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

Plug-in hybrid-electric vehicles (PHEVs) have recently emerged as a promising alternative technology to dramatically reduce fleet petroleum consumption. However, the fuel economy of many recent prototype and theoretical vehicles has varied widely and often been exaggerated in the press. PHEVs present a significant challenge as compared with conventional vehicle fuel economy reporting 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 techniques used to characterize PHEV fuel economy and discusses the relative merits, limitations, and best uses of each. This review will include a discussion of the SAE J1711 Recommended Practice issued in 1999 and will comment on how recent analysis indicates that the described procedures could be improved for reporting PHEV fuel economy. The paper highlights several critical reporting practices accurately captured by SAE J1711: use of standardized drive cycles; inclusion of charge depleting and charge sustaining operation; and using utility-factor weighting to properly combine the vehicle’s operating modes using representative driving statistics. Several recommended improvements to J1711 are also discussed: separate reporting of fuel and electricity use; better determination of the vehicle’s charge depleting performance; and application of a once-per-day vehicle charging assumption. As the U.S. Environmental Protection Agency (EPA) considers changes to window-sticker fuel economy test procedures, and the original issuance of SAE J1711 expires, the authors hope to stimulate the necessary 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.