ASTM D976-21e1

Standard Test Method for Calculated Cetane Index of Distillate Fuels

SIGNIFICANCE AND USE
4.1 The Calculated Cetane Index is one tool available for estimating ASTM cetane number where a test engine is not available for determining this property. It may be employed for approximating cetane number where the quantity of sample is too small for an engine rating. In cases where the cetane number of a fuel has been initially established, the index is useful as a cetane number check on subsequent samples of that fuel, provided its source and mode of manufacture remain unchanged.  
4.2 Test Method D4737 may also be used to approximate the ASTM cetane number of diesel fuels.  
4.2.1 Procedure A of Test Method D4737 was developed as a result of a larger degree of offset between Test Method D976 Cetane Index and the results of Test Method D613 over the entire range of the correlation. Generally, it has been found that use of Test Method D4737 results in less offset than use of Test Method D976, but there can be specific cases where this is not true.  
4.2.2 Procedure A of Test Method D4737 is recommended to estimate the cetane number of diesel fuels with sulfur contents above 500 ppm or No. 1–D diesel fuels.  
4.2.3 Procedure B of Test Method D4737 is recommended to estimate the cetane number of No. 2–D diesel fuels with sulfur contents at or below 500 ppm.  
4.3 Calculated Cetane Index, as described in Test Method D976–80, is recognized by the United States EPA as an alternative method to meet the U.S. Federal Diesel aromatics limit for diesel fuels containing less than 500 ppm sulfur. The equation for Calculated Cetane Index in Test Method D976–80 and in this version (D976–06) of the test method are the same.
SCOPE
1.1 This test method covers the Calculated Cetane Index formula, which represents a means for directly estimating the ASTM cetane number of distillate fuels from API gravity and mid-boiling point. The index value, as computed from the formula, is termed the Calculated Cetane Index.2  
1.2 The Calculated Cetane Index is not an optional method for expressing ASTM cetane number. It is a supplementary tool to estimate cetane number when used with due regard for its limitations.  
1.3 The Calculated Cetane Index formula is particularly applicable to straight-run fuels, catalytically cracked stocks, and blends of the two.
Note 1: This test method is temporarily retained because the proposal to the U.S. EPA to control diesel fuel aromatics concentrations via a 40 Calculated Cetane Index minimum is based on the correlation between Test Method D976 and aromatics concentration. Test Method D4737 is the preferred method as estimator of cetane number.  
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Oct-2021
• ICS: 75.160.20 - Liquid fuels
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.E0 - Burner, Diesel and Non-Aviation Gas Turbine Fuels

Relations

• Refers: ASTM D613-24 - Standard Test Method for Cetane Number of Diesel Fuel Oil
• Effective Date: 01-Feb-2024
• Refers: ASTM D4175-23a - Standard Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 15-Dec-2023
• Refers: ASTM D86-23a - Standard Test Method for Distillation of Petroleum Products and Liquid Fuels at Atmospheric Pressure
• Effective Date: 01-Dec-2023
• Refers: ASTM D86-23ae1 - Standard Test Method for Distillation of Petroleum Products and Liquid Fuels at Atmospheric Pressure
• Effective Date: 01-Dec-2023
• Refers: ASTM D4175-23e1 - Standard Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 01-Jul-2023
• Refers: ASTM D2887-23 - Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography
• Effective Date: 01-Jul-2023
• Refers: ASTM D2887-19 - Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography
• Effective Date: 01-Jul-2019
• Refers: ASTM D613-17c - Standard Test Method for Cetane Number of Diesel Fuel Oil
• Effective Date: 01-Dec-2017
• Refers: ASTM D613-17a - Standard Test Method for Cetane Number of Diesel Fuel Oil
• Effective Date: 01-May-2017
• Refers: ASTM D613-17 - Standard Test Method for Cetane Number of Diesel Fuel Oil
• Effective Date: 01-Mar-2017
• Refers: ASTM D613-16a - Standard Test Method for Cetane Number of Diesel Fuel Oil
• Effective Date: 01-Oct-2016
• Refers: ASTM D86-16 - Standard Test Method for Distillation of Petroleum Products and Liquid Fuels at Atmospheric Pressure
• Effective Date: 01-Jul-2016
• Refers: ASTM D613-16 - Standard Test Method for Cetane Number of Diesel Fuel Oil
• Effective Date: 01-Jun-2016
• Refers: ASTM D2887-15 - Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography
• Effective Date: 01-Jul-2015
• Refers: ASTM D613-15a - Standard Test Method for Cetane Number of Diesel Fuel Oil
• Effective Date: 01-Apr-2015

Overview

ASTM D976-21e1: Standard Test Method for Calculated Cetane Index of Distillate Fuels provides a recognized method for estimating the ASTM cetane number of distillate fuels using the Calculated Cetane Index formula. Cetane number is a critical property influencing diesel engine combustion quality. This standard is widely adopted to offer a practical alternative where direct engine-based determination, such as ASTM D613, is unavailable or sample size is limited.

The Calculated Cetane Index relies on easily measurable parameters - API gravity and mid-boiling point - making it valuable for laboratories and fuel quality assessments. However, it is important to note that it is not a replacement for ASTM cetane number but serves as a supplementary estimation tool.

Key Topics

• Purpose and Scope

  • Assesses the suitability of distillate fuels for diesel engines by providing a calculated estimate of cetane number.
  • Applicable to straight-run fuels, catalytically cracked stocks, and blends.
  • Frequently used when only a small sample is available or for routine cetane number checks of fuels with consistent composition and production.

• Testing Methodology

  • Uses a formula based on the API gravity (measured by ASTM D287, D1298, or D4052) and mid-boiling point (from ASTM D86, or data corrected via ASTM D2887).
  • The resulting value, termed "Calculated Cetane Index," is derived without the need for a test engine.
  • The method features a nomograph for faster calculation and clear rounding/reporting instructions for consistency.

• Limitations

  • Not suitable for fuels containing cetane-improving additives, pure hydrocarbons, synthetic fuels, or products from coal, shale oils, or tar sands.
  • Less accurate for crude oils, residuals, or products with end-point volatility below 500 °F.
  • Primarily optimized for diesel fuels within the typical range of 30-60 cetane numbers.

• Regulatory Recognition

  • Recognized by the United States EPA as an alternative for meeting aromatics concentration limits for diesel fuels with less than 500 ppm sulfur.
  • Temporarily retained for specific regulatory applications, although ASTM D4737 is now the preferred estimator for cetane number, especially for fuels with varying sulfur content.

Applications

• Fuel Quality Control

  • Enables refineries, laboratories, and terminal operators to estimate cetane number during blending and production.
  • Provides a fast check method for routine analysis where reference engine testing is impractical.
  • Useful in verifying consistency in fuel quality for ongoing production batches when source and process remain unchanged.

• Regulatory Compliance

  • Assists fuel suppliers in demonstrating compliance with federal diesel fuel standards, particularly for diesel aromatics and sulfur content as mandated by the EPA.

• Product Development and Blending

  • Essential for formulating diesel blends to meet specific cetane requirements without the resource commitment of engine-based methods.
  • Frequently used for straight-run and catalytically cracked diesel blending.

Related Standards

• ASTM D613: Test Method for Cetane Number of Diesel Fuel Oil (engine method - primary reference test)
• ASTM D4737: Test Method for Calculated Cetane Index by Four Variable Equation (preferred alternative for some fuel types)
• ASTM D86: Test Method for Distillation of Petroleum Products and Liquid Fuels at Atmospheric Pressure (for boiling point)
• ASTM D287, D1298, D4052: Test Methods for Determination of API Gravity and Density
• ASTM D2887: Test Method for Boiling Range Distribution by Gas Chromatography
• ASTM D4175: Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants

Implementing ASTM D976-21e1 ensures streamlined, practical cetane number estimation for distillate (diesel) fuel quality assessment within both regulatory and operational frameworks. For sulfur content above 500 ppm or modern low-sulfur diesels, always confirm preferred standards with current regulatory guidance.

Keywords: cetane index, calculated cetane index, distillate fuels, diesel fuel, ASTM D976, cetane number estimation, fuel quality standards, API gravity, mid-boiling point, petroleum product testing.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
´1
Designation: D976 − 21
Standard Test Method for
Calculated Cetane Index of Distillate Fuels
This standard is issued under the fixed designation D976; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
ε NOTE—Editorially updated adjunct information in February 2023.
1. Scope* 2. Referenced Documents
1.1 This test method covers the Calculated Cetane Index 2.1 ASTM Standards:
formula, which represents a means for directly estimating the D86 Test Method for Distillation of Petroleum Products and
ASTM cetane number of distillate fuels from API gravity and Liquid Fuels at Atmospheric Pressure
mid-boiling point. The index value, as computed from the D287 Test Method for API Gravity of Crude Petroleum and
formula, is termed the Calculated Cetane Index. Petroleum Products (Hydrometer/Method)
D613 Test Method for Cetane Number of Diesel Fuel Oil
1.2 The Calculated Cetane Index is not an optional method
D1298 Test Method for Density, Relative Density, or API
for expressing ASTM cetane number. It is a supplementary tool
Gravity of Crude Petroleum and Liquid Petroleum Prod-
to estimate cetane number when used with due regard for its
ucts by Hydrometer Method
limitations.
D2887 Test Method for Boiling Range Distribution of Pe-
1.3 The Calculated Cetane Index formula is particularly
troleum Fractions by Gas Chromatography
applicable to straight-run fuels, catalytically cracked stocks,
D4052 Test Method for Density, Relative Density, and API
and blends of the two.
Gravity of Liquids by Digital Density Meter
NOTE 1—This test method is temporarily retained because the proposal
D4175 Terminology Relating to Petroleum Products, Liquid
to the U.S. EPA to control diesel fuel aromatics concentrations via a 40
Fuels, and Lubricants
Calculated Cetane Index minimum is based on the correlation between
D4737 Test Method for Calculated Cetane Index by Four
Test Method D976 and aromatics concentration. Test Method D4737 is the
preferred method as estimator of cetane number.
Variable Equation
2.2 ASTM Adjuncts:
1.4 This standard does not purport to address all of the
Nomograph for Calculated Cetane Index
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
3. Terminology
priate safety, health, and environmental practices and deter-
3.1 For definitions of terms used in this test method, refer to
mine the applicability of regulatory limitations prior to use.
Terminology D4175.
1.5 This international standard was developed in accor-
dance with internationally recognized principles on standard-
4. Significance and Use
ization established in the Decision on Principles for the
4.1 The Calculated Cetane Index is one tool available for
Development of International Standards, Guides and Recom-
estimating ASTM cetane number where a test engine is not
mendations issued by the World Trade Organization Technical
available for determining this property. It may be employed for
Barriers to Trade (TBT) Committee.
approximating cetane number where the quantity of sample is
too small for an engine rating. In cases where the cetane
number of a fuel has been initially established, the index is
This test method is under the jurisdiction of ASTM Committee D02 on
useful as a cetane number check on subsequent samples of that
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of
Subcommittee D02.E0 on Burner, Diesel and Non-Aviation Gas Turbine Fuels. fuel, provided its source and mode of manufacture remain
Current edition approved Nov. 1, 2021. Published November 2021. Originally
unchanged.
approved in 1966. Last previous edition approved in 2016 as D976 – 06 (2016).
DOI: 10.1520/D0976-21E01.
2 3
A method of calculating cetane index was developed by the Diesel Fuels For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Division, Coordinating Fuel and Equipment Research Committee of the Coordinat- contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
ing Research Council. See Young, H. D., “Methods for Estimating Cetane Number,” Standards volume information, refer to the standard’s Document Summary page on
Proceedings, PPIRA, American Petroleum Institute, Vol. 30 M [III], 1950. This the ASTM website.
method was revised in 1960 by Research Division I of Committee D02 to conform Available from ASTM International Headquarters. Order Adjunct No.
to the revised Test Method D613. ADJD0976-E-PDF. Original adjunct produced in 1989.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
D976 − 21
4.2 Test Method D4737 may also be used to approximate 5.3 Test Method D2887 may be used as an alternative to
the ASTM cetane number of diesel fuels. Test Method D86 to determine the mid-boiling temperature of
4.2.1 Procedure A of Test Method D4737 was developed as the fuel.
a result of a larger degree of offset between Test Method D976 5.3.1 If Test Method D2887 is used, convert the data to
Cetane Index and the results of Test Method D613 over the estimated Test Method D86 data following Appendix X5,
entire range of the correlation. Generally, it has been found tha
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