This study undertakes an investigation of the effect of battery charge balance in hybrid electric vehicles (HEVs) on EPA fuel economy label values. EPA's updated method was fully implemented in 2011 and uses equations which weight the contributions of fuel consumption results from multiple dynamometer tests to synthesize city and highway estimates that reflect average U.S. driving patterns. For the US06 and UDDS cycles, the test results used in the computation come from individual phases within the overall certification driving cycles. This methodology causes additional complexities for hybrid vehicles, because although they are required to be charge-balanced over the course of a full drive cycle, they may have net charge or discharge within the individual phases. As a result, the fuel consumption value used in the label value calculation can be skewed.
To determine the effect of net battery energy change on fuel consumption using an example vehicle, testing was carried out on a 2010 Toyota Prius at Argonne National Laboratory's vehicle dynamometer test facility. Each phase was driven with various levels of electric charge or discharge, thereby yielding a charge correction line quantifying the tradeoff between electric and fuel consumption for that driving phase. The slopes of these lines will be compared along with discussion of the factors influencing them. The testing effort to produce this information was considerable, with at least 4 test points per driving phase, and the importance of preconditioning and vehicle preparation in ensuring valid results further extended the time investment. The collected data are used to show the effect of phase charge correction on fuel economy label values, and potential methods of implementing the corrections as well as the shortcomings thereof are discussed.
