United States
Environmental Protection
Agency
Office of Transportation EPA420-B-04-005
and Air Quality June 2004
Guidance on Quantifying NOx
Benefits for Cetane
Improvement Programs for
Use in SIPs and Transportation
Conformity
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EPA420-B-04-005
June 2004
Guidance on Quantifying NOx Benefits for Cetane
Improvement Programs for Use in SIPs and
Transportation Conformity
Transportation and Regional Programs Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
NOTICE
This Technical Report does not necessarily represent final EPA decisions or positions.
It is intended to present technical analysis of issues using data that are currently available.
The purpose in the release of such reports is to facilitate an exchange of
technical information and to inform the public of technical developments.
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A. Introduction
(Note: As used in this document, the terms "you" and "your" refer to State or States and the
terms "we," "us" and "our" refer to EPA.)
What is the purpose of this document?
This document provides guidance and sets forth the Environmental Protection Agency's
(EPA) policy and interpretation regarding the granting of State Implementation Plan (SIP) credit
under section 110 of the Clean Air Act (CAA) for emission reductions attributable to the use of
cetane additives in diesel fuel. Cetane improvement programs have the potential to contribute
emission reductions needed for progress toward attainment and maintenance of the National
Ambient Air Quality Standards (NAAQS). EPA believes that SIP credit is appropriate when
there is confidence that cetane improvement programs can achieve emission reductions and meet
any other applicable requirements. This document identifies the terms and conditions for
establishing and implementing a cetane improvement program and the requirements for
approvable SIP submittals and transportation conformity determinations under the Clean Air Act.
This document is intended solely as guidance for SIP credit and does not represent final
Agency action. It does not supersede or change any existing federal or State regulations or
requirements, including those of an approved SIP. If this guidance is followed, all otherwise
applicable CAA requirements pertaining to the crediting of emission reductions for
transportation conformity or SIPs apply, including all requirements pertaining to emission
reductions for Reasonable Further Progress (RFP) plans, attainment or maintenance strategy.
Furthermore, this document does not address the use of NOx reductions from the use of cetane
improvement additives in any form of credit trading program, including programs under EPA's
NOx SIP call. Additional requirements or limitations may apply in these cases. If information is
submitted to EPA that differs from the guidance in this document, EPA will review the
information and make a decision as to the appropriateness of its use on a case-by-case basis.
This guidance does not prohibit nor substitute for implementation of more stringent requirements
if otherwise permitted.
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 cetane number result in reduced emissions of
nitrogen oxides. Common cetane improvement additives include 2-ethylhexylnitrate and di-
tertiary butylperoxide used at diesel fuel concentrations of generally less than 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
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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 D below for more details.
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. (See Section G for additional
discussion.)
How would "federal preemption" apply to cetane improvement programs?
In general, the 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. However, EPA may waive preemption under
certain circumstances, as discussed below.
State adoption of motor vehicle fuel requirements is controlled by section 21 l(c)(4) of the
CAA. Section 21 l(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 21 l(c)(l).
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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.J 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 cetane number rather than cetane index or
aromatics. While cetane number of unadditized diesel fuel has an impact on the cetane index of
that fuel, increasing the cetane number using additives does not 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. EPA believes
there may be State cetane improvement programs that may not be preempted. States should
consult with their regional EPA office regarding preemption of specific State cetane programs.
Section 21 l(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. 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." EPA has provided guidance on the necessity showing that
a State must make in order to meet the preemption waiver requirements of section 21 l(c)(4)(C).
This guidance was distributed in 1997 in the context of State opt-in to RFG and low RVP
requirements in ozone SIPs2, and readers are referred to that document for more information.
States should also consult with their regional EPA office regarding making necessity showings.
A State voluntary or incentive-based cetane control program that did not impose
mandatory requirements or obligations on fuel content, typically would not be considered a fuel
"control or prohibition." Such a program, therefore, would not be preempted under section
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 § 80.520.
2 "Guidance On Use of Opt-In to RFG and Low RVP Requirements In Ozone SIPs," August 1997.
http ://www. epa. gov/otaq/regs/fuels/rvpguide .pdf
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B. Federal Criteria for SIPs and Transportation Conformity
What are the basic criteria for using emission reductions in SIPs or transportation conformity?
In order to be approved as a measure which provides additional emission reductions in a
SIP or transportation conformity determination, a control measure cannot interfere with other
requirements of the CAA, and would need to be consistent with SIP attainment, maintenance, or
RFP/ROP requirements. In addition, the control measure would need to provide emission
reductions that meet the criteria described below.
(A) Quantifiable - The emission reductions from a control measure are quantifiable if they can
be reliably and replicably measured. Emission reductions should be calculated for the time period
for which the reductions will be used. Sections C through E of this document provide a method
which EPA believes is acceptable for quantifying emission reductions.
(B) Surplus - Emission reductions are generally surplus and can be used as long as they are not
otherwise relied on to meet other applicable air quality attainment, reasonable further progress and
maintenance requirements. In the event that the measure is relied on to meet such air quality-
related program requirements, they are no longer surplus and may not be used for additional
credit.
(C) Federally Enforceable - Control measures included in a SIP or SIP revision need to be
enforceable. Where the SIP emission reductions are part of a rule or regulation, they are
considered federally enforceable if they meet all of the following requirements:
• They are independently verifiable.
• Violations are defined, as appropriate.
• You and EPA have the ability to enforce the measure if violations occur.
• Those liable for violations can be identified.
• Citizens have access to all the emissions-related information obtained from the
responsible party.
• Citizens can file suits 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.
Specific elements of a compliance and enforcement program associated with a cetane
improvement program must normally be approved by EPA's Office of Enforcement and
Compliance Assurance. See Section G for additional information.
If a SIP revision is approved under EPA's Mobile Source Voluntary Measures Policy, the
State is responsible for assuring that the reductions credited in the SIP occur. The State would
need to make an enforceable SIP commitment to monitor, assess and report on the emission
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reductions resulting from the voluntary measure and to remedy any shortfalls from forecasted
emission reductions in a timely manner. That is, all reductions would need to be achieved by the
date the SIP relies on those reductions to meet any attainment, RFP/ROP or maintenance needs.
Further, under the policy the total of all voluntary mobile source measures (including cetane-
related measures) may not exceed 3 percent of the total reductions needed to meet any
requirements for RFP/ROP, attainment or maintenance. In the circumstance where the actual
emission reductions achieved are more than the amount estimated in the SIP, you may take credit
for the additional emission reductions provided it does not exceed the 3 percent cap on voluntary
mobile source measures and meets the other provisions of the Mobile Source Voluntary Measures
Policy. If you wish to have a SIP revision approved under the Mobile Source Voluntary Measures
Policy consult that policy for further information.3 Additional discussion of a voluntary cetane
improvement program can be found in Section F. Please note that the Mobile Source Voluntary
Measures Policy is also only guidance and final approval of a program under the policy will be
completed through notice-and-comment rulemaking on a SIP submittal.
(D) Permanent - The emission reduction would need to be permanent throughout the term that
the credit is granted.
(E) Adequately Supported - The State would need to demonstrate that it has adequate funding,
personnel, and other resources to implement the control measure on schedule.
How can the estimated emission reductions be used for SIP purposes?
For SIP RFP/ROP, attainment or maintenance strategies, the emission reductions which
are produced from the cetane-related measure can be used by applying the following criteria:
(A) 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.
(B) 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. In addition, the most recent emissions model approved by EPA should
be used in quantifying reductions from SIP control measures that are under development.
See "Guidance on Incorporating Voluntary Mobile Source Emission Reduction Programs in State
Implementation Plans (SIPs)," Memorandum from Richard D. Wilson, Acting Assistant Administrator for Air and
Radiation to EPA Regional Administrators, October 23, 1997. This memo can be found at
http://www.epa.gov/otaq/transp/traqvolm.htm.
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How can the emission reductions be used for transportation conformity purposes?
EPA's transportation conformity regulation (40 CFR parts 51 and 93) describes the
requirements for including emission reductions from on-road mobile control measures in a
conformity determination for a transportation plan, transportation improvement program (TIP), or
transportation project. If credit is obtained for a cetane improvement measure in a SIP's budget,
this does not preclude it from also being recognized in a transportation conformity determination.
To include NOX emission reductions from a cetane improvement measure in a regional
conformity analysis, the appropriate jurisdictions would need to be committed to the measure.
The appropriate level of commitment varies according to the requirements outlined in 40 CFR
93.122(a) which are described as follows:
(A) If the measure does not require a regulatory action to be implemented, it can be
included in a conformity determination if it is included in the transportation plan and TIP
with sufficient funding and other resources for its full implementation.
(B) If the measure requires a regulatory action to be implemented, it can be included in a
conformity determination if one of the following has occurred:
(1) The regulatory action for the measure is already adopted by the enforcing
jurisdiction (e.g., a State has adopted a rule to require a control measure);
(2) The measure has been included in an approved SIP; or
(3) There is a written commitment to implement the measure in a submitted SIP
with a motor vehicle emissions budget that EPA has found adequate.
(C) If the measure is not included in the transportation plan and TIP or the SIP, and it
does not require a regulatory action to be implemented, then it can be included in the
conformity determination's regional emissions analysis if the conformity determination
contains a written commitment from the appropriate entities to implement the measures.
Whatever the case, the emission reductions can only be applied in a conformity
determination for the time period or years in which the control measure will be implemented.
Written commitments must come from the agency with the authority to implement the measure.
The latest emissions model and planning assumptions must be used when calculating emission
reductions from the measure, according to 40 CFR 93.110 and 93.111.
Areas should utilize the conformity interagency consultation process to discuss the
methods and assumptions used to quantify the reductions from the measure. The conformity
determination should include documentation of the methodology, assumptions, and models that
were used to calculate emission reductions from cetane improvement measures, as well any
commitments that are necessary for implementation, as described above.
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What types of penalties can be assessed for not complying with CAA 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 CAA requirements are
subject to administrative, civil, and/or criminal enforcement under Section 113 of the CAA, as
well as to citizen suits under Section 304 of the CAA. The full range of penalty and injunctive
relief options remain available to the federal or State government (or citizens) bringing the
enforcement action.
What should a State submit to EPA to meet the criteria for incorporating a cetane improvement
control measure in a SIP?
The State should submit to EPA a written document which:
(A) Identifies and describes the cetane-related control measure and its implementation
schedule to reduce emissions within a specific time period;
(B) Contains estimates of emission reductions attributable to the measure, including all
relevant technical support documentation for the estimates. States must rely on the most
recent information available at the time the SIP is developed pursuant to Clean Air Act
§172(c)(3)and40CFR51.112(a);
(C) Contains federally enforceable procedures to implement, track, and monitor the
measure as applicable;
(D) Enforceably commits to monitor, evaluate, and report the resulting emission
reductions of the measure as applicable;
(E) Enforceably commits to remedy any SIP emission shortfall in a timely manner if the
measure does not achieve estimated emission reductions; and
(F) Meets all other requirements for SIP revisions under sections 110, 172 and 175(a) of
the CAA, as applicable.
(G) In the case of a cetane improvement program which is federally preempted, meets the
requirements of section 21 l(c)(4)(C) of the CAA.
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C. Per-vehicle NOx benefits of cetane improvement additives
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 vehicle using cetane-enhanced diesel
fuel. The fleet-wide NOx benefits may differ from the per-vehicle 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 issue will be addressed separately in Section D 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. One ready source is the EPA Technical Report entitled, "The
Effect of Cetane Number Increase Due To Additives on NOx Emissions from Heavy-Duty
Highway Engines."4 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 xRC]}
where
(%NOx)pv = Per-vehicle percent reduction in NOx emissions
k = Constant representing fraction of NOx inventory associated with cetane-sensitive
diesel trucks
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.
The per-vehicle NOx benefits of cetane improver additives 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:
EPA document number EPA420-R-03-002 (February 2003). This document can be downloaded from:
http://www.epa.gov/otaq/models/analysis.htm. See also EPA document number EPA420-S-03-001 (February 2003),
providing a response to comments submitted on the Draft Technical Report by independent peer reviewers.
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Constant 'k'
The default values in Appendix A may be used for highway engines. Alternatively,
you can generate values specific to the program area by calculating the fraction of
the heavy-duty highway diesel NOx inventory that derives from pre-2003 model
year engines for the calendar year of interest. For nonroad engines, constant 'k' is
equal to 1.0 until nonroad engines begin to be designed with cetane-insensitive
technologies.
RC: Reference cetane
This is the average natural (unadditized) cetane number of diesel fuel prior to
implementation of the cetane improvement program. RC does not represent the
cetane number of the unadditized base fuel after the program has been
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 (such as survey data
collected annually by the Alliance of Automobile Manufacturers), 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 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.
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AC: Additized cetane
This is the increase in cetane number that results from the addition of cetane
improvers to an unadditized base fuel. 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 = AC + BC - RC
(EQ2)
where
AC
AC
RC
BC
= 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 D.4)
= Reference cetane; the natural (unadditized) cetane number of diesel fuel prior to
implementation of the cetane program (see discussion above)
= 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
Type 1
Type 2
Type 3
Description
Total cetane
number standard
Cetane number
increase standard
Cetane additive
concentration
standard
How to determine AC prior to the
start of the program
Standard minus RC
Standard
Convert standard into a cetane
number increase using cetane
response functions (see Appendix B)
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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 fuels properties. However,
indirect measurements of fuels properties (i.e. proxy properties) introduce additional
uncertainties which may reduce the fleet-wide NOx benefits that can be claimed (see
Section D.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 ACBxRC]}
[(%NOx)pv]A = k x 100% x (1 . exp[ - 0.015151 x ACA
+ 0.000169 x ACA2
+ 0.000223 x ACA x RC ]}
(%NOx)pv = [(%NOx)pv]A - [(%NOx)pv]B
where
ACB =
ACA =
[(%NOx)pv]B =
[(%NOx)pv]A =
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-vehicle 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 D.4 and Appendix B, the presence of any
pre-existing cetane improver additives must also be taken into account. See Appendix B.
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D. Calculating in-use fleet-wide NOx benefits
As described in Section C, you should first have an estimate of the NOx benefits for a
single vehicle using diesel fuel with cetane improver additives before you can estimate the NOx
benefits for an entire in-use fleet. The per-vehicle benefit is the value (%NOx)pv calculated from
equation (EQ 1).
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):
ta = (%NOx)pv x F! x F2 x F3 x F4 (EQ 3)
where
(%NOx)fo = Fleet-wide percent reduction in NOx emissions
(%NOx)pv = Per-vehicle percent reduction in NOx emissions from equation (EQ 1)
Fj = Program factor representing 2-stroke engines
F2 = Program factor representing nonroad fuel
F3 = Program factor representing vehicle migration
F4 = Program factor representing the use of proxy fuel properties
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).
1. Program factor Fj
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.
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Conditions for determining the default value of program factor F,
Condition
General distribution of cetane improver additives through the
system of terminals, pipelines, and service stations
Use of cetane improver additives in a centrally-fueled fleet with
an identifiable and measurable number of 2-stroke and 4-stroke
engines
Default value of F,
1.0
(number of 4-stroke
engines)/(number of 2-
stroke and 4-stroke engines)
2. Program factor F2
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 Program5.
If the cetane improver additive program applies to both highway and nonroad engines,
then equations 3 (EQ 3) and 4 (EQ4, discussed in Section E below) should each be used twice to
calculate NOx tons reduced separately for both highway and nonroad, and the results summed.
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.
See fuels testing protocol at http://www.epa.gov/otaq/retrofit/retroprotocol.htm
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Conditions for determining the default value of program factor F2
Condition
Use of cetane improver additives in highway diesel fuel
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 supercedes equation (EQ 1)
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
Default value of F2
1.0
(volume of off-highway fuel
used in nonroad engines) /
(volume of off-highway fuel
used in nonroad engines and
heaters6)
0.0
3. Program factor F3
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 additive 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 additive 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.
"Heaters" include any fuel combustion unit designed to produce heat instead of work, including residential
heating units, industrial boilers, etc.
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Conditions for determining the default value of program factor F3
Condition
Use
of cetane improver additives in nonroad engines
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
51-300 mi2
301 - 1200 mi2
1201 -2800 mi2
2801 - 7800 mi2
7801 - 70,000 mi2
Above 70,001 mi2
Default value of F3
1.0
0.37
0.5
0.6
0.7
0.8
0.9
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. Program factor F4
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.
EPA memorandum of January 28, 2004, from D. Korotney to C. Petti, "Estimation of factors representing
vehicle migration for diesel vehicles."
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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 [(T10 - 215)2 - (T90 - 310)2]
+ 107 x [exp(-3.5 x (D - 0.85)) - 1]
+ 60 x [exp(-3.5 x (D - 0.85)) - I]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 C
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 B, 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. Examples include Ethyl's proprietary model SPEC and the Petrospec Cetane
2000 Instrument. 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
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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.
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 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 C
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
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E. Calculating tons of NOx reduced
The reduction in NOx tons that results from the cetane improver additive 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)fo = 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 tons 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.
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Conditions for determining a value for "Volume fraction affected" in equation (EQ 4)
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
Cetane improver additive program applies to specific fleets
within area X
Fuel consumed in area X ^
Fuel consumed in area Y
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. These fuel economy rates can be derived from
MOBILE model runs of the area(s) in question.
F. Mobile source voluntary cetane improvement programs
The discussion of quantification of NOx reductions in Sections C, D, and E above
presumes that the cetane improvement program has been mandated for a specific geographic area.
A State may also establish a mobile source voluntary cetane improvement program. Voluntary
measures are discussed generally in Section B in the context of the criteria that programs be
federally enforceable. In terms of the quantification of NOx reductions, a voluntary program
would be subject to all of the discussion in Sections C through E, with several exceptions.
For instance, equation (EQ 2) is designed to account for the possibility that refiners may
lower the natural cetane of their fuel if they know that cetane improvers will be added to the fuel
downstream of the refinery. This situation is more likely in the case of a mandatory program than
in a voluntary program, since refiners cannot count on the addition of cetane improvers under a
voluntary program. Thus if the cetane improver is being added at the terminal level on a
voluntary basis, it may be reasonable to avoid measuring the base cetane value BC and simply
assume that BC is equal to RC in equation (EQ 2). (However, it may be necessary to measure BC
as a means for calculating ACm, in which case the actual measured value of BC should still be
used in equation (EQ 2). See Section D.4 for additional discussion).
Mobile source voluntary cetane improvement programs introduce an additional level of
uncertainty associated with projecting the level of future participation. States should follow
EPA's Mobile Source Voluntary Measures Policy in making such projections. However, it may
be possible to reduce some of the uncertainty in projecting future participation by including
certain elements in a mobile source voluntary cetane improvement program. These elements
might include specified geographic boundaries within which the cetane improver additives should
be used, or a "standard" of Type 1, 2, or 3 as described in Section A.
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If the NOx inventory impacts of a mobile source voluntary cetane improvement program
are to be measured in real time (for instance, as a check on projections of future participation),
then the calculations discussed in Section E may need to be modified. This might be the case if
real-time tracking of cetane improver use produces measurements of a specific volume of diesel
fuel and a specific increase in cetane number due to the addition of cetane improver to that batch
of diesel fuel. The calculation of NOx tons reduced per equation (EQ 4) may require
modifications such as the following:
• The replacement of "Diesel NOx Inventory" with values representing fleet-wide
grams/gallon factors. Such values could be generated from a combination of
MOBILE model output in g/mi for each vehicle weight class and the application of
the default fuel economy rates given in Section E. MOBILE model output would
necessarily represent the geographic area within which the batch of fuel containing
cetane improver additives was needed and was actually used.
The replacement of "Volume fraction affected" with the volume of the additized
batch in question.
• The inclusion of a factor to convert grams into tons
G. Enforcement of cetane improvement programs
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
document does not specify all elements of such a program that might be necessary, but instead
lists several areas that should be considered. A State's compliance and enforcement program must
normally be approved by EPA's Office of Enforcement and Compliance Assurance as part of the
SIP approval process.
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
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• 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
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.
With respect to voluntary programs, please consult EPA's Mobile Source Voluntary
Measures Policy.
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Appendix A
EPA Technical Report EPA420-R-03-002 concluded that many new highway engine
designs are largely insensitive to changes in cetane. Cetane improver additives may have no
impact on NOx emissions for such engines. To account for this, the Technical Rport included a
factor 'k' in the equation giving the NOx impact as a function of the increase in cetane number due
to the use of additives. This factor 'k' represents the fraction of the highway diesel NOx inventory
that derives from cetane-sensitive engines. The Technical Report estimated values for factor 'k' by
assuming that 2003 and newer model year highway engines are insensitive to cetane. As a result,
the factor 'k' varies by calendar year, as shown in Table A-l. A more complete discussion of how
these factors were derived is given in the Technical Report.
Table A-l
Potential yearly weighting factors 'k' for additized cetane model
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
Fraction of diesel highway NOx inventory
which comes from cetane-sensitive engines
0.93
0.84
0.77
0.70
0.65
0.61
0.57
0.55
0.54
0.53
0.51
0.50
0.48
0.46
0.44
0.41
0.39
0.36
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Appendix B
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 function8
can be used:
CNI = a x BC036 x G°'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, 0.119 for DTBP
BC = Base cetane; cetane number of the unadditized fuel to which cetane improver is added
= RC (reference cetane) prior to implementation of the program
G = API gravity of the fuel to which cetane improver is added
= 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.
For typical in-use conventional diesel fuel, the response function above generates the
following curves9:
Equation 2 from SAE paper number 972901, "Prediction and Precision of Cetane Number Improver response
Equations," Thompson et al.
9 This graph represents an example of a specific base cetane (BC). The shape of the curve will differ
depending on the base cetane of the fuel to which cetane improver is added.
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Increase in cetane number
M M
o to i^ ch co o to
Additive impact on cetane number
^
2-EHN^^
^x^
S^ DTBP
/
/
) 0.1 0.2 0.3 0.4 0.5
Volume percent of additive
The use of a cetane response function such as that shown above is only valid when a
cetane improver is added to a fuel which does not already contain any cetane improver. However,
in a cetane improvement program it is possible that cetane improver may be added to a base fuel
which already contains cetane improver. In such cases, the use of a cetane response function to
predict the incremental increase in cetane number associated with the incremental increase in the
concentration of an additive would require two steps.
Using the cetane response function above as an example, the first application of the cetane
response function would use a value for the additive concentration C representing the pre-existing
cetane improver additives in the fuel, while the second application of the cetane response function
would use a value for the additive concentration C representing the total amount of cetane
improver additives in the fuel after some incremental amount of cetane improver had been added.
The difference between these two applications of the cetane response function would provide an
estimate of CNI that best represents the incremental increase in cetane number brought about
through the incremental addition of cetane improver. Mathematically, this process would appear
as follows:
rnvTn — r, Y T)(^036 Y /-^0.57 Y /-i 0.032 Y \n(-\ _i_ 1 H c Y (^ \
[LxiNlJg — a x jj^ x (j x ^B x m^l ~l~ 1 /.j x ^Bj
[CNI]A = a x BC036 x G°'57 x C/032 x ln(l + 17.5 x CA)
CNI=[CNI]A-[CNI]B
where
The concentration of cetane improver additives before implementation of
the cetane improvement program
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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
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Appendix C
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).
60
JD
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Appendix D
This appendix provides several illustrative examples of calculations of NOx reductions in
the context of State-run cetane improver additive programs. 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 these examples are not indicative of actual reductions that might be
expected, and thus they are not to be used directly in a SIP or for planning purposes. Parties
considering implementing local cetane additive programs should use values for the requisite
inputs that are specific to the program being contemplated.
All examples include the following assumptions:
• Only heavy-duty highway vehicles use cetane improver additives
• Programs apply to the State of Tennessee or the five counties comprising the Nashville
nonattainment area
• Programs apply during the months of May through September
• Example calculations represent calendar year 2007. 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) must be determined for that year.
• NOx inventories attributable to heavy-duty diesel vehicles prior to implementation of the
cetane improvement program are assumed to be
- 180 tons per day for the State of Tennessee
- 30 tons per day for the Nashville nonattainment area
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Example D. 1
Purpose
Mandatory program for generating SIP credits
Applicable area
Nashville nonattainment area
Program description
Fuel mandate applies to every gallon sold within the applicable counties
General distribution of the fuel, not targeted to specific fleets
Minimum total cetane standard of 50
Calculations
Vehicle migration factor: 0.8 (based on an applicable program area of 2804 square miles)
Attainment demonstration in 2007: default k-factor = 0.65
Pre-program natural cetane number is 47
(%NOx)pv = 0.65 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 = 0.81%
(%NOx)fo = (%NOx)pv x vehicle migration factor
(%NOx)fi¥ = 0.81%x 0.8
= 0.65%
NOx benefits of cetane additive program
= Diesel NOx inventory x (%NOx)fiv x Volume fraction affected
= 30 tons/day x 0.65% x 100%
= 0.2 tons/day
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Example D. 2
Purpose
Mandatory program for generating SIP credits statewide
Applicable area
State of Tennessee
Program description
Fuel mandate applies to every gallon sold within the State of Tennessee
General distribution of the fuel, not targeted to specific fleets
Minimum total cetane standard of 50
Calculations
Vehicle migration factor: 0.9 (based on an applicable program area of 41,000 square miles)
Attainment demonstration in 2007: default k-factor = 0.65
Absent survey data, pre-program natural cetane number assumed to be 46, with pre-existing
additives contributing an additional 1 cetane number, for a total cetane number of 47
[(%NOx)pv]B = 0.65 x 100% x (1 . exp[- 0.015151 x (1)
+ 0.000169 x (l)2
+ 0.000223 x (1) x (46) ]} = 0.31%
[(%NOx)pv]A = 0.65 x 100% x (1 . exp[- 0.015151 x (50 - 46)
+ 0.000169 x (50 -46)2
+ 0.000223 x (50 - 46) x (46) ]} = 1.09%
(%NOx)pv = [(%NOx)pv]A - [(%NOx)pv]B = 1.09% - 0.31%
(%NOx)pv = 0.78%
(%NOx)fi¥ = (%NOx)pv x vehicle migration factor
(%NOx)fo = 0.78% x 0.9
= 0.70%
NOx benefits of cetane additive program
= Diesel NOx inventory x (%NOx)fiv x Volume fraction affected
= 180 tons/day x 0.70% x 100%
= 1.26 tons/day
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Example D. 3
Purpose
Mobile source voluntary program for generating SIP credits
Applicable area
Nashville ozone nonattainment area
Program description
Parties may use cetane improver additives at any concentration for any volume of fuel
General distribution of the fuel, not targeted to specific fleets
State has a non-binding commitment from two terminals to use cetane improver additives
- These terminals have historically supplied 20% of Nashville's highway diesel fuel
- These terminals indicate they will additize 80% of their fuel
- The terminals indicate they plan to use 0.05vol% cetane improver additive (2-EHN)
Calculations
Vehicle migration factor: 0.8 (based on an applicable program area of 2804 square miles)
Attainment demonstration in 2007: default k-factor = 0.65
Pre-program natural cetane number is 47
Increase in cetane number assumed to be 3 (based on cetane response function for 0.05vol% of the
additive 2-EHN)
(%NOx)pv = 0.65 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 = 0.81%
(%NOx)fi¥ = (%NOx)pv x vehicle migration factor
(%NOx)fi¥ = 0.81% x 0.8
= 0.65%
NOx benefits of cetane additive program
= Diesel NOx inventory x (%NOx)fo x Volume fraction affected
= 30 tons/day x 0.65% x (20% x 80%)
= 0.03 tons/day
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