Measuring and Reporting Fuel Economy of Plug-In Hybrid Electric Vehicles ^†

Plug-in hybrid-electric vehicles (PHEVs) have emerged as a promising alternative to reduce fleet petroleum consumption. However, quantifying PHEVs’ expected benefit is more challenging than with other vehicle technologies because they receive energy from two distinct sources and exhibit widely varying per-mile consumption based on the drive cycle and distance driven. This paper reviews various PHEV fuel economy characterization techniques, including the procedure formalized in the SAE J1711 Recommended Practice (issued in 1999). SAE J1711 accurately captures several critical reporting practices, including: using standardized drive cycles; considering charge depleting and charge sustaining operation; and using driving-statistic-derived utility-factor weighting to properly combine the vehicle’s operating modes. The authors’ proposed modifications to J1711 include: separately reporting fuel and electricity use; specifically measuring the vehicle’s charge depleting performance; and applying a once-daily charging assumption. As the U.S. Environmental Protection Agency (EPA) begins implementing changes to window-sticker fuel economy test procedures, and the original issuance of SAE J1711 expires, the authors hope to stimulate discussion and contribute to adoption of consensus reporting metrics. In order for the resulting metrics to be useful, stakeholders must be able to translate the numbers into sound predictions of relative vehicle energy cost, petroleum use, and potential carbon dioxide (CO₂) production.