SAE Publishes Revised J1711 Standard to Measure Exhaust Emissions and Fuel Economy of Hybrids and Plug-in Hybrids

A revised J1711 standard from SAE International—J1711 - Recommended Practice for Measuring the Exhaust Emissions and Fuel Economy of Hybrid Electric Vehicles, including Plug-in Hybrid Vehicles (J1711 JUN2010)—provides the procedures to accurately measure exhaust emissions and fuel economy of hybrid (HEV) and plug-in hybrid electric vehicles (PHEV). (Earlier post.) The new version supersedes the J1711 standard originally published in March 1999.

Plug-in hybrid technology has experienced rapid development, the J1711 Task Force noted, and as such, the procedures to address this technology needed to be revisited and modified to accommodate the operational possibilities demonstrated by the diverse set of working prototypes and simulated vehicles in the literature. Also, the list of standard test procedures addressed in SAE J1711 has been expanded to cover all five major test cycle procedures (UDDS, HFEDS, US06, SC03, and Cold FTP) now being used to evaluate vehicle fuel economy.

The procedures are structured so that other driving schedules may be substituted, provided that the corresponding preparatory procedures, test lengths, and weighting factors are modified accordingly.

The standard does not specify which emissions constituents to measure (e.g., HC, CO, NOx, CO2); instead, that decision will depend on the objectives of the tester. J1711 defines an HEV for the purposed of the procedure as a road vehicle that can draw propulsion energy from both of the following sources of stored energy:

  1. A consumable fuel. Consumable fuels that are covered by J1711 are limited to petroleum-based liquid fuels (e.g., gasoline and Diesel fuel), alcohol-based liquid fuels (e.g., methanol and ethanol), and hydrocarbon-based gaseous fuels (e.g., compressed natural gas).

  2. A rechargeable energy storage system (RESS) that is recharged by the on-board hybrid propulsion system, an external electric energy source, or both. The RESSs covered by J1711 include batteries, capacitors, and electromechanical flywheels. The HEVs shall have an RESS with a nominal energy >2% of the fuel consumption energy of a particular test cycle to qualify to be tested with the procedures.

There are two basic exhaust emissions and fuel economy tests described in J1711. One test begins with operation assumed to be CS for HEV operating modes: the Charge-Sustaining Test (CST). The other test begins with a fully charged RESS for capturing CD operating modes for PHEVs: the Full Charge Test (FCT). The CST is administered to all HEVs, including PHEVs; the FCT is administered to only PHEVs.

Calculations are provided that combine the CD and charge-sustaining (CS) behavior according to in-use driving statistics. The weighting placed on FCT data equals the probability that a vehicle (based on national, in-use driving statistics) will be driven that distance segment in a single day. This probability is called the Utility Factor because it indicates the limited utility of a particular operating mode.

An operating mode with a very long range, for example, will have a very high utility and, thus, a Utility Factor that approaches 1.0. J1711’s preferred method of applying the Utility Factor is by using cycle length segments of the Utility Factor and applying that to each cycle in the FCT. The CST data is then weighted to this according to the remainder of the CS utility.

Single-roll, electric dynamometer test procedures are specified to minimize the test-to-test variations inherent in track testing and to conform with standard industry practice for exhaust emissions and fuel economy measurements.

The revised J1711 does not include test procedures for recharge-dependent (RD) operating modes or vehicles. J1711 defines the RD operating mode in a PHEV as an operating mode in which either or both of the following conditions occur while no other mode is selected:

  1. Vehicle propulsion is eventually no longer possible if the RESS is never recharged from an external electric energy source, even though the supply of consumable fuel is continually replenished, and

  2. The driver is eventually warned or instructed by the vehicle to discontinue driving in this operating mode because the RESS contains too low of a supply of energy.

The EV operating modes (with no automatic engagement of the engine or hybrid power unit) in PHEVs are always considered to be RD, according to the standard. On the same HEV, it is possible for one HEV operating mode to be classified as RD and another HEV operating mode not to be classified as RD.

Nor does J1711 address the methods or equations necessary to calculate the adjusted US Environmental Protection Agency (EPA) label miles per gallon (MPG) (sometimes referred to “EPA 5-Cycle” calculations).

Michael Duoba, an automotive engineer with the US Department of Energy’s Argonne National Laboratory and also chair of the SAE International Hybrid J1711 Task Force, which created the revised standard, said the new procedure helps to provide consistent information to the consumer.

Until now, the fuel economy claims for plug-in hybrids were not calculated according to similar procedures, making car-to-car comparisons virtually impossible. What makes this procedure—and other SAE-developed recommended practices—significant is that EPA typically considers them as the basis for the automotive regulations it promulgates. Ultimately, the consumer benefits with valuable vehicle information that can help guide a purchase decision.

—Mike Duoba

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