This Society of Automotive Engineers (SAE) Recommended Practice establishes uniform chassis dynamometer test procedures for hybrid-electric vehicles (HEVs) that are designed to be driven on public roads. The procedure provides instructions for measuring and calculating the exhaust emissions and fuel economy of HEVs driven on the Urban Dynamometer Driving Schedule (UDDS) and the Highway Fuel Economy Driving Schedule (HFEDS), as well as the exhaust emissions of HEVs driven on the US06 Driving Schedule (US06) and the SC03 Driving Schedule (SC03). However, 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.

Furthermore, this document 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. The emissions calculations for plug-in hybrid-electric vehicle (PHEV) operation are provided as inventory results, weighted in the same manner as fuel and electrical energy consumption. Decisions for on-board versus off-board emissions, relative benefits of emissions-free driving, and how best to weight a "cold-start" cycle in charge-depleting (CD) mode must first be made before a certification methodology can be determined. Thus, calculations or test methodology intended to certify a PHEV for compliance of emissions standards is beyond the scope of this document.

For purposes of this test procedure, an HEV is defined as a road vehicle that can draw propulsion energy from both of the following sources of stored energy: (1) a consumable fuel and (2) a rechargeable energy storage system (RESS) that is recharged by the on-board hybrid propulsion system, an external electric energy source, or both. Consumable fuels that are covered by this document 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). The RESSs that are covered by this document include batteries, capacitors, and electromechanical flywheels. Procedures are included to test CD operating modes of HEVs designed to be routinely charged off-board, and calculations are provided that combine the CD and charge-sustaining (CS) behavior according to in-use driving statistics.

The HEVs shall have an RESS with a nominal energy greater than 2% of the fuel consumption energy of a particular test cycle to qualify to be tested with the procedures contained in this document.

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

This document does not include test procedures for recharge-dependent (RD) operating modes or vehicles (see 3.1.2 for the definition).

This document does not address the methods or equations necessary to calculate the adjusted U.S. Environmental Protection Agency (EPA) label miles per gallon (MPG) (sometimes referred to "EPA 5-Cycle" calculations).

Requirements Used to Develop the Recommended Practice

The overall goal in developing this document was to allow the testing of any HEV on a fair and comparable basis with conventional vehicles, electric vehicles (EVs), and other HEVs. Meeting this goal required satisfying the following requirements:

a. This document shall provide a recommended practice to measure the exhaust emissions and fuel economy of any type of HEV design or control strategy, as defined in Section 1.

b. Determination of representative exhaust emissions and fuel economy shall account for the driver's usage of external charging and estimations of driving distance between charging and the usage of driver-selected operating modes, if applicable.

c. The EVs and conventional vehicles tested according to this document shall yield the same results as if tested on the test procedures currently established for such vehicles.

d. Measurement methods and driving schedules shall be consistent with those used in existing test procedures for EVs and conventional vehicles.

e. Testing should not require defeating or otherwise forcing a vehicle's control system to perform differently from how it would perform in the driver's hands.

f. This document shall provide a technical foundation to assist government regulatory agencies in developing emissions and fuel economy certification and compliance tests for HEVs.

g. This document shall be as short and simple as possible.

Overview of the Recommended Practice

This document consists of three basic steps: (1) classifying the HEV, (2) testing the vehicle for each test cycle, and (3) weighting the results.

Classifying the HEV Classify the vehicle by obtaining the following information from the manufacturer:

a. RESS—Identify the RESS (i.e., battery, capacitor, or electromechanical flywheel).

b. External Charge Capability—Determine whether the RESS is designed to be recharged from an external (off-board) electric energy source. All HEVs are tested according to the charge-sustaining tests, only externally charged HEVs (PHEVs) require charge-depleting tests. If the HEV does have external charging capability, but this capability is intended solely for infrequent RESS conditioning and is not recommended for routine use, then the HEV shall not be classified as a PHEV.

Testing to Each Cycle Procedure

Five separate procedures are provided in this document (Section 4). PHEVs require both depleting mode tests and sustaining mode tests. Charge-sustaining HEVs only require charge-sustaining tests.

Weighting the Results

For PHEVs, the charge-depleting results can be merged with the charge-sustaining results according to the estimations of distance between charge and daily driving distances (see Section 6).

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