fuel minus BC, but can
also be measured using additive concentration as a proxy property (see
Section 4.4)
= Reference cetane; the natural (unadditized) cetane number of diesel fuel
prior to implementation of the cetane program (see discussion above)
17
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BC = Base cetane; the cetane number of the unadditized base fuel after
implementation of the cetane program
Prior to program implementation, there are no measurements of ACm or BC. Thus, for
SIP planning purposes prior to program implementation, BC can be assumed to be equal to RC
If the fuel contained no pre-existing cetane improver additives prior to implementation of the
program, then the value of AC is determined by the type and level of standard set by the state:
Program
type
Description
How to determine AC prior to the
start of the program
Type 1
Total cetane
number standard
Standard minus RC
Type 2
Cetane number
increase standard
Standard
Type 3
Cetane additive
concentration
standard
Convert standard into a cetane
number increase using cetane
response functions (see Appendix 3)
Once the program has been implemented, the value of AC should be determined from
equation (EQ 2) as a part of the in-use enforcement program. The values of ACm and BC can be
measured using a variety of techniques involving some combination of direct measurements of
cetane number and indirect measurements of fuel properties. However, indirect measurements of
fuel properties (i.e., proxy properties) introduce additional uncertainties which may reduce the
fleet-wide NOx benefits that can be claimed (see Section 4.4 below).
As described above, diesel fuel may already contain some cetane improver additives prior
to implementation of a cetane improvement program. In such cases the calculation of (%NOx)pv
requires an additional step. Instead of simply using equation (EQ 1) once, equation (EQ 1)
should be used twice. The first application of equation (EQ 1) would use a value for AC
representing the pre-existing cetane improver additives prior to implementation of the program,
while the second application of equation (EQ 1) would use a value for AC representing the total
amount of cetane improver additives in the fuel after implementation of the program (pre-
existing additives plus any additional additives resulting from the program). The difference
between these two applications of equation (EQ 1) would provide an estimate of (%NOx)pv that
best represents the NOx benefits of the cetane improvement program. Mathematically, this
process would appear as follows:
[(%NOx)Pv]b = k x 100% x (1 - exp[ - 0.015151 x ACb
+ 0.000169 x ACb2
+ 0.000223 x ACb x RC]}
18
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[(%NOx)Pv]a = k x 100% x (1 - exp[ - 0.015151 x ACa
+ 0.000169 x ACa2
+ 0.000223 x ACaxRC]}
(%NOx)Pv = [(%NOx)Pv]a - [(%NOx)Pv]b
Where:
ACa
ACb
[(%NOx)Pv]a
[(%NOx)Pv]b
The increase in cetane number due to the use of additives before
implementation of the cetane improvement program
The total increase in cetane number due to the presence of all additives
after implementation of the cetane improvement program
Per-vehicle11 percent reduction in NOx emissions due to the use of
additives
before implementation of the cetane improvement program
Per-vehicle percent reduction in NOx emissions due to the presence of all
additives after implementation of the cetane improvement program
If a cetane response function is used to determine increases in cetane number as a function of
cetane improver concentration as described in Section 4.4 and Appendix 3, the presence of any
pre-existing cetane improver additives must also be taken into account. See Appendix 3.
11 Although equation (EQ 1) is written here and described for highway vehicles, this same equation can be used for
calculating NOx benefits of cetane improver additives for nonroad diesel engines.
19
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Section 4. Calculating in-use Fleet-wide NOx Benefits
4.1 Overview
As described in Section 3, you should first have an estimate of the NOx benefits for a
single vehicle or engine using diesel fuel with cetane improver additives before you can estimate
the NOx benefits for the in-use fleet for onroad or nonroad diesel sources. The in-use fleet of
highway vehicles consists of pre-model year 2003 highway vehicles in regulatory classes 41-49,
and the in-use fleet of nonroad engines consists of pre-Tier 3 engines. The per-vehicle benefit is
the value (%NOx)pv calculated from equation (EQ 1), and this equation can also be used for
calculating nonroad benefits.
If the cetane additive program applies to both onroad and nonroad diesel engines, then
equations 3 (EQ 3) and 4 (EQ 4), discussed below should each be used twice to calculate NOx
emissions reduced separately for both highway vehicles and nonroad engines.
The fleet-wide NOx benefits may differ from the per-vehicle benefits due to a variety of
programmatic issues. In order to account for these issues, the fleet-wide NOx benefit should be
calculated from the following equation, which will be referred to as (EQ 3):
Each of the program factors accounts for a specific element of the program design. The
following subsections describe how to determine the value of each of the program factors for use
in equation (EQ 3).
4.2 How is program factor F1 calculated?
As described in the original Technical Report, equation (EQ 1) does not apply to 2-stroke
engines. Thus 2-stroke engines are assumed to receive no NOx benefit from the use of cetane
improver additives. For general distribution of a cetane improver additive, the in-use fleet can be
assumed to be comprised of only a negligible fraction of 2-stroke engines. However, if cetane
improver additives are being used in identifiable, centrally-fueled fleets and those fleets have a
non-negligible fraction of 2-stroke engines, the NOx benefit should be adjusted downward
accordingly.
(%NOx)fw = (%NOx)Pv XF1XF2XF3XF4
Where:
Fi
F2
f3
f4
(%NOx)fw
(%NOx)pV
Fleet-wide percent reduction in NOx emissions
Per-vehicle percent reduction in NOx emissions from equation (EQ 1)
Program factor representing 2-stroke engines
Program factor representing nonroad fuel
Program factor representing vehicle migration
Program factor representing the use of proxy fuel properties
20
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Conditions for determining the default value of program factor Fi
Condition
Default value of Fi
General distribution of cetane improver additives through the
system of terminals, pipelines, and service stations
1.0
Use of cetane improver additives in a centrally-fueled fleet with
an identifiable and measurable number of 2-stroke and 4-stroke
engines
(number of 4-stroke
engines)/(number of 2-
stroke and 4-stroke engines)
4.3 How is program factor F2 calculated?
Cetane improver additives can be used in nonroad engines in addition to highway
engines, and nonroad engines are likely to produce some NOx benefits as a result. However, as
described in the original Technical Report, equation (EQ 1) was based entirely on emissions data
collected on highway engines. A qualitative argument can be made that nonroad engines will
respond to cetane in the same way that highway engines do, particularly for nonroad engines of a
similarly rated horsepower to highway engines, but there is little analysis to prove this assertion.
In addition, a large fraction of diesel fuel designated as "nonroad" is used in residential and
industrial heaters instead of diesel engines. There is no information to suggest that these heaters
will produce any NOx benefits from the use of cetane improver additives.
In order to account for the paucity of data on NOx benefits for nonroad engines and the
fact that heaters also consume some nonroad diesel fuel, an appropriate value for factor F2 should
be chosen. Additional emissions data on the effects of cetane improver additives on nonroad
engines may be necessary. For instance, data can be generated under the EPA's Emission Test
Verification Program.12
Program factor F2 may need to be set at zero if insufficient data on the potential emission
benefits of cetane improver additives in nonroad engines is available.
12 See fuel additive testing information at: https://www.epa.gov/ve-certification/evaluation-program-aftermarket-retrofit-
devices.
21
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Conditions for determining the default value of program factor F2
Condition
Default value of F2
Use of cetane improver additives in highway diesel fuel
1.0
Use of cetane improver additives in off-highway diesel fuel,
where the NOx benefits of cetane improver additives on
nonroad engines has been measured and used to estimate a
value for (%NOx)pv that supersedes equation (EQ 1)
(volume of off-highway fuel
used in nonroad engines) /
(volume of off-highway fuel
used in nonroad engines and
heaters)13
Use of cetane improver additives in off-highway diesel fuel,
where the NOx benefits of cetane improver additives on
nonroad engines has not been estimated
0.0
4.4 How is program factor F3 calculated?
Many highway diesel vehicles travel long distances on a single tank of fuel. As a result,
many vehicles that refuel within the geographic boundaries of a cetane improver program will
quickly travel outside of those boundaries, while many other vehicles that have refueled outside
of the program boundaries will subsequently travel into the program area. As a result of this
vehicle migration, the total NOx benefits of a cetane improver program will actually occur in a
region that includes but extends beyond the geographic boundaries of the covered program area.
The actual NOx benefits occurring within the program area will be less than the total NOx
benefits produced. The fraction of total NOx benefits occurring within the program area is
generally proportional to the geographic size of the program area.
Some segments of the diesel engine fleet may travel shorter distances than the average
highway diesel vehicle, and therefore may not contribute to migration. For instance, nonroad
engines generally do not travel long distances from their refueling locations like highway
vehicles do. Also, some centrally-fueled fleets may use vehicles that only travel within a small
region and thus do not contribute to migration. The State may account for such centrally-fueled
fleets if it can provide supporting data. Finally, truck operators who actively avoid higher-priced
fuel could cause an additional reduction in the NOx benefits of a cetane improver program. The
effects of this "price aversion" as estimated from available data are small relative to the impacts
of vehicle migration and are here considered to be covered by the default values for program
factor F3 shown below.
13 "Heaters" include any fuel combustion unit designed to produce heat instead of work, including residential heating units,
industrial boilers, etc
22
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Conditions for determining the default value of program factor F3
Condition
Default value of F3
Use of cetane improver additives in nonroad engines
1.0
Use of cetane improver additives in highway engines, where the
total square mileage of the area within which the mandated
cetane improver additive program applies is:
Less than 50 mi2
0.3
51 -300 mi2
0.5
301 - 1200 mi2
0.6
1201 -2800 mi2
0.7
2801 -7800 mi2
0.8
7801 - 70,000 mi2
0.9
Above 70,001 mi2
1.0
The state may propose alternative values for factor F3 if it can provide supporting area-specific
vehicle trip length or migration data.
4.5 How is program factor F4 calculated?
Equations 1 and 2 (EQ 1, EQ 2) require measurements for natural cetane number and
additized cetane number. Generally, this would require the use of ASTM test procedure D613
twice:
• Once to measure the cetane number of the fuel prior to addition of cetane improver
• A second time to measure the cetane number of the fuel after the cetane improver has
been added
The first measurement provides a value for BC in equation (EQ 2), while the second
measurement minus the first measurement provides a value for ACm.
However, a state may wish to permit the use of alternative methods for estimating the
values of natural and additized cetane numbers in order to reduce costs, increase the number of
samples that can be taken, or to simplify the compliance process. The use of these "proxy
properties" introduces additional uncertainties and potential bias into the calculation of
(%NOx)Pv. Thus, the fleet-wide NOx benefits should be adjusted to account for the use of proxy
properties.
23
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The two primary proxy properties available to states include the following:
Cetane index (ASTMD4737)
Used to estimate the natural cetane number. Requires the measurement of fuel
distillation properties T10, T50, and T90, and measurement of fuel density. Cetane index
is then calculated from the following equation:
CI = 45.2
+ 0.0892 x (T10-215)
+ {0.131 + 0.901 x [exp(-3.5 x (D - 0.85)) - 1]} x (T50 - 260)
+ {0.0523 - 0.420 x [exp(-3.5 x (D - 0.85)) - 1]} x (T90 - 310)
+ 0.00049 x [(HO - 215)2 - (T90 - 310)2]
+ 107 x [exp(-3.5 x (D - 0.85)) - 1]
+ 60 x [exp(-3.5 x (D - 0.85)) - l]2
Where:
CI = Cetane index
T10 = Distillation property via ASTM D86: temperature in °F at which 10vol%
has evaporated
T50 = Distillation property via ASTM D86: temperature in °F at which 50vol%
has evaporated
T90 = Distillation property via ASTM D86: temperature in °F at which 90vol%
has evaporated
D = Density in g/ml at 15 °C, via ASTM D1298
In order to use cetane index to represent the natural cetane number of a fuel, any biases
between CI and actual measured natural cetane values should be addressed. Appendix 4
provides a default correlation that can be used for this purpose. Cetane index can only be used to
estimate the cetane number of unadditized fuel, or the natural (not total) cetane number of fuel
containing a cetane improver additive.
Additive concentration
Along with a cetane response function such as those in Appendix 3, additive
concentration can be used to provide an estimate of the increase in cetane number due to
the use of cetane improver additives.
There may be other means for generating proxy properties that avoid the use of ASTM
test procedure D613. If these means of generating proxy properties have not been peer reviewed
in a public process, then their accuracy and precision as predictors of cetane number cannot be
confirmed. As a result, allowing their use as compliance tools in a cetane improver additive
program could compromise the NOx benefits of that program. Since the benefits of a cetane
improvement program must be quantifiable and surplus, potential bias in cetane number
predictions for these proxy properties requires an adjustment to the claimable fleet-wide NOx
benefits. Choosing an appropriate value for program factor F4 may be an appropriate means for
mitigating bias introduced through the use of proxy properties.
24
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Although equations (EQ 1) and (EQ 2) would normally require measurements for natural
cetane number and additized cetane number using ASTM test procedure D613, the simplest
possible compliance scheme would involve measurements of additive concentration and only an
assumption regarding the natural cetane of the base fuel [BC in equation (EQ 2)]. For instance,
the value of BC could be assumed to be equal to RC, the natural cetane number of diesel fuel
prior to implementation of the program. If a cetane improvement program permits this
compliance approach, the fleet-wide NOx benefits are much more uncertain. As a result, they
should be adjusted downward by choosing an appropriate value for program factor F4.
Conditions for determining the default value of program factor F4
Condition
Default value of F4
Program requires the use of ASTM test procedure D613 for
measuring base cetane number (BC) and additized cetane
number (ACm)
1.0
Program allows the use of cetane index (including Appendix 4
correlation) and/or additive concentration (with a known
response function) as proxy properties for representing cetane
number measurements via ASTM D613
1.0
Program allows regulated parties to avoid measuring the base
cetane number (BC) by assuming that BC is equal to RC.
RC > 47
44= RC < 47
RC < 44
0.8
0.9
1.0
Program allows the use of other proxy properties whose
measured values are corrected for known bias in comparison to
D613 cetane number and whose uncertainty is established to be
equivalent to D613
1.0
25
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Section 5. Calculating Tons of NOx Reduced
The reduction in NOx emissions that results from the cetane improvement program
depends broadly on the percent reduction in NOx and that portion of the program area's NOx
inventory that is affected by cetane improver additives. Mathematically, this is represented by
the following equation, which will be referred to as (EQ 4):
NOx tons reduced = Diesel NOx inventory x (%NOx)fw x Volume fraction affected (EQ 4)
Where:
NOx tons reduced = Daily or annual tons of NOx reduced within the
geographic boundaries of the cetane improver program area
Diesel NOx inventory = Total daily or annual tons of NOx generated by diesel
engines within the geographic boundaries of the program
area, assuming the cetane additive program is not in effect
(%NOx)fw = Fleet-wide percent reduction in NOx from equation (EQ
3)
Volume fraction affected = Fraction of the diesel fuel volume which contains cetane
improver additives within the program area
The calculation of NOx emissions reduced using equation (EQ 4) may need to take
into account other factors depending on the form of the cetane improver program. For
instance:
• If the cetane improver program only applies for a portion of the year (e.g., summer
months only), then the "Diesel NOx inventory" should likewise represent only that same
portion of the year.
• If the cetane improver additive program applies to both highway and nonroad
engines, then equation (EQ 4) should be used twice to calculate NOx tons reduced
separately for both highway and nonroad, and the results summed.
• If the cetane improver additive program applies to specific centrally-fueled fleets,
then the "Diesel NOx inventory" in equation (EQ 4) should represent those specific
fleets.
The "volume fraction affected" will generally be equal to 1.0 if the cetane improver
additive program applies to all fuel within specified geographic boundaries. In this case the
"Diesel NOx inventory" should represent that same area. However, if the program does not
apply to all fuel within specified geographic boundaries, or if the "Diesel NOx inventory" must
necessarily represent an area that extends beyond the program area, then the "volume fraction
affected" will be less than 1.0.
Conditions for determining a value for "Volume fraction affected" in equation (EQ 4)
26
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Condition
Volume fraction affected
Cetane improver additive program applies to all fuel within area
X and "Diesel NOx inventory" also represents area X
1.0
Cetane improver additive program applies to all fuel within area
X and "Diesel NOx inventory" represents larger area Y
Fuel consumed in area X
Fuel consumed in area Y
Cetane improver additive program applies to specific fleets
within area X
Fuel consumed by fleets ^
Fuel consumed in area X
For highway diesel vehicles, the fuel consumed within a given area can be
calculated from the diesel engine VMT associated with that area and fuel economy rates
for each diesel vehicle weight class for the calendar year being modeled.
27
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Appendix 1. National Average Distribution of Nonroad Diesel Engines by
Certification Tier
Diesel Engine Total Diesel
Year Engine Technology Population Engine Population Tier Distribution
2025
Baseline Pre-1988 Diesel
31404
7378923
0.004
2025
TierO Diesel
132355
7378923
0.018
2025
Tierl Diesel
413170
7378923
0.056
2025
Tier2 Diesel
587042
7378923
0.080
2025
Tier3 Diesel
227160
7378923
0.031
2025
Tier3 Transitional Diesel
87925
7378923
0.012
2025
Tier4 Final Diesel
4295589
7378923
0.582
2025
Tier4 Transitional Diesel
1604279
7378923
0.217
2026 Baseline Pre-1988 Diesel 26078 7433763 0.004
2026
TierO Diesel
114620
7433763
0.015
2026
Tierl Diesel
359047
7433763
0.048
2026
Tier2 Diesel
532660
7433763
0.072
2026
Tier3 Diesel
208629
7433763
0.028
2026
Tier3 Transitional Diesel
81293
7433763
0.011
2026
Tier4 Final Diesel
4513385
7433763
0.607
2026
Tier4 Transitional Diesel
1598051
7433763
0.215
2030 Baseline Pre-1988 Diesel 10669 7707325 0.001
2030
TierO Diesel
64809
7707325
0.008
2030
Tierl Diesel
180919
7707325
0.023
2030
Tier2 Diesel
310606
7707325
0.040
2030
Tier3 Diesel
139022
7707325
0.018
2030
Tier3 Transitional Diesel
55275
7707325
0.007
2030
Tier4 Final Diesel
5379900
7707325
0.698
2030
Tier4 Transitional Diesel
1566125
7707325
0.203
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Appendix 2. National Average Distribution of Nonroad Diesel Engines by
Certification Tier and Equipment Sector
Year
Sector
Engine Technology
Diesel Engine
Population
Total Diesel
Engine Population
By Sector
Tier
Distribution
By Sector
2025
Agriculture
Baseline Pre-1988 Diesel
13175
2110033
0.006
2025
Agriculture
TierO Diesel
35810
2110033
0.017
2025
Agriculture
Tierl Diesel
145741
2110033
0.069
2025
Agriculture
Tier2 Diesel
225858
2110033
0.107
2025
Agriculture
Tier3 Diesel
159126
2110033
0.075
2025
Agriculture
Tier3 Transitional Diesel
32256
2110033
0.015
2025
Agriculture
Tier4 Final Diesel
1211733
2110033
0.574
2025
Agriculture
Tier4 Transitional Diesel
286334
2110033
0.136
2025
Airport Support
TierO Diesel
8
20937
0.000
2025
Airport Support
Tierl Diesel
95
20937
0.005
2025
Airport Support
Tier2 Diesel
236
20937
0.011
2025
Airport Support
Tier3 Diesel
1210
20937
0.058
2025
Airport Support
Tier3 Transitional Diesel
200
20937
0.010
2025
Airport Support
Tier4 Final Diesel
16723
20937
0.799
2025
Airport Support
Tier4 Transitional Diesel
2464
20937
0.118
2025
Commercial
Baseline Pre-1988 Diesel
7596
1465110
0.005
2025
Commercial
TierO Diesel
36730
1465110
0.025
2025
Commercial
Tierl Diesel
81782
1465110
0.056
2025
Commercial
Tier2 Diesel
153174
1465110
0.105
2025
Commercial
Tier3 Diesel
14800
1465110
0.010
2025
Commercial
Tier3 Transitional Diesel
21044
1465110
0.014
2025
Commercial
Tier4 Final Diesel
571771
1465110
0.390
2025
Commercial
Tier4 Transitional Diesel
578214
1465110
0.395
2025
Construction
Baseline Pre-1988 Diesel
8430
2316576
0.004
2025
Construction
TierO Diesel
41332
2316576
0.018
2025
Construction
Tierl Diesel
146813
2316576
0.063
2025
Construction
Tier2 Diesel
133289
2316576
0.058
2025
Construction
Tier3 Diesel
37144
2316576
0.016
2025
Construction
Tier3 Transitional Diesel
25193
2316576
0.011
2025
Construction
Tier4 Final Diesel
1619064
2316576
0.699
2025
Construction
Tier4 Transitional Diesel
305312
2316576
0.132
2025
Industrial
Baseline Pre-1988 Diesel
940
828857
0.001
2025
Industrial
TierO Diesel
6425
828857
0.008
2025
Industrial
Tierl Diesel
13216
828857
0.016
2025
Industrial
Tier2 Diesel
21035
828857
0.025
2025
Industrial
Tier3 Diesel
5479
828857
0.007
29
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Year
Sector
Engine Technology
Diesel Engine
Population
Total Diesel
Engine Population
By Sector
Tier
Distribution
By Sector
2025
Industrial
Tier3 Transitional Diesel
4083
828857
0.005
2025
Industrial
Tier4 Final Diesel
641599
828857
0.774
2025
Industrial
Tier4 Transitional Diesel
136081
828857
0.164
2025
Lawn/Garden
Baseline Pre-1988 Diesel
994
508235
0.002
2025
Lawn/Garden
TierO Diesel
6832
508235
0.013
2025
Lawn/Garden
Tierl Diesel
19587
508235
0.039
2025
Lawn/Garden
Tier2 Diesel
35665
508235
0.070
2025
Lawn/Garden
Tier3 Diesel
4673
508235
0.009
2025
Lawn/Garden
Tier3 Transitional Diesel
4974
508235
0.010
2025
Lawn/Garden
Tier4 Final Diesel
163698
508235
0.322
2025
Lawn/Garden
Tier4 Transitional Diesel
271812
508235
0.535
2025
Logging
Tier2 Diesel
1
15534
0.000
2025
Logging
Tier3 Diesel
135
15534
0.009
2025
Logging
Tier3 Transitional Diesel
7
15534
0.000
2025
Logging
Tier4 Final Diesel
15292
15534
0.984
2025
Logging
Tier4 Transitional Diesel
100
15534
0.006
2025
Oil Field
TierO Diesel
0
46829
0.000
2025
Oil Field
Tierl Diesel
188
46829
0.004
2025
Oil Field
Tier2 Diesel
983
46829
0.021
2025
Oil Field
Tier3 Diesel
2376
46829
0.051
2025
Oil Field
Tier3 Transitional Diesel
24
46829
0.001
2025
Oil Field
Tier4 Final Diesel
38566
46829
0.824
2025
Oil Field
Tier4 Transitional Diesel
4691
46829
0.100
2025
Railroad
Baseline Pre-1988 Diesel
142
14466
0.010
2025
Railroad
TierO Diesel
525
14466
0.036
2025
Railroad
Tierl Diesel
1324
14466
0.092
2025
Railroad
Tier2 Diesel
2201
14466
0.152
2025
Railroad
Tier3 Diesel
1133
14466
0.078
2025
Railroad
Tier3 Transitional Diesel
99
14466
0.007
2025
Railroad
Tier4 Final Diesel
7335
14466
0.507
2025
Railroad
Tier4 Transitional Diesel
1708
14466
0.118
2025
Recreational
Baseline Pre-1988 Diesel
124
42323
0.003
2025
Recreational
TierO Diesel
4367
42323
0.103
2025
Recreational
Tierl Diesel
4425
42323
0.105
2025
Recreational
Tier2 Diesel
4907
42323
0.116
2025
Recreational
Tier3 Diesel
1084
42323
0.026
2025
Recreational
Tier3 Transitional Diesel
45
42323
0.001
2025
Recreational
Tier4 Final Diesel
9808
42323
0.232
2025
Recreational
Tier4 Transitional Diesel
17563
42323
0.415
2025
Underground
Baseline Pre-1988 Diesel
3
10022
0.000
30
-------
Year
Sector
Engine Technology
Diesel Engine
Population
Total Diesel
Engine Population
By Sector
Tier
Distribution
By Sector
Mining
2025
Underground
Mining
TierO Diesel
326
10022
0.033
2025
Underground
Mining
Tier2 Diesel
9694
10022
0.967
2026
Agriculture
Baseline Pre-1988 Diesel
11017
2101965
0.005
2026
Agriculture
TierO Diesel
30550
2101965
0.015
2026
Agriculture
Tierl Diesel
127170
2101965
0.061
2026
Agriculture
Tier2 Diesel
208322
2101965
0.099
2026
Agriculture
Tier3 Diesel
149893
2101965
0.071
2026
Agriculture
Tier3 Transitional Diesel
30006
2101965
0.014
2026
Agriculture
Tier4 Final Diesel
1270101
2101965
0.604
2026
Agriculture
Tier4 Transitional Diesel
274906
2101965
0.131
2026
Airport Support
TierO Diesel
4
21289
0.000
2026
Airport Support
Tierl Diesel
71
21289
0.003
2026
Airport Support
Tier2 Diesel
160
21289
0.008
2026
Airport Support
Tier3 Diesel
947
21289
0.044
2026
Airport Support
Tier3 Transitional Diesel
143
21289
0.007
2026
Airport Support
Tier4 Final Diesel
17635
21289
0.828
2026
Airport Support
Tier4 Transitional Diesel
2330
21289
0.109
2026
Commercial
Baseline Pre-1988 Diesel
6340
1494890
0.004
2026
Commercial
TierO Diesel
31506
1494890
0.021
2026
Commercial
Tierl Diesel
72813
1494890
0.049
2026
Commercial
Tier2 Diesel
140642
1494890
0.094
2026
Commercial
Tier3 Diesel
14368
1494890
0.010
2026
Commercial
Tier3 Transitional Diesel
19898
1494890
0.013
2026
Commercial
Tier4 Final Diesel
617017
1494890
0.413
2026
Commercial
Tier4 Transitional Diesel
592306
1494890
0.396
2026
Construction
Baseline Pre-1988 Diesel
6910
2313640
0.003
2026
Construction
TierO Diesel
36129
2313640
0.016
2026
Construction
Tierl Diesel
126166
2313640
0.055
2026
Construction
Tier2 Diesel
116758
2313640
0.050
2026
Construction
Tier3 Diesel
30248
2313640
0.013
2026
Construction
Tier3 Transitional Diesel
22677
2313640
0.010
2026
Construction
Tier4 Final Diesel
1681646
2313640
0.727
2026
Construction
Tier4 Transitional Diesel
293105
2313640
0.127
2026
Industrial
Baseline Pre-1988 Diesel
788
859850
0.001
2026
Industrial
TierO Diesel
5765
859850
0.007
2026
Industrial
Tierl Diesel
11716
859850
0.014
31
-------
Year
Sector
Engine Technology
Diesel Engine
Population
Total Diesel
Engine Population
By Sector
Tier
Distribution
By Sector
2026
Industrial
Tier2 Diesel
19410
859850
0.023
2026
Industrial
Tier3 Diesel
4522
859850
0.005
2026
Industrial
Tier3 Transitional Diesel
3555
859850
0.004
2026
Industrial
Tier4 Final Diesel
677377
859850
0.788
2026
Industrial
Tier4 Transitional Diesel
136716
859850
0.159
2026
Lawn/Garden
Baseline Pre-1988 Diesel
801
512345
0.002
2026
Lawn/Garden
TierO Diesel
6061
512345
0.012
2026
Lawn/Garden
Tierl Diesel
15949
512345
0.031
2026
Lawn/Garden
Tier2 Diesel
30030
512345
0.059
2026
Lawn/Garden
Tier3 Diesel
4547
512345
0.009
2026
Lawn/Garden
Tier3 Transitional Diesel
4852
512345
0.009
2026
Lawn/Garden
Tier4 Final Diesel
174780
512345
0.341
2026
Lawn/Garden
Tier4 Transitional Diesel
275325
512345
0.537
2026
Logging
Tier2 Diesel
0
15362
0.000
2026
Logging
Tier3 Diesel
84
15362
0.005
2026
Logging
Tier3 Transitional Diesel
4
15362
0.000
2026
Logging
Tier4 Final Diesel
15218
15362
0.991
2026
Logging
Tier4 Transitional Diesel
56
15362
0.004
2026
Oil Field
Tierl Diesel
142
47289
0.003
2026
Oil Field
Tier2 Diesel
777
47289
0.016
2026
Oil Field
Tier3 Diesel
1844
47289
0.039
2026
Oil Field
Tier3 Transitional Diesel
16
47289
0.000
2026
Oil Field
Tier4 Final Diesel
41193
47289
0.871
2026
Oil Field
Tier4 Transitional Diesel
3316
47289
0.070
2026
Railroad
Baseline Pre-1988 Diesel
114
14508
0.008
2026
Railroad
TierO Diesel
475
14508
0.033
2026
Railroad
Tierl Diesel
1196
14508
0.082
2026
Railroad
Tier2 Diesel
2068
14508
0.143
2026
Railroad
Tier3 Diesel
1110
14508
0.076
2026
Railroad
Tier3 Transitional Diesel
98
14508
0.007
2026
Railroad
Tier4 Final Diesel
7771
14508
0.536
2026
Railroad
Tier4 Transitional Diesel
1676
14508
0.116
2026
Recreational
Baseline Pre-1988 Diesel
105
42663
0.002
2026
Recreational
TierO Diesel
3872
42663
0.091
2026
Recreational
Tierl Diesel
3824
42663
0.090
2026
Recreational
Tier2 Diesel
4792
42663
0.112
2026
Recreational
Tier3 Diesel
1067
42663
0.025
2026
Recreational
Tier3 Transitional Diesel
44
42663
0.001
2026
Recreational
Tier4 Final Diesel
10646
42663
0.250
2026
Recreational
Tier4 Transitional Diesel
18315
42663
0.429
32
-------
Year
Sector
Engine Technology
Diesel Engine
Population
Total Diesel
Engine Population
By Sector
Tier
Distribution
By Sector
2026
Underground
Mining
Baseline Pre-1988 Diesel
2
9961
0.000
2026
Underground
Mining
TierO Diesel
258
9961
0.026
2026
Underground
Mining
Tier2 Diesel
9701
9961
0.974
2030
Agriculture
Baseline Pre-1988 Diesel
4567
2065696
0.002
2030
Agriculture
TierO Diesel
17430
2065696
0.008
2030
Agriculture
Tierl Diesel
65026
2065696
0.031
2030
Agriculture
Tier2 Diesel
113645
2065696
0.055
2030
Agriculture
Tier3 Diesel
103926
2065696
0.050
2030
Agriculture
Tier3 Transitional Diesel
17339
2065696
0.008
2030
Agriculture
Tier4 Final Diesel
1548337
2065696
0.750
2030
Agriculture
Tier4 Transitional Diesel
195426
2065696
0.095
2030
Airport Support
TierO Diesel
0
22757
0.000
2030
Airport Support
Tierl Diesel
15
22757
0.001
2030
Airport Support
Tier2 Diesel
54
22757
0.002
2030
Airport Support
Tier3 Diesel
362
22757
0.016
2030
Airport Support
Tier3 Transitional Diesel
40
22757
0.002
2030
Airport Support
Tier4 Final Diesel
20348
22757
0.894
2030
Airport Support
Tier4 Transitional Diesel
1936
22757
0.085
2030
Commercial
Baseline Pre-1988 Diesel
2589
1622342
0.002
2030
Commercial
TierO Diesel
16578
1622342
0.010
2030
Commercial
Tierl Diesel
37103
1622342
0.023
2030
Commercial
Tier2 Diesel
94206
1622342
0.058
2030
Commercial
Tier3 Diesel
12475
1622342
0.008
2030
Commercial
Tier3 Transitional Diesel
16155
1622342
0.010
2030
Commercial
Tier4 Final Diesel
807664
1622342
0.498
2030
Commercial
Tier4 Transitional Diesel
635574
1622342
0.392
2030
Construction
Baseline Pre-1988 Diesel
2872
2337054
0.001
2030
Construction
TierO Diesel
20698
2337054
0.009
2030
Construction
Tierl Diesel
62300
2337054
0.027
2030
Construction
Tier2 Diesel
64099
2337054
0.027
2030
Construction
Tier3 Diesel
13990
2337054
0.006
2030
Construction
Tier3 Transitional Diesel
15143
2337054
0.006
2030
Construction
Tier4 Final Diesel
1885374
2337054
0.807
2030
Construction
Tier4 Transitional Diesel
272579
2337054
0.117
2030
Industrial
Baseline Pre-1988 Diesel
328
996971
0.000
2030
Industrial
TierO Diesel
3599
996971
0.004
33
-------
Year
Sector
Engine Technology
Diesel Engine
Population
Total Diesel
Engine Population
By Sector
Tier
Distribution
By Sector
2030
Industrial
Tierl Diesel
5416
996971
0.005
2030
Industrial
Tier2 Diesel
12241
996971
0.012
2030
Industrial
Tier3 Diesel
1911
996971
0.002
2030
Industrial
Tier3 Transitional Diesel
2110
996971
0.002
2030
Industrial
Tier4 Final Diesel
819636
996971
0.822
2030
Industrial
Tier4 Transitional Diesel
151730
996971
0.152
2030
Lawn/Garden
Baseline Pre-1988 Diesel
233
529307
0.000
2030
Lawn/Garden
TierO Diesel
3738
529307
0.007
2030
Lawn/Garden
Tierl Diesel
8584
529307
0.016
2030
Lawn/Garden
Tier2 Diesel
11936
529307
0.023
2030
Lawn/Garden
Tier3 Diesel
3858
529307
0.007
2030
Lawn/Garden
Tier3 Transitional Diesel
4357
529307
0.008
2030
Lawn/Garden
Tier4 Final Diesel
211435
529307
0.399
2030
Lawn/Garden
Tier4 Transitional Diesel
285166
529307
0.539
2030
Logging
Tier2 Diesel
0
15053
0.000
2030
Logging
Tier3 Diesel
7
15053
0.000
2030
Logging
Tier4 Final Diesel
15037
15053
0.999
2030
Logging
Tier4 Transitional Diesel
9
15053
0.001
2030
Oil Field
Tierl Diesel
31
49586
0.001
2030
Oil Field
Tier2 Diesel
237
49586
0.005
2030
Oil Field
Tier3 Diesel
612
49586
0.012
2030
Oil Field
Tier3 Transitional Diesel
1
49586
0.000
2030
Oil Field
Tier4 Final Diesel
47755
49586
0.963
2030
Oil Field
Tier4 Transitional Diesel
950
49586
0.019
2030
Railroad
Baseline Pre-1988 Diesel
36
14838
0.002
2030
Railroad
TierO Diesel
305
14838
0.021
2030
Railroad
Tierl Diesel
662
14838
0.045
2030
Railroad
Tier2 Diesel
937
14838
0.063
2030
Railroad
Tier3 Diesel
908
14838
0.061
2030
Railroad
Tier3 Transitional Diesel
89
14838
0.006
2030
Railroad
Tier4 Final Diesel
10340
14838
0.697
2030
Railroad
Tier4 Transitional Diesel
1560
14838
0.105
2030
Recreational
Baseline Pre-1988 Diesel
44
44056
0.001
2030
Recreational
TierO Diesel
2383
44056
0.054
2030
Recreational
Tierl Diesel
1782
44056
0.040
2030
Recreational
Tier2 Diesel
3665
44056
0.083
2030
Recreational
Tier3 Diesel
973
44056
0.022
2030
Recreational
Tier3 Transitional Diesel
41
44056
0.001
2030
Recreational
Tier4 Final Diesel
13973
44056
0.317
2030
Recreational
Tier4 Transitional Diesel
21194
44056
0.481
34
-------
Year
Sector
Engine Technology
Diesel Engine
Population
Total Diesel
Engine Population
By Sector
Tier
Distribution
By Sector
2030
Underground
Mining
Baseline Pre-1988 Diesel
0
9663
0.000
2030
Underground
Mining
TierO Diesel
77
9663
0.008
2030
Underground
Mining
Tier2 Diesel
9586
9663
0.992
35
-------
Appendix 3. Cetane Response Function
The increase in cetane number that results from the use of an additive depends on
the cetane response function for that additive. For the common cetane improver additives
2- ethylhexylnitrate (2-EHN) and di-tertiary butyl peroxide (DTBP), the following
response function14 can be used:
CNI = a x BC036 x G0-57 x C0-032 x ln(l + 17.5 x C)
Where:
CNI = Increase in cetane number due to the use of a cetane improver additive
a = Constant, 0.16 for 2-EHN and 0.119 for DTBP
BC = Base cetane; cetane number of the unadditized fuel to which cetane
improver is added. Equals RC (reference cetane) prior to implementation of
the program
G = API gravity of the fuel to which cetane improver is added. Equals 34.6 prior
to implementation of the program
C = Concentration of the additive in volume percent (equation is valid up to 0.5
volume percent, according to referenced study)
If other cetane improver additives are permitted or required, alternative cetane response
functions should be developed.
Where:
Cb = The concentration of cetane improver additives before implementation of
the cetane improvement program
Ca = The total concentration of cetane improver additives after implementation
of the cetane improvement program
[CNI]b = Increase in cetane number due to the presence of a cetane improver additive
before implementation of the cetane improvement program
[CNI] A = Increase in cetane number due to the presence of all cetane improver
additives after implementation of the cetane improvement program
14 Equation 2 from SAE paper number 972901, "Prediction and Precision of Cetane Number Improver response
Equations," Thompson et al.
36
-------
Appendix 4. Base Cetane Value
Cetane index (CI) is a means for estimating the natural cetane number of a fuel
using fuel properties. It allows one to avoid the more costly and involved engine testing
required for cetane number under ASTM test method D613. However, there is a
measurable bias between CI estimates and actual measurements of natural cetane number
made with ASTM D613. A correlation that corrects for this bias for cetane index
estimates made through ASTM D-4737 is shown below:
Natural cetane number = 1.107 x CI - 5.617
This equation can be used to estimate values for the base cetane (BC) in equation (EQ 2).
37
-------
Appendix 5. Example Emission Reduction Calculation
This appendix provides a hypothetical illustrative example of the calculation of NOx
reductions in the context of state-run cetane improvement program in an area that is a Severe
nonattainment area for the 2008 ozone NAAQS with an attainment date in July 2027. The values
used for the various inputs have been chosen only for purposes of showing how the calculations
would be done. The calculated NOx reductions in the area, and thus they are not to be used
directly in a SIP or for planning purposes. States that have already example are not indicative of
actual reductions that might be expected in a specific nonattainment or maintenance
implemented cetane additive programs and states considering implementing cetane additive
programs should use values for the requisite inputs that are specific to the program being
contemplated.
The example includes the following assumptions:
• Only heavy-duty highway vehicles use cetane improver additives
• Program requirement applies throughout the hypothetical nonattainment area
• Program requirement applies during the months of May through September
• Example calculations represent calendar year 2026. If a cetane additive program is
implemented in some other year, both the relevant emissions inventory and the constant
'k' used in equation (EQ 1) would be determined for that year.
• NOx emissions attributable to heavy-duty diesel vehicles in Regulatory Classes 41-49
in 2026 prior to implementation of the cetane improvement program are assumed to be
30 tons per day.
Example
Purpose
Mandatory program for generating emission reductions for the SIP
Program description
Cetane requirement applies to every gallon of diesel sold within the applicable nonattainment
area
General distribution of the fuel, not targeted to specific fleets
Minimum total cetane standard of 50
38
-------
Calculations
Vehicle migration factor: 0.8 (based on an applicable program area of 2804 square miles)
Attainment demonstration in 2026: area specific k-factor = 0.22
Pre-program natural cetane number is 47
(%NOx)pv = 0.22 x 100% x {1 - exp[ - 0.015151 x (50 - 47)
+ 0.000169 x (50 -47)2
+ 0.000223 x (50 - 47) x (47)]}
(%NOx)pv = 0.27%
(%>NOx)fw = (%NOx)Pv x vehicle migration factor (%NOx)fw = 0.27% x 0.8
(%NOx)fw = 0.22%
NOx benefits of cetane additive program = Diesel NOx inventory x (%NOx)fw x Volume
fraction affected
NOx benefits of cetane additive program = 30 tons/day x 0.22% /100%
NOx benefits of cetane additive program = 0.07 tons/day
39
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