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Fuel Regulations

USA: Fuels: Diesel Fuel

US Federal Diesel Fuel

Regulatory Requirements

Diesel fuel for mobile applications in the US has limits on sulfur and aromatics [6143]. Sulfur is limited to a maximum of 15 ppm while the limit on aromatics requires either a minimum cetane index of 40 or a maximum aromatic content of 35% volume. These limits have applied to on-road diesel fuel since 2006, nonroad diesel fuel since 2010 and locomotive and marine fuel since 2012. The requirements for nonroad, locomotive and marine fuels were adopted with the EPA Tier 4 emission standards for nonroad engines. Diesel fuel not intended for on-road use is required to be dyed red.

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Exceptions to the 15 ppm ultra-low sulfur diesel (ULSD) requirement exist for 500 ppm LM (locomotive and marine) diesel and ECA marine fuel. Exemptions also exist for distillate fuel used in aircraft and fuels for which a national security or research and development exemption has been approved or fuel that is exported from the USA. Heating oil and fuel for stationary engines, distillate and residual fuels or residual fuel blends, are also exempt. Kerosene is also exempt unless it is used in motor vehicles or nonroad mobile equipment.

On-Road Diesel

Effective 1993.10, in parallel with the 1994 emission standards for heavy-duty engines, a sulfur limit of 0.05% (500 ppm) superseded the earlier ASTM specification of 0.5%. The 0.05% sulfur fuel was termed “low sulfur” diesel. In addition, the EPA required either a minimum cetane index of 40 or a maximum aromatic content of 35%. These requirements applied to all areas of the USA except Alaska.

Further significant reductions of sulfur levels in highway diesel fuel were legislated by the EPA as a part of the 2007-2010 emission regulations for heavy-duty engines [66 FR 5135-5193, 18 January 2001]. Fuel of maximum sulfur content of 15 ppm (wt)—called “ultra low sulfur diesel” (ULSD)—was required to be available beginning in mid-2006. Other fuel properties remain unchanged. The ULSD fuel was legislated as a “technology enabler”, to facilitate the use of sulfur-sensitive catalyst-based emission technologies on MY2007 and later heavy-duty engines, as well as on Tier 2 compliant light duty diesel vehicles.

The ULSD implementation schedule was as follows:

  • 1 June 2006: Refinery level and importers
  • 1 September 2006: Terminal level (downstream of the refinery/importer, except for retail outlets and wholesale consumers)
  • 15 October 2006: Retail level (including wholesale consumers)

To ease the transition, diesel fuel meeting a 22 ppm sulfur limit was allowed as ULSD at the pump until October 15, 2006. A number of additional flexibility provisions were included to lessen the impact of the specification change on refiners:

  • Temporary compliance option: Refiners were allowed to produce up to 20% of their output of highway diesel fuel at 500 ppm until 31 December 2009 (31 May 2010 if the refiner banked sufficient credits). The 500 ppm and 15 ppm S fuel batches were to be kept separate in the distribution system.
  • Credits: Early credits could be generated for manufacturing 15 ppm S fuel ahead of the regulatory schedule. Credits could be generated until 31 December 2009, and were to be used by 31 May 2010. Early batches of the 15 ppm S fuel were to be segregated in the distribution system and sold at retail as 15 ppm diesel to be eligible for credits.
  • Averaging, Banking and Trading (ABT) program
  • Alternative requirements for Alaska and Hawaii
  • Increased flexibility for small refineries

Nonroad Diesel

Historically, the maximum sulfur content in nonroad and highway diesel fuels was the same at 0.5%, as specified by ASTM standards. Since the introduction of low sulfur fuel (500 ppm) for highway use by the EPA in 1993, the specifications for highway and nonroad fuels became different. Sulfur in nonroad fuels remained unregulated at the ASTM specification of 0.5% (max) and an average in-use sulfur level of about 0.3% = 3,000 ppm (with the exception of California, where low sulfur CARB diesel has been widely used for nonroad engines).

EPA Tier 4 Fuel Program. In May 2004, EPA adopted the Tier 4 nonroad emission standards and diesel fuel rule, which mandates the following sulfur limits in nonroad fuels:

  • 500 ppm effective June 1, 2007 for nonroad (NR), locomotive and marine (LM) fuels
  • 15 ppm (ULSD) effective June 1, 2010 for nonroad fuel, and June 1, 2012 for locomotive and marine fuels

Heavy fuel oils (HFO) used in Category 2 and Category 3 marine diesel engines are exempt.

The Tier 4 program also extends the cetane index/aromatics specification for highway fuel—a minimum cetane index of 40 or a maximum of 35% (vol.) aromatics—to NRLM diesel fuel.

Implementation and Flexibilities. The above dates applied to the refinery/importer level. Terminals had to meet the 500 ppm deadline by August 2007 and retailers/wholesale customers by October 2007. Likewise, for the ULSD deadlines in June 2010/2012, the deadline for terminals was two months later (August), and retailers four months later (October).

A number of flexibility provisions existed during the transitional period, such as:

  • Diesel sulfur credit banking and trading program—Sulfur credits could be accumulated for early introduction of low sulfur fuels.
  • Small refinery flexibility—Small refiners and suppliers who accumulated sulfur credits were allowed a delay; they had to convert all NRLM fuel to 500 ppm in June 2010, and to 15 ppm in June 2014. An exception was the Northeast/Mid-Atlantic region, where all NRLM fuel had to be ≤ 500 ppm sulfur by June 2007.
  • Interface/transmix fuel provisions—Material generated within the distribution system could be sold into the NRLM diesel fuel markets as follows:
    1. High sulfur NRLM diesel fuel or heating oil from June 1, 2007 through May 31, 2010
    2. 500 ppm sulfur NRLM diesel fuel or heating oil from June 1, 2010 through May 31, 2014
    3. 500 ppm sulfur LM diesel fuel or heating oil after June 1, 2014 (i.e., indefinitely)
  • Special provisions for Alaska and the Territories

Other Federal Regulatory Provisions

Fuel Additives

Diesel fuel additives for use in motor vehicle fuel subject to 15 ppm sulfur standards must have a maximum sulfur content of 15 ppm. Additives with a sulfur content exceeding 15 ppm are allowed provided that (1) the additive is used in the diesel fuel in a quantity less than 1% (vol.), (2) the additive is not used or intended for use by the ultimate consumer and (3) product transfer documentation meets EPA requirements.

Lubricating Oil

The EPA has not mandated any sulfur limits in engine lubricating oils. EPA estimated that the sulfur contribution from normal oil consumption is less than 2 ppm on an equivalent fuel sulfur basis.

Used Motor Oil

Introducing used motor oil, or used motor oil blended with diesel fuel, into the fuel system of MY2007 or later diesel motor vehicles or model year 2011 or later nonroad diesel vehicles or engines (not including locomotive or marine diesel engines) is not allowed.

ASTM D975 Specification

In addition to the federal regulatory requirements, diesel fuel must meet a number of other requirements in order for it to be used successfully in diesel engines. In the USA, these other properties are specified in ASTM D975, Standard Specification for Diesel Fuel Oils. While diesel fuel is not mandated at the federal level to meet this standard, many states have legislation in place that does.

Since 2004, the D975 standard has covered seven grades of diesel, Table 1. Heavier fuel oils Grade 5 and 6 (residual), which are used primarily for heating purposes, are described by ASTM D396.

Table 1
Diesel fuel grades
GradeDescriptionMax Sulfur
No. 1-D S15A special-purpose, light middle distillate fuel for use in diesel engine applications requiring higher volatility than that provided by No. 2-D fuels.15 ppm
No. 1-D S500500 ppm
No. 1-D S50005000 ppm
No. 2-D S15A general-purpose, middle distillate fuel for use in diesel engines, especially in applications with conditions of varying speed and load.15 ppm
No. 2-D S500500 ppm
No. 2-D S50005000 ppm
No. 4-DA heavy distillate fuel, or a blend of distillate and residual oil, for low- and medium-speed diesel engines in applications involving predominantly constant speed and load. 

The Sxxx designation was first adopted in the D975-04 edition of the standard to distinguish grades by sulfur content. The S5000 grades correspond to the “regular” sulfur grades, the previous No. 1-D and No. 2-D. S500 grades correspond to the previous “Low Sulfur” grades (D975-03). S15 grades are commonly referred to as “Ultra-Low Sulfur” grades or ULSD.

Fuel grade 1-D is a special purpose volatile distillate fuel. Fuel grade 2-D is a general-purpose, middle distillate fuel for automotive diesel engines. Table 2 lists the detailed requirements of the No. 1 and No. 2 grades [6195].

Table 2
US diesel fuel specification (ASTM D975)
Fuel PropertyUnitSpecificationASTM Referee Test Method
Grade 1-DGrade 2-D
Cetane Number 40 (min)a40 (min)aD613a
Eitherb (1) Cetane Index 40 (min)c40 (min)cD976/D4737d
Or (2) Aromatics% (vol.)35 (max)c35 (max)cD1319/D5186d
Cloud Point or TFT/CFPP°CeeD2500/D4539/D6371
Distillation 90% (vol.)°C-
288 (max)
282f (min)
338 (max)
Total Sulfur
- Grade S15µg/g15 (max)15 (max)D5453
- Grade S500% (mass)0.05 (max)0.05 (max)D2622
- Grade S5000% (mass)0.50 (max)0.50 (max)D2622
Flashpoint°C38 (min)52f (min)D93
Kinematic Viscosity @ 40°Cmm2/s1.3 (min)
2.4 (max)
1.9f (min)
4.1 (max)
Water & Sediment% (vol.)0.05 (max)0.05 (max)D2709
Ramsbottom Carbon on 10% Residue% (mass)0.15 (max)0.35 (max)D524
Ash% (mass)0.01 (max)0.01 (max)D482
Copper Strip Corrosion (3h @ 50°C) No. 3 (max)No. 3 (max)D130
Lubricity, HFRR @ 60°Cmicron520 (max)520 (max)D6079/D7688
ConductivitygpS/m or Conductivity Units (C.U.)25 (min)25 (min)D2624/D4308
Biodiesel% (vol.)5 (max)5 (max)
a - D4737 can be used as an approximation
b – only applicable in the United States
c – apply to Grades S15 and S500 only
d – these test methods are specified by federal regulation. Refer to 40 CFR Part 1090 to verify the acceptable version of the test methods.
e – agreed upon between buyer and seller for the intended use and expected ambient temperatures
f - if cloud point of less than -12°C is specified, the minimum viscosity at 40°C shall be 1.7 mm2/s, the minimum flash point shall be 38°C and the 90% distillation point shall be waived
g – The electrical conductivity is measured at the time and temperature of the fuel at delivery. The 25 pS/m minimum conductivity requirement applies when instances of high velocity transfer (1.96 to 7.0 m/s depending on transfer conditions) occur into a mobile transport container.

In 2005 a diesel fuel lubricity limit of 520 µm wear scar diameter (HFRR test, ASTM D6079) was added to ASTM D975 to minimize lubricity-related problems with ultra-low sulfur fuels. Meeting this requirement has forced the widespread use of lubricity additives. In 2008, a conductivity requirement was added to minimize the risk of static discharge when transferring fuel into a tank and has increased the use of static dissipater additives.

In 2008, the standard was modified to allow up to 5% biodiesel. No specific labeling of the fuel is required so that the purchaser may not know whether or not the fuel contains biodiesel.

California Diesel Fuel

CARB Diesel

Regulatory requirements for California diesel fuel include a sulfur limit of 15 ppm, a limit on aromatics of 10% or equivalent and a lubricity requirement of a maximum HFRR wear scar diameter of 520 µm. California diesel fuel must also meet the requirements of ASTM D975. Diesel fuel requirements are contained in CCR 13 § 2280 to § 2285 and CCR 4 § 4140 to § 4149 [6196][6197]. This fuel, commonly referred to as CARB diesel, is mandatory for use in a variety of applications including both highway and off-road vehicles. The limits and applicability of CARB diesel fuel have evolved as follows:

  • 1993.10—Sulfur limited to a maximum of 500 ppm and aromatics to 10% or equivalent. Applicable to on- and off-road vehicles but not stationary engines, locomotives and marine vessels.
  • 2004.12—CARB diesel requirement extended to stationary sources (applies to on- and off-road motor vehicles and nonvehicular sources other than locomotives and marine vessels).
  • 2005.01—CARB diesel required to meet a lubricity requirement of a maximum wear scar diameter of 520 microns by ASTM D6079, the High Frequency Reciprocating Rig (HFRR).
  • 2006.06—Sulfur in CARB diesel limited to 15 ppm.
  • 2007.01—CARB diesel requirement extended to intrastate locomotives and marine engines (within the SCAQMD, CARB diesel must be sold to harborcraft operators beginning 2006.01).
  • 2016.01—The definition of CARB diesel expanded to include fuels containing biodiesel blends up to B5, fuels that include hydrocarbons derived from non-petroleum sources and fuels containing additives intended to reduce criteria pollutants and toxic air contaminants.

In a study comparing CARB diesel to federal diesel, CARB diesel was found to provide about a 6% NOx reduction for a range of heavy-duty diesel engines [6200][6198][6199].

By 2004, about 90% of diesel fuel sold in California was Low Sulfur 2-D complying with the CARB diesel requirements. Average properties of diesel fuel in California, before and after 1993 are compared with those of the federal diesel in Table 4.

Table 4
Average properties of reformulated diesel fuel
Fuel Property California USA(1)
Pre-19931999Summer 2007 1999Summer 2007
Sulfur, ppmw 440(2)140(3)4 3606
Aromatic hydrocarbons, % vol. 351916.7 3528.6
Cetane number 435051.8 4546.3
Polycyclic aromatics, % wt. n/a3 n/a
Nitrogen, ppmw n/a150 110
1 - AAMA National Surveys for on-road vehicles only
2 - For Los Angeles area only, greater than 3000 ppmw in rest of California
3 - About 20% of total California volume was less than 15 ppmw

Aromatics. The California fuel regulations allow significant flexibility in meeting the limit on aromatics. These options are:

  • Limit aromatics to a maximum of 10% (vol.).
  • Designated Alternative Limit. Limit aromatics to a designated alternative limit that can exceed 10% (vol.). The volume of aromatic hydrocarbons in the fuel with an aromatic limit above 10%, however, must be fully offset with fuel having an aromatic content of less than 10% within 90 days.
  • Certified Diesel Fuel Formulation. Certify a fuel formulation that meets ASTM D975 by demonstrating that the exhaust emissions are equivalent to those with the reference fuel shown in Table 5. The emission equivalency is determined on a 1991 Detroit Diesel Series 60 heavy-duty diesel engine tested over the FTP transient test.
  • Designated Equivalent Limits. Produce a fuel that meets the designated equivalent limits shown in Table 6. These limits are based on the average properties of certified formulations. Designated Equivalent Limits Diesel may contain 2-ethyl-hexyl nitrate but does not contain biodiesel or di-tert-butyl peroxide (DTBP).
  • Small Refiner Exemptions. Small California refiners (≤ 55,000 bbl/day crude oil capacity) have additional flexibility that allows them to limit aromatics to 20%, use a designated alternative limit only if aromatics exceed 20% or certify a diesel fuel formulation with a reference fuel having properties different from those for larger refiners.
Table 5
California diesel reference fuel
Fuel PropertyUnitSpecification
Cetane Number (natural)48a (min)
Sulfurppmw15e (max)
Aromatics% (vol.)10b (max)
Polycyclic Aromatics% (wt.)1.4c (max)
Nitrogenppmw10d (max)
API Gravity   33-39
Kinematic Viscosity @ 40°C mm2/s 2.0-4.1
Flashpoint °F (°C) 130 (54) (min)
Distillation Range °F (°C)  
  IBP 340-420 (170-215)
  10% point 400-490 (205-255)
  50% point 470-560 (245-295)
  90% point 550-610 (290-320)
  EP 580-660 (305-350)
a - 47 cetane for small refiners
b - 20% aromatics for small refiners
c - 4% polycyclic aromatics for small refiners
d - 90 ppm nitrogen for small refiners
e - 500 ppm sulfur before August 14, 2004
Table 6
Designated equivalent limits
Fuel PropertyUnitSpecification
Cetane Number 53 (min)
Sulfurppmw15a (max)
Aromatics% (vol.)21.0 (max)
Polycyclic Aromatics% (wt.)3.5 (max)
Nitrogenppmw500 (max)
API Gravity36.9 (min)
a - 160 ppm sulfur before June 1, 2006

Given the flexibility options, almost all California diesel fuels are certified alternative formulations or fuels that meet the designated equivalency limits.

Alternative Diesel Fuels

California’s Alternative Diesel Fuel Regulation was phased-in between 2016.01 and 2018.01 with a focus on ensuring there are no unwanted NOx increases from pre-2010 on-road or pre-Tier 4 nonroad diesel engines due to the increased use of biodiesel [6201]. The regulation imposes in-use measures intended to mitigate potential increases in emissions on biodiesel blends above B5. All blends, including B5, are subject to reporting requirements. Exemptions to the regulation are granted to centrally fueled fleets and retail fueling stations that supply fuel primarily to vehicles equipped with new technology diesel engines (NTDE). NTDEs are defined as those meeting 2010 ARB emission standards for on-road heavy duty diesel engines, those meeting Tier 4 emission standards for non-road compression ignition engines or those equipped with a Diesel Emissions Control Strategy (DECS) verified by CARB. The regulation will be sunset when 90% of vehicle operation in California is by NTDEs.

Commercial formulations with biodiesel include blends up to B20 with diesel fuel and/or renewable diesel. Renewable diesel is not considered to be an alternative diesel fuel and is subject to the regulations governing CARB diesel. Blends up to B5 must meet the requirements of ASTM D975 and blends from B6 to B20 must meet the requirements of ASTM D7464.

Commercialization of biodiesel blends involves a three-stage process consisting of a pilot program (Stage 1), the development of fuel specification (Stage 2) and commercial sales (Stage 3). Fuels that reach stage 3 are further categorized depending on the potential emissions impact. Fuels with a potential emissions impact are referred to as Stage 3A fuels and are subject to in-use mitigation requirements to mitigate the NOx emissions impact. Fuels without an adverse NOx emissions impact are referred to as Stage 3B fuels and are not subject to in-use mitigation requirements.

For biodiesel blends up to B20, in-use NOx control measures are needed if the blend level exceeds the values shown in Table 7. The blend level, referred to as the Pollutant Control Level or NOx Control Level, depends on the feedstock used in the biodiesel production process and is characterized by “saturation level”. The saturation level is determined by the unadditized cetane number of the B100 used in the biodiesel blend.

Table 7
Biodiesel blend level above which in-use measures are required
Feedstock SaturationTime of YearBlend Level
Low (biodiesel cetane number < 56)Apr 1 to Oct 31B5
Nov 1 to Mar 31B10
High (biodiesel cetane number ≥ 56)Jan 1 to Dec 31B10

Suitable in-use measures for the blend levels above those shown in Table 7 include:

  • use of an approved additive, di-tert-butyl peroxide (DTBP), at levels detailed in the regulation,
  • use of approved Alternative Diesel Fuel (ADF) formulations,
  • certification of ADFs resulting in emissions equivalence with diesel. The emissions equivalence is determined using a 2004-2006 Cummins ISM370 heavy-duty diesel engine and is relative to the reference fuel in Table 5. Certified alternative fuels can be formulated by using blend stocks or by using additives.

Certified ADFs, fleet exemptions and retail site exemptions are issued an executive order [6202]. Retail site exemptions require a demonstration that the volume ratio of diesel exhaust fluid (DEF) to diesel fuel exceeds 2.70%.