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SAE - J2978

Truck and Bus Coastdown Procedure

active, Most Current
Organization: SAE
Publication Date: 1 June 2021
Status: active
Page Count: 43
scope:

This document describes a rigorous engineering test procedure that utilizes industry-accepted data collection and statistical analysis methods to determine the road load and to estimate the aerodynamic drag area of trucks and buses weighing more than 10000 pounds. The test procedure may be conducted on a test track or on a public road under controlled conditions and supported by extensive data collection and data analysis constraints. The estimated aerodynamic-drag-area result represents a single-speed and single-yaw-angle condition. Test results that do not rigorously follow the method described herein shall not be represented as an SAE J2978 result.

Document Use

Users of this document will generally fall into one of two groups: those seeking to generate road load force data, and others wishing to estimate aerodynamic drag. Because the steps to achieve either objective are similar, this document describes the shared guidelines and processes while notifying the user where differences exist. All users are therefore encouraged to thoroughly review this document before executing a test program.

This procedure provides background information and specific guidance to generate road load data and/or a single-speed single-yaw-angle drag-area value for a truck or bus. While coastdown procedures have been used extensively in characterizing the performance of light duty passenger vehicles, the method was used infrequently for commercial vehicles until the past decade. Specifically, most of the coastdown test experience within the commercial vehicle industry has focused around tractor-trailer combinations related to the introduction of greenhouse gas (GHG) regulations in the U.S. in 2011 [EPA 2011-20740] and 2016 [EPA 2016-21203] for Class 7 or 8 semitrailer trucks with aerodynamic tractors and van type trailers weighing more than 30000 pounds. While this procedure is written with that perspective, the coastdown methodology may apply equally to other vehicle types (such as buses or straight trucks) when the underlying assumptions defined herein regarding the physics of coastdown hold. The equation of motion and the analysis methods presented within this procedure (detailed in Appendix A) are based on several assumptions that are summarized in 11.1. These assumptions are based on experimental data obtained on heavy combination vehicles from the 2010 to 2019 time period.

The requirements and recommendations of this procedure are based on best practices developed through experience with combination-vehicle coastdown testing, and are not necessarily supported by peer-reviewed publications.

Limitations

The limitations of this procedure are defined below. It is important that all test limitations described below in items "a" through "d" be reviewed and rigorously followed. These limitations are applicable to on-track and on-road testing.

a. Only road-load or drag-area data obtained from this procedure may be represented as an SAE J2978 test result. Test results generated from any test that does not rigorously follow this method and the test reporting requirements are not intended for public use and dissemination and shall not be represented as an SAE J2978 test result.

b. All test results must be presented: in the defined format, with the defined statistical analysis, with vehicle configuration description and with environmental test information described in this procedure.

c. Test results are valid only for the vehicle configuration and test conditions under which the test was conducted. Constraints on the state of the test vehicles and on environmental factors are defined so as to minimize differences in results from one test to another of the same vehicle configuration and test conditions.

d. The test procedure does not measure aerodynamic drag or drag area explicitly. Aerodynamic drag-area values obtained from this method are estimated based upon road-load measurements, atmospheric measurements, and numerous assumptions; and therefore shall only be defined as "SAE J2978-derived aerodynamic drag area."

Document History

J2978
June 1, 2021
Truck and Bus Coastdown Procedure
This document describes a rigorous engineering test procedure that utilizes industry-accepted data collection and statistical analysis methods to determine the road load and to estimate the...

References

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